INS updates without landmarks? How to?

[Nealius]

Normally when I plan a mission I put waypoints over some easily recognizable geographic feature that can be seen visually or on radar--river openings, islands, lakes, etc.--so I can easily update the INS. However, when I'm flying missions out to sea my INS will drift 10-20km away and I become lost on RTB. Obviously there are no landmarks over the ocean, so what's the best procedure to keep the INS updated over water?

[Ice_Cougar]

Normally when I plan a mission I put waypoints over some easily recognizable geographic feature that can be seen visually or on radar--river openings, islands, lakes, etc.--so I can easily update the INS. However, when I'm flying missions out to sea my INS will drift 10-20km away and I become lost on RTB. Obviously there are no landmarks over the ocean, so what's the best procedure to keep the INS updated over water?

I try to keep myself close enough to shore to be able to see a large landmark like a busy harbor or river opening or peninsula at maximum radar range to get a rough INS update to get near base for TERNAV updates and/or navigation assistance from the ATC. There is also the strategy of keeping some stationary ships at a waypoint for better precision, but that seems somewhat unrealistic for me.

[Home Fries]

Normally when I plan a mission I put waypoints over some easily recognizable geographic feature that can be seen visually or on radar--river openings, islands, lakes, etc.--so I can easily update the INS. However, when I'm flying missions out to sea my INS will drift 10-20km away and I become lost on RTB. Obviously there are no landmarks over the ocean, so what's the best procedure to keep the INS updated over water?

 

As a guy who flew professionally over water using inertial navigation systems, I'll just say: you don't update over water. The INS isn't so much a precision navigation system as it is something that will get you where you need to go and back. That said, 10-20km is a huge amount of drift over a short flight (the INS I used was rated at 2nm/hour, and the drift was usually a fraction of that). Since there's no spin-up time in the Viggen, I'm wondering if it's actually an INS or a flight computer using Doppler inputs. If it's a Doppler, then switching the Land/Sea switch (in front of the radar stick) to Sea when over water should drastically reduce your drift.

 

Ultimately, the INS should get you back to a place on the shoreline that you can recognize (or a navaid you can dial up). Pick a waypoint that is clearly visible on radar at a distance (like you have mentioned in your post), and use that as a rough correction. Then you can fine-tune your update when you get closer.

[some1]

Viggen does not have an INS. It has a doppler navigation system connected to a rather smart computer. But AFAIK such systems don't work very well over water, as it's harder to get a good ground speed readout over water.

[renhanxue]

The basic position calculation is just dead reckoning. The computer takes course and accelerations in all three axes from the ADI and autopilot, as well as temperature and air pressures from the air data unit, to calculate a true airspeed vector. Then it converts this to an estimated ground speed vector by compensating for the wind speed using either doppler radar-measured speed over the ground if possible, or forecast wind (as input from the data cartridge) if the doppler isn't working. Then it integrates this every 103 ms to continuously calculate the current position. The only kinda inertial components are the accelerometer gyros.

 

Setting the land/sea switch (LAND/SJÖ) to sea mode when over water is advisable to help the doppler out (according to the manual the doppler spectrum tends to be shifted lower over sea and the sea mode is supposed to compensate for this - it also states that in level flight and with moderate waveheights the error is on the order of "a few percent"). The manual also states that the doppler will not work reliably at pitch or bank angles above 15° when in sea mode. If the doppler has failed to track for two minutes straight you should get a NAVSYST warning on the announciator panel.

 

You can override the doppler by inputting forecast wind manually while in the air (input done on the ground does not do this) - input wind direction in degrees with three digits followed by wind speed in km/h with two digits in VIND/RUTA/MÅL IN, confirm with LS/SKU. Input wind direction 000° and speed 00 km/h in the same way to re-enable doppler measurement. You can also see what CK37 thinks the current wind is by going VIND/RUTA/MÅL UT - output is just like input (three digits direction in degrees, two digits speed in km/h) with the addition that the sixth digit is a - (minus) if it's using forecast wind or a 0 (zero) if it's using the doppler.

 

10-20km of drift sounds gigantic, though. Are you flying acrobatics the entire time?

Edited November 8, 2017 by renhanxue

[Nealius]

This happened in the "Hunting the Hunters" SP mission. On RTB, with the computer in AKT/POS the last digit was 4, which according to Chuck's guide indicates estimated error of 4km. Novorossiysk was my L1 and LS waypoints, however my navigation system was leading me somewhere between Novorossiysk and Anapa for both waypoints. Measured on the map it was close to 20-30km off.

[QuiGon]

This happened in the "Hunting the Hunters" SP mission. On RTB, with the computer in AKT/POS the last digit was 4, which according to Chuck's guide indicates estimated error of 4km. Novorossiysk was my L1 and LS waypoints, however my navigation system was leading me somewhere between Novorossiysk and Anapa for both waypoints. Measured on the map it was close to 20-30km off.

Is it possible, that you performed an update fix during the flight that was far off and caused this huge offset?

[Nealius]

Would targeting a ship with the radar cause an update fix?

[some1]

Not if done by the book.

 

But if you did not change the waypoint type over the ship to "Target" then by slewing that waypoint you would change all waypoints in the flight plan as well.

[Nealius]

Ah, I changed the waypoint to a target point after designating the ship. Setting the waypoint as a target point first would prevent this?

[some1]

Yes, that's what it's for. :)

 

If you slew the target waypoint, you're updating the target position.

If you slew the regular waypoint, you're performing fix update, in other words you move all waypoints by the same amount.

[WildBillKelsoe]

follow the compass to the nearest land using cardinal directions.

 

 

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