Relative timing characteristics of the one pulse per second (1PPS) output pulse of three GPS receivers

The atomic clocks orbiting around the earth in GPS satellites can be utilised by anyone to obtain accurate timing. All that is required is an inexpensive GPS receiver and a clear view of the sky. This has enabled a revolution in applications requiring synchronization such as cellular networks, telecommunications timing, digital TV and radio transmission, calibration laboratory systems and the internet. The atomic clocks in the GPS satellites are monitored and compared to 'master clocks' by the GPS Operational Control Segment, this 'GPS time' is steered to within one microsecond of Universal Time. Many GPS receivers provide a 'one pulse per second' (1PPS) output signal, and some receivers are optimised for timing applications. This paper examines the 1PPS output pulse of three inexpensive GPS receivers: Trimble Lassen LP, Motorola M12+ Oncore and Sigtec MG5001. The Trimble and Motorola receivers have an overdetermined clock mode that enables the coordinates of the antenna to be fixed in the solution, leaving the clock offset only to be solved for. Measurements of the offset of the 1PPS pulse of the receivers compared to a Leica CRS1000 where taken. The time offsets were found to be approximately 640ns for the Motorola and Sigtec receivers, and 1500ns for the Trimble receiver. Improvements in the offset standard deviation were observed when operating the Trimble and Motorola receivers in time-only solution mode. The Motorola and Sigtec receivers where found to have smaller offset standard deviations than the Trimble receiver.