GPS – MAIN COMPONENTS
The GPS main components are:
• one receiver (that is a GPS module inside MAU 2)
• one antenna
The GPS receiver uses the Commercial Access (C/A) code of the
NAVSTARGPS satellite constellation and can operate when selective
availability (SA) is activated and deactivated.
GPS – CONTROLS AND INDICATORS
The performance of the GPS can be monitored by the GPS 1 STATUS
pages (1/2 and 2/2) and PREDICTIVE RAIM page (1/1). (RAIM = Receiver
Autonomous Integrity Monitor calculations).
RAIM and FOM indicate current uncertainty of position expressed in nautical
miles. HDOP and VDOP are numbers that rate current satellite geometry in
the horizontal (HDOP) and vertical (VDOP) axis with 1 being the best
geometry. Normally, HDOP and VDOP numbers are below 10.
The fifth line displays the operational mode of the GPS. Possible operational
modes are displayed as follows:
• SELF-TEST
• INITIALIZATION
• ACQUISITION
• NAVIGATION
• DIFFERENTIAL
• ALTITUDE AIDING
• VELOCITY AIDING
• FAILED.
The acquisition mode is used to acquire satellites after power is applied. The
GPS tracks four satellites to acquire its position.
After being in the navigation mode, altitude aiding is the mode entered when
fewer than four satellites are being tracked. In this mode, the GPS uses
altitude from the digital air data computer (DADC) to aid in determining
position.
Predictive RAIM (PRAIM) calculates the estimated value of the Horizontal
Integrity Limits (HIL) at some future place and time. The FMS can interrogate
the PRAIM function of the GPS through the ARINC 429 interface. However,
RAIM integrity performance requirements cannot be selected with the GPS.
GPS – PRINCIPLE OF OPERATION
The GPS receiver operations are transparent to the crew. Each GPS has
RAIM (Receiver Autonomous Integrity Monitor) outputs for the current
position and time in the form of Horizontal and Vertical Integrity Limits (HIL
and VIL). In order to compute RAIM, the GPS must have a minimum of five
satellite signals. The FMS does not accept GPS data unless a valid RAIM
figure is available.
Therefore the GPS RAIM function assures the integrity of the data because
the GPS RAIM function can detect satellite failures. It isolates and removes
failed satellites when it is tracking a sufficient number of satellites for
measurement redundancy.
The FMS uses predictive RAIM to determine the integrity levels at specific
locations/times to support a non precision approach and the flight planning
activities of the pilot.
The GPS has the following types of RAIM predictions:
• destination
• alternate waypoint
The destination and alternate waypoint predictions are made at specific
locations or they are the Estimated Time Of Arrival (ETA) when the FMS
makes the request for flight planning purposes. Satellites can be manually
deselected or enabled for destination and alternate waypoint predictions.