Global Positioning System (GPS) - Theory

Global Positioning System

Syllabus

UNIT 1: FUNDAMENTALS OF GLOBAL POSITIONING SYSTEM

Global Positioning System
Satellite constellation
GPS Segments: Control, Space & User, Functions
Advantages and limitations of GPS

UNIT 2: GPS SIGNAL STRUCTURE & ORBITS

GPS signals, structure
Time systems
Carriers, GPS codes: C/A, P, navigational message
Principles of Geopositioning
GPS antenna, Type of GPS receivers
GPS Orbits, GPS satellite coordinates.

UNIT 3: GPS SIGNAL PROCESSING

Pseudo Range & Phase Difference Measurement
Navigational solution: Code/ phase based
Data Processing Models
Models for single point & differential positioning
GPS data formats: RINEX, SP3

UNIT 4: GPS ERRORS AND ACCURACY

Ephemeris errors and orbit perturbations, forces on gps satellites, effects of orbital bias
Clock bias (satellite & receiver), cycle slip, selective availability
Ionospheric & Tropospheric errors, Multipath, Station coordinates
Geometry dependent (Dilution of Precision), User Equivalent Range Error

UNIT 5: DGPS, GNSS & APPLICATIONS

GPS Positioning Types: Absolute & Differential, Single Point or Point Vs Relative, Static Vs Kinematic, Real time Vs Post mission
GPS Survey Planning, GPS & DGPS Data Processing and Accuracy
GNSS: NAVSTAR, GLONASS, GALILEO, COMPASS, Indian Navigation Satellite Missions
Introduction to Augmentation System, WAAS
GPS Applications in navigation, vehicle tracking, military, tectonic movements, ionosphere studies etc.

History of GPS Development

Stone Age: Man identified and remembered references.
Star Age: Celestial bodies used as references at sea.
Challenges arise in environments lacking reference objects, such as deserts or oceans.

History of Mapping and Surveying

First Maps: Mental maps used by early hunters and gatherers for overland navigation.
Babylonians (5000 years ago): Produced property descriptions and simple maps on stone tablets.
Ancient Egyptians: Used surveying to reestablish property corners after Nile floods.
Greeks and Romans (2000-2500 years ago): Surveyed and mapped new settlements precisely, using methods that remained consistent for centuries, constructing roads and aqueducts.

Overcoming Obstacles in Navigation

Challenges: Clouds, fog, and rain hinder star observation.
Solution: Long Range Navigational system (LORAN) developed, using radio waves for positioning.

GPS Development

First true all-weather system for determining receiver position.
Principle: Two radio transmitters at known positions transmit synchronized short pulses; the time difference in pulse arrival at the receiver determines a line of position.
Evolved through Omega System, LORAN C, etc., initially two-dimensional (latitude and longitude only).
Satellites enabled three-dimensional positioning.
TRANSIT System (1964): First of its kind, but two-dimensional.
NAVSTAR GPS: Development driven by continuous search for three-dimensional positioning.

Mapping and Surveying Advancements

Classical Methods: Triangulation, combined with trilateration and traversing used extensively in the 19th and early 20th centuries.
Limitations: Reliance on "line-of-sight" observations.
Instrument Progression: Chain, Tape, Theodolites, Compass, Levels, Total Stations, Digital levels, and GPS.

Navigation History

PreHistory - Present: Celestial Navigation
- Ok for latitude, poor for longitude until accurate clock invented ~1760
13th Century: Magnetic Compass
1907: Gyrocompass
1912: Radio Direction Finding
1930's: Radar and Inertial Nav
1940's: Loran-A
1960's: Omega and Navy Transit (SatNav)
1970's: Loran-C
1980's: GPS

GPS Timeline

1969: Defense Navigation Satellite System (DNSS) formed.
1973: NAVSTAR Global Positioning System developed.
1978: First 4 satellites launched (Delta rocket launch).
1993: 24th satellite launched, achieving initial operational capability.
1995: Full operational capability.
May 2000: Military accuracy made available to all users.

Global Positioning System (GPS)

A satellite-based navigation and surveying system for determining precise position & time.

The Global Positioning System (GPS) is a Constellation of Earth-Orbiting Satellites for the Purpose of Defining Geographic Positions On and Above the Surface of the Earth.

A network of satellites that continuously transmit coded information, which makes it possible with help of an instrument (hand held or vehicles) to precisely identify locations on earth by measuring distance from the satellites.

e.g., Your location is: 23o 23.323’ N 85o 32.162’ E 532.456 m

GPS records data transmitted by each satellite and processes this data to obtain three dimensional coordinates.

GPS is the only system today able to show you your exact position anytime, in any weather, anywhere.

The Three Parts of GPS:

Satellites
Receivers
Software

GPS calculates latitude, longitude, altitude, and velocity.

NAVSTAR GPS

(Navigation System Time and Ranging Global Positioning System) by USA

It is an all-weather, space based navigation system development by the U.S. DOD to satisfy the requirements for the military forces to accurately determine their position, velocity, and time in a common reference system, anywhere on or near the Earth on a continuous basis.
In 1973 the U.S. DOD decided to establish, develop, test, acquire, and deploy a spaceborne Global Positioning System (GPS), resulting in the NAVSTARGPS (NAVigation Satellite Timing And Ranging Global Positioning System).

Launch of First GPS Satellite

The first GPS satellite PRN 4 was launched on February 22, 1978.

Navigation Accuracy Comparisons

GPS - $15m$
LORAN C - $180m$
Transit - $200m$
TACAN (Tactical Air Navigation) - $400m$
Inertial - $1 km$
Omega - $2 km$

GPS General Characteristics

Developed by the US DOD.
Provides accurate navigation (- $10-20 m$ ).
Worldwide coverage.
24-hour access.
Common Coordinate System.
Designed to replace existing navigation systems.
Accessible by Civil and Military users.

GPS Tidbits

Development costs estimate $12 billion$
Annual operating cost ~ $400 million$
3 Segments:
- Space: Satellites
- User: Receivers
- Control: Monitor & Control stations
Prime Space Segment contractor: Rockwell International (now Lockheed Martin).
Coordinate Reference: WGS-84
Operated by US Air Force Space Command (AFSC).
Mission control center operations at Schriever (formerly Falcon) AFB, Colorado Springs.

Key Components of the GPS System

*Space Segment: Includes the satellite constellation.
*User Segment: Encompasses GPS receivers.
*Control Segment: Consists of monitor & control stations.

The four GPS segments are:

Space Segment: constellation of GPS satellites that transmit signals to users.
Control Segment: responsible for monitoring and operating the Space Segment.
User Segment: includes user hardware and processing software for positioning, navigation, and timing applications.
Ground Segment: civilian tracking networks which provide the User Segment with reference control, precise ephemerides, and real-time services (DGPS) to mitigate "selective availability".

GPS Segments

Space Segment
User Segment
Control Segment
- Ground Antennas
- Master Station
- Monitor Stations

Three Segments of GPS

Space segment:
- Uplink data (Satellite ephemeris position constants, Clock-correction factors, Atmospheric data, Almanac)
- Downlink data (Coded ranging signals, Position information, Atmospheric data, Almanac)
Control segment
- Master control station
- Ground Antenna
- Monitor stations
User segment

System Description

The Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. GPS uses these "man-made stars" as reference points to calculate positions accurate to a matter of meters. The advanced GPS systems can make measurements to better than a centimeter. GPS receivers have been miniaturized to just a few integrated circuits and are becoming very economical. The constellations of 24 satellites are distributed in a manner that ensures at least four satellites are visible almost anywhere in the world at any time. Each satellite receives and stores information from the control segment, maintains very accurate time through onboard precise atomic clocks and transmits signals to the earth. GPS basically consists of three segments.

Satellite Hardware (Space Segment)

There are nominally 24 GPS satellites, but this number can vary within a few satellites at any given time, due to old satellites being decommissioned, and new satellites being launched to replace them.
All the prototype satellites, known as Block I, have been decommissioned.
Between 1989 and 1994, 24 Block II (1989-1994) were placed in orbit.
From 1995 onwards, these have started to be replaced by a new design known as Block IIR.

Satellite nominal specifications:

Life goal: $7.5$ years
Mass: ~ $1 tonne$ (Block IIR: ~ $2 tonnes$ )
Size: $5 metres$
Power: solar panels $7.5 m^2$ + Ni-Cd batteries
Atomic clocks: $2 rubidium$ and $2 cesium$

The orientation of the satellites is always changing, such that the solar panels face the sun, and the antennas face the centre of the Earth. Signals are transmitted and received by the satellite using microwaves. Signals are transmitted to the User Segment at frequencies $L1 = 1575.42 MHz$ , and $L2 = 1227.60 MHz$ . Signals are received from the Control Segment at frequency $1783.74 Mhz$ .

The flow of information is as follows: the satellites transmit L1 and L2 signals to the user, which are encoded with information on their clock times and their positions. The Control Segment then tracks these signals using receivers at special monitoring stations. This information is used to improve the satellite positions and predict where the satellites will be in the near future. This orbit information is then uplinked at $1783.74 Mhz$ to the GPS satellites, which in turn transmit this new information down to the users, and so on
The orbit information on board the satellite is updated every hour.

The space segment consists of 24 space vehicles. The satellites are placed in almost circular in six orbital planes, with an orbital inclination of $55$ degrees. The orbital height is about $20,200 km$ corresponding to about $26,600 km$ for the semi-major axis. The orbital period is $12$ hours of sidereal time, and provides repeated satellite configurations, everyday four minutes earlier with respect to universal time. The techniques for positional accuracy with different types of receivers according to the requirement, is a result of much effort.

Satellites, the source of communication of signals are developed in different blocks. These satellites can be grouped in three types:

Block I: Development or R&D satellites
Block II: Production or operating satellites
Block III: Replenishment satellites

Space Segment

(Initial Operational Capability (IOC)-1993)

(Full Operational Capability (FOC)-1995)

Block I
- First Launch: 22 Feb 78(78-85)
- On-Orbit: None, Total=11
Block II/IIA
- First Launch: 14 Apr 89(89-97)
- Total: 28
Block III
- Block IIR / IIR-M(L2C civil signal & new military code M on both L1& L2)
  - First Launch: 22 Jul 1997/25Sep2005
  - Total=21/8
  - (R: Replenishment; M: Modernized)
- Block IIF
  - First Launch: 2009
  - Acquiring up to 19 SV's

The block I satellites, NAVSTAR numbered from 1 to 11 were launched between 1978 and 1985 into two orbital planes of $63^o$ inclination. These were launched at the stage of initial development for experimental purpose. The design life of these prototype vehicles was only 5 years, but has been exceeded in most cases. One advantage of the prototype satellites was that the navigational signals are not subject to deliberate corruption (No selective availability or antispoofing introduced in their signal). One disadvantage was that it was not possible to observe more than two such satellites at any time.

The first Block II production satellite was launched in February 1989. A total of 28 Block II satellites were planned to support the 24-satellite configuration. The design lifetime of the operational Block II satellites was 7.5 years. The development of the follow-up generation satellites after 1995 include 20 replenishment satellites known as Block IIR satellites that replaced Block II satellites as necessary. Two of the new design features are the ability to measure distances between the satellites (cross link ranges), and to compute ephemeris on-board. The GPS program is currently funded with replacements through 2006.

Each satellite carries high performance frequency standards with an accuracy of between $1 x 10^{-12}$ to $1 x 10^{-13}$ seconds forming a precise time base. The prototype satellites were partly equipped only with quartz oscillators. All Block II production satellites had cesium frequency standards and two rubidium frequency standards. Two carrier frequencies in the L-band are coherently derived from the fundamental frequency $10.23 MHz$ , 154 and 120 times. Each satellite transmits signals on both frequencies. These are the navigation signals (codes) and the navigation and system data message. The codes are modulated on the carrier frequencies as so called Pseudo Random Noise (PRN) sequences.

The GPS satellites are identified by two different numbering schemes, SVN (space vehicle number) or NAVSTAR number based on the launch sequence and by PRN (pseudo random noise) or SVID (space vehicle identification) number, related to the orbit arrangement and the particular PRN segment allocated to the individual satellite.

Carrier Frequency (f) Wavelength ( $\\lambda$ )

L1 $10.23 x 154 = 1575.42 MHz$ $19 Cm$
L2 $10.23 x 120 = 1227.60 MHz$ $24 Cm$

In a nutshell each satellite broadcasts the following information:

Time
Position (referenced to WGS84)
Carrier phase
Code phase
Almanac for all the other NAVSTAR satellites
P code
C/A code

The GPS receiver stores the data about the position of the satellites in space at any given time in its memory. This data is called the almanac data and is received from the satellites. These orbits of satellite are inclined at 55 degree angle to equator of the earth.

Space Segment

24 satellite vehicles
Six orbital planes
- Inclined 55o with respect to equator
- Orbits separated by 60o
20,200 km elevation above Earth
Orbital period of 11 hr 55 min
Five to eight satellites visible from any point on Earth
24 Satellites
Space Segment
4 satellites in 6 Orbital Planes inclined at 55 Degrees
20200 Km above the Earth
12 Hourly orbits
In view for 4-5 hours
Designed to last 7.5 years
Different Classifications - Block 1, 2, 2A, 2R & 2 F

The Space Segment

Comprises of constellation of GPS satellites and broadcasting of signals which allows user to determine position velocity and time
GPS satellites orbit at an altitude of $20, 200 km$
There are 6 orbits. At present there are total 28 operational satellites, out of these 28, 24 are operational and 4 are operational spare satellites. The orbits of satellite are inclined at $55$ degree angle to equator of the earth.
These satellites travel at a speed of $11500 km/hour$ which allows them to circle the earth once every 12 hrs.
The satellites are powered by solar energy and are built to last for ten years, back batteries work in case the solar energy fails during eclipses on board to keep them running

BASIC FUNCTIONS OF SATELLITES

Receiving and storing data transmitted by the control segment stations
Maintaining accurate time
Transmitting information and radio signals to users on two L-band frequencies
Providing a stable platform and orbit for the L-band transmitters

SATELLITE CONSTELLATION CHARACTERISTICS

Constellation was designed in such a way that it should give best possible coverage of the earth with minimum number of satellites
To minimize the effect of gravitation and atmospheric drag and easy upload and monitoring capabilities from all control stations
They provide visibility of minimum of four satellites above 15-degree cut off angle at any location at any time on the earth.

The Control Segment

Run by the US Air Force, is responsible for operating GPS. The main Control Centre is at Falcon Air Force Base, Colorado Springs, USA. Several ground stations monitor the satellites L1 and L2 signals, and assess the “health” of the satellites. The Control Segment then uses these signals to estimate and predict the satellite orbits and clock errors, and this information is uploaded to the satellites. In addition, the Control Segment can control the satellites; for example, the satellites can be maneuvered into a different orbit when necessary. This might be done to optimise satellite geometry when a new satellite is launched, or when an old satellite fails. It is also done to keep the satellites to within a certain tolerance of their nominal orbital parameters (e.g., the semi-major axis may need adjustment from time to time). As another example, the Control Segment might switch between the several on-board clocks available, should the current clock appear to be malfunctioning.

Belonging to the control segment are the Master Control Station (MCS), several monitor stations (MS) located around the world and Ground Antennas (GA) for uploading data into the satellites.

The operational control segment (OCS) for GPS consists of the Master Control Station near Colorado Springs (MCS), three monitor stations and ground antennas in Kwajalein, Ascension and Diego Garcia as well as two more monitor stations in Colorado Springs and Hawaii.

There are five Monitor stations around the world, which continuously track all the satellites and feed the information to the master control station at Colorado.

The monitor stations receive all satellite signals from which they determine the pseudo ranges to all visible satellites, and transmit the range data along with local meteorological data via data link to Master control station. From these data the master control station pre-computes satellite ephemerides and the behaviour of the satellite clocks and formulates navigation data (message).

The message data are transmitted to the ground antennas and up linked via S-band to the satellites in view. Because of global distribution of the upload antennas at least three contacts per day can be realized between control segment and each particular satellite. The cesium oscillator at a selected monitor station defines the GPS system time. No clock parameters are derived for the station.

Main Control Station (MCS)

Ground Antennae (GA)
Monitor Station (MS)
- Prediction of ephemeredes and clock behaviour
- Uplink of navigation Message to satellites
- Control of ephemeredes and satellite clocks

Control Segment

MASTER STATION-1
MONITOR STATION-5
GROUND ANTENNAS-4 DISTRIBUTED AMONG 5 STATIONS ON THE EARTH

Function of Control Segment

IT TRACKS THE GPS SATELLITES
AND UPDATES THEIR ORBITING POSITIONS
CALIBRATES AND SYNCHRONISE THEIR CLOCKS
TO DETERMINE THE ORBIT OF EACH SATELLITE AND PREDICTS ITS PATH FOR NEXT 24 HRS
AND SEND THE ORBITAL DATA TO THE MASTER CONTROL STATION WHICH IN TURN SEND CORRECTED DATA TO THE SATELLITES
THIS CORRECTED AND POSITION DATA IS CALLED THE “EPHEMERIS”WHICH REMAIN VALID FOR 6 HRS AND TRANSMITTED TO THE GPS RECEIVERS IN CODED FORM
THE MASTER CONTROL STATION IS LOCATED AT THE CONSOLIDATED SPACE OPERATION CENTER NEAR COLORADO SPRING (USA)
THE MONITORING STATIONS TRACKS THE NAVIGATION SIGNALS OF ALL THE SATELLITES AND THEN PROCESSED AT MCS. THESE DATA AFTER PROCESSING USED TO UPDATES SATELLITES NAVIGATION MESSAGES.
THE MCS ALSO COMPUTES CLOCK CORRECTIONS DERIEVED FROM GMS DISTRIBUTED AT VARIOUS LOCATIONS IN THE WORLD OF THE WEEKS PERIOD.
GROUND ANTENNAS USED TO TRANSMITTE CAMMANDS TO SATELLITES AND RECEIVE SATELLITES TELEMETRY.
THE GMS ARE LOCATED AT ASCENSION ISLAND, COLORADO SPRINGS,DIEGO GARCIA, HAWAII AND KWAJALEIN ATOLL.

User Segment

The most visible segment - GPS receivers are found in many locations and applications.

Who Uses It?

Everyone!
- Merchant, Navy, Coast Guard vessels forget about the sextant, Loran, etc.
- Commercial Airliners, Civil Pilots
- Surveyors has completely revolutionized surveying
- Commercial Truckers
- Hikers, Mountain Climbers, Backpackers
- Cars now being equipped
- Communications and Imaging Satellites
- Space-to-Space Navigation
- Any system requiring accurate timing

GPS Receiver Components

Antenna
R.F. (Radio frequency) section with signal identification & signal processing
Micro-processor for receiver control, data sampling and data processing.
(Navigation Solution)
Precision oscillator
Power supply
Memory, Data storage
User interface, Command and display channel

User Segment Applications

Military
Search and rescue
Disaster relief
Surveying
Marine, aeronautical and terrestrial navigation
Remote controlled vehicle and robot guidance
Satellite positioning and tracking
Shipping
Geographic Information Systems (GIS)
Recreation

THE USER SEGMENT

Comprises of GPS Receivers.
There are three types of GPS receivers which are available in today’s marketplace. Each of three types offers different level of accuracy.
- C/A code receivers or Navigational : typically provide 1-5 meter GPS position accuracy with differential correction. Such receivers if allowed to occupy for longer duration (Up to 3 minute) give GPS position accuracy within 1-3 meter. Recent advanced C/A code receiver provide accuracy up to sub-meter level.
- Carrier phase receivers or GIS application
  - typically provide $10-30 cm$ GPS accuracy with differential correction.
  - Carrier phase receivers measures the distance from receiver to the satellites by counting the number of waves that carry the C/A code signals.
  - Such receivers provide the higher level of accuracy as demanded in many GIS based applications
  - it does require substantially higher occupation time to get the accuracy of $10-30 cm$ level accuracy and need to be very close to the base station.
- Dual frequency receiver or Survey grade
  - are capable of providing sub-centimeter level accuracy with differential correction.
  - Such receivers provide survey grade level of accuracy. These receivers receive signals from the satellites on two frequencies simultaneously. This characteristics allows the receiver to determine very precise position.
GPS is a widely seen as the most important gift of the DoD to the civil world. It can be utilized in any conceivable project under the sky where position, time and velocity of any object or phenomenon is required.

Why GPS?

Weather Independent
Does not require line of sight
Gives high Geodetic Accuracy
Can be operated day and night
Quicker and requires less manpower
Economical advantages
Common Coordinate System
Wide Range of Applications
Competitively Priced

Use of GPS

Power Grid Interfaces
Railroads
Personal Navigation
Trucking & Shipping
Precision farming
Surveying & Mapping
Communications
Recreation
Offshore Drilling
Fishing & Boating

Automatic Vehicle Locating
Applications.

Topo and Locations
Boundaries
Volumes
■ Mapping
■ Monitoring
Photo control
Construction Control
and Stakeout
Measuring Tectonic Movement
Machine Guidance
Air Navigation
Deformation Monitoring
Missile Guidance
■ Seismic Stakeout
Profiles
Establishing Portable Control
Stations (sharing with Total
Stations)
Agriculture - Slope Staking
Tracking of people, vehicles
Plate movements
Sports (boating, hiking,…)
■ Archeology
Public Transport
Emergency services
Geodynamics
Dams
Bridges,
Buildings.
Oil platforms.

GPS Applications:

GPS for Utility Mapping
GPS for Forestry and Natural Resources
GPS for Land Seismic Surveying
GPS for Precision Farming
GPS for Monitoring Bridge Deformation
GPS for Civil Engineering Applications
Use in Open-Pit Mining
GPS for Marine Seismic Surveying
GPS for Airborne Mapping (Aerotriangulation/LIDAR)
GPS for Seafloor Mapping (Multibeam/Single-beam echo sounding)
GPS for Vehicle Navigation & Transit Systems
GPS Stakeout / Retail / Cadastral Surveying

User requirements in positioning accuracies

Geomatics
- Land (Fault Monitoring, Road Grading)
- Airborne (Cat II/III, Sensor positioning)
- Marine (Dredging, Pylon Positioning)
Geodetic Infrastructure/Earth Moving/Construction Surveys
Engineering Surveys/Mining/Geodynamics/Urban cadastral survey/Resource Mapping/Facility Surveys/GIS Database/Mapping/GIS/Docking
Legal Surveys/Utility Mapping/Highway Surveys/Buoy Position/Highway Surveys/Rural cadastral survey/Precision farming/GIS Data Collection/Automobiles/Site Specific Farming/Emergency/Navigation/Public/Tracking/Mapping
Reconnaissance/Navigation/Area Navigation/Channel Navigation/Sensor navigation/Cabling/Transport Research/Harbor Entry/Oceanic/Harbor Approach/Oceanic
Accuracy requirement depending on application

The Geocentric Cartesian Coordinate System

$AP = \sqrt{(XP-XA)^2 + (YP-YA)^2 + (ZP-ZA)^2}$
A signal is transmitted from each satellite in the direction of the Earth. This signal is encoded with the “Navigation Message,” which can be read by the user’s GPS receivers. The Navigation Message includes orbit parameters (often called the “broadcast ephemeris”), from which the receiver can compute satellite coordinates (X,Y,Z). These are Cartesian coordinates in a geocentric system, known as WGS-84, which has its origin at the Earth centre of mass, Z axis pointing towards the North Pole, X pointing towards the Prime Meridian (which crosses Greenwich), and Y at right angles to X and Z to form a right-handed orthogonal coordinate system. The algorithm which transforms the orbit parameters into WGS-84 satellite coordinates at any specified time is called the “Ephemeris Algorithm,”

GPS Satellite Signals

Atomic Clock (G, Rb) fundamental frequency = $10.23 MHz$
L1 Carrier Signal = $154 X 10.23 MHz = 1575.42 MHz$
- L1 Wave length = $19.05 Cm$
L2 Carrier Signal = $120 X 10.23 MHz = 1227.60 MHz$
- L2 Wave Length = $24.45 Cm$
P-Code Frequency (Chipping Rate) = $10.23 MHz (Mbps)$
- P-Code Wavelength = $29.31 M$
- P-Code Period = $267 days/Satellite$
C/A-Code Frequency (Chipping Rate) = $1.023 MHz (Mbps)$
- C/A-Code Wavelength = $293.1 M$
- C/A-Code Cycle Length = $1 Milisecond$
Data Signal Frequency = $50 bps$
- Data Signal Cycle Length = $30 Seconds$

Generation of GPS Signals

L1 CARRIER $1575.42 MHz$
C/A CODE $1.023MHz$
NAV/SYSTEM DATA $50 Hz$
- L1 SIGNAL
P-CODE $10.23 MHz$
L2 CARRIER $1227.6 MHz$
- L2 SIGNAL

GPS SIGNALS IN DETAIL

Each GPS satellite continuously transmits signals that contain a wealth of information. Depending on the type and accuracy of positioning being carried out, a user may only be interested in a portion of the information included in the GPS signal. Similarly, a given GPS receiver may only enable use of a portion of the available information. It is therefore important for users to understand the content and use of GPS signals. The information contained in GPS signals includes the carrier frequencies, coarse acquisition (C/A) and precise (P) codes and the satellite message.

GPS satellites send two signals: a carrier and a pseudo- random code. The signals are timed by an atomic clock in the satellite, and the GPS receiver generates a matching code timed by its own synchronized clock. The time it takes for the signals to reach the receiver indicates how far away the satellite is. This calculation is generally performed using the pseudo-random code signal, but for better precision, the carrier signal can be used instead.

To make position calculations, GPS receivers use signals from four or more GPS satellites. The first three satellites are used to triangulate a position. The fourth is used to improve the position's accuracy by factoring in the time offset between the satellite system's clock and the GPS receiver's clock.

The signals from a GPS satellite are fundamentally driven by an atomic clocks (usually cesium, which has the best long-term stability). The fundamental frequency is $10.23 Mhz$ . Two carrier signals, which can be thought of as sine waves, are created from this signal by multiplying the frequency by 154 for the L1 channel (frequency = $1575.42 Mhz$ ; wavelength = $19.0 cm$ ), and 120 for the L2 channel (frequency = $1227.60 Mhz$ ; wavelength = $24.4 cm$ ). The reason for the second signal is for self-calibration of the delay of the signal in the Earth’s ionosphere. Information is encoded in the form of binary bits on the carrier signals by a process known as phase modulation. (This is to be compared with signals from radio stations, which are typically encoded using either frequency modulation, FM, or amplitude modulation, AM). The binary digits 0 and 1 are actually represented by multiplying the electrical signals

There are three types of code on the carrier signals:

The C/A code
The P code
The Navigation Message
The C/A (“course acquisition”) code can be found on the L1 channel. This is a code sequence that repeats every $1 ms$ . It is a pseudo-random code, which appears to be random, but is in fact generated by a known algorithm. The carrier can transmit the C/A code at $1.023 Mbps$ (million bits per second). The “chip length”, or physical distance between binary transitions between digits +1 and -1), is $293 metres$ .
The basic information that the C/A code contains is the time according to the satellite clock when the signal was transmitted (with an ambiguity of $1 ms$ , which is easily resolved, since this corresponds to $293 km$ ). Each satellite has a different C/A code, so that they can be uniquely identified.
The P (“precise”) code is identical on both the L1 and L2 channel. Whereas C/A is a courser code appropriate for initially locking onto the signal, the P code is better for more precise positioning. The P code repeats every $267$ days. In practice, this code is divided into 7-day segments; each weekly segment is designated a “PRN” number and is designated to one of the GPS satellites. The carrier can transmit the P code at $10.23 Mbps$ , with a chip length of $29.3 metres$ . Again, the basic information is the satellite clock time or transmission, which is identical to the C/A information, except that it has ten times the resolution