Global Positioning System Reference
In-Depth Information
6.2.7
Dedicated Terrestrial Systems
There are many application-specific and dedicated (often proprietary) radio
positioning systems, some of which use techniques developed during the Second
World War (see Section 1.4.1). Long-range navigation, or LORAN, is still in use
(in the form of the LORAN C standard) and uses VHF radio beacons that transmit
their carriers over coastal regions. LORAN (and the now obsolete Decca system)
uses carrier phase to determine position. The locus of equal phases between two
transmitters is a set of hyperbolae. If several pairs of transmitters are in range, the
intersection of the hyperbolic loci gives position. Accuracy is worse than GPS by
about an order of magnitude (i.e., 10s to 100s of meters) but adequate for many
marine and some land uses. LORAN C is still operational and is being upgraded.
Although prone to radio atmospherics it is a simple system and its robust nature
makes it complementary to GNSS.
In a similar vein are the various aircraft systems such as ADF and VOR.
Automatic direction finder (ADF) allows a pilot to know the bearings of dedicated
VHF navigational beacons so that from several bearings a position can be plotted
on a map. A simple loop antenna can be tuned to find the direction of each signal
to which the receiver is set.
VOR (VHF omnidirectional range) is more complex. The beacon transmits an
omnidirectional reference signal and a second transmission from a rotating highly
directional antenna, the phase of which is electronically varied according to the
absolute direction. The receiver can calculate its heading from the phase
difference it is experiencing, which will change as the flight continues unless
heading directly in line with the beacon.
Another dedicated class of system is that used to track or follow targets that
may be animals, fellow radio amateurs, orienteering enthusiasts, or perhaps stolen
vehicles. In these cases the transmitter is small, simple, and narrowband (to
preserve battery power). A simple pulsed “bleep” will conserve energy further.
When triggered, a moving receiver is used to find the direction (i.e., direction
finding, or “DF-ing”). Although handheld directional antennae are used, another
approach is to use a simple static phased array mounted on the roof of a vehicle.
These can be seen on police vehicles as a square pattern of four quarter-wave
monopoles. These are electronically switched in phase, which can simulate the
effect of rotating the array. The phase switching is synchronized to a display to
show the approximate heading of any received signal. A similar arrangement is
used for coastal maritime radio communication (see Figure 6.10).
The latest manifestations of the DF technique are emerging for use with the
ISM band (the band used by WiFi at 2.45GHz). Small tags can be attached to
objects [10] that can be easily lost about a room (e.g., vehicle keys). When the
small remote controller is activated it “pings” the tag, which is a transponder that
replies. The reply is picked up by the remote, which displays the approximate
direction of the signal and its level.
