2 GPS Satellite Systems Jan 2010

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DP Operator Course DGPS/Glonass

Training Manual

Jan. 2010 Kongsberg Maritime AS Page 6.2.1

Rev. 07 Training


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DGPS/Glonass DP Operator Course

Training Manual

Page 6.2.2 Kongsberg Maritime AS Jan. 2010

Training Rev. 07

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.

Documents

2 GPS Satellite Systems Jan 2010