1. Global Navigation Satellite Systems
(GNSS)
Dr. Mahesh K. Jat
Malaviya National Institute of Technology Jaipur

2. Introduction
Global Navigation Satellite Systems (GNSS) involve satellites, ground stations
and user equipment to determine positions around the world and are now
used across many areas of society
• GNSS GPS (USA), GLONASS (Russia), Galileo(Europe), Augmentation
Systems (SBAS, GBAS), IRNS (India), QuasiZenth (Japan)
• Fuelling growth during the next decade will be next generation GNSS
that are currently being developed.

4. Global Navigation Satellite Systems
(GNSS)
• NAVSTAR
– USA
• GLONASS
– Russians
• Galileo
– Europeans

5. GLONASS from Russia
• GLONASS-M (L1 and L2 bands ) satellites with an improved 7-year design
lifetime.
• 2007 to 2008 planned to launch GLONASS-K satellites with improved
performance, also transmit a third civil signal (L3).
• Stated intention is to achieve a full 24-satellite constellation transmitting
two civil signals by 2010.
• Full constellation is planned to be broadcasting three sets of civil signals
by 2012.
• Indian Government announced at the end of 2004 that it would be
contributing funds to assist Russia to revitalize GLONASS.

6. Galileo from the European Union
• Constellation of 30 satellites, increased altitude (approximately 3000km higher
than GPS) which will enable better signal availability at high latitudes.
• Exact signal structure is still liable to change,
• Galileo satellites broadcast signals compatible with the L1(E5a E5b) and L5 GPS
signals. Galileo will also broadcast in a third frequency band at E6; which is not
at the same frequency as L2/L2C of GPS.

7. • Current plan is to offer 5 levels of service:
o Open Service uses the basic signals, free-to-air to the public with performance similar to
GPS and GLONASS.
o Safety of Life Service allows similar accuracy as the Open Service but with increased
guarantees of the service, including improved integrity monitoring to warn users of any
problems.
o Public Regulated Service is aimed at public authorities providing civil protection and
security (eg police), with encrypted access for users requiring a high level of performance
and protection against interference or jamming.
o Search and Rescue Service is designed to enhance current space-based services (such as
COSPAS/SARSAT) by improving the time taken to respond to alert messages from distress
beacons.
o Commercial Service allows for tailored solutions for specific applications based on
supplying better accuracy, improved service guarantees and higher data rates.

8. GNSS Signal Spectrum
Galileo E5/A
Lower L-Band Upper L-Band C-Band
GPS L2
1151MHz
1300MHz
Galileo E5/B
1164MHz
1188MHz
1215MHz
Glonass
G2
1237MHz
1239MHz
1260MHz
1261MHz
Galileo E6
1559MHz
06/09/07 Veena G Dikshit, Sc 'E' , ADE, Bangalore
ARNS ARNS
1610MHz
GPS L1
1563MHz
1587MHz
Glonass
G2
5010MHz
5030MHz
1591MHz
1254MHz
1258MHz
1593MHz
ARNS
RNSS RNSS* RNSS*
960MHz
RNSS
5250MHz
RNSS
Galileo C1
Galileo E3
GPS L5
RNSS* shared with other services
1214MHz
Galileo E5/A or E5/B frequency band options
Galileo E1
Galileo E2
Galileo E4

9. BENEFITS OF GNSS
• Availability of Signals
• Extra satellites improve continuity
• Extra satellites and signals can improve accuracy
• Extra satellites and signals can improve efficiency
• Extra satellites and signals can improve availability (of satellites at a particular
location)
• Extra satellites and signals can improve reliability
06/09/07 Veena G Dikshit, Sc 'E' , ADE, Bangalore

10. What is the GPS?
• Orbiting navigational satellites
– Transmit position and time data
• Handheld receivers calculate
– latitude
– longitude
– altitude
– velocity
• Developed by Department of
Defense

11. The Global Positioning System
• Baseline 24 satellite constellation in medium earth orbit
• Global coverage, 24 hours a day, all weather conditions
• Satellites broadcast precise time and orbit information on L-band
radio frequencies
• Two types of signals:
– Standard (free of direct user fees)
– Precise (U.S. and Allied military)
• Three segments:
– Space
– Ground control
– User equipment

12. History of the GPS
• 1969—Defense Navigation Satellite System
(DNSS) formed
• 1973—NAVSTAR Global Positioning System
developed
• 1978—first 4 satellites launched
Delta rocket launch

13. History of the GPS
• 1993—24th satellite
launched; initial
operational capability
• 1995—full operational
capability
• May 2000—Military
accuracy available to all
users

17. Three Segments of the GPS
Space Segment
Control Segment
User Segment
Monitor Stations
Ground
Antennas
Master Station

19. Components of the System
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
Block I Satellite Vehicle

20. GPS Satellite Vehicle
• Four atomic clocks
• Three nickel-cadmium batteries
• Two solar panels
– Battery charging
– Power generation
– 1136 watts
• S band antenna—satellite control
• 12 element L band antenna—user
communication
Block IIF satellite vehicle (fourth
generation)

21. GPS Satellite Vehicle
• Weight
– 2370 pounds
• Height
– 16.25 feet
• Width
– 38.025 feet including
wing span
• Design life—10 years
Block IIR satellite vehicle
assembly at Lockheed
Martin, Valley Forge, PA

22. GPS Space Segment
• The space segments nominally consists of 24 satellites, currently:
– 28 (24+4 spares) active GPS satellites (26 Block II, 2 Block IIR)
– Constellation design: at least 4 satellites in view from any location at any time to allow
navigation (solution for 3 position + 1 station clock unknowns)
– “Right Time, Right Place, Any Time, Any Place”
• GPS Orbit characteristics:
– Semi-Major Axis (Radius): 26,600 km
– Orbital Period : 11 h 58 min
– Orbit Inclination: 55 degrees
– Number of Orbit Planes: 6 (60 degree spacing)
– Number of Satellites: 24 (4 spares)
– Approximate Mass: 815 kg, 7.5 year lifespan
– Data Rate (message): 50 bit/sec
– PRN (Pseudo-Random Noise) Codes: Satellite-dependent Codes
– Transmit, Frequencies L-Band L1: 1575.42 MHtz
L2: 1227.60 MHtz

23. GPS Space Segment
Currently: 26 Block II, 2 Block IIR, no Block I satellites are active.
Picture of a Block II Satellite

24. Components of the System
User segment
• GPS antennas & receiver/processors
• Position
• Velocity
• Precise timing
• Used by
– Aircraft
– Ground vehicles
– Ships
– Individuals

25. Components of the System
Ground control segment
• Master control station
– Schreiver AFB, Colorado
• Five monitor stations
• Three ground antennas
• Backup control system

26. GPS Control Segment
US Air Force and NIMA Control and Tracking Stations
MCS Colorado Springs
Hawaii
Hermitage
Ouito
Buenos Aires
US NIMA Tracking Sites
Diego Garcia
Ascension
Bahrain
Kwajalein
Smithfield
US Airforce Tracking Sites
US Airforce Upload Sites
See also map at <http://164.214.2.59/GandG/sathtml>
MCS – Master Control Station

27. GPS Communication and Control

28. GPS Ground Control Stations

31. GPS involves 5 Basic Steps
• Trilateration
– Intersection of spheres
• SV Ranging
– Determining distance from SV
• Timing
– Why consistent, accurate clocks are required
• Positioning
– Knowing where SV is in space
• Correction of errors
– Correcting for ionospheric and tropospheric delays

32. Accurate Timing is the Key
• SVs have highly accurate atomic clocks
• Receivers have less accurate clocks
• Measurements made using “nanoseconds”
– 1 nanosecond = 1 billionth of a second
• 1/100th of a second error could introduce error of
1,860 miles
• Discrepancy between satellite and receiver clocks
must be resolved
• Fourth satellite is required to solve the 4
unknowns (X, Y, Z and receiver clock error)

33. Satellite Positioning
• Also required in the equation to solve the 4
unknowns is the actual location of the
satellite.
• SV are in relatively stable orbits and constantly
monitored on the ground
• SV position is broadcast in the “ephemeris”
data streamed down to receiver

34. How does GPS work?
• Satellite ranging
– Satellite locations
– Satellite to user distance
– Need four satellites to determine position
• Distance measurement
– Radio signal traveling at speed of light
– Measure time from satellite to user
• Low-tech simulation

35. How does GPS work?
Pseudo-Random Code
• Complex signal
• Unique to each satellite
• All satellites use same
frequency
• “Amplified” by
information theory
• Economical

36. Signal Structure
• Each satellite transmits its own unique code
• Two frequencies used
– L1 Carrier 1575.42 MHz
– L2 Carrier 1227.60 MHz
• Codes
– CA Code use L1 (civilian code)
– P (Y) Code use L1 & L2 (military code)

37. How GPS works?
• Range from each satellite calculated
range = time delay X speed of light
• Technique called trilateration is used to determine
you position or “fix”
– Intersection of spheres
• At least 3 satellites required for 2D fix
• However, 4 satellites should always be used
– The 4th satellite used to compensate for inaccurate
clock in GPS receivers
– Yields much better accuracy and provides 3D fix

38. How does GPS work?
• Distance to a satellite is determined by measuring how long a
radio signal takes to reach us from that satellite.
• To make the measurement we assume that both the satellite
and our receiver are generating the same pseudo-random
codes at exactly the same time.
• By comparing how late the satellite's pseudo-random code
appears compared to our receiver's code, we determine how
long it took to reach us.
• Multiply that travel time by the speed of light and you've got
distance.
• High-tech simulation

39. How does GPS work?
• Accurate timing is the key to measuring
distance to satellites.
• Satellites are accurate because they have four
atomic clocks ($100,000 each) on board.
• Receiver clocks don't have to be too accurate
because an extra satellite range measurement
can remove errors.

40. How does GPS work?
• To use the satellites as references for range measurements we
need to know exactly where they are.
• GPS satellites are so high up their orbits are very predictable.
• All GPS receivers have an almanac programmed into their
computers that tells them where in the sky each satellite is,
moment by moment.
• Minor variations in their orbits are measured by the
Department of Defense.
• The error information is sent to the satellites, to be
transmitted along with the timing signals.

41. Position is Based on Time
Signal leaves satellite at
time “T”
T + 3
T
Signal is picked up by the
receiver at time “T + 3”
Distance between satellite and
receiver = “3 times the speed of
light”

42. Pseudo Random Noise Code
Receiver PRN
Satellite PRN
Time
Difference

43. What Time is It?
Zulu Time
Universal Coordinated Time
GPS Time + 13*
Local Time: AM and PM (adjusted for local time
zone)
Military Time
Greenwich Mean Time
(local time on a 24 hour clock)
* GPS Time is ahead of UTC by approximately 13 seconds

44. Signal From One Satellite
The receiver is
somewhere on
this sphere.

45. Signals From Two Satellites

46. Three Satellites (2D Positioning)

47. Triangulating Correct Position

48. Three Dimensional (3D) Positioning

49. GPS Position Determination

50. System Performance
• Standard Positioning
System
– 100 meters horizontal accuracy
– 156 meters vertical accuracy
– Designed for civilian use
– No user fee or restrictions
• Precise Positioning System
– 22 meters horizontal accuracy
– 27.7 meters vertical accuracy
– Designed for military use

51. System Performance
Selective availability
• Intentional degradation of signal
• Controls availability of system’s full capabilities
• Set to zero May 2000
• Reasons
– Enhanced 911 service
– Car navigation
– Adoption of GPS time standard
– Recreation

52. System Performance
• The earth's ionosphere and atmosphere cause
delays in the GPS signal that translate into
position errors.
• Some errors can be factored out using
mathematics and modeling.
• The configuration of the satellites in the sky
can magnify other errors.
• Differential GPS can reduce errors.

53. Differential Correction
• Technique used to correct some of these errors
• Referred to as “differential GPS” or DGPS
• In DGPS, two GPS receivers are used
• One receiver is located at an accurately surveyed point
referred to as the “base station”
• A correction is calculated by comparing the known
location to the location determined by the GPS satellites
• The correction is then applied to the other receiver’s
(known as the “rover”) calculated position

54. DGPS Methods
• Post-processing
– Corrections performed after the data is collected
– Special software required
• Real-time
– Corrections are performed while the data is being
collected
– Need special equipment to receive the DGPS signal

55. Wide Area Augmentation System - WAAS
• New “real-time” DGPS
• Satellite based
• FAA initiative….now fully operational
• Series of ~25 ground reference stations relay info to
master control station
• Master control station sends correction info to WAAS
satellite
– http://gps.faa.gov/programs/waas/howitworks.htm

56. GPS Accuracy Comparison
Some common GPS devices used by FWS:
GPS Device Autonomous WAAS
DGPS
Real-time
DGPS
Post-process
DGPS
Garmin GPSMap 76s ~ 10 - 15 ~3 3 1 - 3
Rockwell – PLGR
Federal Users Only ~ 8 - 15 NA 3 NA
Trimble - GeoXT ~ 10 ~3 1-3 Sub-meter
Accuracy given in meters

57. GPS Accuracy Issues
• Ways to improve the accuracy of your GPS collected data
– Standardize data collection methods
– Establish protocols for your applications
– Employ averaging techniques
– Perform mission planning
– Utilize DGPS
– Understand how the selection of datums and coordinate systems
affect accuracy
• GPS data collected in wrong datum can introduce ~200 meters of error into your
GIS!

58. Application of GPS Technology
• Location - determining a basic position
• Navigation - getting from one location to
another
• Tracking - monitoring the movement of people
and things
• Mapping - creating maps of the world
• Timing - bringing precise timing to the world

60. Application of GPS Technology
• Private and recreation
– Traveling by car
– Hiking, climbing, biking
– Vehicle control
• Mapping, survey, geology
• English Channel Tunnel
• Agriculture
• Aviation
– General and commercial
– Spacecraft
• Maritime

61. GPS Navigation

62. Handheld GPS Receivers
• Garmin eTrex
– ~$100
• Garmin-12
– ~$150
• Casio GPS wristwatch
– ~$300
• The GPS Store

63. Some issues to consider when
purchasing GPS devices
• What is the accuracy level required for your application?
(10 meters or sub-meter)
• How is unit going to be used in field?
– External antenna required, in heavy canopy, ease of use, durability, data
dictionary capability, waterproof…
• Cost…… from $100 to $12K
• Staff expertise..training..support network
• How well does unit interface with GIS?

64. Latest Technology
Mobile mapping software for WindowsCE devices
TerraSync (Trimble)
ArcPad (ESRI)
Multi-path rejection technology
Trimble GeoXT
Bluetooth
Allows for cable free operation

65. ArcPad Software
Bring GIS data into the field!
Custom forms for data collection
Integrate GPS with GIS

66. References
Websites
• GPS Links from IGS: http://igscb.jpl.nasa.gov/overview/links.html
• U.S. Coast Guard Navigation Information Center: http://www.navcen.uscg.mil
• U.S. Department of Transportation: http://www.dot.gov
• NIMA Satellite Geodesy: http://164.214.2.59/GandG/sathtml/
• UNAVCO: http://www.unavco.ucar.edu/
• GPS Environmental & Earth Science Information System: http://genesis/html/index.shtml
• GPS Joint Program Office http://gps.laafb.af.mil/
Books:
• Institute of Navigation, Global Positioning System, Vol. I, Papers published in NAVIGATION,
ISBN: 0-936406-00-3, 1980 (Spilker, Van Dierendonck, etc.
• American Institute of Aeronautics and Astronautics (AIAA), Global Positioning System: Theory and
Applications, Volume I & II, Progress in Astronautics and Aeronautics ISBN or Order Number: 1-
56347-107-8 , 1996
• Kleusberg, A. P. Teunissen,ed. GPS for Geodesy, Lecture Notes in Earth Sciences, Springer-
Verlag, ISBN 3-540-60785-4, 1996
• Springer, T.A., “Modeling and Validating Orbits and Clocks Using the Global Positioning System”,
Doctoral Thesis, University of Bern, Switzerland, November 1999

67. 67
U.S. Augmentations
Nationwide Differential GPS Wide Area Augmentation System
Continuously Operating Reference Stations Local Area Augmentation System

68. Galileo EU/ESA GPS
USA
GLONASS
Russia
Global Navigation Satellite
Systems
GNSS
Accuracy 10m or better
Compass
China
Planned
India, Japan, Korea

69. Space Based Augmentation Systems
Improves GNSS accuracy to 3 metres

70. GNSS
Errors
Canyon Effect – 1 metre
Ionospheric & Tropospheric
diffraction 10 + 1 metres
Part copied from http://www.kowoma.de/en/gps/errors.htm
Timimg errors 4m –
Rounding errors 1m
Geometry up to 100m Orbits up to 5m