simultaneously "in view" of the receiver for 3 D positioning purposes.  The following paragraphs

give a description of the GPS satellite signals and GPS receiver operation.

1.4.1  GPS Satellite Signals

1.4.1.1  C/A Code

The C/A code consists of a 1023 bit pseudorandom noise (PRN) code with a clock rate of 1.023

MHz which repeats every 1 millisecond.  The short length of the C/A code sequence is designed

to enable a receiver to rapidly acquire the satellite signals which helps the receiver transition to the

longer P code.  A different PRN is assigned to each GPS satellite and selected from a set of codes

called Gold codes.  The Gold codes are designed to minimize the probability that a receiver will

mistake one code for another (minimize the cross correlation).  The C/A code is transmitted only

on L1.  The C/A code is not encrypted and is therefore available to all users of GPS.

1.4.1.2  P(Y) Code

The P code is a 10.23 MHz PRN code sequence that is 267 days in length.  Each of the GPS

satellites is assigned a unique seven day segment of this code that restarts every Saturday/Sunday

midnight GPS time (GPS time is a continuous time scale  maintained within 1 microsecond of

UTC, plus or minus a whole number of leap seconds).  The P code is normally encrypted into the

Y code to protect the user from spoofing.  Since the satellites have the capability to transmit

either the P  or Y code, it is often referred to as the P(Y) code.  The P(Y) code is transmitted by

each satellite on both L1 and L2.  On L1, the P(Y) code is 90 degrees out of carrier phase with

the C/A code.

1.4.1.3  Navigation Message

A 50 Hz navigation message is superimposed on both the P(Y) code and the C/A code.  The

navigation message includes data unique to the transmitting satellite and data common to all

satellites.  The data contains the time of transmission of the message, a Hand Over Word (HOW)

for the transition from C/A code to P(Y) code tracking, clock correction, ephemeris, and health

data for the transmitting satellite, almanac and health data for all satellites, coefficients for the

ionospheric delay model, and coefficients to calculate UTC. 

The navigation message consists of 25 frames of data, each frame consisting of 1,500 bits.  Each

frame is divided into 5 subframes of 300 bits each (see Figure 1 5).  At the 50 Hz transmission

rate, it takes 6 seconds to receive a subframe, 30 seconds to receive one data frame, and 12.5

minutes to receive all 25 frames.  Subframes 1, 2, and 3 have the same data format for all 25

frames.  This allows the receiver to obtain critical satellite specific data within 30 seconds. 

Subframe 1 contains the clock correction for the transmitting satellite, as well as parameters

describing the accuracy and health of the broadcast signal.  Subframes 2 and 3 contain ephemeris

(precise orbital) parameters used to compute the location of the satellite for the positioning

equations.

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