What functions would be disabled if the GPS satelites were knocked out?

[Echo38]

As I understand from the manual, several of the functions in the A-10 require GPS input, and other functions require those first functions to be active in order to work. For example, the yaw SAS won't work if a certain switch (the name of which I can't remember at the moment--sorry, I'm new) is off, and I believe that switch is for something that needs GPS. It chains--if the GPS thingy is off, the yaw SAS won't work, and if the yaw SAS won't work, then the EAC won't work, and that would mean that the air-to-air lead-computing gunsight won't work, and so on and so forth.

 

It's a bit perplexing, because in some cases, the mandatory link seems arbitrary; the lead-computing gunsight does not use GPS to calculate anything, so why can't it be used when the GPS link is not active? What would happen if the GPS network somehow went down? Also, some of the functions are said to be reliant on radar, but the A-10 has no onboard radar (as far as I know), so I assume there also must be a radar link maintained for these? Or is there a miniature, specialized radar (as opposed to a radar for air-to-air or air-to-ground) that I don't know about, similar to the tail-warning radar on the P-38L?

 

Pardon any errors I've made in this assessment, and please do correct them with all speed. As I said, I'm new, and a Bear of Very Little Brain who is having a bit of a difficult time making sense of all the systems. : )

[Eddie]

In the event of GPS jamming (or other cause of GPS failure), the only thing that would be affected is the EGI's ability to self update to prevent INS drift and the ability to perform EGI alignment without manually entering the initial aircraft position.

 

Other than that, everything would work exactly the same.

 

Nothing in the aircraft "requires" GPS, things are only assisted by GPS.

[xxJohnxx]

The Radar you mean is probably the Radar Altimater, which meassures distance frome aircraft to ground.

[Echo38]

Can anything in the A-10C be "hacked" by an external source during flight? I'm far from an expert, but it seems that relying on computer data from an external source could leave the aircraft open to digital attack.

[Eddie]

No. Nothing relies on any data from any "external" source.

[Echo38]

The Radar you mean is probably the Radar Altimater, which meassures distance frome aircraft to ground.

 

Yeah. Does this use an on-board radar, like the tail-warning radar on some Second World War fighters, or is it using data transmitted from an AWACS or something? The latter sounds rather risky, but I wasn't aware of any radar at all on the A-10.

[Eddie]

Yeah. Does this use an on-board radar, like the tail-warning radar on some Second World War fighters, or is it using data transmitted from an AWACS or something? The latter sounds rather risky, but I wasn't aware of any radar at all on the A-10.

 

Yes the RADAR altimeter is an actual RADAR in the aircraft looking downwards to measure the distance from the terrain. It's no different in function or design to the rad alt in a 747.

 

It sounds like you're expecting things to be a lot more complicated than they are for some reason.

[Echo38]

I'm just trying to figure out how all this works. My only prior significant area of study was Second World War fighters, and I'm experiencing something of a culture shock.

 

My curiosity about the GPS and such was sparked by those events in previous decades, wherein pilots defected to other countries with an aircraft, e.g. the Soviet pilot defecting to a NATO country with a Mig-29 (I read his autobiography once, but it was a very long time ago). In the A-10's case, the GPS would seem to limit the pilot's ability to do this (as it would provide the angry powers-that-be with exact real-time location, regardless of altitude and cloud cover), and I was wondering if there were any other such devices by which a pilot could be hampered in the event of him going rogue.

 

Another possibility which occurred to me is that of a hostile power somehow gaining control of GPS satellites and transmitting incorrect information (e.g. speed and heading) to the A-10. I wonder what would happen, in that case, to the systems which take GPS data into consideration.

Edited August 11, 2012 by Echo38

[Kaiza]

The EGI (embedded GPS inertial) is a ring laser gyro that is updated by a military P-code GPS to provide a "hybrid" solution. So essentially the system uses two different sources of position information to calculate its position. The hybrid solution uses the GPS and the inertial to come up with its best position and a figure of merit (FOM) which is an assessment of its own accuracy. If the EGI loses GPS it can still fly using only the inertial solution, however the inertial will slowly drift due to errors associated with inertials. This isn't a problem as the drift can be accounted for and updated and is how jets operated before GPS was introduced. So if a pilot went rogue as you say, and GPS satellites were jammed he or she could still use inertial and visual navigation.

 

I am not 100% sure what EGI the A-10C has (LN-100G?) but most EGIs will automatically deselect the GPS from the position solution if it is assessed that the position is erroneous or being jammed. Remember that these GPSs are 'P-coded' so a lot more difficult to jam than a civilian GPS.

Edited August 11, 2012 by Kaiza

[xxJohnxx]

I think your understanding of the GPS is a little bit wrong to! A GPS satelite doesn't transmit speed nor heading nor altitude. It just continusly transmits the time (and some other stuff, which I will skip here) meassured by it's internal atomic clock. The satelite itselfe doesn't transmit your position as it doesn't know your position (your mobile phone can't be bositioned by GPS only as you don't send anything to the satelites).

The accuracy of GPS navigation depends on the number of satelites which send you their time. E.g.:

1 Satelite available: You only get the actual UTC time from the satelite. Your GPS won't do anything, as it doesn't know where it is.

2 Satelites available: Your GPS can now calculate a 2D position which maybe is prett inacurate.

3 Satelites available: Your GPS can now calculate a 3D position which is more accurate.

4 Sateelites or more available: As a GPS device normally doesn't have an atomic clock included the time meassurement is not as exact as it could be. With 4 satelites or more your GPS reciver can use the 4th (or any other of the four satelites) as a aid for more precise timing, which will allows the GPS to calculate your position more actually.

 

The calculation of the position can be explained easily (Altough it is much more difficult):

For example you have 3 satelites available, every one sends you their time. Every time has been send at exactly 08:00:00 o'clock. We recived the signals at that times:

 

1. 08:00:02 The signal traveled 2 seconds from the satelite to you.

2. 08:00:03 This signal even traveled 3 seconds.

 

As the satelites send out electro magnetic waves, which travel with the speed of light, we would be able to calculate the distance ( distance = speed * time ).

Now we know the distance from both satelites, and we somehow (I don't want to exain that here) know their possition. We can now draw two imagine lines from the satelites. Our position is at that point where both ends meet. Now we know our latheral position but not our altitude. That is the momment where the 3rd satelite comes in handy. We recived it's signal at 8:00:05 (5 second of travel). We can also calculate our distance relative to it and we also can draw that line, whic end also is at our position. Now the GPS can calculate our altitude to.

 

Heading and speed aren't transmited by the satelite as stated privously, they are just easy (easier than the calculations bevore :D ) calculations: The GPS device "thinks": One second before I have been there, now I am here, that means I hae traveled so fast in that heading.

 

 

I hope thats understandable, it is not technically perfect and I am pretty tired (1 am in the mornig, lol), but it may helps you to understand the GPS.

If somebody finds any failures in my descripiton, please corrrect me, thanks!

 

Best regards!

John

[Yurgon]

In the A-10's case, the GPS would seem to limit the pilot's ability to do this (as it would provide the angry powers-that-be with exact real-time location...

 

If I read this correctly, you assume that GPS transmits an aircraft's location, which to the best of my knowledge is not correct. The GPS on the aircraft (just as in a car) is merely a receiver that calculates its own position with the help of the signals it receives from multiple satellites at the same time.

 

Of course, being a modern aircraft integrated with the other units on the battlefield, the A-10C has an array of communication devices so that other friendly units can know the aircraft's location (and the aircraft can know other unit's locations, like those shown on the TAD). I'm sure several of the guys on the forum can give a lot more information on the topic, but let's simply subsume all of these devices under the term "data link", and AFAIK the A-10C's datalink capability can be switched on or off with the switch labeled SADL (situational awareness datalink) JTRS (Joint Tactical Radio System) (although just recently I read that the system behind it is not actually called "SADL" "JTRS", they just didn't re-label the switch accordingly).

 

So if a pilot wanted to defect in an A-10C he could still use GPS without using the datalink. Then again, I doubt the Warthog would be a pilot's jet of choice to defect in because it is incredibly slow compared to just about anything that would be sent to intercept it... :megalol:

 

Another possibility which occurred to me is that of a hostile power somehow gaining control of GPS satellites and transmitting incorrect information (e.g. speed and heading) to the A-10. I wonder what would happen, in that case, to the systems which take GPS data into consideration.

 

Faking GPS signals in order to reroute GPS receivers is pretty simple and doesn't even require the GPS satellites themselves to be affected in any way. Well, I couldn't do it, but from a technical point of view it's pretty straight forward, especially with unencrypted civilian GPS. As mentioned above, the US military uses encrypted GPS which is a lot harder to break and I don't know if there are any scenarios right now to perform such an attack in the real world.

 

When a US drone went down in Iran last year, one of the theories was that the military, encrypted GPS had been jammed, which led to a fall-back to the unencrypted and thus vulnerable signal. No clue whether that's what happened or whether the drone's software even uses such a fall-back, but it sounded like a plausible explanation for hijacking a drone.

 

Edit: Leto is right, I mixed up SADL and JTRS. I wish we had a strike-through markup on the forum, or did I just not find it?

Edited August 12, 2012 by Yurgon
Mixed up SADL and JTRS

[Leto]

and AFAIK the A-10C's datalink capability can be switched on or off with the switch labeled SADL (situational awareness datalink) (although just recently I read that the system behind it is not actually called "SADL", they just didn't re-label the switch accordingly)

 

you mix up jtrs and sadl here

[Echo38]

Yep, John, that's very helpful.

 

My next question is related, but not quite on the same tack: how was inertial drift accounted for before the invention of GPS? Specifically, how would one account for inertial drift in the gyro horizon, when one is in zero-visibility conditions? I understand that in VFR conditions, you could level the wings visually and then reset the gyro instrument, but what about IFR?

[PeterP]

but what about IFR?

 

 

you would do it like back in the good'ol days...

you would try to climb above the clouds to be VFR again or go low enough to get a reference point at the ground.

[cichlidfan]

... or go low enough to get a reference point at the ground.

 

Being very careful on that part!;)

[PeterP]

yes :D

 

switching to radar altimeter would be also a very good idea.

[Yurgon]

how was inertial drift accounted for before the invention of GPS?

 

You should get into flying Black Shark. While INU (*) drift isn't actually modeled, you can still perform INU updates with pre-defined INU fix points by pointing at them with the Shkval targeting system or by positioning the chopper right above the point so that the chopper would have a decent position even if GLONASS failed (and INU drift was modeled and your average mission was actually long enough for INU drift to take a noticeable effect).

 

The Shark is somewhat old school in that regard, but you'll learn a thing or two about seat of the pants flying. :)

 

(*) It may not be 100% technically correct, but AFAIK INU (Intertial Navigation Unit) is pretty much the Russian term(inology) for INS.

[Kaiza]

Yep, John, that's very helpful.

 

My next question is related, but not quite on the same tack: how was inertial drift accounted for before the invention of GPS? Specifically, how would one account for inertial drift in the gyro horizon, when one is in zero-visibility conditions? I understand that in VFR conditions, you could level the wings visually and then reset the gyro instrument, but what about IFR?

 

Remember GPS only helps solves drift in position, not attitude. GPS cant help with the attitude displayed on the HUD and ADI, it doesnt know what your pitch and roll is. Thats a pure INS solution, so losing GPS will make no difference.

Edited August 12, 2012 by Kaiza

[PeterP]

kaiza,

GPS tells you your absolute x/y/z coordination within the tracking space of the satellites and so you can calculate your position to a set reverence point you use (your map for instance) - this includes also your altitude.

 

Otherwise I would have enormous difficulties to us a hand-held gps in the mountains to get back to my trail (what I did last year in the Alps without a problem. even if I was drunk and everyting was pitch black around me - I found my tent again :D)...:)

Edited August 12, 2012 by PeterP

[Kaiza]

True PeterP. You may have misread my post I was talking attitude rather than altitude.

 

GPS position can help the INS with correcting transport errors by providing a position, but it provides no information on reference to the horizon (attitude).

Edited August 12, 2012 by Kaiza

[PeterP]

oh well - :) - you are right! Let's talk german next time :D

 

But I hope that the engineers that build in the GPS receivers in the A-10c (I read somewhere that it can have a res of 30centimeters for the military- of course this is all classified... ) ...Used more than only one receivers to determined the absolute position. so lets say a receiver is in each wing and tail - so it would be no problem to get a good guess how you are really aligned.

 

please correct me if I'm wrong. But when you would ask me to build in a GPS in your plane I would suggest to use three or even more receiver:).

Edited August 12, 2012 by PeterP

[peter]

OMG, just fly the airplane, that is what a pilot does. I know from experience.

[PeterP]

OMG, just fly the airplane, that is what a pilot does. I know from experience.

Yes!

This is how it works if you are a pilot and you "just" Fly - this keeps you busy enough...

 

But this is a simulation-forum.

 

Here are also the guys that have not the possibility or just do it as hobby... - so they start to ask funny things you would never ask as a real pilot...:) ( simply because you have to know it as Real Pilot- your life depends on it..)

 

But some of us "desktop jockeys" want to know it to the last pin - ...and only after we got all info we are satisfied...:P

...and when our experience in the Sim doesn't match with what others told us.... we decide to open another thread where we tell the developer what they did wrong...:music_whistling:

 

This is how it works on this FL....:thumbup::P:doh::D

Edited August 12, 2012 by PeterP

[xxJohnxx]

This seems also to be a good read about I.N.Systems.

[amalahama]

INU drift is not modelled, but still you can use SYSTEM/INS/UPDATE page in the CDU, is fully functional. I've not tried to wipe out all GPS satelltes via .LUA and see what happens though. I'm afraid CDU will continue telling you keep GPS link with several satellites. GPS modelling seems more complex in BlackShark than A-10C

 

Regards!