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distortions, a fourth satellite measurement will not intersect with the first three. The com-
puter on board the receiver looks at the offset from universal time and looks for a single
correction factor for the receiver’s clock that it can make to all the timing values to make all
four measurements intersect at a single point. The measured values from the three satellites
do not intersect at one single point for 2D case (four satellites for 3D case). However,
when the clock correction value is applied to all the three measurements, the measure-
ments from the satellites intersect to locate the object. This correction brings the clock into
“synchronization” with the atomic accuracy clock of the satellites.

6.13.2 Sources of Error in GPS
There are five groups of major sources of error in GPS, briefly discussed below:

1. Clock Errors: Any difference between the GPS satellite based clock and receiver
based clock leads to direct position measurement error.
2. Errors due to Atmospheric Conditions: Earth’s atmosphere refracts the GPS signals
and this causes an error in measurement of distance. Furtheremore, the induced error
will be different between different satellites and the receiver.
3. Errors due to Relative Location of GPS Satellites: The relative location of the
satellites with respect to each other and the receiver also plays an important role in
determining the position measurement accuracy. Good satellite distribution geometry
is obtained when the satellites used for measurement are spread out in space.
4. Multipath Error: The basic concept of GPS assumes that a GPS signal travels
straight from the satellite to the receiver. Unfortunately, in the real world the signal
will also bounce around on the local environment and get to the receiver. The result
is many signals arriving at the receiver, first the direct one, then a bunch of delayed
reflected ones. If the bounced signals are strong enough they can confuse the receiver
and cause erroneous measurements.

6.13.3 Differential GPS
Standard GPS signal accuracy is negatively affected by various atmospheric distortions,
hence reducing the GPS positioning accuracy. The GPS accuracy can be improved by the
so called “differential GPS” (DGPS).
The underlying principle of differential GPS (DGPS) is that any two receivers that
are relatively close together on Earth will experience the same atmospheric errors. Let us
assume that one of the receivers is stationary, and the other receiver may be moving. DGPS
requires that one of the GPS receivers be set up on a precisely known location. This GPS
receiver is the base (reference) station whose location on Earth is precisely known relative
to a reference location on Earth, that is control station. The base station receiver calculates
its position based on satellite signals and compares it to the known location. The difference
is applied to the GPS data recorded by the second GPS receiver. The basic assumption is
that since the base station and receiver are closely located on Earth, any GPS error induced
(i.e., due to atmospheric conditions, multi path, or relative location errors) will be induced
by almost the same amount on both signals received at the base station and the receiver. If
we can determine that induced error using the known location of the base station, then we
can cancel the same amount of error from the signal received by the receiver. The corrected
information can be applied to data on the second receiver and transmitted in real-time in
the field using radio transmitters (Figure 6.73).