The Global Positioning System (Gps) - Thales Z-Max Operation And Application Manual

The Global Positioning System (GPS)

The system that makes all this possible is the Global Positioning System or GPS.
GPS consists of three primary components: satellites, ground-based control and
monitoring stations, and receivers.
The control and monitoring stations' main purpose is to monitor and maintain the
satellites. As a user, these stations are invisible to you. You need know nothing else
about them except that they exist to ensure the satellites are functioning properly.
Satellites make up the second primary component of GPS. A full constellation is
defined as 24 satellites, although at the time of this writing, 28 are currently
operational. Each satellite is positioned approximately 20,000 km above the earth's
surface and has an orbital period of slightly less than 12 hours. On board each
satellite, among other things, is a radio transceiver. The transceiver receives
information and instructions from the control station and transmits information about
its identity, location, time, etc. Each satellite is capable of transmissions on two
separate frequencies, L1 at 1575.42 MHz and L2 at 1227.60 MHz. Unlike control and
monitoring stations, it is important to have knowledge regarding the location,
geometry, and number of satellites available during data collection periods. Since the
satellites are not geosynchronous, the number of satellites that are available and the
satellite geometry (PDOP) at any given time will vary significantly. It is important that
you are aware of the favorable times of the day and choose the times when the most
satellites and the lowest PDOP are available to perform your surveys. These
important factors will dictate the reliability and accuracy of a GPS survey.
GPS receivers function exactly as the name implies, they receive and store
transmissions from the GPS satellites. This is their primary function, and for some
receivers, this is their only function. In addition to reception and storage of satellite
transmissions, some receivers perform additional functions such as compute and
display receiver positions in various datums and grid systems, output raw data and
computed position through serial ports, and display satellite availability information.
The more sophisticated receivers are capable of receiving raw data from a second
GPS receiver collecting data simultaneously and combining this data with its own to
compute a very accurate position of its location in real-time. This is how an RTK
receiver functions.
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