GEOFFREY BLEWITT: BASICS OF THE GPS TECHNIQUE ground track is said to repeat. From the user's point of view the same satellite appears in the same direction in the sky every day minus 4 minutes. Likewise, the"sky tracks"repeat. In general, we can say that the entire satellite geometry repeats every sidereal day(from the point of view of a ground user) As a corollary, any errors correlated with satellite geometry will repeat from one day to the next. An example of an error tied to satellite geometry is"multipath, which is due to the antenna also sensing signals from the satellite which reflect and refract from nearby objects In fact, it can be verified that, because of multipath, observation residuals do have a pattern that repeats every sidereal day. As a consequence, such errors will not significantly affect the precision, or repeatability, of coordinates estimated each day. However, the accuracy can be significantly worse than the apparent precision for this reason Another consequence of this is that the same subset of the 24 satellites will be observed every day by someone at a fixed geographical location. Generally, not all 24 satellites will be seen by a user at a fixed location. This is one reason why there needs to be a global distribution of receivers around the globe to be sure that every satellite is tracked sufficiently well We now turn our attention to the consequences of the inclination angle of 55. Note that a satellite with an inclination angle of 90 would orbit directly over the poles. Any other inclination angle would result in the satellite never passing over the poles. From the user's point of view, the satellite's sky track would never cross over the position of the celestial pole in the sky. In fact, there would be a"hole"in the sky around the celestial pole where the satellite could never pass. For a satellite constellation with an inclination angle of 55, there would therefore be a circle of radius at least 35 around the celestial pole, through which the sky tracks would never cross. Another way of looking at this, is that a satellite can never rise more than 55 elevation above the celestial equator This has a big effect on the satellite geometry as viewed from different latitudes. An observer at the pole would never see a GPS satellite rise above 55 elevation. Most of the satellites would hover close to the horizon. Therefore vertical positioning is slightly degraded near the poles. An observer at the equator would see some of the satellites passing overhead, but would tend to deviate from away from points on the horizon directly to the north and south Due to a combination of Earth rotation, and the fact that the GPS satellites are moving faster than the Earth rotates, the satellites actually appear to move approximately north-south outh-north to an oberver at the equator, with very little east-west motion. The north component of relative positions are therefore better determined than the east component the closer the observer is to the equator. An observer at mid-latitudes in the Northern Hemisphe would see satellites anywhere in the sky to the south, but there would be a large void towards the north. This has consequences for site selection, where a good view is desirable to the outh, and the view to the north is less critical. For example, one might want to select a site in the Northern Hemisphere which is on a south-facing slope(and visa versa for an observer in the Southern Hemisphere) 2.2. 3 Satellite hardware There are nominally 24 GPS satellites, but this number can vary within a few satellites at any given time, due to old satellites being decommissioned, and new satellites being launched toGEOFFREY BLEWITT: BASICS OF THE GPS TECHNIQUE 5 ground track is said to repeat. From the user’s point of view, the same satellite appears in the same direction in the sky every day minus 4 minutes. Likewise, the “sky tracks” repeat. In general, we can say that the entire satellite geometry repeats every sidereal day (from the point of view of a ground user). As a corollary, any errors correlated with satellite geometry will repeat from one day to the next. An example of an error tied to satellite geometry is “multipath,” which is due to the antenna also sensing signals from the satellite which reflect and refract from nearby objects. In fact, it can be verified that, because of multipath, observation residuals do have a pattern that repeats every sidereal day. As a consequence, such errors will not significantly affect the precision, or repeatability, of coordinates estimated each day. However, the accuracy can be significantly worse than the apparent precision for this reason. Another consequence of this is that the same subset of the 24 satellites will be observed every day by someone at a fixed geographical location. Generally, not all 24 satellites will be seen by a user at a fixed location. This is one reason why there needs to be a global distribution of receivers around the globe to be sure that every satellite is tracked sufficiently well. We now turn our attention to the consequences of the inclination angle of 55 o . Note that a satellite with an inclination angle of 90 o would orbit directly over the poles. Any other inclination angle would result in the satellite never passing over the poles. From the user’s point of view, the satellite’s sky track would never cross over the position of the celestial pole in the sky. In fact, there would be a “hole” in the sky around the celestial pole where the satellite could never pass. For a satellite constellation with an inclination angle of 55 o , there would therefore be a circle of radius at least 35 o around the celestial pole, through which the sky tracks would never cross. Another way of looking at this, is that a satellite can never rise more than 55 o elevation above the celestial equator. This has a big effect on the satellite geometry as viewed from different latitudes. An observer at the pole would never see a GPS satellite rise above 55 o elevation. Most of the satellites would hover close to the horizon. Therefore vertical positioning is slightly degraded near the poles. An observer at the equator would see some of the satellites passing overhead, but would tend to deviate from away from points on the horizon directly to the north and south. Due to a combination of Earth rotation, and the fact that the GPS satellites are moving faster than the Earth rotates, the satellites actually appear to move approximately north-south or south-north to an oberver at the equator, with very little east-west motion. The north component of relative positions are therefore better determined than the east component the closer the observer is to the equator. An observer at mid-latitudes in the Northern Hemisphere would see satellites anywhere in the sky to the south, but there would be a large void towards the north. This has consequences for site selection, where a good view is desirable to the south, and the view to the north is less critical. For example, one might want to select a site in the Northern Hemisphere which is on a south-facing slope (and visa versa for an observer in the Southern Hemisphere). 2.2.3 Satellite Hardware There are nominally 24 GPS satellites, but this number can vary within a few satellites at any given time, due to old satellites being decommissioned, and new satellites being launched to