The height of the antenna above the vehicle roof may play a role as well. [Caution: what follows is my interpretation of things I've read, but my grasp of the subject matter is very tenuous.] When the antenna is very close to the roof, the roof will tend to act as a ground plane, blocking signals from below and mostly reflecting signals over -- rather than into -- the antenna. But as you increase the distance from the roof to the antenna, signals can reflect off the roof up to the antenna and induce a surface wave on the antenna's built-in ground plane, created a multipath condition.
Let's not ignore the probable shift in phase center when you plop those antenna down on big ground planes.
Mark Silver, post: 423324, member: 1087 wrote: Let's not ignore the probable shift in phase center when you plop those antenna down on big ground planes.
Good point. My guess is millimeters, but without testing, who knows?
Interesting discussion about possible multipath. Other than this recent setup, I can't say that I've discerned any repeatable pattern/problem, and have generally gotten good results. So, I'm inclined to keep using it, keeping this question about multipath in mind.
Thanks,
Al
Just had a follow up thought. If multipath were an issue - it is the case that (in an open sky condition) the receiver would receive the same signal twice? That is, the first time, getting the "straight" signal direct from the satellite, and the second time, just a little later, from a "reflected" signal off the roof? If so, can the receiver sort between the two?
Big Al, post: 423339, member: 837 wrote: If so, can the receiver sort between the two?
That depends on the receiver. Some are better than others at this.
Shift in phase Center would probably be totally irrelevant because there is no Monument underneath the receiver as it is only sitting on a truck. No Monument underneath it. Unless it is Shifting slowly throughout the day.
Nate The Surveyor, post: 423344, member: 291 wrote: Shift in phase Center would probably be totally irrelevant because there is no Monument underneath the receiver as it is only sitting on a truck. No Monument underneath it. Unless it is Shifting slowly throughout the day.
Another good point.
The worst reception I've had out in the wide open was in the dry bed of a shallow flat bottom 2å± acre horseshoe pond with a layer of shiny dead leaves coating the surface of the ground.
Why not stick a nail in the ground or a rebar and set the base on that? Then if you have to go back you have a base point to start from. How long does it take to set up a base, 5 minutes or less?
David Livingstone, post: 423369, member: 431 wrote: Why not stick a nail in the ground or a rebar and set the base on that? Then if you have to go back you have a base point to start from. How long does it take to set up a base, 5 minutes or less?
I've only used my truck for my a base for a production job a couple of times. I've done it a lot in testing. It works well.
On a small job, sometimes it's overkill setting up the base over a physical point. I still process the base to CORS so it's related to the world, but I don't really care where it was setup. If I ever do come back, it's easy enough to shift in on a point I collected with the rover. I've also set the base on brick mailboxes, concrete retaining walls, light pole pedestals, curbs, etc. The world is filled with objects stable enough to set the base on without worrying about driving a marker in the ground and setting up the tribrach and tripod on.
In Big Al's example, there's a lot of snow on the ground. I don't deal with that much here, but I suspect that you do in Illinois. I can imagine messing around with a ground marker in those situations is more hassle than help on some jobs.
On jobs I think I might return to again, I set a mark. On big jobs I set a rebar. On smallish jobs I set a nail.
Big Al, post: 423131, member: 837 wrote: Hey, Moe, that's a good point. I hadn't considered to what extent that might be a problem. Here's a picture of my setup. The base gets mounted on top of the pressure treated board up on the roof rack, via a 5/8-11 galvanized carriage bolt thread (obviously, not where it's located inside the side compartment). I got the concept for this from Javad advertisements, where they shows (and promote) the base mounted on top of a vehicle. I can imagine that signals might be reflected off the top of the cap and upwards toward the receiver. Not sure what effect this might have. Like most GNSS receivers, signals below a certain degree above horizontal will be rejected (i.e. below 10 degrees), but I'm guessing that this is a function of the elevation of the satellite relative to the observed position, i.e. the receiver will not know the difference between a "straight" and a "reflected" signal originating from the same satellite.
If multipath is likely to be a problem with this setup, can you or other suggest a homemade solution? Thanks.
Al
Sure. Just set your base on a permanent control point.
Nate The Surveyor, post: 423344, member: 291 wrote: Shift in phase Center would probably be totally irrelevant
I tend to disagree, because the change in apparent distance to the satellite will depend on the direction of arrival. If the signal arrives from the east, the additional path length will make it look like the antenna is a bit west, and vice versa.
