1.4.2.6  Data Demodulation

Once the carrier tracking loop is locked, the 50 Hz navigation data message can be read.  Each

subframe of the navigation message begins with a preamble contained in the Telemetry Word,

enabling the receiver to detect the beginning of each subframe.  Each subframe is identified by bits

contained in the Handover Word (HOW), enabling the receiver to properly decode the subframe

data.

1.4.2.7  P(Y) Code Signal Acquisition

The one millisecond C/A code length permits a relatively narrow search window for code

correlation even if the receiver must "search the sky" to find the first satellite. However the week

long P(Y) code sequence at 10.23 MHz does not allow the same technique to be used.  Precise

time must be known by the receiver in order to start the code generator within a few hundred

chips of the correlation point of the incoming signal.  The HOW contained in the GPS navigation

message provides satellite time and hence the  P(Y) code phase information.  A P(Y) code

receiver may attempt to acquire the P(Y) code directly, without first acquiring the C/A code, if it

has accurate knowledge of position, time and satellite ephemeris from a recent navigation

solution.  External aiding and/or an enhanced acquisition technique are usually required to

perform direct P(Y) code acquisition.

1.4.2.8  PVT Calculations

When the receiver has collected pseudorange measurements, deltarange measurements, and

navigation data from four (or more) satellites, it calculates the navigation solution, PVT.  Each

navigation data message contains precise orbital (ephemeris) parameters for the transmitting

satellite, enabling a receiver to calculate the position of each satellite at the time the signals were

transmitted.  The ephemeris data is normally valid and can be used for precise navigation for a

period of four hours following issue of a new data set by the satellite. New ephemeris data is

transmitted by the satellites every two hours.

As illustrated in Figure 1 10, the receiver solves a minimum of four simultaneous pseudorange

equations, with the receiver (3 D) position and clock offset as the four unknown variables.  Each

equation is an expression of the principle that the true range (the difference between the

pseudorange and the receiver clock offset) is equal to the distance between the known satellite

position and the unknown receiver position.  This principle is expressed below mathematically

using the same notation as Figure 1 10.

R      C

2

2

2

B

  =  c t 

    C

B

  =   (X     U

X

)   +  (Y     U

Y

)   +  (Z     U

Z

)

These are simplified versions of the equations actually used by GPS receivers.  A receiver also

obtains corrections derived from the navigation messages which it applies to the pseudoranges. 

These include corrections for the satellite clock offset, relativistic effects, ionospheric signal

propagation delays.  Dual frequency receivers can measure the delay between the L1 and L2

P(Y) codes, if available, to calculate an ionospheric correction.  Single frequency (either C/A 

1 14