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MOTOROLA INC. Technical Developments Volume 14 December 1991
GLOBAL POSITIONING SYSTEM DIGITAL SPHERICAL COVERAGE
by S. J. Sheard
PROBLEM SOLVED
Global Positioning System (GPS) receivers operating on an airplane need to be able to receive satellite signals in all tlight modes. Spherical tracking of GPS satellites by a GPS receiver on an airplane is currently accomplished by multi- ple antennae feeding into coupler circuits or special antenna. switching networks. GPS receiver operation without mul- tiple antennae is possible but signal acquisition is lost dur- ing maneuvering due to antennae not having full spherical coverage because of signal blockage by the airframe.
The same problem is encountered in motor vehicles when a single antenna cannot be roof-mounted and so can- not provide a clear line-of-sight to all satellites. Multiple antennae on motor vehicles exhibit the same problems as those on aircrat?, with signals beiig blocked by the vehicle roof canopy.
Current GPS receivers use a single pm-amplifier, down converter, and digitizing network. Digitized signals feed into a Digital Signal Processor, which must lock onto the signal, synchroniziig to it to decode satellite data in order to perform a range measurement. The delay required for these operations, typically 60 seconds, is not acceptable when it is initiated by an airplane or vehicle turning during a maneuver. The receiver design shown in Figure 1 elimi- nates problems with out-of-phase signals bciig combim~ cable losses from excess cable length, and signal strength loss from mixing circuits.
PROBLEM SOLUTION
Figure 1 schematically illustrates a new approach to GPS receiver design which solves the problems described above. In this approach, two identical GPS front end circuits, Fl, F2 comprise two half-spherical antennae Al, A2 mounted on the aircraft top and bottom (or the vehicle front and rear). Two pre-amplifiers/down con- veners Pl, P2 are each coupled to digitizing circuits Dl, D2, and to antennae Al, A2, respectively. GPS signals received by antennae Al, A2 pass through pm-amplifiers/ down converters Pl, P2, respectively, and are then digitized
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by digitizers Dl, D2. The digitized data are output on paths 1, 2 to Digital Signal Processor DSPl including program Pl, where data from each of paths 1, 2 are separately processed.
Figure 2 depicts the relationships between Software Modules Sl, S2 and Control Program Module CPl, all...
