As anyone who's tried to use GPS indoors can tell you, global positioning systems have their limitations. For them to work properly, you have to be outdoors and you need a clear view of the sky. If you’re in the military, you also have to be sure that the enemy isn't jamming the satellite signal. For this reason, the US Air Force has awarded Canberra-based firm Locata a “sole source” contract to install a ground-based version of GPS over 2,500 square miles (6,475 sq/km) of the White Sands Missile Range in New Mexico as part of a program to develop a practical supplement to GPS.

Called LocataNet, the navigation system completed its latest phase of testing at White Sands last December. It uses technology very similar to satellite-based GPS and provides navigational fixes as accurate as GPS in areas where GPS is not available or may be degraded. In fact, to the user it is almost identical to GPS. It’s a receive-only system where the user only needs to receive a signal from the ground-based transmitters, it can serve an unlimited number of receivers, and has an accuracy of 2.5 inches (6 cm) horizontally and 6 inches (15 cm) vertically at a height of 25,000 feet (7,620 m) while travelling 350 mph (304 knots, 563 km/h).

How LocataNet can overcome GPS limitations in built up areas

There are many ground-based radio navigation systems, but they aren’t very accurate and often require data links or reference networks. Operating on WiFi frequencies, the LocataNet does not require reference receivers because its design is very close to that of satellite-based GPS, but without the built-in degradation that the US requires for civilian receivers.

The LocataNet receiver is almost identical to a standard GPS receiver and has the same ability to be installed as a chip in devices like smartphones. It receives signals from LocataLite transceivers, which duplicates all of a GPS satellite’s functions. These are linked together to form the LocataNet.

The secret behind LocataNet is TimeLoc. This is a patented system that synchronizes all the transceivers in the LocataNet to within a nanosecond. Satellite GPS works by sending out a signal telling the transceiver’s precise location and a time signal of atomic clock accuracy. It is this precise time signal that allows the receiver to calculate its position relative to the satellite. In the LocataNet, the transceivers synchronize on a single precise time signal that allows the network to act as a local version of GPS. In addition, the transceivers are fitted with the TimeTenna antenna, which is designed for use underground and indoors.

LocataNet installed at the White Sands Missile range

LocataNet isn’t intended as a replacement for GPS. Its purpose is for local applications where GPS may not be suitable or available. If you’ve ever tried to use a smartphone to navigate in a megastore or among skyscrapers, having ground-based transmitters have obvious advantages in GPS blind spots. Locata sees LocataNet being used in city centers, mining, construction work, managing ports and warehouses, and at airports.

Since the US military developed GPS, it seems odd that it would invest in a ground-based rival, but LocataNet has a strategic advantage over GPS in that the LocaLite transceiver signal is, according to Locata, a billion times stronger. This makes it extremely difficult to jam without a transmitter that would stick out like a sore thumb on a battlefield.

Source: Locata via New Scientist