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Also from the same page: Some modern receivers are able to use both GLONASS and GPS satellites together, providing greatly improved coverage in urb

Also from the same page: Some modern receivers are able to use both GLONASS and GPS satellites together, providing greatly improved coverage in urban canyons and giving a very fast time to fix due to over 50 satellites being available. In indoor, urban canyon or mountainous areas, accuracy can be greatly improved over using GPS alone. Is there any reason, other than the higher number of sources, why GPS+GLONASS is better than just GPS in these situation? I.e. Is GLONASS better in these sorts of environments?

omgtehlion 15y ago
It's all about (visible) satellite count
CWuestefeld 15y ago
In situations like the "urban canyon", where the problem is that only a small slice of sky is visible, I would think that the advantages of using both systems would only be marginal. The most accurate GPS fix is had when the satellites you're seeing are widely separated in the sky. In the urban canyon, the problem isn't a lack of satellites, but the fact that you can only see those that happen to be in a narrow slice. And narrow slices are bad for constraining the solution to the equations. The way I think about it is each satellite being used in the solution is like a rope tied to the satellite (where stretchiness of the rope is analogous to measurement error). Your GPS receiver can calculate your location to within the accuracy of how far you can move while keeping all ropes taut. So if you had a fix from two satellites that are smack next to each other, you really don't have a fix any better than a single sat. If you've got two ropes at 180-degrees opposite each other (i.e., at either end of the street in the urban canyon), you've still got ambiguity due to your potential for lateral movement. For a good fix, you've got to bring in another rope at a wide angle from those -- but the tall buildings interfere with getting that rope, even if there are lots of satellites up there that could be offering it.
- sjwright 15y ago
  Yeah, but if there are more useful satellites in the sky, there's a greater chance of the visible satellites having a wider separation. Even in the 'urban canyon', you still frequently have partial sky visibility in some non-vertical directions. And there will be occasions when a GLONASS satellite will be better placed than a GPS satellite. To use a reductio ad absurdum argument, imagine a satellite positioning network with a thousand nodes. Wouldn't it have a greater chance of providing a useful fix in this environment: http://g.co/maps/sa8zk http://g.co/maps/sa8zk ?

omgtehlion 15y ago

It's all about (visible) satellite count

CWuestefeld 15y ago

In situations like the "urban canyon", where the problem is that only a small slice of sky is visible, I would think that the advantages of using both systems would only be marginal. The most accurate GPS fix is had when the satellites you're seeing are widely separated in the sky. In the urban canyon, the problem isn't a lack of satellites, but the fact that you can only see those that happen to be in a narrow slice. And narrow slices are bad for constraining the solution to the equations. The way I think about it is each satellite being used in the solution is like a rope tied to the satellite (where stretchiness of the rope is analogous to measurement error). Your GPS receiver can calculate your location to within the accuracy of how far you can move while keeping all ropes taut. So if you had a fix from two satellites that are smack next to each other, you really don't have a fix any better than a single sat. If you've got two ropes at 180-degrees opposite each other (i.e., at either end of the street in the urban canyon), you've still got ambiguity due to your potential for lateral movement. For a good fix, you've got to bring in another rope at a wide angle from those -- but the tall buildings interfere with getting that rope, even if there are lots of satellites up there that could be offering it.

sjwright 15y ago
Yeah, but if there are more useful satellites in the sky, there's a greater chance of the visible satellites having a wider separation. Even in the 'urban canyon', you still frequently have partial sky visibility in some non-vertical directions. And there will be occasions when a GLONASS satellite will be better placed than a GPS satellite. To use a reductio ad absurdum argument, imagine a satellite positioning network with a thousand nodes. Wouldn't it have a greater chance of providing a useful fix in this environment: http://g.co/maps/sa8zk http://g.co/maps/sa8zk ?