Gps Accuracy

Civilian users have access to standard positioning service (SPS). SPS has an accuracy of 15 - 25 meters for 95% of the position fixes. For security reasons, the American Department of Defense intentionally degrades the accuracy for civilian users to 100 meters for 95% of the fixes.

Military users have access to precision positioning service (PPS). PPS has an accuracy of 18 meters or less for 95% of the fixes.

GPS - THEORY OF OPERATION – 2

Ranging

The GPS receivers use the principle of ranging to measure the distance between the receiver and the satellites. The receiver always has the location of the satellites in their orbits in memory.

The receiver measures the time it takes for a radio signal to go from a satellite to the airplane. Since the receiver knows the location of the satellite and that the radio signal travels at the speed of light, it can calculate the distance to the satellite. The receiver uses one way ranging. The receiver must know exactly at what time the satellite sent the radio signal. The receiver compares the satellite signal to a signal that the receiver makes at the same time as the satellite. The difference between the two signals (Δt) is the time the satellite signal took to get to the receiver.

Each satellite has an atomic clock to keep accurate time. All the satellites have precisely the same time. The receiver in the airplane has an internal clock but it is not atomic. It is not as accurate. Thus, it is not possible for the receiver to have precisely the same time as the satellite.

The receiver assumes that its internal clock is off by some clock bias (ΔtBIAS). This ΔtBIAS is an unknown that the receiver must calculate. The ΔtBIAS is the difference between the receiver time and GPS time.

To calculate the airplane position (latitude, longitude, and altitude) and the ΔtBIAS, the receiver must know the position of at least four satellites. The receiver then measures the distances to all the satellites at the same time. It then solves for these four unknowns with four range equations:

• Latitude

• Longitude

• Altitude

• ΔtBIAS.

Gps Time

All the satellites synchronize to universal time (coordinated) (UTC). The satellites transmit this time to the receiver. The accuracy of the satellite UTC is approximately 100 nanoseconds. The receiver transmits UTC on an ARINC 429 format. The receiver also transmits a very accurate time mark once per second.

GPS - THEORY OF OPERATION – 3

Gps Modes of Operation

The GPS operates in these modes:

• Acquisition mode

• Navigation mode

• Altitude aided mode

• Aided mode.

Acquisition Mode

The GPS looks for and locks on to the satellite signals. The GPS

must find at least 4 satellites before it starts to calculate GPS

data. The GPS gets data from the air data inertial reference

system (ADIRS). The GPS uses this data from air data inertial

reference unit (ADIRU) when it is in the acquisition mode:

• Present position

• Altitude.

The GPS uses this data and its internal database to calculate which satellites are available at the present airplane position. The GPS takes less time to get to the navigation mode with this data.

If the ADIRU data is not available, the GPS can still acquire satellites signals. It takes longer to acquire the satellite signals because it has to look for all the satellites. When the GPS finds the satellites, it calculates which ones it can use.

The GPS takes approximately 75 seconds to acquire the satellite signals when the ADIRU data is available. The GPS takes approximately 4 minutes (maximum of 10 minutes) to acquire the satellites when the ADIRU data is not available.