4

ഹ്‌

ഹ്‌

ഹ്‌

The size of the error caused by the antenna- phase-center variation depends on the
antenna type, and is typically in the order of a few centimeters.

It is difficult to model the antenna- phase-center variation and, therefore, care has to
be taken when selecting the antenna type.

Mixing different types of antennas or using different orientations will not cancel the
error.

Due to its rather small size, this error is neglecte in most of the practical GPS
applications.

6. Receiver Measurement Noise

ഹ്‌ The receiver measurement noise results from the limitations of the receiver’s electronics.
A good GPS system should have a minimum noise level.

ഹ്‌ The contribution of the receiver measurement noise to the range error will depend very
much on the quality of the GPS receiver.

ഹ Typical average value for range error due to the receiver measurement noise is of the order

of

0.6m

7. Ionospheric Delay

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The ionosphere, which is a zone of charged particles approximately 50-3000 kilometres
above the earth, causes signal delays which vary from day to night and by solar activity.
The altitude and thickness of thelayers vary with time, as a result of the changes in the
sun’s radiation and the Earth’s magnetic field.

Generally, ionospheric delay is of the order of 5m to 15m, but can reach over 150m
under extreme solar activities, at midday, and near the horizon.

The ionosphere is a dispersive medium, which means it bends the GPS radio signal and
changes its speed as it passes through the various Ionospheric layers to reach a GPS
receiver.

Bending the GPS signal path causes a negligible error, particularly if the satellite
elevation angle is greater than 5°.

It is the change in the propagation speed that causes a significant range error.

As the ionosphere is a dispersive medium, it causes a delay that is frequency dependent.
The lower the frequency, the greater the delay.

The ionospheric delay is proportional to the number of free electrons along the GPS
signal path, called the total electron content (TEC).

TEC depends on a number of factors:

(1) the time of day (electron density level reaches a daily maximum in early afternoon
and a minimum around midnight at local time)

(2) the time of year (electron density levels are higher in winter than in summer)

(3) the 1 1-year solar cycle (electron density levels reach a maximum value approx. every
11 years.