Global Positioning System Reference
In-Depth Information
angle of the signal reflection and the transit time for the pulse of energy
transmitted to travel to the target and back again. Doppler RADAR measures
relative velocity and uses the Doppler effect (i.e., the change in frequency when
objects moving relative to each other send and receive propagating wavelike
signals such as the siren pitch change of a police vehicle's siren). Unlike pulse
RADAR Doppler (and the somewhat similar radar altimeter) uses continuous
carrier radars and employ changes in frequency. There are many other types of
RADAR systems beyond the scope of this topic, since many techniques are not
helpful in the highly cluttered radio environment of urban and indoor areas where
the clear lines of sight needed for reflections are rarely available and targets are
not metallic. There is one exception, however: that of secondary radar, which
does not employ reflections and involves active electronics within a transponder
unit attached to the target.
A transponder uses both communications and the directionality of transmitted
signals. The simplest transponder is an RFID tag and has some similarities in
operational mode to an aircraft transponder. The process starts with a request
signal from the base station asking any transponders present to reply (i.e., to
transpond). Any unit within range then initiates a transmission back with a
modulated signal encoded with identity information. The base receiver can then
decode the signal and measure characteristics of the signal. In the case of an RFID
tag, it is simply a case of proximity but with aircraft, the secondary system is
associated with a pulse system that also gives a heading and range. The
transponded code is made to show up on the display next to the blip representing
its target.
There are many other transponder systems used in positioning including
indoor ultrawideband systems and the tags used for vehicle tolling. It is a very
important technique in Whereness and is sometimes present is another form, for
example, within the protocols used to set up mobile phone calls. To recap, the
main principle is essentially for the base station to ask “Is anything or anybody
there?” and if present the person or thing replies with its identity. Smart antennae
can be used to gather angle information (angulation) and signal timings to provide
distances (lateration).
6.1.6
The Mobile Radio Environment
One of the biggest headaches of mobile radio is the extremely variable signal
levels present when radios are moving in a cluttered environment. Although
location by proximity is simple in principle, it is inherently unreliable. Figure 6.8
shows the dominant effects. At the top of the diagram the overall effect of
declining signal level with distance is shown (as described in equation 6.3). In the
center the effect of shadowing due the general topography is shown with effects
such as the bending of signals as they diffract around corners. Any obstacle that
appears as a “knife edge” vertically or horizontally will cause a sharp diffraction
and leads to an unintuitive effect that just because there is a line of sight between
