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Appendices
Test Bed Receiver Subsystem Addendum – Rev 1 41
GCLA/B CLOCK INFORMATION
This log contains the time difference information between GPS and GLONASS time as well as status flags. The status
flags are used to indicate the type of time processing used in the least squares adjustment. GPS and GLONASS time are
both based on the Universal Time Coordinated (UTC) time scale with some adjustments. GPS time is continuous and
does not include any of the leap second adjustments to UTC applied since 1980. The result is that GPS time currently
leads UTC time by 13 seconds.
GLONASS time applies leap seconds but is also three hours ahead to represent Moscow time. The nominal offset
between GPS and GLONASS time is therefore due to the three hour offset minus the leap second offset. Currently this
value is at 10787 seconds with GLONASS leading. As well as the nominal offset, there is a residual offset on the order of
nanoseconds which must be estimated in the least squares adjustment. The GLONASS-M satellites will broadcast this
difference in the navigation message.
This log will also contain information from the GLONASS navigation data relating GLONASS time to UTC.
GCLA
Structure:
$GCLA week
sec nominal offset residual offset
residual offset variance
NA
τc # GPS sv # GLONASS sv time status
*xx [CR][LF]
Field # Field
Description Example
1
$GCLA Log Header $GCLA
2 Week GPS week number 994
3 Sec GPS time into week 149871.00
4 Nominal Offset Nominal offset between GPS and GLONASS time
references, in seconds
10787
5 Residual Offset Residual offset estimated in filter, in meters 10.62179349
6 Residual Offset Variance Variance of residual offset, in meters 167.82950123
7 NA Calendar day number within four year period beginning
since the leap year, in days
1121
8
τc From GLONASS almanac -
GLONASS time scale
correction to UTC(SU) given at beginning of day NA,
in seconds
-3.0544738044739E-007
9 # GPS sv Number of good GPS sv tracked 9
10 # GLONASS sv Number of good GLONASS sv tracked 4
11 Time Status
Time status (
see below)
00000000
12 * xx Checksum *7B
13 [CR][LF] Sentence terminator [CR][LF]