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F-10 Acutime 2000 Synchronization Kit User Guide

Theory of Operation

F.6 System Architecture

The standard Acutime 2000 incorporates a proprietary DSP (Digital

Signal Processor), which operates at the L1 frequency (1575.42 MHz)

and processes the Coarse/Acquisition (C/A) code portion of the GPS

signal. The RF and digital signal processing components of the GPS

module are custom ASICs designed by Trimble. In addition to the

signal processing functions, these ASICs also contain support

circuitry for the microprocessor. The microprocessor performs the

navigation computations in addition to controlling the DSP channels

and managing the I/O operations.

GPS satellite signals are collected by the antenna, filtered and

amplified by the antenna's pre-amp and then fed to the RF down

converter. A highly stable, crystal reference oscillator, operating at

12.504 MHz, supports the down converter in producing the correct

signals for the digital signal processor. The signal processing stage

tracks the GPS satellite signals and extracts the carrier code

information, as well as the navigation data at 50 bits per second.

In addition to supporting the down conversion process, the crystal

oscillator also serves as the reference clock for the navigation

processor. This microprocessor controls the operation of the signal

processing channels by allocating the channels during satellite

acquisition and tracking. In the overdetermined mode of operation,

the smart antenna automatically tracks the highest eight (or twelve)

satellites above the horizon and selects the optimal set of satellites for

the position solution.

In addition to controlling the DSP stage, the navigation processor also

collects the ephemeris and almanac data for all of the satellites and

manages the Universal Asynchronous Receiver/Transmitter (UART

or DUART) interface. The UART supports full-duplex serial

communication with the smart antenna.

The RS-232 and RS-422 versions of the Acutime 2000 block diagram

for the GPS smart antenna are illustrated in Figure F-1 and

Figure F-2.

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Trimble Outdoors 45005-00-ENG Manuel d'utilisateur Page 300