Leica Geosystems GPS Basics - Manual (Page 45)

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GPS Basics -1.0.0en Surveying with GPS
5.1.4 RTK Surveys
RTK stands for Real Time Kinematic. It
is a Kinematic on the Fly survey carried
out in real time.
The Reference Station has a radio link
attached and rebroadcasts the data it
receives from the satellites.
The Rover also has a radio link and
receives the signal broadcast from the
Reference. The Rover also receives
satellite data directly from the satellites
via it’s own GPS Antenna. These two
sets of data can be processed together
at the Rover to resolve the ambiguity and
therefore obtain a very accurate position
relative to the Reference receiver.
Once the Reference Receiver has been
set up and is broadcasting data through
the radio link, the Rover Receiver can be
activated.
When it is tracking satellites and receiv-
ing data from the Reference, it can begin
the initialization process. This is similar
to the initialization performed in a post-
processed kinematic on the fly survey,
the main difference being that it is
carried out in real-time.
Once the initialization is complete, the
ambiguities are resolved and the Rover
can record point and coordinate data. At
this time, baseline accuracies will be in
the 1 - 5cm range.
It is important to maintain contact with
the Reference Receiver, otherwise the
Rover may lose the ambiguity. This
results in a far less accurate position
being calculated.
Additionally, problems may be encoun-
tered when surveying close to obstruc-
tions such as tall buildings, trees etc. as
the satellite signal may be blocked.
RTK is quickly becoming the most
common method of carrying out high
precision, high accuracy GPS surveys in
small areas and can be used for similar
applications as a conventional total
station. This includes detail surveying,
stakeout, COGO applications etc.
The Radio Link
Most RTK GPS systems make use of
small UHF radio modems. Radio
communication is the part of the RTK
system that most people experience
difficulty with. It is worth considering the
following influencing factors when trying
to optimize radio performance:
1. Power of the transmitting radio.
Generally speaking, the more power, the
better the performance. However, most
countries legally restrict output power to
0.5 - 2W.
2. Height of transmitter antenna. Radio
communication can be affected by line of
sight. The higher up you can position the
antenna, the less likely you are to get
line of sight problems. It will also
increase the overall range of radio
communication. The same also applies
to the receiving antenna.
Other influencing factors affecting
performance include the length of the
cable to radio antenna (longer cables
mean higher losses) and the type of
radio antenna used.