【正文】 be very precisely determined for a given time. Chapter Four: Computing the Distance Between Your Position and the GPS Satellites GPS determines distance between a GPS satellite and a GPS receiver by measuring the amount of time it takes a radio signal (the GPS signal) to travel from the satellite to the receiver. Radio waves travel at the speed of light, which is about 186,000 miles per second. So, if the amount of time it takes for the signal to travel from the satellite to the receiver is known, the distance from the satellite to the receiver (distance = speed x time) can be determined. If the exact time when the signal was transmitted and the exact time when it was received are known, the signal39。s clock. The insertion of the drift rate effectively synchronizes all of the GPS satellite clocks. The same procedure cannot be applied to the clock in a GPS receiver. Therefore, a fourth variable (in addition to x, y and z), time, must be determined in order to calculate a precise location. Mathematically, to solve for four unknowns (x,y,z, and t), there must be four equations. In determining GPS positions, the four equations are represented by signals from four different satellites. Chapter Six: The GPS Error Budget The GPS system has been designed to be as nearly accurate as possible. However, there are still errors. Added together, these errors can cause a deviation of +/ 50 100 meters from the actual GPS receiver position. There are several sources for these errors, the most significant of which are discussed below: Atmospheric Conditions The ionosphere and troposphere both refract the GPS signals. This causes the speed of the GPS signal in the ionosphere and troposphere to be different from the speed of the GPS signal in space. Therefore, the distance calculated from Signal Speed x Time will be different for the portion of the GPS signal path that passes through the ionosphere and troposphere and for the portion that passes through space. As mentioned earlier, GPS signals contain information about ephemeris (orbital position) errors, and about the rate of clock drift for the broadcasting satellite. The data concerning ephemeris errors may not exactly model the true satellite motion or the exact rate of clock drift. Distortion of the signal by measurement noise can further increase positional error. The disparity in ephemeris data can introduce 15 meters of positional error, clock drift disparity can introduce meters of positional error and measurement noise can introduce 010 meters of positional error. Ephemeris errors should not be confused with Selective Availability (SA), which is the intentional alteration of the time and ephemeris signal by the Department of Defense. A GPS signal bouncing off a reflective surface prior to reaching the GPS receiver antenna is referred to as multipath. Because it is difficult to pletely correct multipath error, even in high precision GPS units, multipath error is a serious concern to the GPS user. Chapter Seven: Measuring GPS Accuracy As discussed above, there are several external sources which introduce errors into a GPS position. While the errors discussed above always affect accuracy, another major factor in determining positional accuracy is the alignment, or geometr