Viavi 87339, Operation Manual

Page: 128 / 198

Principals of Operation
6 -11
The spectral power density of the received GPS signal lays at approximately 16 dB below
the spectral power density of the thermal or background noise. The demodulation and
de-spreading of the received GPS signal causes a system gain G of:
After de - spreading, the power density of the usable signal is greater than that of the
thermal or background signal noise.
Fig. 6-14 GPS Signal After De-spreading
6.1.4.C Measuring Distance (Pseudo Range)
GPS satellites orbit 200km above the earth and are distributed in such a way that from
any point on the ground there is line-of-sight contact to at least four satellites.
Each one of these satellites is equipped with onboard atomic clocks. In order to make
them even more accurate, they are regularly adjusted or synchronized from various
control points on Earth. GPS satellites transmit their exact position and onboard clock
time to Earth.
These signals are transmitted at the speed of light (300,000 km/s) and therefore require
approximately 67.3 ms to reach a position on the Earth’s surface directly below the
satellite. The signals require a further 3.33 µs for each additional kilometer of travel. To
establish position, all that is required is a receiver and an accurate clock.
By comparing the arrival time of the satellite signal with the onboard clock time the
moment the signal was transmitted, it is possible to determine the signal travel time.
Distance = Velocity * Time: Velocity is 300,000 km/s and Time is the travel time of the
signal.