Is this a bad idea for RTK, they are two receivers about 10 years old? Would there be issues mixing the two brands?
Radio communication is the biggest issue but not really that bad, especially if you are using an external radio at the base.?ÿ RTCM protocol is your friend.?ÿ
As John said radio links are the only issue, I've had a Topcon Base talking to a Trimble rover on a work site for the last 6 months with no issues. As long as the settings between the base and rover are the same they seem to speak ok, just remember to use RTCM to keep it clean cut. Also, I set the base to speak in Trim Talk, I don't know if it really helps but thought whatever could be done to avoid any communication issues would make everyones lives easier.
The biggest problem will be the phase variance in the vertical.?ÿ If both sets use GLONASS it can be as bad as a few tenths.?ÿ When you use matched base/rover the issue compensates.
This is an interesting problem that I have only rarely heard about, but has come up enough that I am curious. I have rarely used mixed receivers. Comm issues are bad enough between same-manufacturer receivers.
For static work, orienting all antennae correctly and using the correct antenna models will generally get good results even when using mixed receivers. But generally speaking, in my experience this is when working at the hundredths level, not tenths.
I am surprised that there would be as much as a tenth or so of error from phase variance alone, even with RTK. I was under the impression that for RTK, the signal processing and PCV adjustments due to signal direction were performed on the boards within the base prior to sending the information out over the comm link, which would suggest that the correct adjustments would be applied if the firmware was up to date in the base receiver.
In years past I have only seen a few scientific articles that addressed phase center variation in RTK, and there was not much discussion of the "why", just reporting of the processing results with PCV and without, across different receivers.
Would be interested in others' experiences and of course any articles/explanations...
If this was the case would it not be a problem for all the VRS/Network RTK units that have many different receiver types accessing the network?
I think the answer is yes and no, as it depends on the RTK initialization approaches used by the receivers involved. GPS doesn't see this issue near as much because it is frequency modulated. Due to GLONASS signals being "noisier" is why most brands used GPS to obtain solutions and GLONASS was "additional". There is a lot to discuss there, but long story short the Topcon receivers mentioned in the original post are basically legacy Javad gear. Javad was pretty well known for embracing GLONASS warts and all. His approach was to treat the "noise" as it cancelled each other out if using identical technology at each end.
Other brands that don't rely as heavily on GLONASS might only see a tenth or so oscillation. This is why on Javad's sales page they still offer the .2mm "dynamic calibration", you don't need to pay for that option if all your equipment is J-green. I was in a class at Trimble Dimensions taught by the Spectra SP-80 and SP-60 development team and they seemed to indicate the legacy Epoch equipment was highly susceptible to phase variance.
I also suspect a lot of people are being affected by this and don't even realize it because the control points are "good enough". Or they observe only one VRS and set their local base on it - then all the phase variance error from the VRS affects the global absolute accuracy, but not the local relative accuracy or precision if using a matched base.
Modern receivers pile in a lot of Glaileo signals which have quality on par with GPS III and superior to legacy GPS SVs, and this becomes almost a non-issue.
This is pretty much as I understand it and how it was relayed to me by people with electrical engineering PHD's. I may have incorrectly filled in some gaps in the knowledge over time.
I'm sure John Hamilton can expound on the conversation and point out if I'm wrong or give us all a free education on the mattter.
@plumb-bill One detail point: GPS signals are not frequency-modulated. The modulation used is binary phase shift keying (BPSK), to generate direct sequence spread spectrum (DSSS) signals. GLONASS uses a combination of BPSK / DSSS with frequency division multiple access (FDMA), which itself is rather different than conventional frequency modulation (FM).
I can't add anything useful as I have been pretty much a single brand (Trimble) user since 1988 (before that I used macrometers and Trimble). And I don't have any insider info. I have rarely used any VRS that wasn't Trimble (Tennessee and NY, maybe one or two others for a limited time).
I do remember using both Keynet and Leica on the same points in Virginia a few years ago, there was a few cm difference occasionally but usually pretty close.
@john-hamilton That's cool that you used Macrometers.
Correct I jumbled terms, they are on different frequencies not that they vary the frequency.
That's cool that you used Macrometers