GNSS Observables (I)
There are two main measurements when it comes to GNSS observables: Pseudoranges based on the code or based on the Carrier phase measurements. At the end of the day both things are estimations of the real radial distance between the receiver and the Satellite.
To those familiar with Deep Space Operations and Navigation the closest thing to Sequential Ranging [1] in Deep Space is not the pseudorange but the carrier phase measurement in the GNSS field. The problem here is that the same ambiguities we have to face in ranging are also present here but, instead of the sequential frequency changes we have in Deep Space ranging we will have a single frequency when we measure the carrier phase in GNSS -well, we can use two frequencies, this will be useful to take the ionospheric effects out up to the 99%, avoiding the need of ionospheric models, but this will not help much with the carrier ambiguities.
Extracted from: http://www.nasa.gov/sites/default/files/thumbnails/image/l1_dscovr_diagram.png
Therefore, although carrier phase measurements are more accurate, at the end of the day we are using the carrier wavelength to measure the distance, this measurement will have an unknown bias due to the fact that we do not know how many cycles the carrier underwent before reaching our antenna.
When we process the output of a receiver we find measurements of both types and, depending on the receiver, at different frequencies. (When I talk about the "output of a receiver" I'm talking about a special receiver, outputting intermediate measurements not the typical Position Velocity and Time (PVT) final measurement. Luckily a RINEX formated file with these measurements).
Therefore we should keep in mind that when we look into Carrier phase measurements we are dealing with an accurate measurement but biased by an entire number of cycles. An entire number that will also change as soon as the receiver gets out of lock, so you should expect bumps in the data.
Following picture shows real GPS carrier (Lx) and pseudorange measurements (Px and Cx). As you can see the distance difference is huge. We know GPS Satellites are placed in MEO orbits so we should expect measurements around 20000 Km. Therefore we know for sure carrier phase measurements are biased.
We will keep talking about GNSS observables and how these measurements are done in following blog entries, stick around!
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[1] Deep Space Ranging http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/40972/1/07-0166.pdf
Delta DOR (This is a different approach to ranging and will provide us with angular measurements (not the distance to the satellite) and combines VLBI measurements. Nonetheless I think these slides are quite good to have some insights on Deep Space Navigation) http://www.slideshare.net/esaops/ops-forum-esa-delta-dor-from-implementation-to-operation-16032007
