— THE GPS SYSTEM HAS THREE SEGMENTS
1.GROUND BASED SEGMENT
2.SPACE SEGMENT
3.RECEIVER SEGMENT
— GPS satellites fly in circular
orbits at an altitude of 20,200 km and with a period of 12 hours.
— Powered by solar cells, the
satellites continuously orient themselves to point their solar panels toward
the sun and their antenna toward the earth.
— Orbital planes are centered on the
Earth
— Each planes has about 55° tilt
relative to Earth's equator in order to cover the polar regions.
— Space Segment (Continued)
— Each satellite makes two complete
orbits each sidereal day.
— Sidereal - Time it takes for the
Earth to turn 360 degrees in its rotation
— It passes over the same location on
Earth once each day.
— Orbits are designed so that at the
very least, six satellites are always within line of sight from any location on
the planet.
—
— There are currently 30 actively
broadcasting satellites in the GPS constellation.
— Redundancy is used by the additional
satellites to improve the precision of GPS receiver calculations.
— A non-uniform arrangement improves
the reliability and availability of the system over that of a uniform system,
when multiple satellites fail
— This is possible due to the number
of satellites in the air today
— Control Segment
— The CS consists of 3 entities:
— Master Control System
— Monitor Stations
— Ground Antennas
— Master Control Station
— The master control station, located
at Falcon Air Force Base in Colorado Springs, Colorado, is responsible for
overall management of the remote monitoring and transmission sites.
— GPS ephemeris is the tabulation of
computed positions, velocities and derived right ascension and declination of
GPS satellites at specific times for eventual upload to GPS satellites.
— Monitor Stations
— Six monitor stations are located at
Falcon Air Force Base in Colorado, Cape Canaveral, Florida, Hawaii, Ascension
Island in the Atlantic Ocean, Diego Garcia Atoll in the Indian Ocean, and
Kwajalein Island in the South Pacific Ocean.
— Each of the monitor stations checks
the exact altitude, position, speed, and overall health of the orbiting
satellites.
—
— The control segment uses
measurements collected by the monitor stations to predict the behavior of each
satellite's orbit and clock.
— The prediction data is up-linked, or
transmitted, to the satellites for transmission back to the users.
— The control segment also ensures
that the GPS satellite orbits and clocks remain within acceptable limits. A
station can track up to 11 satellites at a time.
—
— This "check-up" is
performed twice a day, by each station, as the satellites complete their
journeys around the earth.
— Variations such as those caused by
the gravity of the moon, sun and the pressure of solar radiation, are passed
along to the master control station.
— Ground Antennas
— Ground antennas monitor and track
the satellites from horizon to horizon.
— They also transmit correction
information to individual satellites.
— User Segment
— The
GPS receiver is the USER of the GPS system.
— GPS receivers are generally composed
of an antenna, tuned to the frequencies transmitted by the satellites,
receiver-processors, and a highly-stable clock, commonly a crystal oscillator).
— They can also include a display for
showing location and speed information to the user.
— A receiver is often described by its
number of channels this signifies how many satellites it can monitor
simultaneously. As of recent, receivers
usually have between twelve and twenty channels.
— User Segment (continued)
— Using the RTCM SC-104 format, GPS
receivers may include an input for differential corrections.
— This is typically in the form of a
RS-232 port at 4,800 bps speed. Data is actually sent at a much lower rate,
which limits the accuracy of the signal sent using RTCM.
— Receivers with internal DGPS
receivers are able to outclass those using external RTCM data.
— The
SVs transmit two microwave carrier signals. The L1 frequency (1575.42 MHz)
carries the navigation message and the SPS code signals. The L2 frequency
(1227.60 MHz) is used to measure the ionospheric delay by PPS equipped
receivers.
— Three
binary codes shift the L1 and/or L2 carrier phase.
— The
C/A Code (Coarse Acquisition) modulates the L1 carrier phase. The C/A code is a
repeating 1 MHz Pseudo Random Noise (PRN) Code. This noise-like code modulates
the L1 carrier signal, "spreading" the spectrum over a 1 MHz
bandwidth. The C/A code repeats every 1023 bits (one millisecond). There is a
different C/A code PRN for each SV. GPS satellites are often identified by
their PRN number, the unique identifier for each pseudo-random-noise code. The
C/A code that modulates the L1 carrier is the basis for the civil SPS.
— The
P-Code (Precise) modulates both the L1 and L2 carrier phases. The P-Code is a
very long (seven days) 10 MHz PRN code. In the Anti-Spoofing (AS) mode of
operation, the P-Code is encrypted into the Y-Code. The encrypted Y-Code
requires a classified AS Module for each receiver channel and is for use only
by authorized users with cryptographic keys. The P (Y)-Code is the basis for
the PPS.
— The
Navigation Message also modulates the L1-C/A code signal. The Navigation
Message is a 50 Hz signal consisting of data bits that describe the GPS
satellite orbits, clock corrections, and other system parameters.
— GPS SIGNAL
— THE GPS SIGNAL CONSISTS OF
a)Pseudo
random code:This code identifies the satellite because each satellite has got
unique code
b)Ephemeris
data:this provides the status of the satellite,current date and time
c)Almanac
data:this provides the the status of all satellites including theirs locations through out the
day
— P code and C/A code:
— The C/A code is made up of a
sequence of 0’s and 1’s called the chip,having a frequency of 1.023MBits/sec
and in terms of distatnce ,it is 293m.the duration of each chip is 1µs and
entire sequence is of one millisecond.the code sequence of 1023 such chips arranged in pseudo random order
known to the receiver and repeated every
after one millisecond.
— The C/A code can be decoded by the
civilian GPS
— P code and C/A code
— In case of P code the chip frequency
is 10.23MBits/sec,the duration of each chip is 0.1µs and in terms of distance
29.3m.the full code length is of 267 days and each satellite is allocated only
7days piece of code,during this interval there is no repetition of the
code.Every 6 secs the satellite transmits the time that has lapsed since P code
started.
— Precise position system
— Authorized
users with cryptographic equipment and keys and specially equipped receivers
use the Precise Positioning System. U. S. and Allied military, certain U. S.
Government agencies, and selected civil users specifically approved by the U.
S. Government, can use the PPS.
— PPS
Predictable Accuracy
— 22
meter Horizontal accuracy
— 27.7
meter vertical accuracy
— 200
nanosecond time (UTC) accuracy
— 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) codecapable 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.
— Standard positioning system
— 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).
— These
GPS accuracy figures are from the 1999 Federal Radionavigation Plan. The
figures are 95% accuracies, and express the value of two standard deviations of
radial error from the actual antenna position to an ensemble of position
estimates made under specified satellite elevation angle (five degrees) and
PDOP (less than six) conditions.
— For
horizontal accuracy figures 95% is the equivalent of 2drms (two-distance
root-mean-squared), or twice the radial error standard deviation. For vertical
and time errors 95% is the value of two-standard deviations of vertical error
or time error.
— Receiver
manufacturers may use other accuracy measures. Root-mean-square (RMS) error is
the value of one standard deviation (68%) of the error in one, two or three
dimensions. Circular Error Probable (CEP) is the value of the radius of a
circle, centered at the actual position that contains 50% of the position
estimates. Spherical Error Probable (SEP) is the spherical equivalent of CEP,
that is the radius of a sphere, centered at the actual position, that contains
50% of the three dimension position estimates. As opposed to 2drms, drms, or
RMS figures, CEP and SEP are not affected by large blunder errors making them
an overly optimistic accuracy measure
— Some
receiver specification sheets list horizontal accuracy in RMS or CEP and
without Selective Availability, making those receivers appear more accurate
than those specified by more responsible vendors using more conservative error
measures
— GPS DATA
— The
GPS Navigation Message consists of time-tagged data bits marking the time of
transmission of each subframe at the time they are transmitted by the SV. A
data bit frame consists of 1500 bits divided into five 300-bit subframes. A
data frame is transmitted every thirty seconds. Three six-second subframes
contain orbital and clock data. SV Clock corrections are sent in subframe one
and precise SV orbital data sets (ephemeris data parameters) for the
transmitting SV are sent in subframes two and three. Subframes four and five
are used to transmit different pages of system data. An entire set of
twenty-five frames (125 subframes) makes up the complete Navigation Message
that is sent over a 12.5 minute period.
— Data
frames (1500 bits) are sent every thirty seconds. Each frame consists of five
subframes.
— Data
bit subframes (300 bits transmitted over six seconds) contain parity bits that
allow for data checking and limited error correction.
— Contents of Navigational Message
— The
navigation message is made up of three major components. The first part
contains the GPS date and time, plus the satellite's status and an indication
of its health. The second part contains orbital information called
ephemeris data and
allows the receiver to calculate the position of the satellite. The third part,
called the almanac, contains information and status concerning all the
satellites; their locations and PRN numbers.
— Navigational message
— GPS Frequencies
— L1 (1575.42 MHz) - Mix of Navigation
Message, coarse-acquisition (C/A) code and encrypted precision P(Y) code.
— L2 (1227.60 MHz) - P(Y) code, plus
the new L2C code on the Block IIR-M and newer satellites.
— L3 (1381.05 MHz) - Used by the
Defense Support Program to signal detection of missile launches, nuclear
detonations, and other applications.
— GPS Proposed Frequencies
— L4 (1379.913 MHz) - Being studied
for additional correction to the part of the atmosphere that is ionized by
solar radiation.
— L5 (1176.45 MHz) – To be used as a
civilian safety-of-life (SoL) signal.
— Internationally protected range for
aeronautical navigation.
— The first satellite that using this
signal to be launched in 2008.
— Position Calculation
— The coordinates are calculated
according to the World Geodetic System WGS84 coordinate system.
— The satellites are equipped with
atomic clocks
— Receiver uses an internal crystal
oscillator-based clock that is continually updated using the signals from the
satellites.
— Receiver identifies each satellite's
signal by its distinct C/A code pattern, then measures the time delay for each
satellite.
— Position Calculation (cont’d)
— The receiver emits an identical C/A
sequence using the same seed number the satellite used.
— By aligning the two sequences, the
receiver can measure the delay and calculate the distance to the satellite,
called the pseudorange.
— Orbital position data from the
Navigation Message is used to calculate the satellite's precise position.
Knowing the position and the distance of a satellite indicates that the
receiver is located somewhere on the surface of an imaginary sphere centered on
that satellite and whose radius is the distance to it.
— Position Calculation (cont’d)
— When four satellites are measured at
the same time, the point where the four imaginary spheres meet is recorded as
the location of the receiver.
— Earth-based users can substitute the
sphere of the planet for one satellite by using their altitude. Often, these
spheres will overlap slightly instead of meeting at one point, so the receiver
will yield a mathematically most-probable position.
— ERRORS IN GPS
— 1.IONOSPHERIC AND TROPOSPHERIC DELAY
— As
a GPS signal passes through the charged particles of the ionosphere and then
through the water vapour in the troposphere it gets slowed down
a bit, and this creates the same kind of error as bad clocks.
— There
are a couple of ways to minimize this kind of error. For one thing we can
predict what a typical delay might be on a typical day.
— This
is called modeling and it helps but, of course, atmospheric conditions are
rarely exactly typical.
— ERROR IN GPS
— This “
dual frequency” measurement is very sophisticated and is only possible
with advanced receivers.
— 2.USERS CLOCK ERROR
If the user clock is not perfectly
synchronised with satellite’s clock,the range measurement will be wrong
3.SATELLITE
CLOCK ERROR
This error is caused due to the error in the satellite’s clock wrt
the GPS time.this is monitored by the
ground based segments and any error in the satellite clock forms part of the
navigational message
— ERRORS IN GPS
4.MULTIPLE PATH
— The
GPS signal may bounce off various local obstructions before it gets to the
receiver.
— This
is called multipath erro
5.DEVAITION
OF SATELLITE FROM PREDICTED PATH:the satellite are monitored and their paths
are predicted by the ground based stations.however between the two consecutive
monitoring of the same satellites,their may be minor drifts from their
predicted paths resulting in small position inaccuracy.
— ERROR IN GPS
— 6.GEOMETRIC DILUTION OF PRECISION:
This depends on the number and the geometry of the satellites used.
— If
four satellites are clustered near each other, then one meter of error in
measuring distance may result in tens or hundreds of meters of error in
position.But if many satellites are scattered around the sky, then the position
error may be less than 1.5 meters for every meter of error in measuring
distances.
— The
effect of the geometry of the satellites on the position error is called
Geometric Dilution Of Precision (GDOP), which can roughly be
interpreted as the ratio of the position error to the range error.
— Imagine
the tetrahedron that is formed by lines connecting the receiver to each
satellite used.
— The
larger the volume of this tetrahedron, the smaller (better) the GDOP.
— In
most cases, the larger the number of satellites the smaller the GDOP.
— DGPS
— Working
of DGPS
The reference station established on ground receives signals from the
satellites, after which it calculates the difference in the positioning of its
own location. The reference station provides the users with the necessary
corrections in the distance measured by the GPS system. These corrections are
transmitted by means of ultra high frequency waves (UHF). Only those users,
within the range of 370 km of the reference stations can benefit from the
service. However, even the DGPS can generate errors resulting from the
distortions produced in the troposphere and ionosphere. The ephemeris errors
may also lead to the users receiving incorrect information. Thus, the
information provided by the DGPS loses accuracy as we move away from the
reference station. The errors in the DGPS may range from 0.22 to 0.67 km per
100 km.
— Limitations Of DGPS
— The corrections which will be
applied to the position is valid for particular position i.e. for the reference
station and roving stations close to the reference station.As the distance from
the reference station increases, the
error in the wave will be different .Ionospheric and tropospheric
correction will vary.
— There may be chance that ephemeris
data received at the reference may be different and that will lead to the incorrect
correction
— The multipath error will remain
uncorrected or correction based on the
this will lead to the error in position.
— Some area which is far away from the
transmitting will not be to receive data
from the Reference station.
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