gps – Creating Self-Made RTK Correction Using RTCM Data

gpsrtklib

I would like to explain my situation.

  • I have a GNSS receiver, which outputs me NMEA data (read below).
  • I receive RTCM data via NTRIP from a caster.
  • I would like to correct my GPS position with centimeter-accuracy.
  • I do not want to use factory-made GNSS devices which perform this correction on their own.

I know (from other questions on GIS_STACKEXCHANGE) that with only NMEA data I cannot perform any such correction. So I need to buy a GNSS receiver which gives me raw data (is it right?).

Now some questions:

  1. Do I really need these raw data?
  2. Which kind of data need I own? Ephemeris? Pseudoranges? What else?
  3. Do I need to get these same data from the caster? I studied the format and I got that there are several message codes each of one refers to a particular kind of data. Which message need I look to?

Best Answer

welcome to the forum! Some comments that might help you finding a direction with your questions:

  1. Raw data corrections from a NTRIP caster can be used to improve the position of a standalone GNSS receiver to centimeter (eventually millimeter if using "high-end" receivers and proper processing techniques) level accuracy. The mandatory requirement is to get access to the raw measurements of the GNSS receiver whose position has to be estimated. Hence, the NMEA solutions delivered by your receiver cannot be improved by means of the raw measurements received from the NTRIP caster.

  2. To achieve centimeter level position accuracy you need to process pseudorange and phase measurement data and, depending on the processing technique, either precise satellite products (i.e., ephemeris, clocks, code and eventually phase biases) or raw observations from another receiver that is used as reference and whose position has to be well known.

  3. There are some very well developed and open source packages that implement GNSS positioning techniques and can be used as a starting point for your research/activity. Probably the one that I would suggest for numerous reasons is rtklib. This will give you an understanding about what RTCM messages you need to look at and what observations you need from your receiver.

Good luck and have fun with GNSS!

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