Transcript
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NAVSTAR GPS - Navigation Signal Timing and Ranging Global Positioning System
General description
GPS is a position-reference system based on measuring ranges from satellites. The
NAVSTAR GPS, referred to as GPS, was developed by the US Department of Defence (DoD)
in cooperation with NASA. The system was developed to provide all weather, round the clock
and worldwide navigational capabilities for military ground, sea and air forces. It has
however become an important navigation and positioning system for civilian users around the
world.
Historical highlights:
1960: DoD and NASA start the first project with a satellite based positioning system.
1962: The satellite based positioning system Transit in operational use. The Transit provided
only 2D (two dimensional) positions. Time between position fixes was up to 100
minutes, meaning only usable for quite stationary users.
1969: The NAVSTAR GPS project formed.
1978: The first 4 GPS satellites are launched.
1993: 24 satellites in orbit and Standard Positioning Service (SPS) reached its initial
operating capability.
1994: Anti-spoofing (A-S) operational mode implemented.
1995: The Precise Positioning Service (PPS) for authorized users reached its fully operating
capability.
2000: On May 1st, Selective Availability (SA), (which had degraded civil positioning
accuracy to 100 m), was switched off.
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System overview
The GPS system consists of three segments:
- Space Segment- Control Segment
- User Segment
COLORADO
SPRINGS
SPACE SEGMENT
USER
SEGMENT
CONTROL SEGMENT
MONITOR
STATIONS
H a w
a i i K w a j a l e i n
A s c e n s i o n I s
D i e g o G
a r c i a
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Space segment
The space segment consists of at least 24 operational satellites in six orbital planes (four
satellites in each plane). Additional active spare satellites give normally around 27-30
operational satellites at any time. The inclination angle, (angle between orbital plane andequatorial plane), is 55 degrees. The satellites have an altitude of around 20200 km (orbital
radius of 26560 km). The orbital speed is nearly 14000km/h and orbital period is
approximately 11 hours and 58 minutes. Since the earth is rotating under the satellites, the
same satellite will pass the same point on the earth every 23 hours and 56 minutes. The
satellites are positioned in the orbital planes so that a GPS receiver on earth normally has at
least four satellites with a good geometric relationship available. The satellites are powered by
solar energy, with battery backup, and are built to last about 10 years. The satellites are
equipped with four highly accurate atomic clocks. They also have small rocket boosters to be
able to adjust the orbital position.
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Control segment
The control segment consists of:
• Five Monitor Stations (Ascension Island, Colorado Springs, Diego Garcia, Hawaii and
Kwajalein).• Three Ground Antennas, Up-link stations (Ascension Island, Diego Garcia and
Kwajalein).
• One Master Control Station (MCS) (located at Schriever Air Force Base, Colorado
Springs).
The monitoring stations are constantly tracking all satellites in view, collecting ranging data
from each satellite. This data is sent to the Master Control Station (MCS) for verification and
further processing. Corrected information can then be transmitted from the MCS through oneof the Ground Antennas to the satellites, to correct the navigation message sent from the
satellites. The MCS functions also include control of satellite station-keeping manoeuvres,
reconfiguration of spare satellites and other maintenance activities.
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User segment
The user segment consists basically of antennas and receiver-processors that are able to
compute and present position, velocity and precise time to land, sea and airborne users.
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GPS services
GPS provides two levels of service, the Standard Positioning Service and the Precise
Positioning Service.
Standard Positioning Service (SPS)
The SPS is a positioning, and timing service which will be available to all GPS users on a
continuous, worldwide basis with no direct charge. Until May 1st 2000 the accuracy of the
SPS was degraded to around 100 m horizontal positioning by introducing errors known as
Selective Availability (SA). However, SA was switched off May 1st 2000, and the position
accuracy of the Standard Positioning Service (SPS) is about 10-15 metres, (95% CEP), (CEP-
Circular Error Probability).
Precise Positioning Service (PPS)
The 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. PPS provides apredictable positioning accuracy of around 5-10m, (95% CEP).
Satellite Signal
All GPS satellites transmit signals on two carrier frequencies, the L1=1575.42 MHz and L2=
1227.6MHz. The next generation of satellites will also transmit on L5=1176.45Mhz. Since the
carrier frequency is the same for all satellites, the signal must contain characteristics making it
possible to separate the different satellites from each other. This is achieved using codes on
the signals, called Pseudo Random Noise codes (PRN codes). These codes are unique for each
satellite and modulated on top of the carrier frequency. Apparently these codes will look like
false random noises, hence the name Pseudo Random Noise. In addition to segregating the
different satellites, the PRN codes are also used in the range calculations. This will be covered
later in the chapter. There are different types of Pseudo Random Noise codes, described
below.
C/A-code
The C/A-code (Coarse Acquisition code) is the bases for Standard Positioning Service (SPS),
civilian GPS use. The C/A-code has a frequency of 1.023MHz that repeats itself every 1
millisecond. The short length of the C/A-code sequence is designed to enable a receiver to
rapidly acquire the satellite signals. The C/A-code is not encrypted and is therefore availableto all users of GPS. The C/A-code is transmitted on the L1 frequency.
P(Y)-code
The P-code is the basis for Precise Positioning Service (PPS). The P-code is an encrypted
code for military GPS users. The P-code is a 10.23 MHz PRN code. In January 1994 an Anti-
spoofing mode (A-S) was implemented, with a new code that is even more difficult to jam.
This code is named the Y-code and replaces the P-code when the Anti-spoofing mode is
activated. The code used for the Precise Positioning Service is often referred as P(Y)-code.
The P(Y)-code is transmitted on both L1 and L2 frequencies.
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The data transmitted from the satellite consists of these unique PRN codes, and a navigation
message that basically contains the satellite’s position, time, atmospheric data and an almanac
giving information about all active satellites.
Downlink:
PRN-code (C/A or P(Y) -code)
Satellite Position (ephemeredes)Time (System time and satellite clock correction)
Status health
Atmospheric data
GPS Almanac
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Positioning Principle
The basic positioning principle is based on triangulation of ranges from different satellites.
Normally at least 4 satellite ranges must be measured to calculate a position. To calculate the
ranges, Travel Time must be known, (time from when a signal was sent by a satellite until it isreceived at the GPS receiver). The range from a satellite is then calculated by multiplying
Travel Time with Speed of Light (Range = Speed of Light x Travel Time), where Speed of
Light is approximately 300.000km/s. To triangulate using the ranges, the positions of the
satellites must be known. Satellite position is part of the navigation message sent from the
satellites. How Travel Time is determined is described below.
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Travel Time
To measure correct travel time is crucial. For instance if there was an error in the travel time
of only one millisecond, that would result in an error in the range of 300km. Such errors are ofcourse not acceptable.
The time measurement is based on the assumption that both the satellite and the receiver are
generating the same PRN-code at exactly the same time. The travel time is found by
comparing how late the satellite’s PRN-code appears compared to the receiver’s code.
The satellites have four highly accurate atomic clocks, and all satellites are synchronized with
a GPS system time. The receiver though, does not have the same accurate clock. Since the
principle above is based on the satellite and receiver generating the PRN-code exactly at the
same time, we need to have a correct time also in the receiver. This receiver clock offset is
corrected by measuring an extra satellite range. That is the reason why we need at least foursatellites to calculate a position, to calculate Latitude, Longitude, Altitude and receiver clock
offset.
0 01 01 1 1010
0 01 1010
t1t0
DARPS100SEATEX
PRN code transmitted
PRN code received
0 1 1
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Sources of Error
There are various sources that influence the signal accuracy:
Troposphere
The troposphere is the lower part of the earth’s atmosphere. This is where changes in
temperature, pressure and humidity associated with weather changes occur. These factors
cause varying degrees of delays to the signals.
Ionosphere
The ionosphere is the layer of the atmosphere ranging in altitude from 50 to 500 km and
consists largely of ionised particles, which also causes a delay to the signals.
200 km
50 km
Ionosphere
Troposphere
Particles
Earth
Clouds
Multipath effects
These are caused by reflected signals from surfaces near the receiver that can either interfere
with, or be mistaken for, the signal that follows the straight-line path from the satellite. If the
reflected signal is very strong, the GPS receiver might loose lock on the satellite.
Multipath is difficult to detect and sometimes hard to avoid.
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Effects of Geometry
Basic geometry can itself magnify other errors with a principle called Geometric Dilution of
Precision - GDOP.
When the user is at a point where the lines drawn from the satellites are nearly perpendicular
to each other, the point of intersection is well defined.
Good Geometry
Range errors
from eachsatellite
Region of positionuncertainty
When the angle either becomes very large or very small, the point of intersection is blurred
and positioning degrades.
Poor Geometry
The effects of geometry vary with time of day and number of satellites that are available. Poor
geometry can also be caused by obstructions, for example, when a vessel is close to the
platform structure, the correction signals may easily be blocked.
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Dilution of Precision (DOP)
The position calculation is basic geometry. When the satellites are in specific configurations
with respect to the observer, it is possible for small errors to be magnified. The dilution of
precision (DOP) is a dimensionless number indicating how much geometry magnifies theerror.
DOP can be broken into categories:
∗ Horizontal DOP (HDOP)
∗ Vertical DOP (VDOP)
∗ Geometric DOP (GDOP)
∗ Time DOP (TDOP) ∗
The most commonly used DOP value is called Position DOP (PDOP), which is HDOP and
VDOP in combination.
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DP Requirements for Accuracy
Very accurate measurements of a vessel’s position are necessary for precise dynamic
positioning (DP). Standard GPS is often not good enough as input to a DP-system.
To improve the accuracy of GPS, differential GPS (DGPS) is used.
DGPS (Differential GPS)
Inmarsat
Reference
Station
Correction data(Network)
Correction data
A shore-based reference station is established at a known location, monitoring GPS
transmissions from the satellites. The reference stations constantly compare their known
position against the computed GPS position, calculating the errors in each satellite’s signals
and transmitting error corrections to GPS users. The correction message format follows the
standard established by the Radio Technical Commission for Maritime Services (RTCM-
SC104).
In addition to a GPS system, the user requires a DGPS antenna and a DGPS receiver unit. The
correction signals can be received via different methods, for example IALA radio link (range
approx. 200 km) or dedicated satellite systems, Spotbeam or Inmarsat (range approx. 2000km).
These differential corrections are then applied to correct the pseudo ranges received by the
vessel’s GPS receiver prior to using them for the calculations, thus removing most of the
satellite signal errors and improving accuracy.
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The coverage map for differential signals distributed by Fugro SeaSTAR when using
Inmarsat.
The Coverage map for differentials signals distributed by IALA. using marine radio beacons.
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Relative Positioning- DARPS
Some DP functions require the positioning of a vessel relative to a moving, rather than fixed,
position. An example of this is the operation of a DP shuttle tanker loading via a bow loading
hose from the stern of a floating production vessel. Extra equipment needed is UHF linkantenna and UHF transceiver and modem.
For the measurements of a relative position, differential corrections are not used, as the errors
would be the same for both vessels. A transponder is placed on the point of reference and re-
transmits received GPS data to the UHF transceiver onboard the shuttle tanker. A computer
onboard the shuttle tanker utilises GPS measurements from both vessels to derive a range/
bearing vector which may be input to the DP system as position reference.
GLONASS
The Global Navigation Satellite System (GLONASS) is the Russian counterpart to the
American GPS system.
GLONASS has much in common with NAVSTAR GPS in terms of the satellite constellation,
orbits and signal structure. All errors that influence GPS will also apply for GLONASS.
Separate GLONASS differential correction signals are offered commercially.
The current GLONASS constellation consists of 11 operational satellites (February 2007).
Three satellites are newly launched, and will be operational in the nearest future. Still,
GLONASS is far from being fully operational. The plan is to be fully operational with 24
satellites in 2010.
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