GPS 101

[weirdjim]

I'm teaching a freshman class in very basic electronics. One of the requirements is for a technical paper with the usual academic BS --- MLS format, full references, bibliography, figures, etc..

One of the students wants to do a paper on Aviation GPS. I took a look at where I'd go to start my research (FAA, Trimble, Garmin...) and to a fault each of them had something that isn't true in today's world. One of them even talked about the military dither (SA) that makes the GPS inaccurate. I thought that went away 15 or 20 years ago.

Anyway, anybody got a good aviation GPS site that does a decent semi-technical explanation of how GPS works in TODAY'S aviation world? Remember, this is a fresh(wo)man with no electronics OR aviation experience and the paper is more to introduce the method of writing a formal paper in a technical field than to be absolutely technically precise to the fourth decimal place.

Thanks,

Jim

 

[MAKG1]

I wouldn't go into the technicals given what the goals are. There are a fair number of weeds to get stuck in. It is a VERY large topic.

Maybe how GPS is used in aviation, e.g., LPV approaches from the pilot's perspective.

SA was turned off in 2000.

Maybe a better idea is to get the student to narrow down the topic a bit.

 

[Stewartb]

http://www.mwri.gov.eg/PDF_files/GPS-for-Beginners.pdf

 

[flyingron]

Alas, that Garmin doc falls into the same trap of trying to simplify the GPS "how it works" to the point of it being wrong. They start out right by tell you that your GPS doesn't really know the exact time, but it then goes and does the intersecting spheres explanation which can't possibly work if you don't have the time. You don't know how far you are from satellite A and satellite B, just the relative difference in the distances between them. This gives a hyperboloid surface and you intersect those from different pairs of satellite to get a decent fix (need at least three). Note that once you have a pretty good guess as to your position, you can then go back and compute the time. No I don't know anybody who explains it right simply enough for people to understand.

It's sort of like telling people that the VOR determines position by a pulse that goes out at 360 omnidirectional and then some time later a directional signal, when in fact the signals are transmitted continuously and you're using the phase. Nobody whose not an engineer probably understands the latter explantion.

 

[weirdjim]

It's sort of like telling people that the VOR determines position by a pulse that goes out at 360 omnidirectional and then some time later a directional signal, when in fact the signals are transmitted continuously and you're using the phase. Nobody whose not an engineer probably understands the latter explantion.

Sitting the ground school students around you in a circle, using a bright strobe flashlight pointing up and a weak flashlight that you slowly rotate in a circle (with appropriate timing) is very effective. You are right, trying to explain phase angles until about the second or third course in electronics just phases up the works.

I'll never forget my embarrassment ... First semester in college, never heard the phase story, hired by the airlines, boss asks me if I'm flying TO a VOR and I turn around, what does the flag in the VOR head do. I said it flipped to FROM. And I was a student pilot at the time. Never lived that down in the five years I spent there.

If my student does a decent job of it, I'll post her paper on the RST website for you all to use when you try and explain it.

jw

 

[Velocity173]

While SA was removed in 2000, military GPS are still required to operate in the PPS or encrypted P (Y) code. That is until all are full up (M) code. Reason is for better accuracy (some cases) and anti-jamming / spoofing purposes.