The Global Positioning System (Gps); Real-Time Kinematic (Rtk) Surveying - ashtech Z-Xtreme System Manual

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. 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 27 are currently opera-
tional. 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, loca-
tion, time, etc. Each satellite is capable of transmissions on two separate frequen-
cies, L1 at 1575.42 MHz and L2 at 1227.60 MHz. Unlike control and monitoring
stations, you are required to have knowledge regarding the location, geometry, and
number of satellites available during data collection periods. 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 transmis-
sions 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 transmis-
sions, 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. 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.

Real-Time Kinematic (RTK) Surveying

RTK mode of GPS surveying requires the use of at least two GPS receiver systems
functioning simultaneously, as shown in Figure 1.1. In fact, this is true for any mode
of GPS use where better than 100 meters of positional accuracy is required. In a two-
receiver GPS RTK system, one receiver subsystem is designated as the base, and
the other is designated the rover. The base receiver system is usually located on a
known position, i.e. known latitude, longitude, height or easting, northing, height.
Once operational, the base system collects raw data from all available satellites. This
raw data is packaged and sent out the serial port to an awaiting radio transmitter. The
transmitter broadcasts the packaged raw data to anyone who wants to listen. This is
2 Real-Time Kinematic Surveying