Orolia GSG-5 Series, User Manual With Scpi Manual, Page: 57 / 392

GPS time and leap seconds
The Start time is based on GPS time, i.e. the displayed time is always GPS time. Unlike
UTC time – which is frequently displayed by GNSS receivers – GPS time does
not
include
leap seconds.
NTP real time and downloaded Ephemeris
Using NTP as start time in conjunction with Ephemeris set to Download is subject to
licensing options, as it requires the Simulate Now option to be present. In this con-
figuration, the GSG unit will simulate the sky as it is in that start position at current time.
This functionality is currently only available for the GPS constellation. Please also note that
the availability of good ephemeris data cannot be guaranteed, and periods where no data is
found and hence no signals can be generated, may occur.
About GPS time and GPS week number
In the GPS data format, there are 10 bits reserved to represent the GPS week number,
which leads to a modulo 1024 ambiguity in the week number and hence the GPS date:
The GPS week number count began at midnight of January 5/6, 1980. Since then, the
count has been incremented by "1" every week, and broadcast as part of the GPS mes-
sage. Consequently, at the completion of week 1023, the GPS week number will roll-over
to week number 0.
This means that if looking only at the week number (WN) parameter in the GPS data mes-
sage, it is impossible to determine if WN 1023 corresponds to August 1999, or April 2019,
etc. GPS receivers must therefore account for this roll-over problem, and use other means
to decide on which 1024 week period they currently are in.
The designers of GPS receivers have a number of ways of ensuring that the WN is inter-
preted correctly. These techniques range from keeping GPS week numbers in non-volatile
memory, keeping a real-time clock, etc.
One popular method involves resolving the year period ambiguities with software revision
dates. For example: Since the GPS software knows that it was made on February 11, 2011
(corresponding to GPS week number 1622, and in the data message WN 598), this inform-
ation can be used to map the WN to a year by concluding that e.g., WN 597 cannot cor-
respond to early February 2011, but rather to mid-September 2030.
This in turn, means that when simulating scenarios using a simulator, going back and forth
in time and in GPS week numbers, you may see unexpected behavior in how the WN is
interpreted. This could result in a scenario that worked ‘correctly’ in the past, starts out-
putting a different date that is 19.7 years forward in time.
The GLONASS system does not have the week roll-over problem that GPS has. When sim-
ulating scenarios with historical dates, however, it is likely that a receiver that is trying to
compensate for the week roll-over based on the firmware build date mentioned above, will
3.5 "Select" Menu
User Manual GSG-5/6 Series Rev. 27 41