The Global Positioning System (Gps); Surveying With Gps - ashtech Z-Xtreme System Manual

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 sat-
ellites. 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 constella-
tion 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 satel-
lite 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 availabil-
ity information, etc.

Surveying with GPS

A GPS surveying system consists of at least two GPS receivers. The receivers col-
lect data simultaneously to determine the location of one receiver relative to the
other(s). The positional relationship between the receivers is presented in the form of
a vector, i.e. (delta X, delta Y, delta Z) or (delta N, delta E, delta H) between the
receivers. This is a 3-dimensional relationship. It is analogous to a conventional
observation of (horizontal angle, vertical angle, slope distance). Vectors are pro-
duced between all combinations of GPS receivers used during the survey.
2 Post-Process GPS Surveying