L1 vs L1/L2 Post processing

A different solution is expected, but that's quite a difference. How long is the vector between points?

FWIW,

The reason for multi-frequency combinations, in addition to solving for ionospheric effects, is to allow the combination of frequency data in order to accurately determine the integer unknowns. While your L1 solution may indicate it is fixed, the correct integers may not have been determined.

For long baselines frequency combinations are needed. For short baselines an L1 solution is ideally better as the frequency combination process introduces noise into the solution. What is a short baseline? Some say 5 others 10 kilometers.

When I did processing, we always computed an iono-free solution to assist in integer fixing and then used the solved integers in an L1-only.

HTH,

DMM

For Short vectors L1 is sometimes better, but I have had few cases where I could not live with L1 only solutions. At times I have discarded the L1/L2 solution.

I do not think your big difference had to do with the fact that it was L1 only. I would suspect that you had better orbit data by the time you got back in the office. The biggest difference being between broadcast and post orbits, igu, igr or igs.

Paul in PA

If you shot it with a total station, I would suspect the baseline length was suitable for L1 only. Perhaps your observation time was too short? I've had my complaints with GNSS Solutions in the past, but the processor itself has always been impressive. Can you repeat the bad solution now?

The vector was approximately 185.452m from the L1/L2 & 185.163m for the L1 only.

Paul, I processed the L1 & L1/L2 vector using the same downloaded gps observation.
I use the gps results to orient my survey control network to the right orientation.
I would have caught the error eventually by comparing with the TS network.

My only concern & this is a BIG concern, shouldn't the software have detected this discrepancy?
Ashtech or whoever is now releasing the GNSS Solutions software could have processed the data
using L1 & L1/L2 methods. Compare the results internally & give a red flag if the 2 results are
not the same. If the dongle was not present then give the L1 results only but have a red flag
that says something like - "Please check your observation data....software detected discrepancies
in processed results..." or something similar.

There is no way the precise versus broadcast orbit will result in a 0.30 m difference for any normal length lines. The most likely explanation is that there was not enough L1 data to properly resolve the integers. My experience with L1 is that you need at least 45 minutes of relatively clean data to get a good solution. I am not familiar with the software you used to process the data, but what were the statistical indicators? Ratio (if used)? rms? I haven't used L1 only in many years (20+), so algorithms, etc may have improved since then.

Yes, I believe the time duration is the main reason for this discrepancy. The 2 points were observed for ~45 minutes. Even for L1 data only this is usually sufficient. Anyway just a heads up for those that have GNSS Solutions & are using it to process L1 using L1/L2 data.

This is strange. I have been using GNSS Solutions as one of my post processing options for many years. My horizontal solutions have always been pretty good. My issues have only been with the vertical.

I always observe for longer than necessary.

I have also found, after suggested by Shawn Billings, that if you have to delete data, it is best to just start a new project. There are gremlins that seem to creep in if you start deleting data, and reimporting.

I generally process L1 only in GNSS Solutions, and L1/L2 in Topcon Tools. I have only had the L1/L2 option in Topcon Tools for about a year and a half. Before that, I was using the L1 option only in both programs.

Good Luck!

L1 vs L1/L2 Post processing Orbits?

John, I was comparing the broadcast orbits to the various better orbits, ultra-rapid, rapid and precise for which there is very little difference.

What I forgot was that especially with the broadcast orbit and L1 only solutions you can get a fixed position which has a way off elevation. To match that with total station observations you have to bring the GPS solution down to the surface of the earth where your total station was. Way, way back when I was using Ashtech Solutions and L1 only I would sometimes do that by holding a point elevation fixed from USGS topo data. Just holding a close elevation fixed greatly improved the results.

L1/L2 results in better elevation fixes.

If you were to remove the L2 dongle and reprocess L1 only you most likely could not repeat your earlier solution, because Solutions would remember where it was.

Paul in PA

45 minutes over such a short baseline is way more than plenty. In reality, when we tested L1 several years ago, we could get reliable fixes on baselines less than 300m with five minutes of data. We seldom did this in practice because we were generally doing other things while the receivers were set up.