What Is an Elevation Mask for GPS?
The Global Positioning System consists of a network, or constellation, of 24 satellites placed in fixed orbit above the surface of the earth. Each satellite transmits radio signals containing information about itself, which are detected by GPS receivers on the ground and used to calculate their three-dimensional position on the surface of the earth. Most GPS receivers include an elevation mask, which filters out signals from satellites below a certain angle of elevation above the horizon.
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Atmospheric Noise
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Satellites that are very close to the horizon, from the point of view of a GPS receiver, experience more errors, or noise, caused by the atmosphere than satellites high in the sky. By setting the elevation mask, GPS receiver users can effectively mask noisy signals, so that they are not used to calculate the latitude, longitude and altitude of the GPS receiver.
Typical Value
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According to the North Dakota Department of Transportation, the elevation mask of most GPS receivers is set at an angle of between 10 and 15 degrees. A GPS receiver requires signals from a minimum of four satellites to calculate its precise geographical position, so setting the elevation mask angle too high introduces the (albeit rare) possibility of too few satellites being high enough in the sky to provide sufficient positional data.
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Lowering Elevation Mask
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In certain situations, such as at sea, it may be necessary to set the elevation mask of a GPS receiver to a lower angle -- perhaps 5 degrees, or even zero -- in order to obtain a position. The signals received by the GPS receiver will, by definition, be noisy and provide inaccurate positional data as a result, but inaccurate data may be better than no data at all. Note that most GPS processing software allows the elevation mask angle to be increased, but not decreased, once processing is under way.
Benefits
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One of the benefits of setting the elevation mask angle to around 15 degrees is that it increases the chances of a GPS receiver using the same satellites as a GPS base station, or reference station. A GPS base station is a GPS receiver whose geographical position is known precisely. The base station calculates its position from incoming satellite signals, compares that position with its real position and sends a correction, if necessary, to any mobile GPS receivers within range. The elevation mask of base stations is typically set to 10 degrees, so it “sees” more satellites than a GPS receiver with an elevation mask angle of 15 degrees.
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References
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