3 Global Positioning System (GPS)

Dr. Puneeta Pandey

CONTENTS

1.  Learning Objectives

2.  Introduction

3.  Segments of GPS

4.  GPS data types

5.  Working of GPS

6.  Locks in GPS

7.  GPS Satellite System around the World

8.  Application of GPS

9.  Summary

10.  References

1.  Aim of the Module

• To understand the basic concepts of GPS
• To understand the working principle of GPS 
• To study the navigational satellite systems around the world. To study the applications of GPS 

2.  Introduction

The Global Positioning System (GPS) is officially called as Navigational Satellite Timing And Ranging Global Positioning System (NAVSTAR GPS), owned by United States Department of Defence (DoD). This navigation system provides location and time information in all weather conditions. GPS project was developed in 1973 and was originally run with 24 satellites. Roger L. Easton is credited as the inventor of GPS. The first GPS satellite PRN 4 was launched on February 22, 1978 and became fully operational in 1994. Currently, GPS system is maintained by the United States government and is accessible to anyone with a GPS receiver.

3. Segments of GPS

The segments of GPS are three: Space, Control and User segment.

3.1 Space segment- deals with GPS satellites systems

3.2 Control Segment- describes ground based time and orbit control prediction

3.3 User segment- various types of existing GPS receiver and its application is dealt

Figure 1: The Space, Control and User segment of GPS

3.1 Space Segment

The space segment consists of 21 GPS satellite with an addition of 3 active spares. These satellites are placed in almost six circular medium Earth orbits (MEO) with an inclination of 55 degree to the equatorin order to cover the polar regions. There are six orbital planes A to F with a separation of 60 degrees at right ascension (crossing at equator). Orbital height of these satellites is about 20,200 km; while the orbital period is 12 hours of sidereal time (i.e., the time takenby the Earth to turn 360 degrees in its rotation.This provides repeated satellite configuration every day advanced by four minutes with respect to universal time. Each satellite makes two complete orbits each sidereal day; and 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.

Figure 2: Space Segment (www. brainkart.com)

GPS satellites are broadly divided into four blocks (each generation of satellites with similar characteristics is called a block): Block-I satellite pertains to development stage, Block II represented production satellites and block IIR were replenishment/spare satellite. The fourth block consists of newer satellites and is called Block-III.

3.1.1 Block I, Navigation Development Satellites:This was a test constellation of Block I satellites built by Rockwell International; and inclined by 63o to the equator instead of the current specification of 55o. Between 1978 and 1985, 11 satellites were launched with an average life of 4.5 years. They were capable of giving positioning service for 3 or 4 days without any contact with the control center.The latest satellite of Block I was retired in 1995.

3.1.2 Block II and IIA, Operational Satellites:The next generation of GPS satellites consisting of 28 satellites is known as Block II satellites. They were launched from 1989 onwards and the last was decommissioned in 2007. These satellites were also built by Rockwell International.

3.1.3 Block IIR, Replacement Operational Satellites:The first satellite that was launched in January 1997, was unsuccessful; however, the next in July of 1997 succeeded. They have an average lifespan of 10 years. They have the capability to measure distances between themselves and transmit data to other satellites or to the control segment.

3.1.4 Block IIR-M, Modernized Satellites:Block IIR-M is a modification of IIR satellites that was launched in September 2005. The satellites were upgraded two new codes; a new military code (the M code), a new civilian code (the L2C code) and demonstrate a new carrier, L5.

3.1.5 Block IIF, Follow-on Operational Satellites:Block IIF is the most recent group of GPS satellites launched in May, 2010. Their theoretical average life is about 12 years. Block IIF satellites have faster processors and more memory onboard.

3.1.6 Block III: This is the latest generation of GPS satellites with advancements in navigation capabilities. The first launch was expected in 2017; but has been delayed to May 2018.

3.2 Control Segment (CS)

This segment deals with monitoring and controlling the satellite system continuously, predicting the satellite ephemeries and the behaviour of each satellite clocks; while updating periodically the navigation message for each particular satellite.

The Control Segment consists of 3 components:

–  Master Control System

–  Monitor Stations

–  Ground Antennas

Figure 3: Control Segment (http://www.gps.gov/multimedia/images/GPS-control-segment-map.pdf)

3.2.1 Master Control Station:The master control station is located at Falcon Air Force Base in Colorado Springs, Colorado; and is responsible for overall management of GPS satellites. It receives information pertaining to navigation from the monitor stations, to determine the precise locations of the GPS satellites and uploads this data to the satellites.It is also responsible for activation of spare satellites; resolving satellite anomalies; and passive tracking of the satellites.

3.2.2 Monitor Stations: There are six monitor stations for GPS satellites thatare located at Falcon Air Force Base in Colorado, Cape Canaveral in Florida, Hawaii, Ascension Island in the Atlantic Ocean, Diego Garcia Atoll in the Indian Ocean, and Kwajalein Island in the South Pacific Ocean. These monitor stations check the behaviour, position, speed, and overall health of the orbiting satellites by performing the routine tests of these satellite twice a day.

3.2.3 Ground Antennas:Ground antennas monitor and track the satellites from horizon to horizon; and transmit information pertaining to ephemerides and clock correction to individual satellites; besides the navigation messages.They are used to communicate with the GPS satellites for command and control purposes via S-band radio signals. There are four dedicated GPS ground antenna sites co-located with the monitor stations at Kwajalein Atoll, Ascension Island, Diego Garcia, and Cape Canaveral.

3.3 User Segment: The user segment involves GPS receiver based on US GPS system; which is composed of an antenna, receiver-processors and a stable crystal oscillator clock. In general, a GPS receiver can monitor between twelve and twenty channels simultaneously, indicating the number of satellites that can be monitored simultaneously.

4. GPS data types

GPS satellites broadcast two different types of data:

4.1 Almanac – sends time and status information about the satellites.

4.2 Ephemeris – has orbital information that allows the receiver to calculate the position of the satellite.

5. Working of GPS

Global Positioning System (GPS) is actually a constellation of solar-powered Earth-Orbiting satellites that orbit the earth at about 19300 km, making two rotations per day. These orbits are arranged such that at any point on the earth and time, at least four satellites are “visible” in the sky. So, a GPS receiver locates four or more of these satellites, calculates the distance of each satellite from the receiver and then deduces its own location based on the principle of ‘trilateration’ (Figure 4).

Figure 4: (https://spotlight.unavco.org/how-gps-works/gps-basics/decoding-the-gps-signal.html)

A GPS receiver’s job is to locate four or more of these satellites, figure out the distance to each, and use this information to deduce its own location. This operation is based on a simple mathematical principle called trilateration. Hence, the entire mechanism of working of GPS can be summed up under the following steps:

A. GPS transmits a radio signal to each satellite at a known time.

B. These radiosignals travel at the speed of light.

C. The system measures the time delay between the signal transmission and signal reception

D. This time difference helps in finding information about the satellite’s location.

E. Similar process is repeated for each of the visible satellites in the sky to determine the position of, and distance to, at least three satellites.

F. The receiver computes the position using mathematical principle of trilateration.

G. A GPS requires at least 3 satellites to calculate 2-D position (latitude and longitude); while at least 4 satellites to determine receiver’s 3-D position (latitude, longitude and altitude).The more the number of satellites visible, higher would be the accuracy of location determined by GPS receiver.

Figure 5: Working of GPS

(http://www.geneko.rs/uploads/content/images/Technology/gps_system_how_it_works.png)

6. Locks in GPS

Based on how a GPS receiver starts, a lock in GPS is carried out. This process of locking GPS usually takes time in a moving vehicle or in dense urban areas. Accordingly, there are three types of start – hot, warm and cold.

6.1 Hot Start: In this type, the GPS device remembers its last calculated position,the almanac used, the satellites in view, the UTC Time and makes an attempt to lock onto the same satellites and calculate a new position based upon the previous information. This is the quickest GPS lock if the receiver is in the same location when the GPS was last turned off.

6.2 Warm Start:The warm start is when the GPS device remembers its last calculated position, almanac used, and UTC Time, but not which satellites were in view. It then performs a reset and attempts to obtain the satellite signals and calculates a new position.This takes longer than a hot start but not as long as a cold start.

6.3 Cold Start: In this start, the GPS device dumps all the information, attempts to locate satellites and then calculates a GPS lock. This takes the longest because there is no known information.

7. GPS Satellite System around the World

Besides, United Sates, several countries have launched their own navigation system, which shall be discussed in this section:

•      Russian Global Navigation Satellite System (GLONASS)

•      European Union Galileo positioning system

•      Chinese Compass navigation system

•      Indian Regional Navigational Satellite System

7.1 GLONASS

This is the navigation system of Russia, known as the Russian Global Navigation Satellite System (GLONASS). The design and development for GLONASS dates back to the 1970s and is similar to GPS. It includes 24 active satellites and 3 spare satellites; unlike GPS by USA which has a network of 31 satellites.GLONASS gives better accuracy in places such as between huge buildings or subways, but lacks global coverage. Therefore, when both GPS and GLONASS are used together, higher coverage with accuracy can be obtained.

7.2European Union Galileo positioning system

Galileo of European Union is operable with GPS and GLONASS; however, it is under civilian control. As far back as the early 1990s, the European Union saw the need for a European-controlled global satellite navigation system. It will help in accurate and more reliable positioning, even in high-rise cities and at high latitude regions.Prague in theCzech Republic is the headquarter of the Galileo project and has two ground operations centres, Oberpfaffenhofen near Munich in Germany and Fucino in Italy. Galileo #1 and #2 were launched on 21 October 2011; followed by Galileo #3 and #4 on 12 October 2012. Two more satellites, Galileo #5 and #6, were launched on 22 August 2014, from French Guiana but were injected into an incorrect orbit, which has now been corrected. On 27 March 2015 the next two (Galileo #7 and #8) satellites were launched successfully from Guiana Space Centre.

7.3 BeiDou Navigation Satellite System

The BeiDou Navigation Satellite System is a Chinese satellite navigation system, which consists of more than 30 satellites. It consists of two separate satellite constellations – a limited test system that has been operating since 2000, and a full-scale global navigation system that is currently under construction. It is expected to begin serving global customers upon its completion in 2020.

7.4 Indian Regional Navigational Satellite System (IRNSS)

The Indian Space Research Organisation (ISRO) has developed an autonomous regional satellite navigation system called as Indian Regional Navigational Satellite System (IRNSS). It is a constellation of 7 satellites, and three satellites are in geostationary orbit over the Indian Ocean. Besides military applications, the satellite would help in navigation, disaster management and vehicle tracking. It is designed to provide accurate position information service to users in the country as well as the region extending up to 1,500 km from its boundary, which is its primary service area.

7.4.1 IRNSS-1A: The IRNSS-1A is the first of the 7 satellites developed by India, has a mission life of 10 years and was launched successfully at 23:41 hrs. on 1st July 2013 SDSC Centre, Sriharikota, India using PSLV – C22 vehicle.

7.4.2 IRNSS-1B: This satellite was successfully launched on Apr 04, 2014 and is placed at 55 deg East longitude, collocated with IRNSS-1A and GSAT-8 satellites. The navigational system would provide two types of services — Standard Positioning Service, which is provided to all the users and Restricted Service, which is an encrypted service provided only to the authorised users.

7.4.3 IRNSS-1C: It was launched successfully on 16 October 2014 at 1:32 am IST from Satish Dhawan Space Centre in Sriharikota and placed in geostationary orbit. It has a lifespan of 10 years.

7.4.4 IRNSS-1D:It waslaunched on March 28, 2015 with a lifespan of 10 years.

7.4.5 IRNSS-1E:It was launched on January 20, 2016 with a lifespan of 10 years.

7.4.6 IRNSS-1F:It was launched on March 10, 2016with a lifespan of 10 years.

7.4.7 IRNSS-1G:It was launched on April 28, 2016 with a lifespan of 10 years.

7.4.8 GPS Aided GEO Augmented Navigation (GAGAN)

GAGAN is a Satellite Based Augmentation System (SBAS) implemented jointly with Airport Authority of India (AAI) to provide Satellite-based Navigation services with accuracy and integrity required for civil aviation applications and to provide better Air Traffic Management over Indian Airspace. The first GAGAN navigation payload was flown on GSAT-8 which was launched on May 21, 2011 and the second on GSAT-10 launched on Sep 29, 2012.

8. Applications of GPS

The GPS was originally developed for military purpose; but later extended to civilian applications.Some of the applications of GPS are as follows:

8.1 Road Transport

GPS is used for commercial fleet management, navigation in private cars, freight tracking, taxi services, public transport monitoring and emergency vehicle location.

8.2 Aviation

GPS finds application in real-time aircraft position, flight’s tracking, commercial aviation, for navigation of unmanned aerial vehicles (UAVs) and aerial surveying.

8.3 Marine

Maritime applications include ocean and inshore navigation, dredging, port approaches, harbor entrance and docking, Vessel Traffic Services (VTS), Automatic Identification System (AIS), hydrography, and cargo handling.

8.4 Rail Transport

Rail systems throughout the world use GPS to track the movement of locomotives, rail cars, maintenance vehicles, and wayside equipment in real time.

8.5 Heavy Vehicle Guidance

GPS is being used increasingly to guide and track heavy vehicles in engineering applications such as mining and construction activities for vehicle guidance and tracking, and mine asset management systems.

8.6 Farming

Farmers rely on repeat planting season after season to maximize their crop productions. By putting GPS receivers on tractors and other agricultural equipment, farmers can map their plantations, work in low visibility conditions such as fog and darkness and map soil sample locations.

8.7 Science

Scientists use GPS technology to conduct a wide range of experiments and research, tracking animal’s movement usingGPS collars or tags, studying changing landscape as well as tectonic plate motion.

8.8 Surveying

Surveyors are responsible for mapping and measuring features on the earth’s surface, determining land boundaries, monitoring changes in the shape of structures or mapping the sea floor.

8.9 Mapping and Geophysics

Applications involve positioning survey marks, buildings and bridges, aerial mapping, mapping seismic activity and to monitor changes in volcanoes and earthquake fault lines.

8.10 Security

Security applications include tracking of vehicles, containers, other valuable cargoes and covert tracking of suspects.

8.11 Telecommunication

GPS timing is important for telecommunications applications, particularly for mobile telephone networks. GPS allows the derivation of synchronised UTC time through resolving the signals from a number of atomic clock sources at known locations.

8.12 Financial Services

Global financial systems require GPS to schedule and prioritise local and international money transfers, settlements and trades and to provide an audit trail for financial transactions.

8.13 Social Activities

Social applications include GPS-based social networking, geotagging photographs, cross country cycling, hiking, skiing, paragliding, skydiving, geocaching, geocaching and other gaming activities.

8.14 Tracking of wild life

GPS is used by scientists/ forest official etc for tracking the movement and behaviour of wild animals.

8.15 Forest health and forest fire monitoring

GPS is used for insect and diseases tracking in forests as well as for forest fire monitoring. GPS is also used to determine the location and areal extent of forest stands

8.16 Survey landfill sites and unauthorized solid and hazardous waste disposal sites

9. Summary

At the end of the module, the reader would have gained an idea about:

• Basic concepts of GPS – segments of GPS along with their roles and types of GPS locks
• The working principle of GPS – principle of trilateration
• The navigational satellite systems around the world.
• Applications of GPS

10. References

• http://www.insidegnss.com/auto/IG0607_CompassFinal.pdf https://www.liveviewgps.com/blog/gps-satellite-systems-world/ https://spotlight.unavco.org/how-gps-works/gps-basics/decoding-the-gps-signal.html
• brainkart.com
• https://www.gsmarena.com/glossary.php3?term=gps