GPS - Global Positioning System

Frequency Range   0 Hz

How it works

Basic GPS theory is rather simple. First of all, when your GPS receiver picks up a satellite, it figures out how far away the satellite is. It does this using pulsed RF transmissions. Now, let's say that you have found a satellite and figured out you are 11,000 miles from the satellite:

1. Your possible position is limited to any point on the surface of a 11,000 mile diameter sphere.

2. With 2 satellites, your position is limited to a circle points.

3. With 3 satellites, your position is limited to 2 points, one of which will be unstable and is discarded since it will be moving rapidly in comparison to the other which is your position.

Remember that these spheres that represent your possible positions, will contract and expand as the satellites move away and toward you. If you were to increase the size of the sphere without the satellites moving away from you, an inaccurate nav solution would be resolved. Since the system relies on distance between the receiver and satellite and the distance relies on time based calculations, inaccurate time calculations would will produce inaccurate distance calculations, which will result in an inaccurate navigation solution. This is why the satellites are equipped with multiple atomic clocks and the receiver, if not equipped with an atomic clock, is equipped with a very accurate clock.

ARN-151 LRUs

R-2325 - The RPU (Receiver-Processor Unit)
AE - The AE

Basic Troubleshooting

The ARN-151 system is pretty simple. Most problems can be solved by changing either the RPU or even the AE, and the GPS test can help steer you toward the right LRU. Just be sure to check out your antenna lines and connections if you're getting an AE fault.

And remember, just because the GPS test comes up with a fail, this doesn't mean the system isn't good for flight. More often than not, GPS will fail, yet the system isn't written up and the FOM drops to 1 quickly. When checking out a system that's been written up, the first thing you should do is enter accurate date and GMT information, then run the GPS test, and then finally, go to the DATA page (since test will put it in NAV automatically) and watch. If you don't get any satellites whatsoever after 15-20 minutes, you may have a problem. I say may, because you must remember that GPS is a line of sight system. If the aircraft is parked too close to a hangar, you may not be able to get any satellites. Not to mention, your geographical position may not be suitable for proper satellite communication. And on a smaller scale, anything immediately blocking the antenna (like rotor blades) can cause problems as well.

Take the GPS test
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