I-Lotus M12M Quick Start Manual - page 12
01 May 2008
M12M
™ Quick Start Guide (403-TTN-001)
Page 12
The 1PPS Offset and 1PPS Cable Delay features work the same in 100PPS mode as they
do in 1PPS mode. In 100PPS mode, these commands are used to accurately control the
placement of the pulse after the long pulse.
8
One Pulse Per Second (1PPS) Timing
Measurement Epoch Timing
The M12M receiver timing is established relative to an internal, asynchronous, 1 kHz clock
derived from the local oscillator. The receiver counts the 1 kHz clock cycles, and uses each
successive 1000 clock cycles to define the time when the measurement epoch is to take
place. The measurement epoch is the point at which the receiver captures the pseudorange
and pseudorange rate measurements for computing position, velocity, and time.
When the receiver starts, it defines the first clock cycle as the measurement epoch. Every
1000 clock cycles from that point define the next measurement epoch. Each measurement
epoch is 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
s/s) and the oscillator's inherent instability (+/-30 ppm over specified
temperature range).
When the M12M 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.