User's Manual

Galaxy Tracking Systems, L.L.C. VTU User’s Guide

Use or disclosure of data contained on this sheet is subject to the restriction on the title page of this report.

Broadcasting Unit (CBU), and a host of fixed receivers, Fixed Tracking Units (FTUs),

placed strategically throughout the area of operations to ensure uninterrupted

communications to member vehicles. The CBU operates under the direct control of the

Central Command Center (CCC) which provides the primary operator interface to

command and control the system. The CBU transmits aiding data, system timing data

and VTU commands at precise intervals synchronized to GPS time.

Every VTU command requires an acknowledgment back to the CCC. The exact time

and interval at which the VTU is to transmit its response(s) is contained in the original

VTU command. Vehicle (VTU) transmissions are received by FTU(s) and forwarded by

direct cable connection or telephone line back to the CCC.

5.3 RF Transmission Scheme

Galaxy’s system currently supports sixteen radio channels. Each channel is comprised

of a receive / transmit frequency pair within the radio’s operating frequency range. Each

channel is divided using a Time Division Multiple Access (TDMA) transmission scheme

that begins on a one-hour boundary and repeats every 30 seconds. Each 30-second

cycle is divided into five six-second frames. The frames are further divided into 100

msec “slots”.

The first six time slots of each frame are allocated to CBU transmissions. The

remaining time slots are allocated dynamically to specific VTUs under control of the

command center.

5.4 System Timing

Accurate time is required to maintain the TDMA transmission scheme employed for

UHF communications. All system components are synchronized to within a few

milliseconds of each other using the GPS 1 PPS output pulse.