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Question about Nav Fix (recent Wags video)


twistking

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I absolutely do not udnerstand how you would accumulate drift with GPS on? Should the GPS not automatically correct for drift?
If you do a fix with an offset of miles like in the video, would you not create a conflict within the nav system where the system would either have to reject your fix OR reject the gps data completely?
I can't wrap my head around how this would not imemdiately create a master warning and fall back to ins nav only or similar.
How would you be able to use a gps-guided bomb after declaring to the system that your gps is inaccurate by a mile?
After the last videos which had some major headscratchers in them (offset aimpoints...), i do wonder if this is a half-baked implementation again. I find this very, very confusing.

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Nav fixes are generally for when you don't have GPS.

GPS guided bombs are actually GPS assisted INS guided bombs. They have ways of accounting for launch position errors for when they acquire the GPS after like 18 seconds or something iirc. Its complicated, but it works.

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10 minutes ago, twistking said:

I absolutely do not udnerstand how you would accumulate drift with GPS on? Should the GPS not automatically correct for drift?

It shouldn't. This functionality is for missions where GPS is unavailable due to various reasons. The most common use case for a large percent of DCS playerbase is using newer modules as stand ins for older blocks and variants on servers where the time frame predates GPS integration. Not having a way of fixing your nav drift is a huge PITA in those situations.

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thanks for the answers.
i already suspected that this was for when gps wasn't avaialble. however the questions still stands, how the avionics would deal with the conflict in positional data, if a fix would lead too such an offset while gps signal was good (like shown in the video). how could the system not completely reject one position source? both cannot exist at the same time, can they?


Edited by twistking
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That's the job of the master navigation filter. Horizontal and vertical position is being influenced all the time from a lot of sources. The filter (which is configurable in the DED to a degree) has to balance and evaluate the data from several sources and blend/prioritize which it uses to affect the sysalt/ownship position. It's a somewhat complicated subject.

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Is the current INS drift model at all well modelled?

Last I heard the INS was drifting at insane rates in any pre-GPS mission. 

If a proper probabilistic based INS drift simulation isn’t included, then it makes NO sense to bother with the FIX page. Other modules such as the M2000 and F-14 have properly researched and carefully implemented INS simulations. The ED Viper does not.  Any simulation and/or gameplay immersion is totally bogus unless the underlying systems behave correctly.  


Edited by AvroLanc
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41 minutes ago, twistking said:

thanks for the answers.
i already suspected that this was for when gps wasn't avaialble. however the questions still stands, how the avionics would deal with the conflict in positional data, if a fix would lead too such an offset while gps signal was good (like shown in the video). how could the system not completely reject one position source? both cannot exist at the same time, can they?

 

To add over @Frederf 's very good answer, the very basis of an INS is integration of multiple state data sources with varying degrees of accuracy and precision. This is generally done through state estimators/filters (with the most widely used family called Kalman Filters/Extended Kalman Filters), which allows the estimation of the state (such as the position) coming from multiple noisy data sources. I'd recommend reading on those if inertial navigation systems are of interest to you, but the details of it doesn't serve a real purpose in DCS though! :)

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  • ED Team

INS drift is present in the viper. 

Although our Viper does not have EGI, it does have both INS and GPS systems. based on multiple reference documents, even though it has both systems, steerpoint drift can happen after extended flights.

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1 hour ago, BIGNEWY said:

INS drift is present in the viper. 

Although our Viper does not have EGI, it does have both INS and GPS systems. based on multiple reference documents, even though it has both systems, steerpoint drift can happen after extended flights.

What's an "extended flight"? My regular flights are about 90 minutes. I guess an "extended flight" is more than 2 hours?


Edited by Ignition
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50 minutes ago, NineLine said:

It still assists with navigation, plus it's needed for all those fancy GPS weapons.

Fancy, depends on what you call it fancy. The weapons of the DCS F-16C have some decades.

If I shoot an AGM-88C in PB EOM it will use the GPS coordinates or INS?

If I dorp a JDAM it will use GPS or INS coorinates?

If I drop a JSOW it will use GPS or INS coordinates?

If I drop a CBU-105/103 it will use the GPS or INS coordinates?

How does the aircraft know the "sys aircraft accur" is High?

How much drift per hour can accumulate? So now not even the TGP coordinates are accurate?

This looks like a pre 2000 like the viggen or the F-14 and not a 2007 aircraft.

I'm not against the INS FIX though, but this looks really old and confusing.

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Maybe I can help here. 

Your position is a BLEND of INS and GPS. The GPS is accurate for sure but it also takes the INS into the position formula. Well....the INS drifts as we know. These POSITION UPDATES help correct the INS drift part of the solution so when mixed with the GPS it is all that much more accurate.

We cannot JUST rely on the GPS because... 

1. GPS has error as well (small).

2. GPS can go away at any moment.

So we do a blend. Sort of an average.


Edited by Lanzfeld113
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Just now, Lanzfeld113 said:

Maybe I can help here. 

Your position is a BLEND of INS and GPS. The GPS is accurate for sure but it also takes the INS into the position formula. Well....the INS drifts as we know. These POSITION UPDATES help correct the INS drift part of the solution so when mixed with the GPS it is all that much more accurate.

MMMmmm this makes more sense.

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2 hours ago, Ignition said:

Fancy, depends on what you call it fancy. The weapons of the DCS F-16C have some decades.

If I shoot an AGM-88C in PB EOM it will use the GPS coordinates or INS?

If I dorp a JDAM it will use GPS or INS coorinates?

If I drop a JSOW it will use GPS or INS coordinates?

If I drop a CBU-105/103 it will use the GPS or INS coordinates?

How does the aircraft know the "sys aircraft accur" is High?

How much drift per hour can accumulate? So now not even the TGP coordinates are accurate?

This looks like a pre 2000 like the viggen or the F-14 and not a 2007 aircraft.

I'm not against the INS FIX though, but this looks really old and confusing.

All modern USAF/USN (and every other branch and nation I can think of) tactical jet airplanes are INS-based. They may or may not have GPS as an assisting sensor.

  • AGM-88C is entirely INS based without GPS
  • JDAM uses INS relative offset until it has picked up a GPS track mid flight which filters in, takes 15-20 seconds so short duration trajectories are 100% INS
  • JSOW should be the same as JDAM from memory
  • WCMD is entirely INS based (the extra precision of a GPS sensor is not needed in area weapons)
  • Accuracy is known by the number of satellites it can see and their relative geometry from the almanac, also having verified secure keys or not will affect expected accuracy
  • Standard performance of the F-16's unaided INS is I believe 80% of the time less than 0.8nm/hr for a "10" alignment. In DCS the TGP is always perfectly accurate even when airplane INS is not (not fully simulated)
  • Your expectations of "it's 2007 so it should be perfect" may not be entirely accurate
  • It's fundamental to how the navigation system works, you probably won't ever worry about it in a GPS environment. F-16 pilot knowledge borders on PhD levels of education. It's going to be confusing if you haven't dedicated 10+ years of your life to it as a career.

This F-16 has an INS-GPS system which is more loosely coupled than later genuine EGI systems but performance and operation are largely similar. I wish I knew more to more fully explain the difference between the two but it's best to think of INS-GPS as early or proto EGI-type systems, an earlier inferior design that has mostly been replaced by the improved product by now. In INS-GPS both halves come up with a position solution independently which may be blended. An EGI has the GPS and the INS "share notes" before they come up with a cooperative position. An EGI has the option of providing INS-only or GPS-only position solutions if desired.

From limited testing DCS unaided INS drift is within limits for the various alignment qualities (stored align is inferior to normal even status 10 vs status 10).

GPS is not a satisfactory device for sole reference in anything which is flying at multiples of 100s of knots. Everything that flies has an INS: AIM-9X, AIM-120, JSOW, HARM, JDAM, WCMD, the airplane. Getting a data point every 1 second or so just isn't enough. Your HUD would have just awful lag and jitter like playing Crysis 3 on a Pentium 2. GPS (without differential receivers) only really does position (and thus velocity, track history) but has no idea about pitch or bank or heading. INS isn't "old news", it's fundamental highest quality device for its purpose and at the heart of all of these systems. GPS-aiding is a nice luxury addition with its own unique benefits when it is applied on top of an INS.

https://www.sto.nato.int/publications/STO Educational Notes/RTO-EN-SET-116-2010/EN-SET-116(2010)-05.pdf

Seems like decent reading about the differences between INS-GPS and EGI.


Edited by Frederf
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2 hours ago, Frederf said:

This F-16 has an INS-GPS system which is more loosely coupled than later genuine EGI systems but performance and operation are largely similar

This is the million dollar question here. Does the GPS in the Viper give automatical corrections to the INS to combat drift? The way I understand it that if GPS is available, it will calculate a delta value between the GPS position and the INS one and have the Kalman filter apply this value to correct the INS if the delta exceeds 300 feet. The interesting part here is that BN's answer seems to indicate that in certain situations (like long flights) the INS will drift even if the GPS is correcting it. Is that accurate?

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1 hour ago, Fromthedeep said:

...The interesting part here is that BN's answer seems to indicate that in certain situations (like long flights) the INS will drift even if the GPS is correcting it. Is that accurate?

Well, every INS will drift with time, otherwise a correction by GPS or any other means wouldn't be necessary.

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4 hours ago, Frederf said:

From limited testing DCS unaided INS drift is within limits for the various alignment qualities (stored align is inferior to normal even status 10 vs status 10).

Do I understand this correctly, that in DCS with a stored heading alignment, the INS will accumulate drift faster than with a nornal alignment, even if both alignments are done to a status of 10? Is this confirmed to work like this in DCS or is it just an assumption based on how it works IRL?

I'm asking, because if this is actually the case, I will start doing normal alignments more often instead of stored alignments, especially when flying missions without GPS.

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thanks to all for the interesting answers!

however i still don't understand what would happen in a case like in wags video, where gps and ins are both fine, but the pilot designates a huge offset with the fix function. if the gps keeps up the good quality, shouldn't it at some point overrule the offset from the fix?
if you designate an offset of more than a mile, it cannot be that both ins and gps are correct. i would assume that the avionics have to reject either of the two sources (meaning, either override your "fix" with gps if gps is deemed valid, or reject gps implying that gps is faulty even though the signal appeared to be fine). wouldn't that warrant a master caution or similar?

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18 hours ago, Frederf said:

All modern USAF/USN (and every other branch and nation I can think of) tactical jet airplanes are INS-based. They may or may not have GPS as an assisting sensor.

  • AGM-88C is entirely INS based without GPS
  • JDAM uses INS relative offset until it has picked up a GPS track mid flight which filters in, takes 15-20 seconds so short duration trajectories are 100% INS
  • JSOW should be the same as JDAM from memory
  • WCMD is entirely INS based (the extra precision of a GPS sensor is not needed in area weapons)
  • Accuracy is known by the number of satellites it can see and their relative geometry from the almanac, also having verified secure keys or not will affect expected accuracy
  • Standard performance of the F-16's unaided INS is I believe 80% of the time less than 0.8nm/hr for a "10" alignment. In DCS the TGP is always perfectly accurate even when airplane INS is not (not fully simulated)
  • Your expectations of "it's 2007 so it should be perfect" may not be entirely accurate
  • It's fundamental to how the navigation system works, you probably won't ever worry about it in a GPS environment. F-16 pilot knowledge borders on PhD levels of education. It's going to be confusing if you haven't dedicated 10+ years of your life to it as a career.

This F-16 has an INS-GPS system which is more loosely coupled than later genuine EGI systems but performance and operation are largely similar. I wish I knew more to more fully explain the difference between the two but it's best to think of INS-GPS as early or proto EGI-type systems, an earlier inferior design that has mostly been replaced by the improved product by now. In INS-GPS both halves come up with a position solution independently which may be blended. An EGI has the GPS and the INS "share notes" before they come up with a cooperative position. An EGI has the option of providing INS-only or GPS-only position solutions if desired.

From limited testing DCS unaided INS drift is within limits for the various alignment qualities (stored align is inferior to normal even status 10 vs status 10).

GPS is not a satisfactory device for sole reference in anything which is flying at multiples of 100s of knots. Everything that flies has an INS: AIM-9X, AIM-120, JSOW, HARM, JDAM, WCMD, the airplane. Getting a data point every 1 second or so just isn't enough. Your HUD would have just awful lag and jitter like playing Crysis 3 on a Pentium 2. GPS (without differential receivers) only really does position (and thus velocity, track history) but has no idea about pitch or bank or heading. INS isn't "old news", it's fundamental highest quality device for its purpose and at the heart of all of these systems. GPS-aiding is a nice luxury addition with its own unique benefits when it is applied on top of an INS.

https://www.sto.nato.int/publications/STO Educational Notes/RTO-EN-SET-116-2010/EN-SET-116(2010)-05.pdf

Seems like decent reading about the differences between INS-GPS and EGI.

 

Thanks, this clarifies a lot more. I think ED should do these explanations BEFORE an inportant update like this because it generates a lot of confusion. Also why some modules like the A-10C or the F-18C work like they do vs the F-16C. A simple explanation is enough, it doesn't need to be very complex just so most of the people can understand.

 

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I have made a test, I hope this is representative.

I turn off GPS, flew for a while and as you can see the SP drifted slightly

turn the GPS back on, well and the SP is almost perfect in this scenario

Spoiler

 

 

 


Edited by Hobel
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With the video about A-Cal now being up, i wonder what purpose Fix has anyhow. A-Cal seems to be able to do position fixes AND altitude fixes seperately or at the same time.
Why is FIX needed int eh first place when you have A-Cal. What am i missing?

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I see people confusing aspects about how an INS works. One thing is position and another different thing is orientation.

For the INS to know its orientation in space, it needs an alignment, which is a incredible complex process to understand, but for all intended purposes let's say it just needs to learn what the north is and how it is positioned in relation to it, as well as knowing its absolute position in 3D space. Absolute, not relative.

Next we have the position part of the INS which it can never learn on its own. That's why it needs a manual input of relative position like coordinates, just XY. Or it can have it input by an external source like GPS.

Now, drift in an INS means the accumulated errors made by dead reckoning. Minimal errors that get accumulated and bigger as time goes by.

So drift only affects the position keeping abilities of an INS, but no its orientation part.

Therefore, an INS that is able to suck data from an external source like a GPS can have its position updated each N period of time, and calculate for other purposes what the accumulated error was.

Wags video only applies for GPS denied situations. That's all.

Hope it helped.

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On 6/21/2022 at 11:58 PM, Frederf said:

An EGI has the GPS and the INS "share notes" before they come up with a cooperative position. An EGI has the option of providing INS-only or GPS-only position solutions if desired.

 

 

Not entirely true.

Legacy Hornets that don't have EGI but separate modules for INS and GPS can also provide position based on only GPS or only INS.

 

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