INS update/realignment - Page 3 - ED Forums


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Old 02-12-2019, 05:41 PM   #21
corvinus's Avatar
Join Date: May 2018
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Originally Posted by Airhunter View Post
It pretty much uses ADR and acceleration data + a doppler radar at low altitudes + a TERNAV system using the radar altimeter to compare the terrain below to stored map data in order to update its position.
Originally Posted by QuiGon View Post
The Viggens NAV system is pretty much a INS, but without the gyro. Instead of getting movement inputs from the gyro like an INS does, the Viggens NAV system gets supplied with those inputs from the normal flight sensosr (pitot tube, barometric altitude, etc). There are some other systems that assist here, like the doppler system that adjusts for wind and (since the AJS upgrade) the TERNAV system as Airhunter said. But because the Viggens NAV system uses normal flight sensors instead of highly precise gyros, it is more susceptible to drift than a regular INS.
Thanks for the explanations!
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Old 02-14-2019, 04:33 PM   #22
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For clarity: it does have gyros, but only for attitude sensing. The nav system is just dead reckoning based on initial position, attitude, course, airspeed and wind speed. The position is calculated cumulatively every 110 milliseconds or something like that. What you're doing when pressing the reference button on the stick has more to do with getting an accurate initial position than anything else, AFAIU.

It's kind of an oddball solution, but it did have some advantages in the 1960's. Just like true INS (but unlike radio beacon-based systems like TACAN that were common at the time) it can't be jammed and isn't reliant on infrastructure that can be destroyed, while offering far faster cold starts than INS, and avoids an additional complicated and expensive gyro installation. The downsides were considered to be possible to live with in the tactical context the Viggen was designed to operate in.

Last edited by renhanxue; 02-14-2019 at 04:46 PM.
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