Thought this was neat (GTN750 related)

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Orlaam
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Thought this was neat (GTN750 related)

Post by Orlaam »

Flying north towards KPRC, I was poking around and have often noticed the obstacle icons on the map. I doubt many of them are actually displayed in the sim, but since it was on my mind, I zoomed in to the mountain in question. What do I see? A tower! Same thing near KSEZ as well I noticed. Now my time will be looking for sim-displayed obstacles that match the GTN, lol

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MarcE
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Re: Thought this was neat (GTN750 related)

Post by MarcE »

In some areas (mostly USA though) there is an amazing precision in detail you would never expect to be when you look at the default terrain and mesh..

I was on a trip from the east coast to the west coast with the 182 equipped with the GTN750.. Just over North Dakota I flew in IMC conditions and suddenly got an obstacle warning. I ignored it ( :oops: ) as I really wasn't expecting an obstacle in 3000ft when North Dakota is around 1000ft elevation and... well... ND being some pretty flat prairie.. Thanks to the crash detection being deacticated I flew just through a radio tower... jep, in 3000ft.. :P

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AKar
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Re: Thought this was neat (GTN750 related)

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MarcE wrote:I was on a trip from the east coast to the west coast with the 182 equipped with the GTN750.. Just over North Dakota I flew in IMC conditions and suddenly got an obstacle warning. I ignored it ( :oops: ) as I really wasn't expecting an obstacle in 3000ft when North Dakota is around 1000ft elevation and... well... ND being some pretty flat prairie.. Thanks to the crash detection being deacticated I flew just through a radio tower... jep, in 3000ft.. :P
How come you were in IMC without doing IFR? :mrgreen:

These GTN series are quite neat, I've done some serious terrain avoidance with them when my VFR's gone bad! :mrgreen:

Image

I am, though, a bit in thought whether the sim presentation gives too good a picture of how well these things function in reality. The obstacles in database are likely well-cautioned, for they are single points with good definition, but I'd expect the TAWS resolution to be somewhat worse in reality. I don't know how jumpy the GNS's GPS altitude is when not coupled to a barometric source.

-Esa

MarcE
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Re: Thought this was neat (GTN750 related)

Post by MarcE »

AKar wrote:How come you were in IMC without doing IFR? :mrgreen:
These GTN series are quite neat, I've done some serious terrain avoidance with them when my VFR's gone bad! :mrgreen:
Don't fly anywhere your mind hasn't been 2 minutes before :mrgreen: :mrgreen:

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Re: Thought this was neat (GTN750 related)

Post by shortspecialbus »

AKar wrote:
MarcE wrote:I was on a trip from the east coast to the west coast with the 182 equipped with the GTN750.. Just over North Dakota I flew in IMC conditions and suddenly got an obstacle warning. I ignored it ( :oops: ) as I really wasn't expecting an obstacle in 3000ft when North Dakota is around 1000ft elevation and... well... ND being some pretty flat prairie.. Thanks to the crash detection being deacticated I flew just through a radio tower... jep, in 3000ft.. :P
How come you were in IMC without doing IFR? :mrgreen:

These GTN series are quite neat, I've done some serious terrain avoidance with them when my VFR's gone bad! :mrgreen:

Image

I am, though, a bit in thought whether the sim presentation gives too good a picture of how well these things function in reality. The obstacles in database are likely well-cautioned, for they are single points with good definition, but I'd expect the TAWS resolution to be somewhat worse in reality. I don't know how jumpy the GNS's GPS altitude is when not coupled to a barometric source.

-Esa
Part of how GPS determines your position is by also determining your altitude. If you think about it, because the whole thing is 3-dimensional, it would pretty well have to. Combination of triangulation and atomic clocks measuring time delay and such, although I'm grossly oversimplifying.

-stefan

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Re: Thought this was neat (GTN750 related)

Post by n421nj »

It is still cool
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Re: Thought this was neat (GTN750 related)

Post by ryanbatc »

MarcE wrote:In some areas (mostly USA though) there is an amazing precision in detail you would never expect to be when you look at the default terrain and mesh..

I was on a trip from the east coast to the west coast with the 182 equipped with the GTN750.. Just over North Dakota I flew in IMC conditions and suddenly got an obstacle warning. I ignored it ( :oops: ) as I really wasn't expecting an obstacle in 3000ft when North Dakota is around 1000ft elevation and... well... ND being some pretty flat prairie.. Thanks to the crash detection being deacticated I flew just through a radio tower... jep, in 3000ft.. :P
I bet it was the Hillsboro antennas (I guess it's the KVLY tv station antennas) - I got my pilot's license in Grand Forks ND, just north of there - and sure enough, they are there. Actually the western portion of ND is very scenic, and not so flat... many hilly portions of the state and buttes takes the elevation up to around 3500 msl ft in places.

https://en.wikipedia.org/wiki/KVLY-TV_mast

Used to be the world's tallest man made structure. I believe it's second now at 3125 ft MSL

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Orlaam
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Re: Thought this was neat (GTN750 related)

Post by Orlaam »

An amateur radio operator's dream tower! Good luck climbing it to fix something! :shock:
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AKar
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Re: Thought this was neat (GTN750 related)

Post by AKar »

shortspecialbus wrote:Part of how GPS determines your position is by also determining your altitude. If you think about it, because the whole thing is 3-dimensional, it would pretty well have to. Combination of triangulation and atomic clocks measuring time delay and such, although I'm grossly oversimplifying.

-stefan
It is actually rather complicated. Determining the pseudoranges is the easy part when speaking of precision applications. The issue is that this position fix often is somewhat jumpy, depending on the conditions. Position shown to the user is typically a solution produced by some significant Kalman filtering, which essentially works by predicting how the position fix would evolve and by constantly updating that prediction by observed (but noisy) fix. Essentially this process is what you see when you bring your phone's GPS on for instance, when the position first drifts around a little before stabilizing to more or less where you are. It is a property of Kalman filter that it is somewhat jumpy, and tends to drift around the true value (which is obviously unknown). This filtering is often improved based on certain assumptions putting physical constraints to the receiver's location: a hand-held device for instance can typically be assumed to remain close to Earth's surface, nailing down the solution pretty firmly. In aviation GPSes, I understand the classic way to help with a good solution in aviation use where the fix is moving in three dimensions over time, is to restrict the allowed evolution of the position fix in coordinates where little change in motion is expected. How the exact filtering process is constructed can mean that one coordinate is sacrificed in stability or accuracy to make the rest more stable. I understand that at least the early GPS devices were less solid in their vertical accuracy than in latitude+longitude.

At least in one aviation-related case I am aware of where the accuracy of the GPS was found insufficient on its own, the accuracy was improved by having several independent units installed to the extremities of an airplane, so that their geometry in relation to each other was known, and these were fed into another round of filtering.

It would be interesting to monitor the "raw" data from a modern GPS to see how they hunt you down!

-Esa

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