Lets say that GPS Base unit is sitting comfortable in "X" amount of height, 2 meters for example. Satelites Orbiting around that base and giving their positions will be recordered based on angle of elevation mask (usually set between 10 and 15 Degrees). Still lets say angle is set to zero, meaning it will include satelite sat the level the unit is stting, 2,00.
Now lets say there are obstructions below the level of the unit. Concrete, walls, stone fences, etc
Will these affect measurements anyhow. And Specifically, if Base is sitting on a roof, and you build a wall around the boundaries of the rooftop surround the unit up and until the height it is set. Will this yield any negative repercussions
There will be reflections off whatever is around - ground, walls, whatever,?ÿ and those?ÿ low-angle reflections combine with the direct signals coming from the satellite above the mask angle, creating a multipath situation. The mask angle is compared with the satellite angle, and the receiver doesn't know about any other angles.?ÿ Some modern receivers may have some ability to pick out the earliest arrival in order to minimize the multipath effects.
The degree the reflections affect the results will depend on how reflective those surfaces are and how well the antenna rejects those reflections. The lower the angle to the reflection area, the better most antennas will reject it. There is some variation in the pickup patterns of various models; larger ground planes incorporated in the antenna help, and choke ring designs are said to be better.?ÿ In most cases I think averaging for some time washes out most of the multipath effect unless it is a really bad situation.
Sorry that those are all general statements with no hard data.
First off I always use a mask angle of some kind, usually 10 or 12 degrees.?ÿ I have seen multipath give bad results using RTK, but it was always in a situation it didn't surprise me, there was something in the way, trees, building etc.?ÿ On open ground I think in theory multipath can occur but unless its set up near something obvious, like a lake, there is little chance multi path will be a problem with objects located below the GPS unit.
Whatever mask angle you use cannot prevent multipath. There is always the possibility that the multipath signal is stronger than the true signal or it may be the only signal. Mask angle is based on the satellite calculated position above the horizon, not the actual angle of the?ÿ recieved signal. I have done some "What if I did something stupid" tests and have found that I can get a GPS position with my antenna inverted and been able to get an OPUS solution to boot. OPUS does give you indicators that the position is funky, but hey it is all just a mathematical crapshoot.
The worst thing I have seen is chain link fence, if I am forced to occupy a point near a fence I use extra rod to get as high as possible. I find it is better to get a meaned solution from a cyclically moving high antenna than to have it fixed and too close to a GPS reflector/deflector/absorber.
Paul in PA
Use a?ÿ GROUND PLANE?ÿ ?ÿFollow at a minimum the manufacturer's recommendations for use of the equipment. There is a scientific reason for?ÿ El masks to be set between 10 - 20. Save yourself time and headache follow them.?ÿ?ÿ
SOP for RTK is to have different elevation mask settings for the base and rover. The base is set lower than the Rover. My Base is set at 10 while the rover is normally set at 15. If you get in a situation where you may need more satellites then it's an easy switch to lower your Rover as you survey to get more common satellites to include in your solution.?ÿ
The point is that the lower satellites at the base are not looked at for the calculations when the Rover is at 15 so they don't impact the accuracy of the Rover's location. But consider the very low satellites (0-10degrees) near the horizon, the angle the signal is approaching the receiver means it can reflect off many more "obstructions" than one approaching near the zenith. Also the angle means the signal is travelling through the atmosphere much farther than one overhead.?ÿ
It was always been a bad idea to access these very low satellites, and over time it's become less of an issue as more satellites are available.?ÿ
Sometimes when we have a base on a high hill and are going into canopy we have set the base at 5, but frankly, it never seemed to be useful.?ÿ
What do you mean by ground plane
What do you mean by ground plane
Older antenna designs were focused on capturing the weak GPS signals and were promiscuous in that regard, happily accepting signals bounced off the ground as well as those coming directly from the SVs.?ÿ For critical applications the manufacturers put the antenna in a choke ring, an aluminum housing with carefully-designed machined rings that trapped reflected signals.?ÿ But choke rings are very expensive, so for less-demanding but still high-precision applications they offered?ÿ a detachable ground plane, a relatively inexpensive aluminum disk that mated to the bottom of the antenna and did a decent job of rejecting ground reflections.
As antenna designs advanced, ground planes got smaller and were built into the antenna package.?ÿ I think all modern geodetic antennas incorporate a ground plane.?ÿ But note that a larger ground plane is still better than a small one, and I see that Trimble recommends their big Zephyr 3 Base antenna (13.5" in diameter) for RTK base station use as well as use in high-multipath environments.
a ground plane is attached to the ant to block multipath from below the ant.?ÿ?ÿ
@danemince@yahoocom
I use Trimble R8-s with internal radio and a small antenna. Not sure i can even use this plate then
all modern geodetic antennas incorporate a ground plane.?ÿ
All antennas incorporate a ground plane. Originally they were huge. Now they are much smaller. But they have always been there.
I suppose it might be theoretically possible for signals to reflect off something below the level of the receiver, then again off something above the masking angle on it's way to the receiver. But that would be so much of a time offset that the filtering would easily identify it.?ÿ ?ÿ ?ÿ
Guys i know this is a stupid question but can the signal come from lower than the antenna like so. Ok not that steep like the satelite is orbit on Earth but an angle lower than the antenna height
Theoretically possible, but not in practice. First, the ground plane in the receiver will physically block any signal from below. Second, the masking angle you have set in your software will reject any signal from below that angle.?ÿ
Any such signal from a very low angle like that would not be much use to you any way. It's path to your receiver would have spent a large percentage of its travel passing through the earth's atmosphere, which is analogous to trying to read this posting through a coke bottle.?ÿ ?ÿ ?ÿ ?ÿ?ÿ
Remember that the GPS signal is RHCP which is used in multi-path rejection.?ÿ
