Leica Geosystems GPS Basics - Manual (Page 20)

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GPS Basics -1.0.0en How GPS works
4
3.2.1 The Reference Receiver
The Reference receiver antenna is
mounted on a previously measured
point with known coordinates. The
receiver that is set at this point is known
as the Reference Receiver or Base
Station.
The receiver is switched on and begins
to track satellites. It can calculate an
autonomous position using the tech-
niques mentioned in section 3.1.
Because it is on a known point, the
reference receiver can estimate very
precisely what the ranges to the various
satellites should be.
The reference receiver can therefore
work out the difference between the
computed and measured range values.
These differences are known as correc-
tions.
The reference receiver is usually at-
tached to a radio data link which is used
to broadcast these corrections.
3.2.2 The Rover receiver
The rover receiver is on the other end of
these corrections. The rover receiver has
a radio data link attached to it that
enables it to receive the range correc-
tions broadcast by the Reference
Receiver.
The Rover Receiver also calculates
ranges to the satellites as described in
section 3.1. It then applies the range
corrections received from the Reference.
This lets it calculate a much more
accurate position than would be possi-
ble if the uncorrected range measure-
ments were used.
Using this technique, all of the error
sources listed in section 3.1.3 are
minimized, hence the more accurate
position.
It is also worthwhile to note that multiple
Rover Receivers can receive corrections
from one single Reference.
3.2.3 Further details
DGPS has been explained in a very
simple way in the preceding sections. In
real life, it is a little more complex than
this.
One large consideration is the radio link.
There are many types of radio link that
will broadcast over different ranges and
frequencies. The performance of the
radio link depends on a range of factors
including:
• Frequency of the radio
• Power of the radio
• Type and ‘gain’ of radio antenna
• Antenna position
Networks of GPS receivers and powerful
radio transmitters have been estab-
lished, broadcasting on a “maritime
only” safety frequency. These are known
as Beacon Transmitters. The users of
this service (mostly marine craft navigat-
ing in coastal waters) just need to
purchase a Rover receiver that can
receive the beacon signal. Such sys-
tems have been set up around the
coasts of many countries.