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Extending the Global Positioning System to within buildings and tunnels

The Global Positioning System (GPS) is being used more and more in a variety of specialist and consumer applications. This use has increased markedly in the past few years due to the reduction in price of GPS receivers, and looks set to increase even more in the coming years.

GPS technology has one major flaw, however. A GPS receiver needs to be able to receive signals from at least 4 satellites, and this is usually impossible within urban canyons and adjacent to, or inside buildings. This means that GPS receivers generally do not work indoors or underground.

High sensitivity GPS receivers

Technology has moved forward, and higher sensitivity GPS receivers are available, sacrificing location accuracy for limited in-building work.  These high sensitivity receivers tend to use onboard CPU processing time ( and battery consumption ) to decode the received signal, and they are able to claim remarkable sensitivities. However, the operation of the enhanced sensitivity receiver is limited by the attenuation of the building materials, signals taking non-direct paths to the receiver ( multipath problems ) causing error in the location, and an inability to decode almanac/ephemeris data from the GPS broadcasts.

In-building 'beacons'

Other solutions to in-building GPS exist, such as WiFi location and other proprietary devices that send a signal from a number of known locations within the target building. The custom location receiving equipment will only work within the equipped building.

GPS repeaters

A GPS repeater ( not an NMEA repeater ) is an on-frequency repeater that uses a well positioned outside aerial, amplifies the signal and then re-radiates this signal to the target GPS receiver. It is possible to buy simple GPS repeaters that re-radiate over a few inches to enable operation within a vehicle.

GPS repeaters reply on a separation between the receive and re-radiation aerials. The greater the gain of the GPS repeater amplifier, the greater the range of the re-radiated signal, but also the greater isolation requirements.

Leaky Feeder systems

TBC