GPS Q&A
By Naush Ladha

Q. What is GPS satellite prediction software and what is it used for?
R.D. Phoenix, Ariz.

A. GPS satellite prediction software allows users of the NAVSTAR GPS constellation of satellites to view relevant information about these satellites. This information typically consists of:
• number of satellites visible,
• which specific satellites are visible,
• satellite configuration information (DOP values), and
• where the satellites are in the sky (graphically and in table form).
      This type of software generally requires a GPS almanac file. An almanac file contains information about the satellites including their position and motion with respect to time. This file is obtained from the satellites and does change. So it is advisable to use the most current almanac file which is available.
     The user then enters parameters to simulate the working environment including:
• site location (geographic or mapping plane coordinates),
• elevation mask angle (how low the satellites can be with respect to the horizon), and
• local time offset from universal time (UT).
      It is important to know how many satellites are visible as a minimum of four satellites are needed to obtain a good GPS position. It is also important to know the configuration of the satellites (DOP) as this will have a bearing on the positioning accuracy.
      Prior to the full deployment of the GPS constellation (24 satellites), there were occasions where receivers could not track enough satellites to obtain a good position. Today, with the full constellation having been deployed, there should be a minimum of at least four satellites visible at all times anywhere in the world, unless there are obstructions (buildings, mountains, trees, etc.) which block the view of the sky. Planning software can anticipate possible problems including obstructions and shows the best times to perform field work.

Q. Why is it recommended not to use low elevation satellites?
B.H. New Brunswick, NJ

A. A low elevation satellite is a satellite which your GPS receiver is tracking that is "just" above the horizon. Generally, a satellite which is considered low elevation is anywhere between the horizon and up to 15 degrees above the horizon. This satellite is usually just setting or just rising.
      There is generally nothing wrong with the data that are being transmitted by these satellites. In fact, the data are the same as a high elevation satellite or a satellite right above you. However, the path that the GPS signal has to take from the satellite to your receiver is different. The signal from low elevation satellites has to go through more atmosphere and the atmosphere it has to go through is usually "thicker." This is important as we always assume that your GPS signal behaves like a typical electro-magnetic signal and travels at the speed of light in a vacuum. All calculations including computing your position are based on this assumption and the fact that the speed at which a GPS signal travels in air is almost the same as it travels in a vacuum. Thus, if the signal is going through more atmosphere and does not travel the same in this "thicker" atmosphere as in a vacuum (or air) then our position calculation is going to be more inaccurate. Also, as the satellite signal has to go through more atmosphere, it is noisier and is not as clean. This will again affect how good the data are.
      Most GPS receivers have a default of anywhere between 5 to 15 degrees set as an "elevation mask" or "cut-off." This value is used so that satellites below the mask or cut-off are not used in computing position. Thus, noisy low elevation satellite data are not used. For users who post-process data, it is best to collect all data (even below the mask). Thus, when post-processing, you will have all data and can experiment at what mask you can use to give you your best results. Usually, the default value is fine. however, in cases where there are not enough satellites visible, a low elevation satellite may actually help in providing a useful solution.

Q. What types of uses are there for GPS? M.A. Tampa Fla.

A. Land uses include: surveying; GIS/LIS; mining; construction; vehicle navigation; fleet management; machine control; crustal deformation; recreation; subsidence monitoring; seismic; and forestry.
      Marine uses include: navigation; seismic; search and rescue; and platform positioning.
      Aviation uses include: navigation; collision avoidance (air traffic control); take-offs/landings; automation; and search and rescue.
      GPS can also be used for precise timing.

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