Kshnandi

Hi!
I'm a student learning more about the GPS nav systems of a commercial aircraft. I want to know more about the accuracy of the location tracking systems when the flight is below 500 ft. How accurate can we get when it comes to long and lat? What are the Thresholds?

Texandrvr

Check out http://www.usno.navy.mil/USNO/time/gps for all your gps info.

JohnBurke

I spend a lot of time below 500' and use GPS frequently to find fires in the mountains, and targets. GPS works just fine, right down to the ground. It's accurate enough when spraying that spray swath widths, dispensing chemicals on crops, are extremely accurate and uniform for every pass down the field. We use GPS for navigation, delivery, even weapons delivery; it's quite accurate (in some applications more than others).

GPS can also be degraded by the user, or by others.

yerongard

When I used to fly a very small aircraft with very modern avionics, it was within 3 meters 99.9% of the time. That was with WAAS and also inside of the coverage area.
Now I'm in a large aircraft with people in the back. It's not as reliable nor accurate. Can't give specifics about accuracy because the avionics suite does not have the information readily available. But I will say that it has malfunctioned a few times.

rickair7777

There is nothing about terrain proximity which would degrade GPS.

Ground based NAVAIDS get degraded because their signal gets distorted as it travels over the terrain. GPS signals (from satellites) typically will not be traveling over terrain

If you're in a deep mountain valley or below large mountain peaks, the GPS system might lose LOS view on some satellites. In an extreme case, it could drop below the minimum four satellites needed for GPS NAV redundancy so you would lose RAIM and be required to abort/switch to other NAV systems.

But RAIM is pretty black-and-white...you either have RAIM and the required GPS accuracy, or you do not. The system knows and will tell you.

Ground-based signals could be distorted but still strong enough to be detected so it would be possible to experience NAV errors without realizing it. But those systems have published service volumes for that reason. Looking at those you'll see a smaller range at low altitudes. Some procedures will have special notes if NAVAID reception is a problem in certain areas or altitudes.

2old2fly

It works fine on my cell phone at ground level. Accurate enough to know which lane I'm in on the freeway. I would hope that planes have a better system than I do.