GPS-GLONASS biases … a receiver issue

GNSS receivers decode all the signals ‘more or less’ at the same time, the time is really set by the GPS but the other systems (Glonass) have a bias so that apart from the clock bias per satellite there is an inter-system bias between the constellations (at least that is what we thought!! here is a paper I wrote sometime ago on the subject: GLONASS_POD

The plot below shows the state of the art when I was still in charge of glonass processing at ESOC some years ago!! you can see that we estimated one bias per receiver and per arc (2 days of data) and the biases are very stable each day and they are ‘grouped’ per receiver type. all the Javads tend to behave ‘similar’ and all the Ashtech Z-18 the same … but the estimated clocks were still not very good so then we estimated ‘one bias per satellite per data arc per receiver’ this is more reasonable, as the previous was an average over all the glonass satellites. The average (in the plot) is enough to get reasonable orbits.

InterSystem Bias

Now at the ESA/ESOC IGS Analysis Center we estimate the intersystem biases per receiver and satellite pair, and we have excellent orbits and clocks for glonass, the size of the intersystem biases is similar to what you see in the plot for each receiver type except each satellite/receiver pair varies a bit around those initial averages per site.

The explanation is that internally the receiver takes longer to decode the glonass satellite signals and since glonass works in different frequencies for each satellite (FDMA and not CDMA) then the bias must be frequency dependent.

Certain ‘batches’ of receivers using the same electronics produce very similar biases per satellites but it still has to be estimated all the time.

For Galileo I would assume a general intersystem bias with GPS will work but this is still unclear to me at this time, maybe many biases are need per satellite and per measurement type?? … not sure yet!!

Happy positioning!
Ignacio Romero (Nacho of the IGS!)

One Response to “GPS-GLONASS biases … a receiver issue”

  1. Felix Perosanz Says:

    Very interesting !
    Thanks a lot Nacho.

    Felix

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