Greetings everyone. So I've been reading GNSS (for self-improvement) and did some let's just call it 'experimentation' with RINEX files using the CSRS-PPP online service. I'm wondering if someone could assist me interpreting the result (see attached file) given to me by the said online software. I was trying to detect if there was any cycle slips in the observation. But all I see are plots of carrier-phase residuals, and I don't really understand how it would describe any presence of cycle slips. Do enlighten me.
FWIW,
I have not used the CSRS-PPP tool in years. I have yet dug out data for a submission. Nonetheless, I will say that your carrier phase residual plot does not show evidence of an unresolved cycle slip. I base this on the X-axis scale of +5 cm to - 3cm.
Do you suspect that a cycle slip exists in the file? You can always run your RINEX file through teqc using the +QC option. Search this site for posts on teqc
Not knowing any better, I would expect that the increased scatter at the middle and end of the plot are rising and setting of SVs.?ÿ
As you express an interest in the underlying science of GNSS and PPP, I recommend a U of Maine doctoral dissertation by Boonsap Witchayangkoon:
It is readable and nicely presented.
Also, have you also tried the NASA PPP tool at: https://apps.gdgps.net/apps_howtouse.php
?ÿ
HTH,?ÿ
DMM
Thanks @geeoddmike. I was actually playing with tecq too, a friend suggested this WinTEQC Editor saying it is suitable for a GNSS newbie like me. One more thing if you don't mind my asking, are carrier phase residuals caused by cycle slips?
P.S. I'm reading the attached dissertation right now. It's a good read. Thanks again.
Glad you find the recommended dissertation useful. There is so much information available nowadays. Finding reliable information can be a challenge. Dissertations from good programs are my go-to resource.
As for your direct question: “...are carrier phase residuals caused by cycle slips?”, my answer is that residuals result from a number of sources. Our measurements, models and understanding are all imperfect.
To over explain:
First of all, read this definition of residual error:
The data reduction performed on GPS data is checked for accuracy and precision by comparing the expected results with those achieved (residuals). Residuals exist due to the observations data and the mathematical and stochastic models used during processing. Remember that before any adjustment is performed all non-random errors in data should be identified and corrected.
Introducing observations contaminated by cycle slips will result in larger residuals.
Recall that a cycle slip will occur in the data logged by a receiver when data from a tracked SV is lost then later regained. There are then two sets of data for the SV recorded by the receiver. These two sets of data can be combined by applying the difference in cycles thereby joining the two sets of data.
Data quality checking programs like teqc can be used to plot the observables made at an individual receiver. The plots generated by the +qc option in teqc will show the slips. There are two types of quality checking outputs a “lite” version and a more detailed version. The “lite” option is implemented when a RINEX navigation message consistent with the RINEX observation file is not available.
In general, GPS data processing works by combining data from multiple receivers to form one database of data. One of the matter first steps in the processing process is triple differencing which identifies and (nowadays) fixes cycle slips automatically.
As more and better data is available from the SVs and given improvements to receivers, antennas and processing software, I consider cycle slips to be a solved or solvable problem.
In conclusion, if I expect that a cycle slip occurred at a station I would run the teqc program option +qc. I would analyze processing results via post-fit plots looking at that station particularly to see if the range of residual values are within the bounds I expect. If not, I would exclude the SV from processing.
HTH,
DMM