Galileo Satellite Navigation System and Signal Acquisition

GNSS (Global Navigation Satellites System) is the general meaning of the satellites-based navigation system. The available two systems include GPS (Global Positioning System) of the United States and GLONASS (Global Navigation Satellites System) of the Russia right now; and another system, Galileo, will be set up well in 2008.

Galileo is a global, all-weather and twenty-four-hours, precise three-dimensional navigation system, which developed by JU (Joint Undertaking) of ESA.GTRF (Galileo TeRrestrial Frame System) coordinate and GST (Galileo System Time) time system are adopted in Galileo. The signal transmitted by the techniques of CDMA (Code Division Modulation Access), BOC (Binary Offset Carrier) Modulation, and pilot channel. And there are two or three channels existed for one particular service; user can recover signal from these channel to reduce ionosphere and troposphere effects. The Galileo space segment is composed of a constellation of 27 satellites with 3 operational in-orbit spares.

It is difficult to acquisition of and tracking Galileo signal because of BOC modulation and longer code length. We do GPS signal acquisition each 500Hz in 5k Hz of carrier frequency; but we must do Galileo signal acquisition each 125Hz in L1B and L1C channel. In other words, in GPS system, the acquisition times is 21 in 5k Hz, but it is 80 in Galileo system; it is a large sum.

Then we think about why we must search the frequency area in 5k Hz? For GPS system, we do acquisition each 500Hz in GPS signal, so we careless of search frequency area. But we do acquisition each 125Hz in Galileo signal; if we reduce search frequency area, such as 1k Hz, we can decrease acquisition 8 times.

We calculate doppler frequency which cause by Galileo satellite moves in it’s orbit relative to fixed position in earth. And we want to find the reasonal search frequency area in GPS and Galileo system.

Finaly, we drew some conclusions about reducing search frequency area.