Global Positioning System Reference
In-Depth Information
A more complex broadcasting arrangement is used in GPS and similar
systems where a number of separate broadcasts must be picked up simultaneously.
To achieve this, a receiver is needed that is effectively a bank of receivers, one per
satellite. In reality, only certain parts of the receiver need to be multiplied.
The most complex system arrangements are where there are a host of mobile
two-way transceivers, for example, as used in cellular radio. Another class of
system of high complexity (but not yet to the degree used in cellular radio) is the
hotspot Internet access system (such as WiFi). One of the most common aspects
of complex communication systems involves “trunking” (i.e., the tree and branch-
like approach that is used to configure communications networks). Trunking can
be very useful to Whereness since part of the task of a computer system managing
trunking is to keep account of which end terminal is using which common
resource that usually has a physical location.
6.2.2
Global Navigational Satellite Systems (GNSS)
GNSS is a generic term that covers systems like GPS, GLONASS, Galileo, and
their predecessors and successors. The phenomenal uptake of GPS for business
and consumer use has brought the former military technology into the public's
mind so that it is likely that the overall future of GNSS will be for both military
and nonmilitary applications. This is a situation not unlike the evolution of two-
way digital radios which has now evolved to the stage where the nonmilitary
systems are ahead in capabilities.
6.2.3
GPS Infrastructure
We will start with an overview of GPS but it is worth noting that it is not so much
about how GPS works but more about what can be expected by users from the
system. If a more in-depth explanation is needed, there are many excellent
publications, of which Kaplan [7] is particularly comprehensive.
GPS is a complex system and the receiver module, embedded within a
domestic satellite navigation (sat-nav) unit or cell phone, is only one small part.
Overall the system consists of a constellation of 24 operational satellites (plus
spares and any failed units) in mid-Earth circular orbit, controlled by a network of
ground stations using two-way radio links. The height is such that each satellite
orbits once every 12 hours and the configuration of the satellites is such that they
are arranged to be well spaced in the sky so that from the ground one can expect
to “see” up to an average of 12 satellites above the horizon at any time.
A receiver needs to be able to receive signals from at least three satellites to
find a two-dimensional position (or four for a position that includes height). If
any local buildings, tree foliage, or anything else opaque to the microwave radio
signals is present and occludes a clear line of sight, signal reception is degraded
limiting the potential for a position to be computed. This is a frequent problem in
dense urban areas where “urban canyons” prevent enough of the sky being
