The Development of Multi-channel Gps Timing Receivers

In 1998 the Time and Frequency (TF) project of the CSIR National Metrology Laboratory (CSIR-NML) had to decide on its replacement strategy for the Allan Osborne and Associates (AOA) TTR5 and TTR5A single channel Global Positioning System (GPS) timing receivers. These receivers would have stopped working at the GPS week rollover, which occurred on 22 August 1999 at 0h Universal Coordinated Time (UTC). It was extremely important to replace these receivers, as they form the primary traceability link for time in South Africa. Several options were available to the CSIR-NML. These included replacing the firmware of the AOA receivers, purchasing new timing receivers and designing and building receivers in-house. The replacement of the firmware of the AOA receivers was ruled out due to costs and the age of the receivers. The purchase of new timing receivers was ruled out due to cost. This meant that receivers had to be designed and built at the CSIR-NML. These receivers were put into operation in the week preceding the GPS week rollover. Introduction The TF project of the CSIR-NML has the responsibility for the maintenance of the time scale of South Africa. This is done in coordination with the International Bureau for Weights and Measures (Bureau International des Poids et Mesures BIPM) in Sevres, France. The time scale for South Africa is generated in the TF laboratories. The primary reference clocks are cesium beam atomic clocks. The offset between the TF clocks and UTC is determined using common view time transfer via the GPS constellation. The GPS timing receivers of the TF project form the primary time link between South Africa and the rest of the world. Early in 1998, the TF project approached AOA in connection with the GPS End-of-week (EOW) compliance of the GPS timing receivers in use at the NML. (These receivers are either model TTR5 or TTR5A from AOA.) AOA informed the TF project that both these models would not survive the GPS EOW rollover (which occurred on 22 August 1999 at 0h UTC). There were several possibilities for overcoming the problem as stated above. New receivers from AOA could be acquired, but the costs involved were prohibitive. The current receivers could be upgraded, but once again the costs were high, and the risk of failure of these receivers are high due to their age. New receivers from another manufacturer could be acquired; high costs once again excluded this option. Another option would be to construct GPS timing receivers at the CSIR-NML to replace the AOA receivers. The most cost-effective solution to this problem was the development of new GPS timing receivers at the CSIR-NML. An additional benefit of this solution is the development of expertise in the operation of GPS receivers, generation of Consultative Committee on Time and Frequency (CCTF) data files and interpretation of GPS timing results. Utilizing this solution also means that the GPS receiver could be upgraded as new hardware becomes available. An additional motivation for choosing this solution is that several other laboratories around the world has decided to follow this route [1,2]. Hardware Motorola has developed a GPS module specifically for use in timing applications, the model Oncore UT plus. These modules form the basis for the GPS timing receivers designed and operating at the NML. Most of the other timing centers use the older (and now discontinued) Motorola Oncore VP receivers. In addition to these modules, each receiver consists of a power supply and data interface and a Pentium class computer for downloading and computing results. The receivers also contain time interval counters for measuring the offset between the GPS system time and the CSIR-NML reference clock once per second. The computer systems used are Pentium based, running the Windows NT4 or Windows 95 operating systems. The software development language used is