DCS: AJS-37 Viggen Discussion

[amb]

The reverser doors start closing when the main gear is compressed, but will abort if the nose gear is not compressed 2 seconds later. So that would account for your missing ~2 seconds. At least that's what I have understood from previous discussion. :-)

 

That would explain it! I had understood it as that it didn't start to close before the front gear touched down. Thanks! :)

[Agremont]

Just a little thing I'm wondering. Does the Viggen have any air brakes? Or is there just flaps?

[mattebubben]

Just a little thing I'm wondering. Does the Viggen have any air brakes? Or is there just flaps?

 

Yes the AJS 37 Has Air Brakes.

 

All Viggen variants had Air Breaks to start with but the JA 37 (fighter variant) had its Air Brakes deactivated during a upgrade program

(they needed the button in the cockpit for something else and the Air Breaks had little effect at lower speeds so JA 37 pilots could life without them)

 

But all other Viggen variants Kept the Air Breaks Throughout their Service time.

Edited March 23, 2016 by mattebubben

[shaggy]

Just a little thing I'm wondering. Does the Viggen have any air brakes? Or is there just flaps?

you can clearly se the airbrake on this model just below the start of the tail fin I believe

 

http://s219.photobucket.com/user/Aigore/media/AJ%2037%20Viggen/Finished/DSC_4449_zpsc6ac66db.jpg.html

[Agremont]

Yes the AJS 37 Has Air Brakes.

 

All Viggen variants had Air Breaks to start with but the JA 37 (fighter variant) had its Air Brakes deactivated during a upgrade program

(they needed the button in the cockpit for something else and the Air Breaks had little effect at lower speeds so JA 37 pilots could life without them)

 

But all other Viggen variants Kept the Air Breaks Throughout their Service time.

 

Thanks for the answer.

 

you can clearly se the airbrake on this model just below the start of the tail fin I believe

 

http://s219.photobucket.com/user/Aigore/media/AJ%2037%20Viggen/Finished/DSC_4449_zpsc6ac66db.jpg.html

 

Ah yes. Kinda embarrassing as I'm currently building one. :D

 

Found this picture of a Viggen with its vertical stabilizer folded. I thought it was cool so I'll post it here.

 

[mattebubben]

Thanks for the answer.

 

 

 

Ah yes. Kinda embarrassing as I'm currently building one. :D

 

Found this picture of a Viggen with its vertical stabilizer folded. I thought it was cool so I'll post it here.

 

 

You can also see the Air brake in this picture (The Belly Air Brakes)

 

you only see a part of it but if you look under the wing.

 

The Air Brakes are pretty small for the size of the aircraft but there are two sets of them.

 

One pair on the sides of the engine (that can be seen in the Model kit Shaggy linked ) and one pair under the aircraft close to the tail. (under the engine)

 

 

They are folded in this picture but they are very visible anyway due to the different color.

Edited March 24, 2016 by mattebubben

[El Hadji]

Better shots of the Viggen airbrakes:

 

 

[renhanxue]

I got around to translating a bit more of the seminar on the development of the JA 37's avionics. The previous parts are here and here.

 

 

Göran Tode: This thing with fighters and the JA 37 in general, it sounds great. But no combat aircraft is good if it doesn't have armament. Haven't you been itching to work on the air-to-air munitions? Picking from the shelf is one thing, but did you ever get to influence radar and IR missile development so you could fit them into the system? Or did you just get to choose from what was available on the market?

 

 

Gunnar Linqvist: Well, really, you should ask yourself that, as a staff officer. But I think that there might be certain things that maybe could've been done differently when it cames to weapons. We had the best fighter gun in the world on the aircraft, in fact significantly better than the one that's on the JAS 39. The gun had a kind of renaissance, as I understand it. And we had the best radar missile that could be found in the 1970's (the Skyflash), and later we added an even better radar missile (he's talking about the addition of the capability to carry AMRAAM that the JA 37D got in the late 90's), so that was good too. But when it came to heat seekers, maybe we should've chosen another option. But that's where the economy comes in, it's a dilemma. Should you buy something decent off the shelf for a reasonable price, or start your own development?

 

We had the choice between the Sidewinder 9L from the US or developing a Swedish missile with the development designation Robot 72-12 (in military Swedish, a missile is for historical reasons called a "robot" - which in ordinary Swedish means the same thing as it does in English). The difference was that the latter would have become a good missile if we had spent a ton of money on it. But we thought the development would take too long, so we bet on the Sidewinder instead. The thing was that we had to wrangle with the Americans for nine years before we were finally allowed to buy it, and in the meantime it became obsolete. That's a representative example of a bad decision, I admit that. But other than that, I don't think you can complain about the armament of the JA 37.

 

 

Moderator: All right. We had a few things left to talk about regarding man/machine interfaces. Lennart, if you would?

 

 

Lennart Alfredsson: We had another bit of functionality in the 37 that was unique in the world for many, many years. And that was the data recording we did in the aircraft. We could record everything, every single variable and every system that was in the aircraft, from the start. And then you could play this back at the base and you could go through the mission and see exactly where you had flown, exactly what the pilot had done, which buttons had been pressed and everything that went on in the aircraft. This was only possible because we had chosen to use a single central computer. SRA tried to sell this idea on export, but we couldn't find a single aircraft that we could even fit the equipment into. Because they all had one weapons management system, one navigation system, one aiming system and the engine of course did its own thing. And all of them, every single one was different. You couldn't fit them all together at a single point without remaking the entire aircraft from scratch.

 

But we did this, and we had it from the start. The recorder was probably the only device at the active service units that wasn't painted gray, I seem to recall it being blue or possibly orange. Those of you who flew with it, I hope it was useful to you out in the field because I never got any feedback. But it was pretty fun to design it. The reason it was coupled to the user interface equipment was that of the roughly 900 signals that we recorded, everything the pilot did and all variables that changed, around 700 were used in the electronic displays. The majority of everything could be shown to the pilot in some form.

 

I did a small survey of how many such signals we've used in different aircraft. In the 35F there were seven symbols that could move or change when you were looking at the air-to-air radar display. It's possible that others might have a different opinion, but that's my count, I got seven. In the AJ 37, there were exactly 23 symbols that could change. And in the JA 37 there were 700! That was a huge expansion in pilot supporting functionality, he could do a lot more than in earlier aircraft. And that was all because the system became digital, it was easy to do it because you had access to everything in all the systems. And it continued in the same vein. When we got to the JAS we thought that it might be a good idea to go big from the start - every one of the four displays got a thousand variables. But it wasn't long before Saab came complaining that that wasn't enough for the tactical display and we had to increase it to 2000. The only thing that's still at a thousand is the HUD, and that's just because it's hard to display too much on it, it's the high brightness that sets the limits. Roughly speaking you can only draw and see about 10 diameters of lines (unclear what this means even in Swedish).

 

 

Göran Tode: There was a saying in the air force, that before the UTB, before the UTA (UTB and UTA were the recording systems for the Viggen and for the Gripen respectively), more air combat exercises had been won in the break room than anywhere else. Whoever talked his performance up the best won.

 

 

Moderator: I want to ask you, Lennart. Wasn't this a result of having this central computer with all of this information readily available? Wasn't that an imporant prerequisite?

 

 

Lennart Alfredsson: Yes. We talked to Englishmen, Germans and Americans about this, and none of them could fit this functionality into their aircraft.

 

 

Moderator: Gösta Elg, you wanted to get a word in?

 

 

Gösta Elg: I was going to comment on what Lennart talked about. Because this is a matter of having an integrated system. There are two other important examples of this, and the first one is data recording in general, not just flight data for training purposes but also recording test results and functionality monitoring, which were both available in central units and helped a lot with troubleshooting. I think that was a big strength. The yankees I talked to at various times had never heard of anything like that either. And they simply couldn't do it, as Lennart points out. So we were ahead there.

 

The other strength we had was that we all knew each other. I graduated from the Royal Institute of Technology in Stockholm in 1961. Lennart was my classmate. I had to find some nice place to work where I could do both computer technology and control engineering at the same time, because I thought that was fun. And then Lennart said "I think I know a place, we could at least go take a look, right?". And that was the radar bureau at what was then the Air Force Administration. They were very grateful that anyone at all was interested in coming to visit. You walked up to Westergård's office and talked to him for a bit and then you were hired at the Air Force Administration. Just like that. You get to talk to people in an entirely different way when you work in integrated systems and I think that's a practical experience that we should pass on to engineers and tech people. That's it. Thank you.

 

 

Moderator: Kim, you wanted to comment?

 

 

Kim Bengtsson: Just a few words on the UTB/UTA. It was a very powerful tool when you started flight testing new software editions to certify them and get them out into service. SYSIM (the system simulator) was for program testing and verification, for approval, it was the absolutely necessary device or piece of equipment, which was complemented well by this data recording equipment.

 

 

Moderator: OK, who else? Leif?

 

 

Leif Åström: Yeah, I can only agree. I'd also like to get back to where Göran was just now - like he said, before the UTB and the UTA, many air duels were won in the break room. The guy who argued the best, talked the loudest and showed the best with his hands what he had done, he won the air combat exercise that had just been fought in reality. But with these recording systems, you could see exactly how it had gone down. And that led to a really commendable improvement in discipline, I'd say. On top of that it was huge for our tactics development. We could actually see exactly what worked and what didn't, and we could reward good behavior and get rid of bad behavior. It should not be underestimated. At the same time it's a bit surprising that we were alone in doing it. Among the pilots, there was a lot of interest for it. After each flight, you went to the UTB as a matter of course, to check out how you had done. It was very exciting to see. But we noticed that many other air forces, they were a bit hesitant in the face of this. They seemed to look at it as the scary big brother watching you. So maybe it's a different attitude here that caused it. But that doesn't diminish its importance, it was immensely valuable.

 

 

Lennart Alfredsson: Just want to clarify a bit here. What you do is that you record the nav system's data in the UTB/UTA for each aircraft, so you now how each aircraft has moved in all three dimensions. Then you can present this information for several aircraft at the same time, so you can show their tracks in relation to each other. The alternative approach is to have a lot of big honking radar stations on the ground that track the aircraft positions from there and then add all of that data together. If you're coming from that background you can see that what we did wasn't so bad at all. It was a lot cheaper.

 

 

(...)

 

 

(Translation note: the seminar starts to get increasingly technical at this point. If you have trouble following, I recommend brushing up on your radar understanding. The Wikipedia page on pulse repetition frequency might be of particular interest, and radartutorial.eu might also be quite helpful.)

 

 

Moderator: I would like to segue over to a few other areas. We still have the radar to talk about, and development methods and software. I think I'd like to move on to the radar, if there isn't anyone who has anything important to say about these things we discussed earlier? In that case I'd like to pass the word to Ingvar for a short introduction. The radar was very special because it incorporated a computer, which was very important if I understand it correctly?

 

 

Ingvar Sundström: (b. 1937, electronic engineer, responsible for developing the digital signal processing in the PS-46 radar for the JA 37, later head of the development team for the PS-05/A radar for the Gripen) I was going to talk for a little bit about how things were around 1968 and a few years into the 1970's. As I experienced it. I worked at an engineering division at LM Ericsson in Mölndal and I was responsible for servo technology and some signal processing. In production at the time was the last bit of deliveries for the Draken series production. There was both the IRST, which we built on license in Mölndal, and the radar, which we had developed ourselves. And we had just started delivery of the series production of the AJ 37's radar in the early 70's. Both the Draken and the AJ 37 had entirely analog radar systems, pulse radar systems, you only used the information contained in the amplitude of the pulse. And the communication between the radar and the CK 37 (the AJ 37's central computer) was entirely analog too.

 

There was computer activity close to me, we were working on a computer for the tele-test truck which was used for the AJ 37 (the tele-test truck was a truck containing automated testing equipment for the AJ 37; you hooked it up to the aircraft and it would test all the onboard systems; there's a long article in Swedish about it). LM Ericsson prototyped it and delivered five sets of equipment to FMV for evaluation, and I think that was the first computer stuff we did in Mölndal.

 

Tele-test truck on the left.

 

Then it continued with process control, close to us was an in-house-designed computer called UAC 1601 which was used for that, and it later became UAC 1610. They tried to get into a lot of fields - train signal control, hospitals, banks, things like that. And when it came to the radar in the JA 37 there were intensive studies and experiments during most of the 1960's. I think they had gotten started for real around 1967. Is that right, Jörgen? Jörgen Nilsson here in the audience was head of the systems division back then. He was very open to new ideas that came up and the requirements for the JA 37's radar were set very high. I think Gunnar said at some point that if we can't make the detection range requirement we're not gonna have a project. Was that right?

 

 

Gunnar Lindqvist: Yes, the radar was a key piece of equipment in the aircraft, which made it the best in Europe for a while.

 

 

Ingvar Sundström: And it was obvious that we were going to use pulse-doppler technology in order to be able to fight at all altitudes. When you're looking down towards the ground with a radar you get a ton of ground returns with all kinds of amplitudes and frequencies. That makes it very hard to see flying targets. But if you use doppler, you can separate flying objects from the ground. You talk about something called waveform, how you modulate your outgoing microwave signal in order to get desirable properties. And there were studies conducted over several years on what this waveform should look like. After a lot of work and talking to American experts we figured out a solution that should be possible, the details of which I don't have time to go into now.

 

The only possible way to do signals processing in a radar like this was to do it digitally. You handle the complex numbers which you get when you start using both the phase and the amplitude in the microwave signal. To begin with you need an extremely "pure" microwave signal for it to be possible at all, and when receiving the signals you must use both the real and the imaginary part - or phase and amplitude, if you want - when processing the signals. And you need to get this into the digital system and then you need an A/D converter and those things were not readily available back then. You either had to build them yourself or pay for someone to develop them for you. It was a pretty big thing. And this complex signals processing, I think we who were working on radar were among the first to do anywhere. These days it's completely ordinary technology that's everywhere in communications equipment: digital television, satellite phones, cell phones and so on. Everything relies on using complex signals processing, but back then it was completely new.

 

Digital signals processing became possible because of the development of "medium scale integration", like the shift registers Lennart was talking about. They became useful for DSP as well. We didn't need quite as many bits in the radar, but those shift registers were among the first components that became interesting to us. And of course the radar was a tracking one, where you had to filter out good target data. Earlier, this had been accomplished by hooking up gyros to the antenna. There were three such gyros in the Draken system and they built servo systems that kept the antenna pointing in the target direction regardless of how you flew and which way the aircraft was pointing. These were pretty "jumpy" systems and it wasn't all that pleasant to mount sensitive gyros to a moving antenna that banged into things pretty hard sometimes. But you wanted to go as fast as possible.

 

In the later half of the 1960's it suddenly became really hard to read magazines on automatic control engineering, you started seeing a lot of weird matrixes. In March of 1969 Karl Johan Åström (b. 1937, became a professor of automatic control engineering in 1965 and is the moderator of this seminar) held a course in modern control engineering that really opened our eyes in Mölndal. Suddenly you got a completely different understanding of the control theory when you could start using these state variables. For the first time, you heard the words "Kalman filtering". It was a stable theoretical model for how to build filters, it was as if it was made for the explicit purpose of filtering radar signals.

 

Suddenly you could use the uncertainity measurements that are a part of Kalman filtering and suddenly you understood what you were really doing. Earlier, we had worked a lot on our intuitition and by practical trials. But if you want a Kalman filter for a radar, you need a computer. You can't do it without one, and that's where the idea of putting a computer in the radar came from. Bengt Sjöberg was a bit hesitant at first. He did some simulations, got a bug in the program and the results were no good, but then he got on board with the idea too. I suggested to Jörgen that we should have a computer and he too agreed. When you started looking at what the radar really needed, the computer became obvious, really. In order to manage the signals processing you have to constantly keep track of how you're flying and you kinda have to calculate what the ground clutter will be like and compensate for that and so on and so on.

 

Another thing that was very nice was that we could use data from the inertial navigation system in the radar directly. We subscribed to updates on roll, pitch and course data and could calculate ourselves, inside the radar, where the antenna should be pointing. And make a Kalman filter. The coordinate transforms that were needed from radar data in polar coordnates were to a ground-fixed coordinate system where the target aircraft resided. We could do that ourselves and didn't need any gyros. We worked with very high data rates, the shortest cycles in the computer were just under three milliseconds long. The control of the DSP was slightly slower.

 

Of course, FMV knew the technical risks of a complicated radar and wanted an American company that could verify the system and cooperate with us on the development. That's how Hughes Aircraft Company entered the picture. In 1972, both parties had proposals ready for the radar architecture. It turned out when we visited them in the US in 1972 that we were both thinking the same things about computers and DSP, and it was pretty easy to cooperate and make a basic proposal for what this radar was going to look like together. HAC also worked with us for a few prototype versions for the first prototype generation of the DSP software. We built a computer with a core memory of eight kilo-words - 16-bit words - (16 kilobytes, in modern parlance) for the first prototype generation. It was an assembly machine, pretty similar to the minicomputers that existed at the time. Then Ingemar Carlsson came with an expert from the Chalmers Institute of Technology, Gunnar Carlstedt was his name, who took a look at our solution. He said it was way too clumsy, what we had done. Too much hardware, he said.

 

Microprogramming (referring to programming a CISC processor) of computers had become pretty hot at the time. And that was what he suggested, that we should use microprogramming and do less in hardware. And that's actually what ended up happening. When we entered series production there was FAMOS memory on the market. That is EPROMs where you could write a program electrically and then erase it with ultraviolet light. The first units were shipped with those and it lasted throughout the entire 1980's. The radar was delivered with a 32 kilo-word program memory and a data memory of 2 kilo-words. Then there were program cycles with lengths from 3 milliseconds and up to 72 milliseconds. It was a bit different depending on a number of factors. Internationally they called this a "software controlled radar", which was used in the marketing.

 

As a host computer, we bought a Honeywell H316 mini-computer early on. We used it for most of the 1970's as a host computer for the software development, until we got our VAX system in the late 1970's. One important part of the computer was that we had trigonometry lookup tables which meant we could do sine and cosine operations and such things extremely quickly. A lookup only took a few microseconds and I don't think I've seen any computer after that with such fast trigonometry. Later on the computer was replaced by a Texas Instruments signals processor, TMS320, with performance in a completely different league. That came in the 1990's. Suddenly radar functionality became easy, suddenly you had plenty of memory and processing capacity. Maybe there's questions?

 

 

(...)

 

 

Moderator: I'll pass the word on to Jörgen, who as we've heard was a benign boss.

 

 

Jörgen Nilsson: (b. 1926, electronic engineer, head of the PS-46/A development project at Ericsson) It was very nice to hear what Ingvar had to say. If you want to criticize or show you a different aspect of it. It wasn't all that easy as a project manager, to keep track of what everyone was doing. Sometimes they just came to you and told you what they had done. Sometimes it could be very surprising. Acke Axelsson was involved in air-cooling the transmitter, which meant finding an entirely new transmitter tube. I was completely against that. But our guy who worked on it, he simply said, well I talked to Acke, and it's gonna have to be air-cooled. Then we had several years of issues with trying to find tubes that could handle air cooling. Sometimes it was like that, you just had to roll with it. But it was very nice of Ingvar to call me a benign boss - it was a very interesting job for me.

 

To us down in Mölndal, it was a very big step, in a lot of ways. In the 1950's and 1960's we were very dependent, we more or less did what FMV told us to do. But then we came to stand on our own legs and I think that had a lot to do with the JA 37 project. We discovered that we were actually pretty good at this radar business and we started getting more customers, we could export and so on. And we could manage ourselves, we came up with new things to do on our own.

 

 

Ulf Frieberg: To me, getting to the MPD thing (MPD is short for "medium pulse doppler") was more or less a detective thriller. Because without MPD we would've had nothing. Can you tell that story?

 

 

Moderator: Well, Jörgen, can you?

 

 

Jörgen Nilsson: Yes. I was working with radar development during the 1960's. Pulse radar, pretty simple things you could say, looking back. The Air Force Administration contracted studies of pulse-doppler radar to a whole bunch of companies, to us, to SRA, Saab, AGA and Teleutredningar AB. We got our share of the money. The contract was split into parts, we got to do pulse-doppler and some others should work on continuous wave (CW) or intermittent CW, and yet others on other things. We got pulse-doppler and many regarded that as being pretty hopeless with a lot of stability issues. We didn't care all that much about those naysayers and built a prototype for our own money that we tested on the ground around 1965. And it turned out it actually did work pretty well.

 

That gave us the courage to work on aircraft-mounted radar. The big problem there was that the radar was mounted in an aircraft that was rushing along above the ground returns at a certain speed. There were vibrations and a horrible noise up there, we were in the very tip of the nose. And that affected the radar, the stability and everything and we were very worried about that. I asked one of my engineers to make a study of the movements between the radome and the antenna, if that was going to affect anything. He concluded that it didn't look good at all, that it probably wasn't going to work. But I realized that it had to work, so I took his report and put it at the bottom of my desk drawer. Then I don't know what happened to it, I think it laid around there for many years.

 

The big issue was the ground returns, you had to track the speed of them. The thing we heard was that we had to approach a resolution of 50 Hz with a 9 GHz carrier, which seemed pretty hopeless. But we tested it in a flying prototype and to our surprise it actually worked. We got a 1-sigma of about 50 Hz. And then we weren't so afraid anymore. Then the work started for real and it was very good that we got to meet with the Americans and figure out that they knew about the same things as we did. They had tested a bit more than we had though, and that was good. I don't know if I should go on?

 

 

Ingvar Sundström: Then there were a lot of discussions about whether we should use low or high PRF?

 

 

Jörgen Nilsson: Oh yeah, that was what we were going to talk about. Using low PRF was a very attractive option, since you could just take the radar we had had in the AJ 37 and with some small changes, adding some equipment, make it into a pulse-doppler radar. But then we figured out that that wouldn't work very well and that we should switch to what was a called a coherent transmitter system. But many argued in favor of keeping the low PRF idea like an ordinary pulse radar, but with a coherent transmitter that was nice and pure. But it turned out that such a radar would get us in trouble with moving targets on the ground, such as cars, trains and other vehicles. We did some studies of it, I think we even test flew something. Volkswagen Beetles resulted in big targets and we weren't interested in that.

 

Eventually it became apparent that in order to be able to use a low PRF on doppler radar you had to approach the target in a very specific way, because you could get into "blind spots" for the radar. And there was quite a bit of discussion in these project groups if you could mandate certain attack paths and so on. But when looking back on that today I'll have to say that was very unrealistic with regards to the pilot. The pilot wanted as little trouble as possible and a great deal of tactical freedom, of course. This led us onward to what is called MPD, which did give you that tactical freedom. You can approach the target from the front, from the rear and so on. There was only one downside, that the range was reduced somewhat, but that was acceptable. Then the range has increased a lot during the work on the radar, so it was a pretty fortunate choice. And the pilot did get his tactical freedom of choice, didn't he?

 

 

Leif Åström: Yes, definitely.

 

 

Moderator: Gunnar wanted something.

 

 

Gunnar Lindqvist: Well, Jörgen kinda talked about this. But there were high PRF radars back then, in the F-4 Phantom, I think it was a Westinghouse radar in there. And we chose, for the tactical freedom among other things, a MPD. But it might be a good idea to clarify two things further. When we went on to the JAS 39 we got a multi-mode radar, where you can mix both high and medium PRF. So the first question is, why couldn't we do that on the JA 37? And secondly I think you should talk a bit about the importance of the antenna studies you've done. Building a low-sidelobe antenna and a transmitter with a spectrally pure signal are the two fundamentals for a doppler radar.

 

 

Ingvar Sundström: When it comes to multi-mode radar, if you have a high PRF you need a shorter pulse length. There are various types of traveling wave tubes you can use. They're usually designed for a certain work factor, that is to say the relation between the pulse length and the pulse repetition frequency. In the JAS 39 they solved that with pulse compression in the transmitter and the receiver, pulse expansion when you transmit and pulse compression when you receive. And that kind of technology wasn't very well developed in the early 1970's, I think it would've been very hard to do that. Later transmitter tubes with two different work factors, "dual mode tubes", and that would also have solved the problem. But those didn't exist in the 1970's either. It was hard, really, you had to chose one or the other.

 

When it comes to the antenna, the side lobes are looking straight down into the ground in level flight. They're usually very powerful and you can't filter them away like you do with the main lobe clutter. Instead, you simply have to design an antenna that minimizes the side lobes, and there was a fantastic amount of work done on designing a Cassegrain antenna with small side lobes. Olle Dahlsjö was responsible for managing that. Additionally they did some things to the radome, they called it the "necktie". It was a grid of radar-absorbing stuff in the lower part of the radome that helped minimize the side lobes. In the JAS 39 they have a different type of antenna, a phased array antenna with a fixed grid of about a thousand radiating elements, which also gives very small side lobes.

 

 

Acke Axelsson: (b. 1922, so 85 years old at the time of the seminar, radar engineer at FMV) You've probably already mentioned what I was going to say, I'm a bit hard of hearing and can't really follow the discussion. But I wanted to say something about the choice of medium PRF. We chose to use a magnetron for the trials. You take the signal that you just transmitted, you take the phase of that and make a reference signal, and you use that in the receiver. Then you get all the echoes from the target within an unambiguous distance so you can give them the doppler treatment. But when it comes to the second, third, fourth time around, ambiguous echoes, you can't handle those. So we just skipped those immediately. Other than that, Hughes Aircraft did signals processing just like we did at the air force test center.

 

Then there was the question of medium or high PRF. The tactical methods of the time were either the gun, or that the air combat controllers guided you to a distance of about 10 or 15 kilometers from the target, and then you approached from there to attack with radar or IR missiles. High PRF wasn't suited to that, so medium PRF which could be used as low PRF if necessary was chosen. And that's the background for that choice. I don't know if it's been a downside really, I think it's worked out pretty well.

 

Then there was another factor, and that was the result of the Air Defense Investigation of 1967, LFU 67, which among other things studied ground-based missiles. In a search radar for a system like that all the clutter, all the side lobe clutter, it has a frequency of zero, so it was easy to get rid of it. On the other hand, if you mounted the radar in an aircraft, the clutter spread out over basically the entire doppler spectrum. Since it's impossible to filter out all the clutter you have to try to choose an antenna that prioritizes the direction the target is in. And it's really tricky to both get a good antenna and to do the signal processing right.

 

When we visited Hughes Aircraft in 1972 Ericsson presented their radar and Hughes presented their proposal for signals processing. They were trying to develop a PRF pattern, the standard was to choose three main PRF's, each of which had two partial PRF's, in order to sort through all the ground clutter. And then you ran that pattern in a few variations. But eventually it was concluded that it's really only one PRF that discovers the target, or maybe two at the most. Then you just waste a bunch of time transmitting the other PRF's! In the last stage of development for the JA 37 around the year 2000 they figured out that you ran with one PRF until you found the target and then added a sub-PRF to determine the range. That was mostly based on the fact that the target fluctuated a lot in amplitude, depending on its path through the air flow and whatnot. The theory was that if the target happened to be in a position which gave a very strong echo, you kept it for a few milliseconds, or maybe even about a dozen milliseconds. If you had one PRF that discovered the target you should quickly take the opportunity to choose a sub-PRF to determine the range. And that was one of the last software modifications that was done at Ericsson for system 37.

 

 

Ingvar Sundström: Yes. It wasn't long ago at all.

 

 

Acke Axelsson: No. And that, together with some other things, resulted in roughly doubling the detection range. Doubling the detection range, that's a lot, that's 12 decibels. But there were some other things involved too, a microwave amplifier and so on. Well, that's what I had to say.

 

 

Moderator: Thank you. Kim?

 

 

Kim Bengtsson: I was thinking about the radar and how it hooked up to SYSIM. In the beginning, we only had a model that provided radar output. But with the development of computers, we could later build a radar simulator, RADSIM, which generated simulated inputs to the radar's computer and gave us the possibility to run the real radar software in a realistic environment, with the actual flight-verified programs running against each other, which was pretty valuable. And the engineers could sit there and discuss what was going on with the pilot.

 

 

Ingvar Sundström: Yes, when they came out with TTL-series multiplicators it became possible to make a signal generator that could create the signal vectors we needed inside the radar. And then you could put the real devices, the real flying ones in the SYSIM, and test the real flying software together with the entire system.

 

 

Kim Bengtsson: That was in the mid-70's?

 

 

Ingvar Sundström: Yeah, mid or late 70's I think. And that was a big step forward.

 

[mattebubben]

I got around to translating a bit more of the seminar on the development of the JA 37's avionics. The previous parts are here and here.

 

 

Göran Tode: This thing with fighters and the JA 37 in general, it sounds great. But no combat aircraft is good if it doesn't have armament. Haven't you been itching to work on the air-to-air munitions? Picking from the shelf is one thing, but did you ever get to influence radar and IR missile development so you could fit them into the system? Or did you just get to choose from what was available on the market?

 

 

Gunnar Linqvist: Well, really, you should ask yourself that, as a staff officer. But I think that there might be certain things that maybe could've been done differently when it cames to weapons. We had the best fighter gun in the world on the aircraft, in fact significantly better than the one that's on the JAS 39. The gun had a kind of renaissance, as I understand it. And we had the best radar missile that could be found in the 1970's (the Skyflash), and later we added an even better radar missile (he's talking about the addition of the capability to carry AMRAAM that the JA 37D got in the late 90's), so that was good too. But when it came to heat seekers, maybe we should've chosen another option. But that's where the economy comes in, it's a dilemma. Should you buy something decent off the shelf for a reasonable price, or start your own development?

 

We had the choice between the Sidewinder 9L from the US or developing a Swedish missile with the development designation Robot 72-12 (in military Swedish, a missile is for historical reasons called a "robot" - which in ordinary Swedish means the same thing as it does in English). The difference was that the latter would have become a good missile if we had spent a ton of money on it. But we thought the development would take too long, so we bet on the Sidewinder instead. The thing was that we had to wrangle with the Americans for nine years before we were finally allowed to buy it, and in the meantime it became obsolete. That's a representative example of a bad decision, I admit that. But other than that, I don't think you can complain about the armament of the JA 37.

 

 

Moderator: All right. We had a few things left to talk about regarding man/machine interfaces. Lennart, if you would?

 

 

Lennart Alfredsson: We had another bit of functionality in the 37 that was unique in the world for many, many years. And that was the data recording we did in the aircraft. We could record everything, every single variable and every system that was in the aircraft, from the start. And then you could play this back at the base and you could go through the mission and see exactly where you had flown, exactly what the pilot had done, which buttons had been pressed and everything that went on in the aircraft. This was only possible because we had chosen to use a single central computer. SRA tried to sell this idea on export, but we couldn't find a single aircraft that we could even fit the equipment into. Because they all had one weapons management system, one navigation system, one aiming system and the engine of course did its own thing. And all of them, every single one was different. You couldn't fit them all together at a single point without remaking the entire aircraft from scratch.

 

But we did this, and we had it from the start. The recorder was probably the only device at the active service units that wasn't painted gray, I seem to recall it being blue or possibly orange. Those of you who flew with it, I hope it was useful to you out in the field because I never got any feedback. But it was pretty fun to design it. The reason it was coupled to the user interface equipment was that of the roughly 900 signals that we recorded, everything the pilot did and all variables that changed, around 700 were used in the electronic displays. The majority of everything could be shown to the pilot in some form.

 

I did a small survey of how many such signals we've used in different aircraft. In the 35F there were seven symbols that could move or change when you were looking at the air-to-air radar display. It's possible that others might have a different opinion, but that's my count, I got seven. In the AJ 37, there were exactly 23 symbols that could change. And in the JA 37 there were 700! That was a huge expansion in pilot supporting functionality, he could do a lot more than in earlier aircraft. And that was all because the system became digital, it was easy to do it because you had access to everything in all the systems. And it continued in the same vein. When we got to the JAS we thought that it might be a good idea to go big from the start - every one of the four displays got a thousand variables. But it wasn't long before Saab came complaining that that wasn't enough for the tactical display and we had to increase it to 2000. The only thing that's still at a thousand is the HUD, and that's just because it's hard to display too much on it, it's the high brightness that sets the limits. Roughly speaking you can only draw and see about 10 diameters of lines (unclear what this means even in Swedish).

 

 

Göran Tode: There was a saying in the air force, that before the UTB, before the UTA (UTB and UTA were the recording systems for the Viggen and for the Gripen respectively), more air combat exercises had been won in the break room than anywhere else. Whoever talked his performance up the best won.

 

 

Moderator: I want to ask you, Lennart. Wasn't this a result of having this central computer with all of this information readily available? Wasn't that an imporant prerequisite?

 

 

Lennart Alfredsson: Yes. We talked to Englishmen, Germans and Americans about this, and none of them could fit this functionality into their aircraft.

 

 

Moderator: Gösta Elg, you wanted to get a word in?

 

 

Gösta Elg: I was going to comment on what Lennart talked about. Because this is a matter of having an integrated system. There are two other important examples of this, and the first one is data recording in general, not just flight data for training purposes but also recording test results and functionality monitoring, which were both available in central units and helped a lot with troubleshooting. I think that was a big strength. The yankees I talked to at various times had never heard of anything like that either. And they simply couldn't do it, as Lennart points out. So we were ahead there.

 

The other strength we had was that we all knew each other. I graduated from the Royal Institute of Technology in Stockholm in 1961. Lennart was my classmate. I had to find some nice place to work where I could do both computer technology and control engineering at the same time, because I thought that was fun. And then Lennart said "I think I know a place, we could at least go take a look, right?". And that was the radar bureau at what was then the Air Force Administration. They were very grateful that anyone at all was interested in coming to visit. You walked up to Westergård's office and talked to him for a bit and then you were hired at the Air Force Administration. Just like that. You get to talk to people in an entirely different way when you work in integrated systems and I think that's a practical experience that we should pass on to engineers and tech people. That's it. Thank you.

 

 

Moderator: Kim, you wanted to comment?

 

 

Kim Bengtsson: Just a few words on the UTB/UTA. It was a very powerful tool when you started flight testing new software editions to certify them and get them out into service. SYSIM (the system simulator) was for program testing and verification, for approval, it was the absolutely necessary device or piece of equipment, which was complemented well by this data recording equipment.

 

 

Moderator: OK, who else? Leif?

 

 

Leif Åström: Yeah, I can only agree. I'd also like to get back to where Göran was just now - like he said, before the UTB and the UTA, many air duels were won in the break room. The guy who argued the best, talked the loudest and showed the best with his hands what he had done, he won the air combat exercise that had just been fought in reality. But with these recording systems, you could see exactly how it had gone down. And that led to a really commendable improvement in discipline, I'd say. On top of that it was huge for our tactics development. We could actually see exactly what worked and what didn't, and we could reward good behavior and get rid of bad behavior. It should not be underestimated. At the same time it's a bit surprising that we were alone in doing it. Among the pilots, there was a lot of interest for it. After each flight, you went to the UTB as a matter of course, to check out how you had done. It was very exciting to see. But we noticed that many other air forces, they were a bit hesitant in the face of this. They seemed to look at it as the scary big brother watching you. So maybe it's a different attitude here that caused it. But that doesn't diminish its importance, it was immensely valuable.

 

 

Lennart Alfredsson: Just want to clarify a bit here. What you do is that you record the nav system's data in the UTB/UTA for each aircraft, so you now how each aircraft has moved in all three dimensions. Then you can present this information for several aircraft at the same time, so you can show their tracks in relation to each other. The alternative approach is to have a lot of big honking radar stations on the ground that track the aircraft positions from there and then add all of that data together. If you're coming from that background you can see that what we did wasn't so bad at all. It was a lot cheaper.

 

 

(...)

 

 

(Translation note: the seminar starts to get increasingly technical at this point. If you have trouble following, I recommend brushing up on your radar understanding. The Wikipedia page on pulse repetition frequency might be of particular interest, and radartutorial.eu might also be quite helpful.)

 

 

Moderator: I would like to segue over to a few other areas. We still have the radar to talk about, and development methods and software. I think I'd like to move on to the radar, if there isn't anyone who has anything important to say about these things we discussed earlier? In that case I'd like to pass the word to Ingvar for a short introduction. The radar was very special because it incorporated a computer, which was very important if I understand it correctly?

 

 

Ingvar Sundström: (b. 1937, electronic engineer, responsible for developing the digital signal processing in the PS-46 radar for the JA 37, later head of the development team for the PS-05/A radar for the Gripen) I was going to talk for a little bit about how things were around 1968 and a few years into the 1970's. As I experienced it. I worked at an engineering division at LM Ericsson in Mölndal and I was responsible for servo technology and some signal processing. In production at the time was the last bit of deliveries for the Draken series production. There was both the IRST, which we built on license in Mölndal, and the radar, which we had developed ourselves. And we had just started delivery of the series production of the AJ 37's radar in the early 70's. Both the Draken and the AJ 37 had entirely analog radar systems, pulse radar systems, you only used the information contained in the amplitude of the pulse. And the communication between the radar and the CK 37 (the AJ 37's central computer) was entirely analog too.

 

There was computer activity close to me, we were working on a computer for the tele-test truck which was used for the AJ 37 (the tele-test truck was a truck containing automated testing equipment for the AJ 37; you hooked it up to the aircraft and it would test all the onboard systems; there's a long article in Swedish about it). LM Ericsson prototyped it and delivered five sets of equipment to FMV for evaluation, and I think that was the first computer stuff we did in Mölndal.

 

Tele-test truck on the left.

 

Then it continued with process control, close to us was an in-house-designed computer called UAC 1601 which was used for that, and it later became UAC 1610. They tried to get into a lot of fields - train signal control, hospitals, banks, things like that. And when it came to the radar in the JA 37 there were intensive studies and experiments during most of the 1960's. I think they had gotten started for real around 1967. Is that right, Jörgen? Jörgen Nilsson here in the audience was head of the systems division back then. He was very open to new ideas that came up and the requirements for the JA 37's radar were set very high. I think Gunnar said at some point that if we can't make the detection range requirement we're not gonna have a project. Was that right?

 

 

Gunnar Lindqvist: Yes, the radar was a key piece of equipment in the aircraft, which made it the best in Europe for a while.

 

 

Ingvar Sundström: And it was obvious that we were going to use pulse-doppler technology in order to be able to fight at all altitudes. When you're looking down towards the ground with a radar you get a ton of ground returns with all kinds of amplitudes and frequencies. That makes it very hard to see flying targets. But if you use doppler, you can separate flying objects from the ground. You talk about something called waveform, how you modulate your outgoing microwave signal in order to get desirable properties. And there were studies conducted over several years on what this waveform should look like. After a lot of work and talking to American experts we figured out a solution that should be possible, the details of which I don't have time to go into now.

 

The only possible way to do signals processing in a radar like this was to do it digitally. You handle the complex numbers which you get when you start using both the phase and the amplitude in the microwave signal. To begin with you need an extremely "pure" microwave signal for it to be possible at all, and when receiving the signals you must use both the real and the imaginary part - or phase and amplitude, if you want - when processing the signals. And you need to get this into the digital system and then you need an A/D converter and those things were not readily available back then. You either had to build them yourself or pay for someone to develop them for you. It was a pretty big thing. And this complex signals processing, I think we who were working on radar were among the first to do anywhere. These days it's completely ordinary technology that's everywhere in communications equipment: digital television, satellite phones, cell phones and so on. Everything relies on using complex signals processing, but back then it was completely new.

 

Digital signals processing became possible because of the development of "medium scale integration", like the shift registers Lennart was talking about. They became useful for DSP as well. We didn't need quite as many bits in the radar, but those shift registers were among the first components that became interesting to us. And of course the radar was a tracking one, where you had to filter out good target data. Earlier, this had been accomplished by hooking up gyros to the antenna. There were three such gyros in the Draken system and they built servo systems that kept the antenna pointing in the target direction regardless of how you flew and which way the aircraft was pointing. These were pretty "jumpy" systems and it wasn't all that pleasant to mount sensitive gyros to a moving antenna that banged into things pretty hard sometimes. But you wanted to go as fast as possible.

 

In the later half of the 1960's it suddenly became really hard to read magazines on automatic control engineering, you started seeing a lot of weird matrixes. In March of 1969 Karl Johan Åström (b. 1937, became a professor of automatic control engineering in 1965 and is the moderator of this seminar) held a course in modern control engineering that really opened our eyes in Mölndal. Suddenly you got a completely different understanding of the control theory when you could start using these state variables. For the first time, you heard the words "Kalman filtering". It was a stable theoretical model for how to build filters, it was as if it was made for the explicit purpose of filtering radar signals.

 

Suddenly you could use the uncertainity measurements that are a part of Kalman filtering and suddenly you understood what you were really doing. Earlier, we had worked a lot on our intuitition and by practical trials. But if you want a Kalman filter for a radar, you need a computer. You can't do it without one, and that's where the idea of putting a computer in the radar came from. Bengt Sjöberg was a bit hesitant at first. He did some simulations, got a bug in the program and the results were no good, but then he got on board with the idea too. I suggested to Jörgen that we should have a computer and he too agreed. When you started looking at what the radar really needed, the computer became obvious, really. In order to manage the signals processing you have to constantly keep track of how you're flying and you kinda have to calculate what the ground clutter will be like and compensate for that and so on and so on.

 

Another thing that was very nice was that we could use data from the inertial navigation system in the radar directly. We subscribed to updates on roll, pitch and course data and could calculate ourselves, inside the radar, where the antenna should be pointing. And make a Kalman filter. The coordinate transforms that were needed from radar data in polar coordnates were to a ground-fixed coordinate system where the target aircraft resided. We could do that ourselves and didn't need any gyros. We worked with very high data rates, the shortest cycles in the computer were just under three milliseconds long. The control of the DSP was slightly slower.

 

Of course, FMV knew the technical risks of a complicated radar and wanted an American company that could verify the system and cooperate with us on the development. That's how Hughes Aircraft Company entered the picture. In 1972, both parties had proposals ready for the radar architecture. It turned out when we visited them in the US in 1972 that we were both thinking the same things about computers and DSP, and it was pretty easy to cooperate and make a basic proposal for what this radar was going to look like together. HAC also worked with us for a few prototype versions for the first prototype generation of the DSP software. We built a computer with a core memory of eight kilo-words - 16-bit words - (16 kilobytes, in modern parlance) for the first prototype generation. It was an assembly machine, pretty similar to the minicomputers that existed at the time. Then Ingemar Carlsson came with an expert from the Chalmers Institute of Technology, Gunnar Carlstedt was his name, who took a look at our solution. He said it was way too clumsy, what we had done. Too much hardware, he said.

 

Microprogramming (referring to programming a CISC processor) of computers had become pretty hot at the time. And that was what he suggested, that we should use microprogramming and do less in hardware. And that's actually what ended up happening. When we entered series production there was FAMOS memory on the market. That is EPROMs where you could write a program electrically and then erase it with ultraviolet light. The first units were shipped with those and it lasted throughout the entire 1980's. The radar was delivered with a 32 kilo-word program memory and a data memory of 2 kilo-words. Then there were program cycles with lengths from 3 milliseconds and up to 72 milliseconds. It was a bit different depending on a number of factors. Internationally they called this a "software controlled radar", which was used in the marketing.

 

As a host computer, we bought a Honeywell H316 mini-computer early on. We used it for most of the 1970's as a host computer for the software development, until we got our VAX system in the late 1970's. One important part of the computer was that we had trigonometry lookup tables which meant we could do sine and cosine operations and such things extremely quickly. A lookup only took a few microseconds and I don't think I've seen any computer after that with such fast trigonometry. Later on the computer was replaced by a Texas Instruments signals processor, TMS320, with performance in a completely different league. That came in the 1990's. Suddenly radar functionality became easy, suddenly you had plenty of memory and processing capacity. Maybe there's questions?

 

 

(...)

 

 

Moderator: I'll pass the word on to Jörgen, who as we've heard was a benign boss.

 

 

Jörgen Nilsson: (b. 1926, electronic engineer, head of the PS-46/A development project at Ericsson) It was very nice to hear what Ingvar had to say. If you want to criticize or show you a different aspect of it. It wasn't all that easy as a project manager, to keep track of what everyone was doing. Sometimes they just came to you and told you what they had done. Sometimes it could be very surprising. Acke Axelsson was involved in air-cooling the transmitter, which meant finding an entirely new transmitter tube. I was completely against that. But our guy who worked on it, he simply said, well I talked to Acke, and it's gonna have to be air-cooled. Then we had several years of issues with trying to find tubes that could handle air cooling. Sometimes it was like that, you just had to roll with it. But it was very nice of Ingvar to call me a benign boss - it was a very interesting job for me.

 

To us down in Mölndal, it was a very big step, in a lot of ways. In the 1950's and 1960's we were very dependent, we more or less did what FMV told us to do. But then we came to stand on our own legs and I think that had a lot to do with the JA 37 project. We discovered that we were actually pretty good at this radar business and we started getting more customers, we could export and so on. And we could manage ourselves, we came up with new things to do on our own.

 

 

Ulf Frieberg: To me, getting to the MPD thing (MPD is short for "medium pulse doppler") was more or less a detective thriller. Because without MPD we would've had nothing. Can you tell that story?

 

 

Moderator: Well, Jörgen, can you?

 

 

Jörgen Nilsson: Yes. I was working with radar development during the 1960's. Pulse radar, pretty simple things you could say, looking back. The Air Force Administration contracted studies of pulse-doppler radar to a whole bunch of companies, to us, to SRA, Saab, AGA and Teleutredningar AB. We got our share of the money. The contract was split into parts, we got to do pulse-doppler and some others should work on continuous wave (CW) or intermittent CW, and yet others on other things. We got pulse-doppler and many regarded that as being pretty hopeless with a lot of stability issues. We didn't care all that much about those naysayers and built a prototype for our own money that we tested on the ground around 1965. And it turned out it actually did work pretty well.

 

That gave us the courage to work on aircraft-mounted radar. The big problem there was that the radar was mounted in an aircraft that was rushing along above the ground returns at a certain speed. There were vibrations and a horrible noise up there, we were in the very tip of the nose. And that affected the radar, the stability and everything and we were very worried about that. I asked one of my engineers to make a study of the movements between the radome and the antenna, if that was going to affect anything. He concluded that it didn't look good at all, that it probably wasn't going to work. But I realized that it had to work, so I took his report and put it at the bottom of my desk drawer. Then I don't know what happened to it, I think it laid around there for many years.

 

The big issue was the ground returns, you had to track the speed of them. The thing we heard was that we had to approach a resolution of 50 Hz with a 9 GHz carrier, which seemed pretty hopeless. But we tested it in a flying prototype and to our surprise it actually worked. We got a 1-sigma of about 50 Hz. And then we weren't so afraid anymore. Then the work started for real and it was very good that we got to meet with the Americans and figure out that they knew about the same things as we did. They had tested a bit more than we had though, and that was good. I don't know if I should go on?

 

 

Ingvar Sundström: Then there were a lot of discussions about whether we should use low or high PRF?

 

 

Jörgen Nilsson: Oh yeah, that was what we were going to talk about. Using low PRF was a very attractive option, since you could just take the radar we had had in the AJ 37 and with some small changes, adding some equipment, make it into a pulse-doppler radar. But then we figured out that that wouldn't work very well and that we should switch to what was a called a coherent transmitter system. But many argued in favor of keeping the low PRF idea like an ordinary pulse radar, but with a coherent transmitter that was nice and pure. But it turned out that such a radar would get us in trouble with moving targets on the ground, such as cars, trains and other vehicles. We did some studies of it, I think we even test flew something. Volkswagen Beetles resulted in big targets and we weren't interested in that.

 

Eventually it became apparent that in order to be able to use a low PRF on doppler radar you had to approach the target in a very specific way, because you could get into "blind spots" for the radar. And there was quite a bit of discussion in these project groups if you could mandate certain attack paths and so on. But when looking back on that today I'll have to say that was very unrealistic with regards to the pilot. The pilot wanted as little trouble as possible and a great deal of tactical freedom, of course. This led us onward to what is called MPD, which did give you that tactical freedom. You can approach the target from the front, from the rear and so on. There was only one downside, that the range was reduced somewhat, but that was acceptable. Then the range has increased a lot during the work on the radar, so it was a pretty fortunate choice. And the pilot did get his tactical freedom of choice, didn't he?

 

 

Leif Åström: Yes, definitely.

 

 

Moderator: Gunnar wanted something.

 

 

Gunnar Lindqvist: Well, Jörgen kinda talked about this. But there were high PRF radars back then, in the F-4 Phantom, I think it was a Westinghouse radar in there. And we chose, for the tactical freedom among other things, a MPD. But it might be a good idea to clarify two things further. When we went on to the JAS 39 we got a multi-mode radar, where you can mix both high and medium PRF. So the first question is, why couldn't we do that on the JA 37? And secondly I think you should talk a bit about the importance of the antenna studies you've done. Building a low-sidelobe antenna and a transmitter with a spectrally pure signal are the two fundamentals for a doppler radar.

 

 

Ingvar Sundström: When it comes to multi-mode radar, if you have a high PRF you need a shorter pulse length. There are various types of traveling wave tubes you can use. They're usually designed for a certain work factor, that is to say the relation between the pulse length and the pulse repetition frequency. In the JAS 39 they solved that with pulse compression in the transmitter and the receiver, pulse expansion when you transmit and pulse compression when you receive. And that kind of technology wasn't very well developed in the early 1970's, I think it would've been very hard to do that. Later transmitter tubes with two different work factors, "dual mode tubes", and that would also have solved the problem. But those didn't exist in the 1970's either. It was hard, really, you had to chose one or the other.

 

When it comes to the antenna, the side lobes are looking straight down into the ground in level flight. They're usually very powerful and you can't filter them away like you do with the main lobe clutter. Instead, you simply have to design an antenna that minimizes the side lobes, and there was a fantastic amount of work done on designing a Cassegrain antenna with small side lobes. Olle Dahlsjö was responsible for managing that. Additionally they did some things to the radome, they called it the "necktie". It was a grid of radar-absorbing stuff in the lower part of the radome that helped minimize the side lobes. In the JAS 39 they have a different type of antenna, a phased array antenna with a fixed grid of about a thousand radiating elements, which also gives very small side lobes.

 

 

Acke Axelsson: (b. 1922, so 85 years old at the time of the seminar, radar engineer at FMV) You've probably already mentioned what I was going to say, I'm a bit hard of hearing and can't really follow the discussion. But I wanted to say something about the choice of medium PRF. We chose to use a magnetron for the trials. You take the signal that you just transmitted, you take the phase of that and make a reference signal, and you use that in the receiver. Then you get all the echoes from the target within an unambiguous distance so you can give them the doppler treatment. But when it comes to the second, third, fourth time around, ambiguous echoes, you can't handle those. So we just skipped those immediately. Other than that, Hughes Aircraft did signals processing just like we did at the air force test center.

 

Then there was the question of medium or high PRF. The tactical methods of the time were either the gun, or that the air combat controllers guided you to a distance of about 10 or 15 kilometers from the target, and then you approached from there to attack with radar or IR missiles. High PRF wasn't suited to that, so medium PRF which could be used as low PRF if necessary was chosen. And that's the background for that choice. I don't know if it's been a downside really, I think it's worked out pretty well.

 

Then there was another factor, and that was the result of the Air Defense Investigation of 1967, LFU 67, which among other things studied ground-based missiles. In a search radar for a system like that all the clutter, all the side lobe clutter, it has a frequency of zero, so it was easy to get rid of it. On the other hand, if you mounted the radar in an aircraft, the clutter spread out over basically the entire doppler spectrum. Since it's impossible to filter out all the clutter you have to try to choose an antenna that prioritizes the direction the target is in. And it's really tricky to both get a good antenna and to do the signal processing right.

 

When we visited Hughes Aircraft in 1972 Ericsson presented their radar and Hughes presented their proposal for signals processing. They were trying to develop a PRF pattern, the standard was to choose three main PRF's, each of which had two partial PRF's, in order to sort through all the ground clutter. And then you ran that pattern in a few variations. But eventually it was concluded that it's really only one PRF that discovers the target, or maybe two at the most. Then you just waste a bunch of time transmitting the other PRF's! In the last stage of development for the JA 37 around the year 2000 they figured out that you ran with one PRF until you found the target and then added a sub-PRF to determine the range. That was mostly based on the fact that the target fluctuated a lot in amplitude, depending on its path through the air flow and whatnot. The theory was that if the target happened to be in a position which gave a very strong echo, you kept it for a few milliseconds, or maybe even about a dozen milliseconds. If you had one PRF that discovered the target you should quickly take the opportunity to choose a sub-PRF to determine the range. And that was one of the last software modifications that was done at Ericsson for system 37.

 

 

Ingvar Sundström: Yes. It wasn't long ago at all.

 

 

Acke Axelsson: No. And that, together with some other things, resulted in roughly doubling the detection range. Doubling the detection range, that's a lot, that's 12 decibels. But there were some other things involved too, a microwave amplifier and so on. Well, that's what I had to say.

 

 

Moderator: Thank you. Kim?

 

 

Kim Bengtsson: I was thinking about the radar and how it hooked up to SYSIM. In the beginning, we only had a model that provided radar output. But with the development of computers, we could later build a radar simulator, RADSIM, which generated simulated inputs to the radar's computer and gave us the possibility to run the real radar software in a realistic environment, with the actual flight-verified programs running against each other, which was pretty valuable. And the engineers could sit there and discuss what was going on with the pilot.

 

 

Ingvar Sundström: Yes, when they came out with TTL-series multiplicators it became possible to make a signal generator that could create the signal vectors we needed inside the radar. And then you could put the real devices, the real flying ones in the SYSIM, and test the real flying software together with the entire system.

 

 

Kim Bengtsson: That was in the mid-70's?

 

 

Ingvar Sundström: Yeah, mid or late 70's I think. And that was a big step forward.

 

Thanks for the Translation =)

 

Its a very interesting read.

[Dejjvid]

Don't worry, I know some of the AJS-37 devs. They all share one thing in common, some sort of letter combination diagnose. Otherwise they wouldn't have started, and pushed this project, inventing new features before ED managed to do. They won't rest until all the obscure features of the Viggen is implemented and working. I rather have the finished product delayed 6 months then wait 7 patches for everything to work. It's gonna be awesome! :)

[MA_Goblin]

+1 :)

[dartuil]

Will wee see march update?

[Agremont]

I think the march update will be mostly about the F-14.

 

It would be great if Cobra could confirm, and also if it's looking like the update will make it in time. :)

[Badger1-1]

I think the march update will be mostly about the F-14.

 

It would be great if Cobra could confirm, and also if it's looking like the update will make it in time. :)

 

+1

[Muppetlord]

An update from Leatherneck...? Is that a thing?

 

Jk. But srsly tho. It would be nice to get something.

How am i suppose to maintain my hype otherwise?

[Cobra847]

I think the march update will be mostly about the F-14.

 

It would be great if Cobra could confirm, and also if it's looking like the update will make it in time. :)

 

Yes, it's F-14 focused.

[Paradox]

Will it happen in March?

[MoGas]

Yes, it's F-14 focused.

Cant wait ;)

[Cobra847]

Will it happen in March?

 

Yes.

[Paradox]

Wheeeeey!

 

Sorry to be all doubting Thomas on you Cobra!

 

I believe in you really!

 

I hope you guys are enjoying your easters

[GaryIKILLYOU]

Will it happen in March?

 

lol

[Darkbrotherhood7]

Yes, it's F-14 focused.

 

Yes.

 

YES!!!:lol::lol::lol::lol:

[Agremont]

Yes, it's F-14 focused.

 

Cool!

 

And hopefully something about the Viggen soon?

[dartuil]

I think they will talk viggen quickly.

[Tirak]

I think they will talk viggen quickly.

 

Why would you think this? :huh: