P1C1 biases from different sources

Coming back to this issue we have encountered a significant problem.

As remarked before in this blog P1C1 biases we have to be able to correct the C1 measurements from GPS to mix them correctly with the P1 measurements. This comes from the fact that when the signal is encoded in the satellite there is a delay between the ‘C/A’ and the ‘P’ code encoding.

Of course we detect this offset between signals (differential code bias: DCB) in the RINEX data which is recorded by a receiver after traveling from the satellite to the antenna through the atmosphere, and from the antenna to the receiver through the cable , amplifiers, etc. Internally in the receiver the electronics can also play a role of course. In any case what we see in the IGS is the RINEX data and this is what we have to work with.

As mentioned in the previous blog entry P1C1 biases the Analysis Center CODE (Center for Orbit Determination Europe) at Univ Berne publishes the P1C1 biases as an average over many brands of receivers available in the IGS network, and generally it is accepted that these averages are to be used when doing IGS-type processing or using the IGS products.

We have found that when an IGS-type solution uses a network of receivers with only Trimble receivers (which need the P1C1 bias) the CODE values may not be the best suited since the Trimble receivers do not show the same DCBs as other receiver brands. We have reached this conclusion after MANY internal investigations and testing. It is clear that depending on what receiver types we test the DCBs are slightly different from the accepted CODE averages, but when the receivers are only Trimble then the differences for certain satellites can be very significant:

in the table above you can see the differences of the P1C1 bias from CODE, as calculated using only Ashtech receivers and as calculated using only Trimble receivers, note the differences for satellites such as G07, G12, G29, etc. in plot form this shows the differences more clearly:

This generally means that the clock solutions from using the incorrect P1C1 bias will be biased versus the satellite clock solutions from a mixed network or as published in the IGS clock combination.

Therefore to conclude, be careful when calculating satellite clock solutions when all the receivers are of a certain brands and the data has to be corrected with the P1C1 bias! This is if you are interested in the satellite clock solutions or on resolving the maximum number of ambiguities with fixed satellite clocks!

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