Commercial GPS Receiver for Time and Frequency Equipment Applications

The NAVSTAR Global Positioning System (GPS) currently being readied for production by the Department of Defense will provide significant improvements in the accuracy and cost-ofownership features of future electronics systems used for navigation, positioning, and time and frequency reference applications. The Collins Avionics Group of Rockwell International has developed a commercial version of a modular sensor unit called the NAVCORE 1." GPS receiver which derives position. velocity and time data from the satellite signals. Digital data outputs are updated at the rate of one complete solution per second making the sensor function applicable to a wide range of dynamic and static product applications. This paper describes the features and capabilities of the timing version of the NAVCORE 1'" receiver and describes the interfaces required for its use as a sensor in time and frequency product applications. Introduction With the recent contract award by the Department of Defense for the first production GPS user equipment, the production phase of the third element of the NAVSTAR GPS program has begun implementation. The first two elements. satellite vehicles and ground command stations, have been underway for over a year, with the first production satellites scheduled for launch starting in October of next year. Based on the current launch schedules worldwide satellite coverage will be available for 2D navigation by late 1987, with full 3D navigation coverage by mid-1988. While the primary focus of the GPS program is on its revolutionary improvements in accuracy and cost-of-ownership for navigation and positioning systems, it offers the same kind of benefits for time and frequency reference product applications. Furthermore, these benefits can be realized several years sooner, with precision time signals already available for 16-20 hours per day from the development satellites, and full worldwide time coverage expected to be available by the beginning of 1987. Commercial Applications Collins Government Avionics has been actively involved in the development of military GPS user equipment since 1974. During that time as we have participated in the transition from advanced technology concepts to application hardware, we have also participated in the promotion of the system benefits for worldwide commercial applications. Two years ago, when we became convinced that the CIA code signals would ultimately be made available for commercial uses without restrictions or usage charges, we initiated product plans to apply the applicable parts of our hardware and software development work to non-military applications. This paper describes the first commercial product resulting from that effort, with emphasis on its possible time and frequency reference system applications. Time & Frequency System Applications Theoretically, GPS time receivers would be able to continuously deliver precision timing signals within the average of the CH2186-0/85/0000-0150$1.00019851EEE 150 eighteen atomic clocks on board the satellites, if satellite positions were always precisely known and if there were no ionospheric refraction to degrade signal transmission. For each user application the architectural complexities required to reduce the effect of these error sources to yield the desired accuracy must be weighed against the mechanization cost of the function. In Collins developmental programs during the past two years, timing signal accuracies of less than I O nsec have been demonstrated using both the NBS-designed common view receiver and a modified military P-code GPS receiver equipment which provides both position and time outputs. Timing signal accuracies of less than 100 nsec are now being consistently demonstrated using stand-alone C/A code receivers. NAVCORE 1'" Product Features Although Collins Government Avionics is not directly involved in the development of time and frequency reference products. time information is a standard output of the navigation solution from the recently introduced NAVCORE 1'" commercial navigation sensors. The NAVCORE 1'" receiver is a single-channel, sequentialtracking receiver that computes position, velocity and time solutions from the C/A code using the L1 frequency. It is designed to perform the navigation sensor function for vehicles with speeds up to 600 knots and accelerations of I g. Through software modifications to force the velocity terms to zero and optimize the Kalman filter parameters, a customized version of the system software has been implemented to specifically meet the requirements of a basic GPS time sensor for use as a building block in a wide range of time and frequency reference products. Operating Features A single channel sequential design is used with the tracking circuitry dwelling upon each of four satellites for one fourth second. thus making measurements for a complete solution update once per second. After the receiver antenna position has once been established through an internal four-satellite position solution, integrity of the time transfer function is maintained with single satellite signal reception. Tracking is interrupted only to acquire satellites (as needed) and to collect ephemeris data periodically. During these intervals tracking is halted for periods of six seconds. which is a standard data subframe interval, with velocity terms held constant to span the data collection intervals. For time applications, this update interval has negligible effect on system performance since the Kalman filter models the frequency error of the receiver's internal frequency standard. Operating software is designed for minimal demands on the user. For example, under normal operation the receiver provides a l-pulse-per-second (pps) signal which is automatically slaved to UTC and a digital data message which identifies time at the pulse. No operator inputs are needed under normal startup conditions because the receiver has "keep alive" memory and a low-power coarse time reference source. At power-down, the last computed position and almanac parameters for all satellites are retained in random access memory. When power is returned, the stored data and approximate knowledge of time from the internal reference allow satellite acquisition to begin automatically. The NAVCORE 1'" receiver implementation uses a TCXO rather than an ovenized frequency standard, so no warm-up time is needed, and the acquisition process begins immediately with the application of power. The first solution is typically obtained in two and one half minutes. The operating software also has a "cold start" mode which is used when critical memory has been lost through battery replacement or other equipment repair. In this mode the operator is requested to designate a satellite for acquisition and to enter the receiver's position within the nearest degree of latitude and longitude. Using this data the receiver is able to bootstrap itself into normal operation within approximately 20 minutes, since almanac data for all satellites must be collected before the normal acquisition procedure can begin. System Architecture The NAVCORE 1'" receiver architecture has been specifically devised to achieve low production implementation cost. Special effort was made to utilize off-the-shelf commercial components and low-cost technologies wherever possible without compromising operational reliability. For example, along with the selection of a TCXO, several stages of conversion were employed to distribute system gain at a number of frequencies. This approach has reduced the risks of regeneration and yielded significant producibility benefits. Figure I shows the detailed frequency plan which is implemented.