A brief explanation of GPS
utilises 24 medium Earth orbit satellites and is currently the
only fully functional Global Navigation System (GNSS). Precise
microwave signals are transmitted from the satellites, allowing
a GPS receiver to determine its location, speed and direction.
GPS reciever calculates its position by measuring the distance
between itself and three or more GPS satellites. The distance
to each satellite can be calculated by measuring the time delay
between transmission and reception of each GPS microwave signal.
This is possible beacause the signal travels at a known speed.
reciever can compute its position using trilateration. Three
spheres intersect at a point, there are 3 distances or ranges
to resolve, latitude, longitude and height.
typically do not have very accurate clocks and therefore tracking
four satellites is required in order to correct the receiver's
clock error. Here there are four ranges to resolve, latitude,
longitude, height and time.
just the receiver, accuracies are about +/- 10metres. The
accuracy can be improved by using two or more receivers
which will give ‘differential’ position (DGPS). It is possible
to know the position of B in relation to A when the co-ordinates
of A are known and the satellites are tracked simultaneously.
In this situation, A would be called a reference station,
or base station and B would be called a Rover.
The data gathered at the base station can be broadcast by
terrestrial radio to the rover and processed almost instantaneously.
Thus positions and heights can be obtained in ‘real time’
- often known as Real Time Kinematic (RTK). It is possible
to obtain millimetric accuracy using this technique.
brief explanation of GPS leaves out some of the more technical
issues and solutions. For a more detailed description, please
use Accuratemaps Survey Partnership?
Survey Partnership was one of the first land survey companies
in the India to acquire GPS. We have wide ranging experience in the
application of this technology. For example, providing coastal
monitoring services, ground control work, pipeline mapping, consultancy
work and software solutions for transforming GPS data.
an incorrect geoid model can result in large mistakes. High accuracy
results are possible but only with great care. Spirit levelling
measures heights relative to the Geoid (mean sea level). GPS measures
heights relative to the ellipsoid (mathematical model approximating
to the Earth’s shape). The geoid/ellipsoid separation must be
determined and modelled to enable conversion of GPS (ellipsoid)
heighting to geoid heights. (eg above OS datum).
It is important to use the correct ellipsoid so that positions
and heights are accurate. The World Geodetic System (WGS84) is
an ellipsoid which provides a best mean fit to the earth, however
other elipsoids can be used such as the North American Vertical
Datum (NAVD83), based on the GRS80 ellipsoid; an ellipsoid derived
from satellite geodesy.