Technical data

2 Post-Process GPS Surveying
considerably so. RTK systems are somewhat more complex, requiring
management of a communication link between GPS receivers. Finally, RTK GPS
surveying is slightly less accurate than post-process.
Even with the introduction of RTK GPS surveying in the mid 1990s, post-process
GPS surveying has continued to be the most popular method of surveying with
GPS. It remains the most accurate and reliable method to survey with GPS.
The Global Positioning System (GPS)
Let’s take a quick look at what makes this all possible, the Global Positioning
System (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. These stations are invisible to the user. You need know nothing else
about them except that they exist to ensure that the satellites are functioning
properly.
Satellites make up the second primary component of GPS. The full satellite
constellation is defined as 24 satellites, although at the time of this writing 27 are
currently operational. Each satellite is in an orbit approximately 20,000 km above
the earth’s surface and has an orbital period of slightly less than 12 hours. On
board each satellite 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 transmission on two
separate frequencies, L1 at 1575.42 MHz and L2 at 1227.60 MHz. While you do
not have to be concerned with the control and monitoring stations, you are
required to have knowledge regarding the location, geometry, and number of
satellites available during your data collection. These important factors dictate
the reliability and accuracy of a GPS survey.
GPS receivers receive and store transmissions from the GPS satellites. This is
their primary function, and for some receivers, their only function. In addition to
reception and storage of satellite transmissions, some receivers perform
additional functions such as compute and display receiver position in various
datums and grid systems, output raw data and computed position through serial
ports, display satellite availability information, etc.
Surveying with GPS
A GPS surveying system consists of at least two GPS receivers. The receivers
collect data simultaneously to determine the location of one receiver relative to
630045.book Page 2 Sunday, February 11, 2001 11:38 AM