In a digital receiver, analog to digital (A/D) conversion takes place at the receiver

IF. Code correlation and further signal processing occurs digitally.  Since the input

signals remain code division multiplexed throughout the front end, this portion of

the receiver can accommodate either a sequential or multiplexed tracking

configuration for any number of satellites.  Thus a digital receiver can easily be

structured as an "all in view" receiver, whereas an analog equivalent would require

a dedicated hardware correlation channel for each satellite in view.  The digital

architecture illustrated in Figure 2 2 also provides for a great reduction in

complexity of the analog portion of a receiver.  This in turn results in lower

production costs for test, calibration, and maintenance.

ANALOG

DIGITAL

DIGITAL

SIGNAL

PROCESSOR

CONTROL

PRESELECTOR

DOWN

AND

GAIN

CONVERSION

A/D

CHANNEL 1

DATA

PROCESSOR

CHANNEL 2

CHANNEL n

Figure 2 2.  Digital GPS Receiver Architecture

2.8  GPS AS A PSEUDORANGE/DELTA RANGE SENSOR

A GPS receiver need not necessarily be used as a PVT sensor in an integrated

navigation system. An integrator may instead wish to use GPS to supply satellite

pseudorange and deltarange measurements to an inte grated positioning solution.

Additional measurements may be provided to the integrated solution by systems or

equipment such as an Inertial Navigation System (INS) (position, velocity,

acceleration, attitude), Doppler Navigation System (position, velocity), Inertial

Aircraft Heading Reference System (velocity, acceleration, attitude), or a Central

Air Data Computer (CADC) (baro altitude and airspeed).  The positioning processor

can combine all the measurement data into one Kalman filter, to generate a system

positioning solution.

Such a solution requires a sophisticated integration scheme.  In order to use the

pseudorange and deltarange data, additional information is needed such as

satellite ephemeris and GPS receiver clock biases.  Accurate system and

subsystem clocks are needed to correct for differences between the time the

calculations are performed and the

2 8