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15: 1PPS Timemark Generator
152 GP4020 GPS Baseband Processor Design Manual
TIMEMARK
1ms
1ms
1us
delay
2us
delay
UTC
012
10 TIC Periods
= 0.999999s
TIC
ARM_TIMEMARK
RAW_TIMEMARK
TIMEMARK DELAY
SECONDS
Figure 15.3 Typical timing relationship between UTC, TIC and Timemark, for small Timemark Delay values
The GP4020 employs two separate sets of logic to allow the Timemark output to be aligned to UTC to a resolution
of 25ns:
1) TIC period slewing – refer to Section 15.4 on page 155
2) Timemark Delay Counter – refer to Section 15.5 on page 159
15.2 Issues To Consider When Aligning Timemark To UTC
There are two main issues, which effect the ability to set Timemark to be 1PPS aligned with UTC (Universal Time
Co-ordinated).
1) The resolution and value of the TIC period; is TIC stable or does it vary in length.
2) The stability / accuracy of the receiver clock reference, which also impacts the stability / accuracy of TIC. The
spec for the 10.000000MHz TCXO in the GPS system is
±2.5ppm, which means that the TIC period is also
susceptible to a
±2.5ppm variation – equivalent to the ±250ns for the default TIC period of 0.0999999
secs.
Both of these effects have long-term timing bias implications, unless the software can account for this difference.
To determine a UTC aligned 1PPS TIMEMARK, the receiver will need to be able to PREDICT the corrections
needed to a TIC pulse to produce a forthcoming Timemark output pulse. At the time that the next Timemark pulse
is required, the GPS receiver needs to know:
i) Absolute value of GPS System Time (i.e. number of GPS seconds which have elapsed in the space of a
GPS Week (7days))
ii) The Receiver Clock offset (i.e. the difference in the clock frequency of the Receiver Clock to UTC). This is
effectively an offset which shows the time error between a "supposed" one-second period, as timed by the
GPS receiver clock, sourced from the receiver TCXO, and an "actual" one-second period as transmitted by
the GPS satellites.
iii) The corrections due to ionospheric signal delays and offsets between UTC and GPS System time.