Navman 11 - Manual (Page 76)

76 MN002000A © 2004 Navman NZ Ltd. All rights reserved. Proprietary information and specifications subject to change without notice.
Appendix B: References
This appendix provides a list of documents that
may help a user of Navman’s GPS receivers to
learn more about the way the GPS can be used.
Not all of these documents have been referred to
in the text of this document.
1. Global Position System Standard Positioning
Service Signal Specification, United States
Department of Defense.
2. Standard For Interfacing Marine Electronic
Devices, NMEA 0183, National Marine Electronics
Association.
3. RTCM Recommended Standards for Differential
NAVSTAR GPS Service, Radio Technical
Commission for Maritime Services.
4. Principle of Operation of NAVSTAR and
System Characteristics, Milliken and Zoller, Global
Positioning System, Vol 1, 1980, pp. 3-14.
5. Department of Defense World Geodetic System
1984, DMA TR 8350.2.
6. Internet web site for the National Geophysical
Data Centre (NGDC): “http://julius.ngdc.noaa.
gov/seq/potfld/geomag.html”
APPENDIX C: NAVSTAR GPS
operation
NAVSTAR GPS is a space-based satellite
radio navigation system developed by the U.S.
Department of Defense (DoD). GPS receivers
provide land, marine, and airborne users with
continuous 3D position, velocity, and time data.
This information is available free of direct charge
to an unlimited number of users. The system
operates under all weather conditions, 24 hours
a day, anywhere on Earth. There are three major
segments:
• space segment
• control segment
• user segment
The space segment
This segment consists of a nominal constellation
of 24 operational satellites (including 3 spares).
These satellites have been placed in 6 orbital
planes (see Figure C-1) about 20 200 km
(10 900 miles) above the Earth’s surface. The
satellites are in circular orbits with a 12-hour orbital
period and inclination angle of 55 degrees. This
orientation normally provides a GPS user with a
Figure C-1 NAVSTAR GPS operational satellite
constellation
minimum of 5 satellites in view from any point on
Earth at any time.
Each satellite continuously broadcasts an RF
signal at a centre frequency of 1575.42 MHz
(Ll band). This signal is modulated by a 10.23 MHz
clock rate precise ranging signal and by a
1.023 MHz clock rate coarse acquisition (C/A)
code ranging signal. Each of these two binary
signals has been formed by a precision code
(p-code) or a C/A code which is modulo-2 added
to 50 bps navigation data.
This navigation data, which is computed and
controlled by the GPS Control Segment, includes
the satellite’s time, its clock correction and
ephemeris parameters, almanacs, and health
status for all GPS satellites. From this information,
the user computes the satellite’s precise position
and clock offset. Currently, the DoD encrypts the
P-code ranging signal and thus denies access
to the Precise Positioning Service (PPS) by
unauthorised users. The Standard Positioning
Service (SPS) uses the C/A code ranging signal
and is intended for general public use.
The control segment
This segment consists of a master control station,
located in Colorado Springs, and a number of
monitor stations at various locations around
the world. Each monitor station tracks all the
GPS satellites in view and passes the signal
measurement data back to the master control
station, where computations are performed to
determine precise satellite ephemeris and satellite
clock errors. The master control station generates
the upload of user navigation data for each
satellite. This data is subsequently re-broadcast by
the satellite as part of its navigation data message.
The user segment
This segment is the collection of all GPS receivers
and their application support equipment such