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When a rover receives observations from a base station (e.g. through NTRIP from a CORS network) it can perform RTK to reduce ambiguities in matching observations to get a more accurate fix.

What happens when the set of visible satellites for the rover have little overlap with the base station? Will only the common observations be used? Or will other observations be used, just with more ambiguity?

For example, how would the performance of an L1/L5 capable rover receiving L1/L2 observations from an NTRIP server compare to the same rover with only L1 observations enabled, receiving the same NTRIP observations as the other?

2 Answers 2

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From my own testing, I've observed that an L1/L2 rover is considerably more accurate with just L1 corrective data (no L2 data) than without any corrections, and that it is also much better than with the rover set to just observe L1 with L1 correction.

To my mind this implies that both the limited corrections and the second frequency measurements are being used to improve the solution. However, the details of the RTK solution are receiver/library specific, with a range of variables even for the same library. I could not therefore comment on the general case.

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  • What module/s have you tested on?
    – ljden
    Commented Sep 22, 2023 at 5:16
  • 1
    Just the u-blox ZED-F9P.
    – colintd
    Commented Sep 22, 2023 at 6:48
  • Perfect, thanks!
    – ljden
    Commented Sep 22, 2023 at 9:31
  • Glad to help. If that answers your question, please mark as answered to help others looking for the same info. I should say, however, that with just an L1 RTK correction stream I couldn't get better than 150mm with my L1/L2 rover, whereas with L1/L2 correction I reliably get a repeatable 10mm.
    – colintd
    Commented Sep 22, 2023 at 9:35
  • Was waiting to see if anyone else was going to comment on the more general case. Do you have approx numbers of performance comparison of L1 observations with L1 corrections vs L1/L2 observations with L1 corrections?
    – ljden
    Commented Sep 23, 2023 at 21:37
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In general, RTK or DGPS will only correct signals and satellites which are shared between the rover and the base. Other signals may be used in the position solution, but because they are uncorrected they will be given lower priority in the solution.

I can't think why supplying L1 + L5 corrections to a L1 + L2 receiver would be any different from supplying L1-only measurements - there's no way to map L5 corrections to the received L2 signals even if the satellites match.

Note that SBAS corrections (WAAS, EGNOS, etc.) supply corrections (Code level, not RTK / Carrier level) which are satellite-specific (GPS-only, L1-only only for an L1 SBAS correction signal) as well as Ionospheric corrections which apply to any satellites in the region.

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  • "I can't think why supplying L1 + L5 corrections to a L1 + L2 receiver would be any different from supplying L1-only measurements" my question is the other way around. Sending L1/L2 corrections to a rover observing L1 only vs a rover observing L1 + L5. i.e. the L1 observations will be the only ones getting corrected, but does the presence of additional L5 observations aid in the accuracy of the solution (and by how much if that is answerable)
    – ljden
    Commented Sep 25, 2023 at 0:54
  • I would not expect an L1 + L2 receiver to behave differently from an L1-only receiver when using L1 + L5 corrections - the L1 signals will be corrected, L2 will not be. Theoretically dual frequency receivers (L1 / L2 or L1 / L5) can be more accurate WITHOUT CORRECTIONS than L1-only receivers (from memory, something about allowing reduction of Ionospheric errors because of different frequencies) but in practice I have not seen this effect.
    – Trams
    Commented Sep 25, 2023 at 22:50
  • You're still not getting the example scenario right. I'm saying the corrections are always L1+L2 and the rover is observing either L1, or L1+L5. Obviously the L2 corrections don't make a difference, so the question is does the presence of dual band L5 observations improve the solution over the single band observations, when considering the recieved (useful) correction are (effectively) L1 only
    – ljden
    Commented Sep 26, 2023 at 23:13
  • I would not expect an L1 + L5 receiver to behave differently from an L1-only receiver when using L1 + L2 corrections - the L1 signals will be corrected, L5 will not be. But the L1 + L5 receiver may be more accurate (uncorrected) than L1-only (reduction of ionospheric effects.)
    – Trams
    Commented Sep 27, 2023 at 19:23
  • That doesn't seem to line up with the behaviour both myself and colintd have observed. I suspect the dual band rover observations improves the solution despite receiving (effectively) single band corrections as it allows for a better 3D solution
    – ljden
    Commented Sep 28, 2023 at 23:49

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