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GPSr Quirk


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I use a Garmin 60CSx for my caching activities. When I'm searching for a cache, sometimes I can see a landmark (large tree, etc.) in the distance that the GPSr is pointing directly at. As I get close to the coordinates, I often find that the heading has changed and the GPSr is no longer pointing toward the landmark but off to the side of it, sometimes by a considerable margin.

 

Does anyone else notice this? Does anyone know what causes it? I'm talking about open ground - not under trees, etc.

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Can't say that I've noticed the deflection as you describe it, but you are probably experiencing multi-pathing, or "signal bounce", as most of us call it. I found a site that does a pretty good job of explaining it, if you want to get more into the technical details : http://www.cmtinc.com/gpsbook/

 

GPS receivers require a line of sight to the satellites in order to obtain a signal representative of the true distance from the satellite to the receiver. Therefore, any object in the path of the signal has the potential to interfere with the reception of that signal. Objects which can block a GPS signal include tree canopy, buildings and terrain features.

 

Further, reflective surfaces can cause the GPS signals to bounce before arriving at a receiver, thus causing an error in the distance calculation. This problem, known as multipath, can be caused by a variety of materials including water, glass and metal. The water contained in the leaves of vegatation can produce multipath error. In some instances, operating under heavy, wet forest canopy can degrade the ability of a GPS receiver to track satellites.

There are several data collection techniques which can mitigate the effects of signal blockage by tree canopy or other objects. For example, many GPS receivers can be instructed to track only the highest satellites in the sky, as opposed to those satellites which provide the best DOP. Increasing the elevation of the GPS antenna can also dramatically increase the ability of the receiver to track satellites.

 

Unfortunately, there will be locations where GPS signals simply are not available due to obstruction. In these cases, there are additional techniques which can help to solve the problem. Some GPS receivers have the ability to collect an offset point, which involves recording a GPS position at a location where GPS signals are available while also recording the distance, bearing and slope from the GPS antenna to the position of interest where the GPS signals are not available. This technique is useful for avoiding a dense timber stand or building.

 

Further, a traditional traverse program can be used to manually enter a series of bearings and ranges to generate positions until satellite signals can again be received. This position data can then be used to augment position data collected with the GPS receiver.

Edited by knowschad
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Well there is always the fact that depending on what map you are using, how accurate it is. For instance, if you have an sat image map, they are only so accurate. Between 20 to 300 ft accuracy depending on the area. Somewhere on the forum is a post where people are checking accuracy for google maps vs gps.

 

Then there is the signal bounce as knowschad has explained. There should be a accuracy reading that will tell you approximately how accurate your reading is. If its saying 40 ft, then its possible either you have a lot of bounce or you are tracking less satellites.

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I'm using the compass pointer on the GPSr to home in on a distant landmark (distant being maybe 1/8 mile or so). The times this has happened have been when I was in open areas (fields, etc.) where I can see far ahead and trees aren't a factor.

Edited by bikebill77
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I use a Garmin 60CSx for my caching activities. When I'm searching for a cache, sometimes I can see a landmark (large tree, etc.) in the distance that the GPSr is pointing directly at. As I get close to the coordinates, I often find that the heading has changed and the GPSr is no longer pointing toward the landmark but off to the side of it, sometimes by a considerable margin.

 

Does anyone else notice this? Does anyone know what causes it? I'm talking about open ground - not under trees, etc.

If your compass is pointing at the object while you are stationary and deflects to the side while moving, the compass may need calibrating.

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