Real-time APL prototype of a GPS system

Abstract

The Global Positioning System (GPS) consists of a constellation of 24 high-altitude satellites with very accurate atomic clocks, along with a global network of satellite tracking stations and sophisticated ground processing stations, that together provide precise navigation coordinates to any user who possesses a small, readily available GPS receiver.The precision that is achieved depends on[1] the number of GPS satellites in view of the user and the geometries involved;[2] the design of the user's receiver (e.g., military or civilian);[3] the presence of nearby objects that may interfere with or degrade the reception of signals from one or more satellites;[4] and other factors.In most cases civilian users (whose receivers can read only unencrypted, intentionally corrupted GPS signals) can expect navigation accuracy on the order of 50-100 meters. The advertised accuracy for users with military receivers, which read the encrypted GPS signals, is 16 meters. Coalition forces in Desert Shield / Desert Storm relied heavily on GPS, and U.S. military authorities praised GPS effusively for its effectiveness as a force multiplier in that conflict.Loral Federal Systems (formerly IBM Federal Systems Division) in Gaithersburg, Maryland has been intimately involved with GPS from its concept validation phase through operations and maintenance. People in this group analyzed the physical dynamics, designed the mathematical algorithms and associated software, and performed the system integration work that culminated in the GPS ground control segment.APL was used over the years in most major aspects of GPS design and analysis. Early in the full-scale development phase, APL was used to model the initial GPS system design, to perform design tradeoffs, to model and test candidate algorithms, and to perform error analyses to estimate future system performance. In later phases APL was used to monitor and analyze actual GPS system performance, to compare it with theoretically achievable performance, and to analyze and isolate anomalies [Allan].