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:-
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).