The GPS is a United States Department of Defense funded and maintained military navigation system that has been in use since 1995. Full Operational Capability (FOC) of the GPS requires 24 blocks of II/IIA or later satellites functioning in their assigned orbits. Though GPS lacks the accuracy of aircraft inertial navigation systems to be used in all phases of flight, it is often used in oceanic and/or remote airspace travels and as a supplementary system when backed by an alternate means of navigation. This system was immediately made available to civilians at no cost, though civil users are denied access to the highest military accuracies. Despite this restriction, the system that civilians are permitted to use is extremely precise for integration in all modes of transportation. This article serves as a guide to GPS aircraft installation and operation.
Installing GPS involves the assembly of several components. To start, aircraft GPS receivers are embedded in the Multi-Mode Receiver (MMR), which also houses ILS-, MLS-, and VOR-receivers. The GPS receiver is a standalone sensor that complies with ARINC 743B characteristics, providing an ARNIC 429 Data Bus (DITS) output to interface with the aircraft systems that require a GPS-pvt input. Data buses are often interfaced with FMC’s for FMS present position updates, EGPWS as PPOS input for terrain avoidance, clocks to synchronize time with GPS derived UTC, and emergency locator transmitter (ELT) beacon for crash site reporting.
Next, a GPS antenna is located on top of the fuselage; though, it is quite small, and is hard to spot due to the array of various antennas sticking out of the aircraft. This antenna is active to compensate for the coax cable that runs to the MMR, and the Low Noise Amplifier (LNA) is then powered through the coaxial center conductor that carries the rf-signal to the receiver.
Some of the first GPS installations featured a self-contained receiver located in the aircraft ceiling near a passive GPS antenna on top of the fuselage. These types of installations were referred to as the Global Navigation Satellite Sensor Unit (GNSSU) and are no longer in use. However, some constructions may find a GPS receiver as a part of a Flight Management Computer System (FMCA) or the Enhanced Ground Proximity Warning System (EGPWS) in retrofit installations without an MMR.
When electrical power is available and the circuit breaker is closed, the GPS receiver is ready for use and will provide output without additional controls. Time-To-First-Fix (TTFF) can be reduced if one inputs their present position into the receiver. As such, the PPOS can be derived from the IRS or memorized by the GPS from the prior flight; this IRS input can aid the receiver during brief periods of GPS signal loss so that re-acquisition can occur more quickly. When the GPS position becomes available, it can be used as an IRS alignment PPOS input to align the IRS’s with accuracy. The GPS receiver features built-in test equipment that will alert the pilot and crew to any receiver failures through messages on the applicable aircraft warning and caution systems.
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