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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?
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  • Pseudoranges are between the satellites and the receiver, so the caster wouldn't know them, you have to get them from your receiver.
    – Dave X
    Dec 14, 2020 at 18:45

2 Answers 2

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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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Great answer from "fastest".

I have no experience with RTKLIB, my company uses almost exclusively Trimble equipment so I've only worked with professional GIS applications like Trimble TerraFlex for data capture and Trimble Pathfinder Office for post-processing.

Based on my experience, I can add that getting "centimeter-accuracy" from a GNSS receiver is a lot easier with a dual-frequency receiver than with an L1-only receiver, and a good antenna is critical. Even a professional GIS device like Trimble Geo 7x with the CM option will only give centimeter-level accuracy if it is used with an external Zephyr 2 antenna or similar, the internal antenna is only good for sub-decimeter accuracy.

Looking at the lower price point than Geo 7x, commercially-available single frequency receivers like uBlox M8T are capable of giving RTK post-processed accuracy in the sub-foot range in good conditions with a good antenna, using professional applications like Trimble Pathfinder Office (but PFO doesn't generally allow RTK post-processing of uBlox data files.)

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