Is anyone aware of any specific guidelines/testing regarding RTK GNSS (standard multi frequency survey grade gear) and how far away from a steel roof/car you ought to have your receiver/base before it affects precision? Not talking urban canyon here just suburban work.
I'm aware the correct answer is as far as possible but in the urban environments in particular you do not always have the luxury.
I've never seen any studies that really quantified this.
I'd say it's darn near impossible without specifying both the exact multipath environment as well as the receiver being tested. There's a huge range in receiver quality, from budget models without any multipath rejection, to continuous-evaluation models running multipath rejection across multiple RTK engines and multiple signal combinations, comparing them in realtime while automatically tossing poor signals on the fly to maintain a quality solution.
Time of day plays a role too, since constellations and incoming signal directions are changing constantly.
I don’t know if you can find them on the web. But University of Texas at Austin has done a lot of testing on multipath in general both near far etc. different antenna types to nesting materials etc. maybe the google scholar search might get you some information. Also Geo++ website and some of their papers address some of this from different studies as well rtk and static effects at both rover and base.
One thing not very academic but multipath is a sneaky slimey pig. It can bight you in the rear. One of the best things you can do is use good field procedures. The base should be set up in the most multipath free environment as possible. This can be obtained by the use of a clinometer and compass and get a plot of obstructions that are above the elevation mask. This is only a preventative measure. The other is observe points at different times preferably 3 to 4 hour gaps. Move the base to a different point and observe again. Time apart time on station helps to identify those times when something could have happened.
You can pre plan as well for the best times to do work in a given area. While satellite signals will work under water so rivers oceans ponds etc. those receivers and antenna’s usually are designed for that environment. I say this because when working around canopy or in canopy after leaves have been drenched by rain will effect you in a totally different way than that same spot same time frame when they are dry. Multipath. Now like @rover83 stated every receiver antenna and the firmware that operates the magic are all different. Even the same exact receiver with different firmware has effect. On how well it can mitigate the positioning. When or if you read the GEO++ articles or even look at an antenna like a choke ring with and without a random or different radomes on NGS website those effects change the antenna phase center. Before antenna calibrations a person that used all same antenna models could do pretty well relative but say a antenna model A from company A with a antenna B from company B in same static session could have introduced more error than one could realize. Now we can model that and it doesn’t matter as much. Then we get to relative vs absolute antenna calibrations. A totally different issue to deal with. I know this is not all necessarily multipath related but you need to understand some of these things as they all tie together for signals being brought to the antenna phase center correctly and also when these things are taken care of multipath is a tiny bit more easier which is not a good word to detect and mitigate.
Get an old sheet pan (cookie sheet). Set up the rover and start walking around it and experimenting.
Not standardized testing...and a quality receiver with a well designed antenna made in this century by a reputable brand should be able to deal with multi-path from directly below (like if mounted on roof or if mounted on a car roof).
But, multi-path is difficult. There are times you can tell it is happening, but there are simple (may not be easy, fast, cheap) ways to mitigate/check for it.
Critical measurements should be repeated at different times of day so that satellite geometry is different. This won't solve everything, so critical measurements should also be checked by terrestrial measurements between multiple points. The saying 2 = 1 and 1 = zero is applicable, but I would say 3 = 1 and 2 =0 and 1 = ?.
Truth is, after a few years of working with it, you kind of get a sixth sense about what is working and what is not. But measurements without checks is not surveying.
Just picked up this document on multipath mitigation, from a Leica webinar, this morning. Not exactly what you are looking for, I guess. It seems that they are working on multipath from the standpoint of hardware and signals rather than through use.
