[BUG ?] Inconsistency between CK-37 and WP true position

Hello all !

During a little training session, I noticed that there was a little inconsistency between the data provided by the CK-37 in AKT/POS-Output and the real position of my attack waypoint.

The CK-37 estimate a 2Km error while the waypoint displayed on the HUD is right where it is supposed to be.

See the screenshot attached... The 6th digit indicate ''2'', you may not see it properly on the screenshot, but the circle on the HUD is pretty much at the same position in mission editor (on the targets). So the estimated error of the CK-37 is wrong.

Is it a bug or a normal behaviour ? I estimate the distance between the circle on the HUD and the true waypoint position on the ME at around 0,1 to 0,2km, not 2km.

Thanks.

IRC Ck37 estimates the accumulated error over time. 2 on the last digit doesn't mean that the system is off by exactly 2 km.

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IRC Ck37 estimates the accumulated error over time. 2 on the last digit doesn't mean that the system is off by exactly 2 km.

 

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Yes but he's saying that the HUD does not reflect the 2km drift.

Hi catt42 and LevelPulse,

Thanks for your answer. I don't understand the manual this way.

Page 188 Output AKT/POS :

(...)

''The sixth (last) digit indicates the navigation systems estimation of its position error (the distance

between the own position and the “true” position) in kilometres.''

So, when the sixth digit shows the value ''1'', the CK-37 estimates that the delta between the true waypoint position and the displayed waypoint position is off by 1 km... This is how I understand this, hence this bug report.

Thanks again.

I'm not an expert on the CK37 and maybe somebody at Heatblur could better explain how it works, but what I meant is that an estimate is just an estimate. If the system could know exactly how much it is off, it could also adjust itself. I think that the last digit represents only a statistical expected drift, given how much time passed since the last fix.

So if it says that it's 2km off, as an example, the waypoint dot on the hud can be anywhere from 1m to 2km from where it's supposed to be.

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I'm not an expert on the CK37 and maybe somebody at Heatblur could better explain how it works, but what I meant is that an estimate is just an estimate. If the system could know exactly how much it is off, it could also adjust itself. I think that the last digit represents only a statistical expected drift, given how much time passed since the last fix.

 

So if it says that it's 2km off, as an example, the waypoint dot on the hud can be anywhere from 1m to 2km from where it's supposed to be.

That is true, but according to the OP there is always zero drift, no matter what number the CK37 is displaying as estimated drift. This would indeed be a bug if that is true (I haven't tested it myself).

catt42 : yes, I understand what you mean, but as I said in the first post : the drift between the true position of the waypoint (in the mission editor) and the waypoint position displayed on the HUD are pretty much the same (maybe 0.1 to 0.2km of drift) while the CK-37 estimates a 2km dirft. I can understand that it's just an estimation, but this estimation is really not reliable.

If I post a bug report now it's because I havn't noticed such a drift in the CK-37 last digit in previous versions (I may be wrong about this though)

Qui Gon : yes you get it. Please keep me informed if you have the possibility to test this.

Sorry for my english, I don't use translator.

Thanks all.

I am not sure I follow what is being asked here, but if it is that the HUD doesnt display a drift I tried just now and at least for me it does:

This was after a long high-altitude flight over the ocean (with doppler in "LAND"-mode). The waypoint true position is in the middle of the cross and the estimated position error in AKTPOS was a little overestimated but nevertheless there. Almost regardless of what it says on the last digit of AKTPOS the true drift could be anywhere from 0 to infinity.

I am not sure I follow what is being asked here, but if it is that the HUD doesnt display a drift I tried just now and at least for me it does:

[ATTACH]195899[/ATTACH]

This was after a long high-altitude flight over the ocean (with doppler in "LAND"-mode). The waypoint true position is in the middle of the cross and the estimated position error in AKTPOS was a little overestimated but nevertheless there. Almost regardless of what it says on the last digit of AKTPOS the true drift could be anywhere from 0 to infinity.

Interesting. What was the drift error indicated by AKTPOS?

I also have to say, that the actual drift (waypoint position next to the crossed runways) is much smaller than I would have expected from a long flight over the ocean and thus without TERNAV support. I expected the Viggens nav system to be much more unprecise tbh.

It uses doppler though, also there are ways to get a good estimate. The nav drift is modeled very in depth I may say so (actually, "drift" isnt modeled but true position estimation through a pitot measuring dynamic and static pressure coupled with a gyroscope). You can play around with drift in the game and have a lot more than this 10 minute flight.

It uses doppler though, also there are ways to get a good estimate. The nav drift is modeled very in depth I may say so (actually, "drift" isnt modeled but true position estimation through a pitot measuring dynamic and static pressure coupled with a gyroscope). You can play around with drift in the game and have a lot more than this 10 minute flight.

Interesting, that's indeed much more in depth than I thought :thumbup:

Indeed I can confirm that the error which is output by the CK (here 5 km), is in fact nearly 0.

After flying half an hour outside of land, and coming back to land with RHM set to OFF in order to avoid any data input to TERNAV (in fact RHM shouldn't indicate or send something in this altitude), the real position error is negligible.

Indeed I can confirm that the error which is output by the CK (here 5 km), is in fact nearly 0.

After flying half an hour outside of land, and coming back to land with RHM set to OFF in order to avoid any data input to TERNAV (in fact RHM shouldn't indicate or send something in this altitude), the real position error is negligible.

Hmm, thanks for testing this :thumbup:

That reinforces my thought, that the Viggens nav system in DCS might be too resiliant to drift. The drift error that is displayed by the CK in TAKT mode seems much more reasonable than the drift that is actually beeing cumulated.

But that's more of a feeling than something backed by hard data, as I don't know how fast drift got acumulated in the real Viggen :dunno:

It makes nav fixes kinda unnecessary though :(

Well I definitely had times where I was 5k or more off and it was accurate.

So that doesn't really tell us anything except err on the side of accuracy (nav fixes)

I don't know if it's been said before, but the error shown in the last digit of the CK37 is an estimate only. And that estimate is purely time based.

What you must realize is that if the system knew how large the error was, then it would be able to correct for the error and thus never have a drift error. So with an estimated 5 km error, the true error can be anything from 5 to a gazillion. I've had cases where my error was 20 km with a low digit showing.

Also, there is no need for a manual fix if you have TERRNAV working (a 5 in the second last digit). The TERRNAV is a separate system that was plugged in to the AJS update, and what it does is it sends fixes ever second or so to the CK37 (which remained largely unchanged). But if you haven't had a TERRNAV for a while, then a manual fix can come in handy

Also, there is no need for a manual fix if you have TERRNAV working (a 5 in the second last digit). The TERRNAV is a separate system that was plugged in to the AJS update, and what it does is it sends fixes ever second or so to the CK37 (which remained largely unchanged). But if you haven't had a TERRNAV for a while, then a manual fix can come in handy

This relates especially for situations when your RHM is OFF for any reason, or you are flying above 600m above ground (as stated somewhere in the manual).