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This article relates to Spirent GNSS customers who have a GLONASS-capable simulator with the SimGEN mode of PosApp software.

The GLONASS ICD classifies the N4 word as ‘non-immediate’ data. This means that in the live GLONASS navigation message the N4 word will not necessarily be updated 'immediately' following the changeover from one 4-year period to another. Instead it will change on the next 'appropriate' satellite upload.

In PosApp uploads are controlled manually via the constellation file (the SimGEN user manual, DGP00686AAA, contains further information on Uploads - but of particular note is the fact that the default timing for Upload 2 is at 12 hours into the scenario run).

This means that creating a scenario where the start date and duration takes you 'through' a 4-year changeover will not automatically cause an 'immediate' change to the N4 word in the broadcast navigation message. You would only see the change after running the scenario for a further 12 hours (due to the default Upload timing). To observe a more 'immediate' change to the N4 word in the broadcast navigation message you must modify the Upload details in the constellation file accordingly. Let us take the following example:-

1. Set your scenario start time to be some time before a 4-year changeover, e.g. 31/12/2011 20:45, with a duration of 30 minutes
       - Note that PosApp scenario start times are based on GPS time. GPS is ahead of UTC time by the 'UTC offset' specified in the scenario. GLONASS is 3 hours ahead of UTC time. Hence, 20:45 GPS time is approximately 23:45 GLONASS time.
       - Using 15 minutes before GLONASS midnight should provide sufficient time for your receiver to begin navigating and also extract the full navigation message prior to the 4-year rollover
       - The date used above is simply the most recent 4-year rollover that occurred at the time of writing. You can of course choose any other date, which falls on a 4-year rollover, to suit your test purposes
2. From the scenario tree go to [GLONASS constellation-->GLONASS signal sources file-->Signal control-->Upload details]
3. Leave the ‘Upload 1’ upload time = 00:00:00
4. Change the ‘Upload 2’ upload time = 00:16:00, i.e. 16 minutes into the scenario, so 1 minute after GLONASS midnight on 1/1/2012.
5. Make this change for as many SVs as you require or click the “ALL” checkbox next to the ‘Upload 2’ upload time to ensure all SVs are uploaded at the same time.
       - In reality it is unlikely that all satellites will receive an upload at exactly the same time. When tracking live signals a receiver will therefore rely on the most recently uploaded satellite signal that it is tracking. 

The attached text file is the resulting decoded navigation message from the above example, for SVID1. The N4 word updates from 4 to 5 at "Seconds Into Run: 984" (which is the first broadcast of the N4 word after the Upload which was scheduled for 960 seconds).