satellite
GPS Systems
uk
We
supply Dyslexic
satellite GPS Systems to dyslexic
and disabled students.
The Global Positioning Systems Intresting
Reading
General GPS information links
UK: The Ordnance
Survey of Great Britain
Maps on CD-ROM & Map Vendors
Airport and other Waypoint Files
Internet Mapping Programs
GPS CAPABILITIES
The GPS is a DoD developed, worldwide, satellite-based radionavigation
system that will be the DoD's primary radionavigation system
well into the next century. The constellation consists of 24
operational satellites. The U.S. Air Force Space Command (AFSC)
formally declared the GPS satellite constellation as having
met the requirement for Full Operational Capability (FOC) as
of April 27, 1995. Requirements include 24 operational satellites
(Block II/IIA) functioning in their assigned orbits and successful
testing completed for operational military functionality.
Prior to FOC an Initial Operational Capability (IOC) was declared
on December 8, 1993 when 24 GPS satellites (Block I and Block
II/IIA) were operating in their assigned orbits, available for
navigation use and providing the Standard Positioning Service
(SPS) levels specified below.
GPS provides two levels of service, Standard
Positioning Service and the Precise Positioning Service .
The Standard Positioning Service (SPS) is
a positioning and timing service which will be available to
all GPS users on a continuous, worldwide basis with no direct
charge. SPS will be provided on the GPS L1 frequency which contains
a coarse acquisition (C/A) code and a navigation data message.
SPS provides a predictable positioning accuracy of 100 meters
(95 percent) horizontally and 156 meters (95 percent) vertically
and time transfer accuracy to UTC within 340 nanoseconds (95
percent).
The Precise Positioning Service (PPS) is a highly accurate military
positioning, velocity and timing service which will be available
on a continuous, worldwide basis to users authorized by the
U.S. P(Y) code capable military user equipment provides a predictable
positioning accuracy of at least 22 meters (95 percent) horizontally
and 27.7 meters vertically and time transfer accuracy to UTC
within 200 nanoseconds (95 percent). PPS will be the data transmitted
on the GPS L1 and L2 frequencies. PPS was designed primarily
for U.S. military use. It will be denied to unauthorized users
by the use of cryptography. PPS will be made available to U.S.
and military and U.S. Federal Government users. Limited, non-Federal
Government, civil use of PPS, both domestic and foreign, will
be considered upon request and authorized on a case-by-case
basis, provided:
It is in the U.S. national interest to do so.
Specific GPS security requirements can be met by the applicant.
A reasonable alternative to the use of PPS is not available.
For questions regarding GPS policy, the user
is advised to refer to the regularly appearing FRP. The FRP
is published every 2 years and is available from the National
Technical Information Service, Springfield, VA 22161. The latest
report number is DOT-VNTSC-RSPA-95-1/DOD-4650.5 for report date
1994.
--------------------------------------------------------------------------------
GPS SIGNAL CHARACTERISTICS
The satellites transmit on two L-band frequencies: L1 = 1575.42
MHz and L2 = 1227.6 MHz. Three pseudo-random noise (PRN) ranging
codes are in use.
The coarse/acquisition (C/A) code has a 1.023 MHz chip rate,
a period of 1 millisecond (ms) and is used primarily to acquire
the P-code.
The precision (P) code has a 10.23 MHz rate, a period of 7 days
and is the principal navigation ranging code.
The Y-code is used in place of the P-code whenever the anti-spoofing
(A-S) mode of operation is activated.
The C/A code is available on the L1 frequency and the P-code
is available on both L1 and L2. The various satellites all transmit
on the same frequencies, L1 and L2, but with individual code
assignments.
Due to the spread spectrum characteristic of the signals, the
system provides a large margin of resistance to interference.
Each satellite transmits a navigation message containing its
orbital elements, clock behavior, system time and status messages.
In addition, an almanac is also provided which gives the approximate
data for each active satellite. This allows the user set to
find all satellites once the first has been acquired.
--------------------------------------------------------------------------------
SELECTIVE AVAILABILITY, ANTI-SPOOFING
Selective Availability (SA), the denial of full accuracy, is
accomplished by manipulating navigation message orbit data (epsilon)
and/or satellite clock frequency (dither). Anti-spoofing (A-S)
guards against fake transmissions of satellite data by encrypting
the P-code to form the Y-code.
SA will be implemented on Block II at the SPS levels, as soon
as each Block II satellite is operational. SA was activated
July 4, 1991 at 0400 UT (ref: Notice Advisory to NAVSTAR Users
121-92282 DTG 011354Z JUL 91 ). A-S was exercised intermittently
through 1993 and implemented on January 31, 1994 (ref: Notice
Advisory to NAVSTAR Users 050-94042, DTG 112054Z FEB 94).
--------------------------------------------------------------------------------
GPS SYSTEM SEGMENTS
The GPS consists of three major segments: SPACE, CONTROL and
USER.
The SPACE segment consists of 24 operational satellites in six
orbital planes (four satellites in each plane). The satellites
operate in circular 20,200 km (10,900 nm) orbits at an inclination
angle of 55 degrees and with a 12-hour period. The position
is therefore the same at the same sidereal time each day, i.e.
the satellites appear 4 minutes earlier each day.
The CONTROL segment consists of five Monitor
Stations (Hawaii, Kwajalein, Ascension Island, Diego Garcia,
Colorado Springs), three Ground Antennas, (Ascension Island,
Diego Garcia, Kwajalein), and a Master Control Station (MCS)
located at Schriever AFB in Colorado. The monitor stations passively
track all satellites in view, accumulating ranging data. This
information is processed at the MCS to determine satellite orbits
and to update each satellite's navigation message. Updated information
is transmitted to each satellite via the Ground Antennas.
The USER segment consists of antennas and
receiver-processors that provide positioning, velocity, and
precise timing to the user.
--------------------------------------------------------------------------------
GPS SYSTEM TIME
GPS system time is given by its Composite Clock (CC). The CC
or "paper" clock consists of all operational Monitor
Station and satellite frequency standards. GPS system time,
in turn, is referenced to the Master Clock (MC) at the USNO
and steered to UTC(USNO) from which system time will not deviate
by more than one microsecond. The exact difference is contained
in the navigation message in the form of two constants, A0 and
A1, giving the time difference and rate of system time against
UTC(USNO,MC). UTC(USNO) itself is kept very close to the international
benchmark UTC(BIPM), and the exact difference, USNO vs. BIPM
is available in near real time.
The latest individual satellite measurements are updated daily.
(Data format explanation.)
The best current measure of the difference,
UTC(USNO MC) - GPS is based on filtered and smoothed data over
the past two days.
--------------------------------------------------------------------------------
GPS TIME TRANSFER
GPS is at the present time the most competent system for time
transfer , the distribution of Precise Time and Time Interval
(PTTI). The system uses time of arrival (TOA) measurements for
the determination of user position. A precisely timed clock
is not essential for the user because time is obtained in addition
to position by the measurement of TOA of FOUR satellites simultaneously
in view. If altitude is known (i.e. for a surface user), then
THREE satellites are sufficient. If time is being kept by a
stable clock (say, since the last complete coverage), then TWO
satellites in view are sufficient for a fix at known altitude.
If the user is, in addition, stationary or has a known speed
then, in principle, the position can be obtained by the observation
of a complete pass of a SINGLE satellite. This could be called
the "transit" mode, because the old TRANSIT system
uses this method. In the case of GPS, however, the apparent
motion of the satellite is much slower, requiring much more
stability of the user clock.
