Overview - How GPS Tracking Device Was Designed to Work
GPS Tracking Devices was designed to work outdoors. The signal from the GPS satellites is extremely weak, and the satellites are far away. The total radiated power from each satellite is just 27W, about the same power from a dim lightbulb. The satellites are more than 20,000-km high (about 55 times higher than the space shuttle flies). When the signal reaches the GPS receiver on the Earth, the received signal power is about 100 attowatts; “atto” means 10–18, and it is not a commonly used prefix. We typically express such low powers in terms of decibels. But it is useful to mention the attowatt just this once, to get a feel for how weak the received GPS signal really is.
The received signal power is 100 attowatts when the receiver is outdoors; when the receiver moves indoors, the signals rapidly get weaker, by 10–100 times in a house,and by 100–1,000 times or more in a large building. However, it is not just indoors where GPS has signal problems; the weak signal is a problem outdoors, too, and standard GPS receivers have trouble acquiring satellites with even the slightest interference or blockage (from buildings, trees, or even the roof of a car). GPS was also designed for periods of continuous operation following a relatively slow startup sequence. The startup of a standard GPS receiver typically includes several seconds to acquire the signal, then 30s to decode required satellite data, for a total time to first fix of approximately 1 min. Thereafter, the typical receiver can compute a new position every second. The required satellite data is known as ephemeris data, and it describes the satellite orbits and clocks. So, as you may have explained to lay people, GPS satellites do not track you, you track them. The basic position computation comprises these steps:
1. Measure the ranges from you to several satellites;
2. Compute the satellite positions (using the ephemeris data);
3. Solve the equations linking the satellite positions, ranges, and your position, thus producing your position.
It can take just milliseconds to measure the range to a satellite, but it is the delay in initial acquisition and the time required to decode ephemeris data that makes standard Portable GPS slow to produce a first fix. It is these two limitations, weak signals and slow time to first fix, that are overcome with A-GPS. The rest of this chapter addresses the details of standard GPS that are necessary to understand before delving into the details of A-GPS.
Many commercial GPS devices will display signal-strength bars and, if you are outside, you can observe many signals from –123 to –129 dBm. You may notice two other things: the signal-strength bars often show carrier-to-noise ratio (C/N0), and the satellites with the lowest elevations will often have lower signal strengths. In practice, the receive antenna gain is often lowest at low elevations, particularly for patch antennas on a ground plane, resulting in several decibels lower received power at 5° elevation. Also, the satellites with the lowest elevation angles may be affected by partial blockage from trees or other elements of the landscape, further reducing the received signal strength.
More information at http://www.jimilab.com/ .
The received signal power is 100 attowatts when the receiver is outdoors; when the receiver moves indoors, the signals rapidly get weaker, by 10–100 times in a house,and by 100–1,000 times or more in a large building. However, it is not just indoors where GPS has signal problems; the weak signal is a problem outdoors, too, and standard GPS receivers have trouble acquiring satellites with even the slightest interference or blockage (from buildings, trees, or even the roof of a car). GPS was also designed for periods of continuous operation following a relatively slow startup sequence. The startup of a standard GPS receiver typically includes several seconds to acquire the signal, then 30s to decode required satellite data, for a total time to first fix of approximately 1 min. Thereafter, the typical receiver can compute a new position every second. The required satellite data is known as ephemeris data, and it describes the satellite orbits and clocks. So, as you may have explained to lay people, GPS satellites do not track you, you track them. The basic position computation comprises these steps:
1. Measure the ranges from you to several satellites;
2. Compute the satellite positions (using the ephemeris data);
3. Solve the equations linking the satellite positions, ranges, and your position, thus producing your position.
It can take just milliseconds to measure the range to a satellite, but it is the delay in initial acquisition and the time required to decode ephemeris data that makes standard Portable GPS slow to produce a first fix. It is these two limitations, weak signals and slow time to first fix, that are overcome with A-GPS. The rest of this chapter addresses the details of standard GPS that are necessary to understand before delving into the details of A-GPS.
Many commercial GPS devices will display signal-strength bars and, if you are outside, you can observe many signals from –123 to –129 dBm. You may notice two other things: the signal-strength bars often show carrier-to-noise ratio (C/N0), and the satellites with the lowest elevations will often have lower signal strengths. In practice, the receive antenna gain is often lowest at low elevations, particularly for patch antennas on a ground plane, resulting in several decibels lower received power at 5° elevation. Also, the satellites with the lowest elevation angles may be affected by partial blockage from trees or other elements of the landscape, further reducing the received signal strength.
More information at http://www.jimilab.com/ .
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