Symmetricom ET6000, Руководство пользователя

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CHAPTER ONE
E. CHAPTER FIVE - MAINTENANCE/TROUBLESHOOTING
Provides a guide to the maintenance and troubleshooting of this instrument. A
description of the available adjustments is also provided.
F. APPENDIX A - ASCII CHARACTER CODES
Provides the cross reference of the ASCII character set to decimal, octal, and
hexadecimal numbers.
G. APPENDIX B - ACRONYMS AND ABBREVIATIONS
Provides a list of acronyms and abbreviations used in this User’s Guide.
H. APPENDIX C -OPTION DESCRIPTIONS
The Option Descriptions that are available with this instrument are located in this
Appendix.
I. APPENDIX D –ANTENNA REPLACEMENT KIT
Provides
superseded model information
.
1.2 NAVSTAR/GPS DESCRIPTION
The Navstar/GPS satellite-based timing and navigation system consists of a constellation of high
altitude satellites orbiting the earth every twelve sidereal hours, a group of ground-based
control/monitoring stations and the user equipment which may be located on land, sea and/or air.
The GPS System was completed in the early 1990’s and provides three dimensional positioning,
velocity, and time, on a continuous world-wide basis. The constellation is comprised of twenty-one
satellites and three spares. The satellites are located in six different orbital planes inclined
approximately sixty degrees to the equator at altitudes of 10,400 miles above the earth.
The GPS Timing Unit determines time and frequency by measuring the time of arrival of the
precise timing mark and measuring the Doppler effect from one satellite. A previously entered or
determined position allows computation of the receivers time offset. An accurate timing mark
(1pps) can be set, and an input 1pps pulse can be measured with respect to UTC. The satellite
positions are known within a few meters and the satellite clocks are calibrated within a few
nanoseconds so position can be computed within an absolute accuracy of better than 120 meters
(with current selective availability).
The GPS signal transmitted from a satellite consists of two carrier frequencies. L1 at a frequency of
1575.42 MHz and L2 at a frequency of 1227.6 MHz. The L1 signal is modulated with both a
precision (P) code and a coarse acquisition (C/A) code. The precision (P) code is available to
authorized users only. The GPS Timing Unit operates on the C/A code.
Each satellite transmits a unique C/A code that reflects the satellite identity for acquisition and
tracking. The C/A PRN code is a gold code of 1023 bits repeating at a one-millisecond rate.
1-2 ET6xxx ExacTime GPS TC & FG (Rev C) Symmetricom Inc