View Full Version : Can't get a straight answer on GPS question

17th Jan 2017, 04:36

Some publications state that your GPS receiver needs a minimum of 3 satellites to be received and 4 satellites at any given time in a (geometrical) plane.

However, I've read some interview gouges that state you need a minimum of 4 satellites and 5 in a (geometrical) plane.

Which one is correct? Cathay interview, Emirates interview, etc.


Check Airman
17th Jan 2017, 04:39
My understanding is that you need 4 to get a signal for IFR. The 5th is for RAIM.

17th Jan 2017, 04:57
The GPS signal is a basic time/distance equation from a minimum of 3 satellites. Unfortunately, the time in the GPS receiver is not as accurate as the time in the satellite clock, which introduces a 4th variable.

1) latitude
2) longitude
3) height above the geoid

So a minimum of 4 satellites for a GPS position. If you have a known, say barometric height, it drops to 3 satellites.

For RAIM, we need 5 satellites. This allows the receiver to detect a problem with one of the satellites, but not which one is faulty. For that, we need 6.

17th Jan 2017, 06:57
Yeah depends on the question.

We need 4 satellites in good geometry to resolve a 3D position, but, with only 4 we can't be sure of the solution's integrity (i.e., to be alerted if one of the satellites is faulty and transmitting bad data).


With 5 satellites, the extra satellite can be used to detect a fault. This is the RAIM Fault Detection system (aka traditional RAIM). RAIM FD can alert us if there's a problem, although it can't tell which of the 5 satellites is faulty.

With 6 or more satellites, a more advanced RAIM function can detect a faulty condition, determine which satellite is at fault, and exclude it from calculations. (FDE: fault detection and exclusion). The assumption here is that there's at most one faulty satellite.

AAIM: some systems can use the barometric altimeter, INS, or other aircraft sensors as a substitute for one satellite in RAIM calculations. I.e., with baro-aiding, we only need 4 satellites to get RAIM FD, and 5 satellites for FDE.


WAAS uses a series of ground stations to detect GPS faults and compute various error parameters. The results are then transmitted via additional satellites in geosynchronous orbit. So if WAAS is available, it can be used instead of RAIM to provide integrity alerts.

In most jurisdictions we need either RAIM or WAAS to use GPS as the primary means of navigation. So that can be a minimum of 5 GPS satellites (to get RAIM FD), or 4 GPS satellites + AAIM (also RAIM FD), or 4 GPS satellites + a WAAS satellite.