Global Positioning System Reference
In-Depth Information
8.4.3 Airborne Application
Flight reference systems (FRSs) using carrier phase or interferometric GPS (IGPS)
techniques have been implemented and flight-tested on transport-category aircraft.
The underlying principle of operation is similar to that used for kinematic survey-
ing and is also referred to as differential carrier-phase tracking. Figure 8.18 depicts
such a system where differential techniques are employed. In this case, raw
observables from all SVs in view are transmitted from a ground subsystem via
datalink. The carrier-cycle ambiguity resolution is done on the fly aboard the air-
craft. Onboard the aircraft, position relative to the runway touchdown point is cal-
culated in near real time and provided to the aircraft autoland system [21]. The
objectives of this IGPS FRS included such things as 0.1-m accuracy RMS (each
axis), one or more updates per second, UTC (USNO) time synchronization better
than 0.1 ms real time, all-weather operation, and repeatable flight paths. The latter
requirement calls for full aircraft integration and coupled flight. Specific applica-
tions for such systems include evaluation of approach/landing systems [such as
instrument landing system (ILS), microwave landing system (MLS), and tran-
sponder landing system (TLS)], and test range instrumentation calibration (e.g., all
manner of tracking systems: laser, infrared, optical, and radar). Precision instru-
mentation of test ranges themselves can be accomplished with special GPS
receiver/datalink equipment aboard aircraft using the test range. Such a system
would perform the ambiguity resolution at an appropriate ground site for each
vehicle using the test range and provide position data to designated test range track-
ing facilities. In addition, feasibility studies in the areas of precision landing/
autoland, low-visibility surface operations (taxiing, docking), high-speed turn-off,
parallel runway operations, input to electronic charts, and four-dimensional navi-
gation can also be supported.
b
Time, SVID
Code phase
Integrated carrier phase
Site information
o
Figure 8.18
Interferometric GPS flight reference system.
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