Trimble Outdoors AcutimeTM2000 Synchronization Kit, Part Number 45005-00-ENG Timing Operation

5 System Operation

GPS time accuracy is affected by the same major source of error that affects position accuracy: Selective Availability (S/A). The position and time errors are related by the speed of light. Therefore, a position error of 100 meters corresponds to a time error of approximately 333 nanoseconds. The GPS receiver's clocking rate and software affect PPS accuracy. The Acutime 2000’s 12.5 MHz clocking rate enables a steering resolution of 80 ns (±40 ns). Using software algorithms like an overdetermined clock solution, the Acutime 2000 mitigates the effects of S/A to achieve a PPS accuracy of 50 ns (one sigma) after survey is complete.

5.4.1Timing Operation

The Acutime 2000 automatically outputs a PPS and time tag. With an accurate reference position, the Acutime 2000 automatically switches to an overdetermined clock mode, activates its T-RAIM algorithm and outputs a precise PPS. The overdetermined clock solution mitigates the effects of S/A. Using a simple voting scheme based on pseudo-range residuals, the Acutime 2000's integrity algorithm automatically removes the worst satellite with the highest residual from the solution set if that satellite’s residual is above a certain threshold.

The Acutime 2000's default configuration provides optimal timing accuracy. The only item under user or host control that can affect the Acutime 2000’s absolute PPS accuracy is the delay introduced by the interface cable. For long cable runs, this delay can be significant (1.25 nanoseconds per foot). TSIP packet 8Ex4A sets the cable delay parameter, which is stored in non-volatile memory. For the best absolute PPS accuracy, adjust the cable delay to match the installed cable length. The cable delay is 1.25 nanoseconds per foot of cable. To compensate for cable delay, use a negative offset to advance the PPS output.

Acutime 2000 Synchronization Kit User Guide 5-13