Rinex Observables

What an issue!!

We are facing a very significant problem in the current Rinex data files , and I need to tell you about it!

The Rinex 2.11 format is defined without regard to where the phase is actually coming from (the C/A or the P code tracking). This has for the longest time not been an issue because it could only be a problem in L1 since it was the only signal with both C/A and P code. But it wasn’t a problem because the receiver manufacturers would align the L1 phase so that a user did not need to know whether the L1 phase came form one tracking or the other.

As the L2C signal spreads through the GPS constellation with the newer satellites we have encountered this pesky problem more and more. Now there are two signals on L2 as well, as we have always had in L1, the ‘P’ code and the ‘L2C’ code … so potentially the L2 phase could come from either signal tracking. In the IGS community we expected that the receiver manufacturers would again align the phase measurement transparently … but they did NOT!

Therefore we are now left with L2 measurements in Rinex 2.11 files that have a 1/4 cycle shift for certain satellites (the IIR-M and IIF ones!) from certain receivers. A clue comes from receivers that do not give you the P2 measurement, only the C2! these data are not useful for the IGS processing and I am pretty sure that the L2 phase reported therein is an L2C phase with a 1/4 cycle offset from the L2P phase. For example:

Rinex file with C2 data

Rinex file with C2 data

You can see that for satellites G05, G25, G29 and G15 (all Block IIR-M satellites) there is a C2 measurement but no P2 measurement! Therefore almost half of the data from this sample station cannot be used in IGS processing. We would need to apply a P2C2 bias as we do to C1, but no such agreement exists yet in the IGS to do so.

In the IGS we are working to solve this issue but it will not be easy, at least we have defined a format (not actually used by anyone!! :( ) which includes a compulsory line in the header to report the offset between phases on the same frequency, Rinex 2.12 format.

Solving this problem will involve the Infrastructure Committee and the Real-Time Pilot Project in direct connection with the receiver manufacturers. more to come in a future post!!

All the best!

Nacho

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