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I used to think that the higher GPS antenna position the better until I read the following on GPSd FAQ:

One common error is to place the GPS or antenna as high as possible. This will increase multipath effects due to signal bounce from the ground or water, which can cause the GPS to mistake its position and the time signal. The correct location for a boat GPS antenna is on the gunwale rail or pushpit rail, close to the water and as far from the mast as possible (to reduce signal bounce from the mast). If you're outside or in a fixed location, put the GPS antenna as far from buildings as possible, and on the ground.

If you're in a car, don't put the GPS antenna on the roof, put it on the towbar or some similar location. If you're driving in a heavily built up area, you're going to get signal bounce off buildings and reduced accuracy. That's just how the physics works. Note, however, that as your velocity goes up it becomes easier for the convergence filters in your GPS to spot and discard delayed signal, so multipath effects are proportionally less important in fast-moving vehicles.

Does anyone has experience placing GPS antenna on a towbar of the car as suggested? Does it give reasonable effect?

My concern is that placing antenna there will not reduce an error that much, but will expose the device (antenna) to possible mechanical damage.

So, are there any better positions apart from roof and towbar?

Thanks

Community
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Boris
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  • Get a mag-mount and give it a whirl? Another problem with the towbar-mount is that the vehicle will block out half the sky, which will be an issue at times. – TimWescott Aug 20 '13 at 18:10
  • I hoped someone tried it already. In our system antenna is bolted now, so I would be happy to find additional proof before moving it at all. – Boris Aug 23 '13 at 10:25
  • I'm curious why you don't trust the GPSd FAQ to be giving you the best possible advice in this situation. – Ian Aug 27 '13 at 18:45
  • @Ian, it is easier to ask rather than move the device (including all the wiring). Also it seems to be rather an exotic place for antenna - if you check some of autonomous cars, e.g. Stanford's Stanley (which won DARPA Grand Challenge), they place antennae on the roof. Same did the other teams. So there is a big question if it is worth the effort. – Boris Aug 28 '13 at 03:57
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    Autonomous vehicles are a different story because they combine their GPS with a host of other sensors (most notably, LIDAR) to get a much more accurate real-time position. If you're planning to do the same thing, you should update your question with that information. – Ian Aug 28 '13 at 19:08
  • Well, the question's title telling about autonomous vehicle. So I thought it is sufficient. – Boris Aug 29 '13 at 03:34
  • How much do you care about accuracy?

    The reality with any GPS system is that you are only going to be accurate to about 1-2 meters unless you use Differential GPS (or some other correction signal). It is usually much better to tie the GPS position to some sort of Inertial measurement system. While I agree multipath is a huge issue for robots... you need to think of what your real needs are. What kind of autonomous vehicle are you trying to make? From that, you could decide how much accuracy you really need from your GPS solution.

     – Aerophilic Oct 07 '13 at 21:47

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I work on race cars which occasionally race in built up areas such as street circuits. As part of the mandatory electronic systems we fit a GPS antenna to the car to allow vehicle tracking.

Some of the guidance we have for the installation is

  • Fit the antenna to the upper surface of the car
  • Keep the antenna within 5 degrees of horizontal
  • Do not place metal objects above the antenna
  • Guarantee the direct visibility of the sky from the antenna, with a minimum of 170º cone unimpeded

So I would have expected that mounting a GPS antenna on a towbar where, potentially, half of the visibility would be obstructed by the metal structures at the rear of the vehicle would not be a good idea and would almost certainly be worse than fitting it to the roof

Matthew Gordon
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Inaccuracy due to multipath bounces is negligible compared to inaccuracy due to seeing fewer satellites, as Matthew Gordon answers. So on a land vehicle, roof is best. Towbar is very poor.

Camille Goudeseune
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