What Is an Elevation Mask for GPS?by Kevin Lee
The U.S. Department of Defense makes it possible for you to drive to unknown locations without using a map. The government's network of GPS satellites beams down information that GPS receivers use to pinpoint precise locations on earth. As these satellites orbit the planet, some may be in locations that render them unsatisfactory as signal sources. By learning to use your GPS unit’s elevation mask, you can ignore those satellites.
GPS satellites orbit the planet at an altitude of about 11,000 miles. GPS receivers anywhere on the Earth can pick up signals from these satellites because they encircle the globe. A GPS receiver uses complex calculations to interpret satellite information and pinpoint latitude and longitude coordinates. A receiver must receive signals from four satellites before it can to determine its location and the location of the person using the device.
If you could see GPS satellites from your back yard, you would notice that they appear at different angles in the sky. Those directly above you would be at 90 degrees while others near the horizon would have lower angles. Most GPS receivers, regardless of price, have a setting you can tweak to adjust your elevation "mask angle." This setting is important because you can use it to tell your GPS receiver to ignore satellites below a certain angle.
Elevation Mask Principles
It may seem odd that you would want to eliminate a satellite if it can help you pinpoint your location more accurately; however, you'll want to ignore satellites that appear near the horizon because those at low angles are subject to signal fade. They also pick up more atmospheric noise than satellites orbiting higher above the horizon. The North Dakota Department of Transportation notes that most GPS programs allow you to increase an elevation mask but you cannot lower it.
Optimizing your mask angle can be tricky. For example, if you set the value too high, your GPS unit might eliminate satellites that would be useful in determining your coordinates. The North Dakota Department of Transportation recommends keeping your mask angle "at the upper end of around 15 to (at most) 20 degrees." A typical mask angle is between 10 and 15 degrees. You can also set your angle to lower values, such as 5 or zero degrees, if you need to get a reading quickly and you don't have time to wait for other satellites to position themselves higher above the horizon. When you choose to use a low elevation mask setting, your GPS unit still reports your location, but the reading might not be as accurate as one you'd get if you used a higher mask angle setting.
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