Time offset constants in sdr_nav.c

[ellarizza]

Hello Takasu-san,

May I clarify about the meaning of time offset constants in sdr_nav.c?
Are these time offset between satellite reference time (for example, galileo standard time) and GPS time?
Or UTC time? or something else?
When I check referenced ICD, I don't see the same values so I assume it is computed values from equations presented in the ICD.

Thank you so much.

[tomojitakasu]

The bit timing of navigation messages is complex and often difficult to determine from ICDs, which can be ambiguous. Therefore, in the development of Pocket SDR, I initially used the ICD to establish a fixed time offset for GPS L1C/A LNAV message (TOFF_L1CA = 0.160 s). The offsets for all other navigation messages were determined empirically using live observation data, based on the principle that their pseudoranges should show no offset relative to the GPS L1C/A pseudorange. Since these offsets must be integer multiples of 1 ms (corresponding 300 km), they can be determined unambiguously. All other TOFF_* constants were derived in this manner.