Figure 1 2. GPS Satellite Constellation

The satellites transmit ranging signals on two D band frequencies: Link 1 (Ll ) at 1575.42  MHz

and  Link  2 (L2) at 1227.6 MHz. The satellite signals are transmitted using spread spectrum

techniques, employing two different ranging codes as spreading fictions, a 1.023 MHz

coarse/acquisition code (C/A code) on L1 and a 10.23 MHz precision code (P code) on both L1

and L2. Either the C/A code or the P code can be used to determine the range between the

satellite and the user, however, the P code is normally encrypted and available only to authorized

users. When encrypted, the P code is known as the Y code. A 50 Hz navigation message is

superimposed on both the P(Y)  code and the C/A code.

The navigation message includes

satellite clock bias data, satellite ephemeris (precise orbital) data for the transmitting satellite,

ionospheric signal propagation correction data, and satellite almanac (coarse orbital) data for the

entire constellation. Refer to paragraph 1.4 for additional details regarding the ranging codes and

navigation message.

1.2.2 

Control Segment

The Control Segment primarily consists of a Master Control Station (MCS), at Falcon Air Force

Base (AFB) in Colorado Springs, USA, plus monitor stations (MS) and ground antemas (GA) at

various locations around the world. The monitor stations are located at Falcon  AFB, Hawaii,

1  3