Navigation Mode

The GPS enters the navigation mode after it acquires and locks on to at least 4 satellites. When the GPS is in the navigation mode, it calculates GPS data.

Altitude Aided Mode

With four satellites available, the GPS stores the difference between the ADIRU inertial altitude and the GPS altitude. The GPS stores the difference between inertial and GPS altitude so that it can estimate the GPS altitude when only three satellites are available.

In the altitude aided mode, the GPS sums the airplane altitude from the ADIRU and the length of the earth radius as the fourth range.

The GPS enters the altitude aided mode only after these three conditions are true:

• The GPS was in the navigation mode

• There are only 3 satellites available with good geometry for position fixes

• The GPS stored the difference between inertial and GPS altitude in memory.

The GPS starts normal operation again when a fourth satellite comes into view.

Aided Mode

The GPS enters the aided mode during short periods (less than 30 seconds) of bad satellite coverage. An example of bad satellite coverage is where at least four satellites are available but the airplane banks and loses satellite reception.

In the aided mode, the GPS receives inertial altitude, track angle, and groundspeed from the ADIRU. The GPS uses the ADIRU data to go back quickly to the navigation mode when there is good satellite coverage again. The GPS output is NCD in this mode.

If the GPS cannot track any satellites for 30 seconds or more, the GPS goes to the acquisition mode.

GPS Frequencies

The satellites transmit to the GPS in the airplane on the L1 (1575.42 MHz) and L2 (1227.6 MHz) frequencies. The satellites downlink satellite status data to the monitor stations on 1783.74 MHz.

The upload stations send information to the satellites on 2227.5 MHz.

Autonomous Integrity Limit

The GPS has a receiver autonomous integrity monitor (RAIM) function. The RAIM monitors the status of the satellites that the GPS uses for calculations. The output of the RAIM function is an estimate of the GPS position error. The autonomous integrity monitor value goes to the FMC. The FMC uses the autonomous integrity monitor to determine if it can use GPS data for navigation.

Values Calculated by the GPS

These are some of the values that the GPS calculates:

• Latitude

• Longitude

• Altitude

• Universal time (coordinated) (UTC)

• Date

• North/south velocity

• East/west velocity

• Vertical velocity

• Track angle

• Autonomous integrity limit

• Satellite position

• GPS status.

GPS - FUNCTIONAL DESCRIPTION

Power

The power supply makes various dc voltages from the 115v ac input to the multi-mode receiver (MMR).

Satellite Signal Processing

The GPS antenna first amplifies the satellite signals and then sends them to the MMR. The low noise amplifier (LNA) in the MMR receives and amplifies the satellite signals from the GPS antenna. The receiver detects the satellite signal and sends it to an analog to digital converter (A/D). The A/D sends the digitized signal to the microprocessor. The microprocessor calculates the airplane position and other GPS data. The GPS data goes to these units:

• Ground proximity warning computer (GPWC)

• Clocks

• Flight management computers (FMC 1 and 2).

The IRS master caution unit receives digital fault data from GPS systems 1 and 2.

Inputs

The ADIRU sends inertial reference data on output bus 4 for initialization. The MMR uses this data in the aided and altitude aided modes.

The program pins tell position to the MMR. This is used for the source destination identifier (SDI) code in ARINC 429 data.

GPS - GPS FAULT MONITORING - FUNCTIONAL DESCRIPTION