User`s guide

Chapter 3 - Receiver Descriptions

about one second later than the previous measurement epoch, where any difference from

1.000000000 seconds is the result of the receiver local oscillator intentional offset (about +60

µs/s) and the oscillator's inherent instability (+/-30 ppm over specified temperature range).

When the M12+ processor computes receiver local time, this time corresponds to the time of the

last receiver measurement epoch. The Oncore process precisely determines this time to an

accuracy of approximately 20 to 300 ns depending on satellite geometry and the effects of

Selective Availability (if Selective Availability were to ever be reactivated by the DoD.)

The computed time is relative to UTC or GPS time depending on the time type as specified by the

user using the Time Mode command (@@Aw). The Oncore system timing is designed to slip

time when necessary in discrete one millisecond intervals so that the receiver local time

corresponds closely to the measurement epoch offset. The Oncore observes the error between

actual receiver local time and the desired measurement epoch offset and then slips the

appropriate integer milliseconds to place the measurement epoch to the correct integer

millisecond. When a time skew occurs (such as after initial acquisition or to keep time within limits

due to local oscillator drift), the receiver lengthens or shortens the next processing period in

discrete one millisecond steps.

The rising edge of the 1PPS signal is the time reference. The falling edge will occur

approximately 200 ms (+/-1 ms) after the rising edge. The falling edge should not be used for

accurate time keeping.

Output Data Timing Relative To Measurement Epoch

Figure 3.8: Output Signal Timing

The 12 Channel Position/Status/Data Messages (@@Ha and @@Hb), the T-RAIM Setup and

Status Message (@@Hn), and the Time Message (@@Gb) are the only output messages

containing time information. If enabled, these messages will be output from the receiver shortly

after a measurement epoch. Generally, the first data byte in the first message will be output

Motorola GPS Products - M12+ User's Guide Revision 6.X 09FEB05