How GPS Was Designed to Work

GPS 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 electronic tracking devices 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. GPS signals are noticeably stronger than signals in houses made of brick or stone. (A drywall panel is made of a paper liner wrapped around an inner core made primarily from gypsum plaster.)

·  In houses made mostly made of wood and drywall (such as the typical Californian house), GPS signals are noticeably stronger than signals in houses made of brick or stone. (A drywall panel is made of a paper liner wrapped around an inner core made primarily from gypsum plaster.)

·  In office buildings, a one-wall rule applies. If you are within one wall of the outside, there are probably several satellites detectable above –160 dBm, but the signals drop off dramatically as soon as you move within two or more walls.

·  In personal navigation devices ( such as rearview mirror GPS navigation , 3G navigator) for in-car navigation, the antennas are usually ceramic pads with close to the ideal 3-dBi gain, and you will often see signal strengths in the expected range of –123 to –130 dBm.

·  In cell phones, GPS antennas are often very small, sometimes too small (cellphone industrial design aesthetics currently have far higher priority than GPS efficiency). Depending on the phone, the average antenna gain can be as much as 10-dB below the ideal 3 dBi. Also, the antennas in phones often exhibit significant directionality, and the gain pattern in certain directions may be more than 15-dB below 3 dBi. This means that 90–97% of the GPS signal power is lost before ever reaching the LNA, and the effective signal strengths in the Table 6.14 must all be adjusted accordingly.