Using the Canadian Active Control System (CACS) for Real-Time Monitoring of GPS Receiver External Frequency Standards

The Canadian Active Control System is a national network of twelve continuously operating geodetic quality GPS receivers equipped with external atomic frequency standards. Each GPS tracking station provides dualfrequency pseudorange and carrier phase measurements in real-time at 1 Hz to a central computer where wide area GPS corrections are generated using filtered ionospherefree combined pseudoranges and predicted ultra-rapid GPS orbits. In contrast to the technique of fixing one of the station clocks, the central process estimates satellite and station clock phase offsets with respect to a Virtual Reference Clock (VRC) that is maintained as a weighted mean of all estimated receiver clocks. The VRC is related to the smoothed mean GPS system time as provided by the broadcast clock parameters of tracked satellites, and its alignment is maintained by regular steering operations. This approach mitigates the effects of instabilities of individual station frequency standards and provides continuity of the time reference. This paper discusses the use of this system to monitor GPS satellite and receiver clocks. First, the central computing system provides continuous synchronisation of the network of GPS receiver clocks and GPS satellite clocks at the ns level. Second, using the wide area satellite orbit and clock corrections, users can synchronise dual-frequency GPS receivers in real time at a precision consistent with the satellite clock estimates. INTRODUCTION The Geodetic Survey Division (GSD) operates, in conjunction with the Geological Survey of Canada, the Canadian Active Control System (CACS). It comprises of a Master Active Control Station (MACS) and a network of continuously operating GPS data acquisition stations, called Active Control Points (ACPs), which are distributed across the Canadian landmass and track all GPS satellites in view. The GPS data from several ACPs are contributed to the International GPS Service (IGS). The NRCan Analysis Centre (EMR), also located at GSD, processes the Canadian and a portion of the global IGS data to generate daily precise GPS satellite ephemerides and clocks, earth orientation parameters (EOP), ionospheric, tropospheric and terrestrial reference frame information, as well as rapid GPS orbit predictions [Tetreault et al, 1998]. Twelve of the CACS tracking stations (Figure 1) facilitated by real-time data communication form the Canada-wide network of Real-Time ACPs (RTACPs). Another RTACP (USN2) is located at the U.S. Naval Observatory, in Washington, DC, and an additional one (RAYM) is located at the Ottawa GSD offices and serves for development purposes. Data from these RTACPs are communicated to the Real-Time Master Active Control Station (RTMACS) in less than two seconds for processing and storage. The RTMACS combines the tracking data and forms ionosphere-free, carrier-phase filtered pseudoranges to compute in real-time, clock phase offsets for all available satellites and receivers, using the predicted IGS ultra-rapid GPS satellite ephemerides and fixed RTACP coordinates. Corrections to the broadcast satellite orbits and a grid of vertical ionospheric delays for a single layer model of the ionosphere are also generated. The corrections to the broadcast satellite clocks and orbits along with the ionosphere model make up the GPS corrections service (GPS•C), which facilitates real-time GPS positioning of about half a meter horizontally and a meter vertically (1σ), when using geodetic quality receivers. The system architecture, data communication infrastructure and the real-time application software are described in Caissy et al [1996]. The RTMACS configuration, processes and the GPS•C positioning performance are described in Skone et al [1996] and Lahaye et al [1997]. Solving for all available clocks at each epoch requires a time reference and usually, as most IGS Analysis Centres do, this reference is achieved by removing a selected (reference) clock from the system of equations. All clock phase offsets are thus estimated with respect to the selected clock. In the context of a real-time system, with potential communication interruptions and variable delays, this technique proved inadequate, especially in the light of two requirements of the GPS•C service: maintaining continuity of the reference clock and maintaining its alignment to GPS time within reasonable bounds. To meet these requirements, we have implemented a so-called Virtual Reference Clock (VRC) that is maintained as a weighted mean of selected clocks in the system. Since the clock weights need to be controlled automatically, performance measures based on clock models were developed. Specifics of the VRC implementation and clock models were described in Lahaye et al [1998] and are summarised in a section

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