GPS surveying

1. Global Positioning Satellite System
(GPS)
- Prakash Kumar Sekar

2. Overview
 The Navigation Problem
 Earlier Approaches
 GPS Description
 How does GPS Work?

3. Acknowledgement
Many of the graphical images used in this
talk are courtesy of Peter H. Dana of the
Department of Geography, University of
Texas at Austin -
http://www.utexas.edu/depts/grg/gcraft/
notes/gps/gps.html
The best web site for GPS

4. The Navigation Problem
 The ancient question:
Where am I?
 Earth coordinates:
latitude and longitude
Lafayette: N40/W86
 Latitude can be
determined by Sun
angle
 What about longitude?

5. Latitude and Longitude

6. Latitude and Longitude

7. Longitude Problem
 No easy way to determine longitude
 On July 8, 1714 the Longitude Act was
established in England to solve the
“longitude problem”
 Two solutions were proposed
-- use of stars and moons
-- the “time” solution

8. Longitude
Longitude : The True Story of a Lone
Genius Who Solved the Greatest Scientific
Problem of His Time
Dava Sobel

9. Longitude Problem: References
 http://www.pbs.org/wgbh/nova/longitude/

10. The “Time” Solution
 Where am I? <=> What time is it in
Greenwich, England?
 The ability to find one’s position is based
on how well one can tell what time is it?
 The development of the chronometer
 To find longitude to within 0.5 degree
requires a clock that loses or gains no more
than 3 seconds/day

11. Longitude
 How does this work?
 The earth turns 360 degrees in 24 hours:
15 degrees = 1 hour
 If you know the time in Greenwich when it
is local noon at your location one can find
your longitude relative to Greenwich
 Must know “datum” reference to use maps

12. Datum Reference
Lone Pine Cemetery - N400 13.8’’ E260 17.24”

13. Satellite Navigation
 US Department of Defense has need for
very precise navigation
 In 1973, the US Air Force proposed a new
system for navigation using satellites
 The system is known as: Navigation System
with Timing and Ranging: Global
Positioning System or NAVSTAR GPS

14. NAVSTAR GPS Goals
 What time is it?
 What is my position (including attitude)?
 What is my velocity?
 Other Goals:
- What is the local time?
- When is sunrise and sunset?
- What is the distance between two points?
- What is my estimated time arrival?

15. GSP System
Simply stated: The GPS satellites are nothing
more than a set of clocks in the sky

16. GPS Segments
 Space Segment: the constellation of
satellites
 Control Segment: control the satellites
 User Segment: users with receivers

17. Space Segment

18. Space Segment
 System consists of 24 satellites in the
operational mode: 21 in use and 3 spares
3 other satellites are used for testing
 Altitude: 20,200 Km with periods of 12 hr.
 Current Satellites: Block IIR- $25,000,000
2000 KG
 Hydrogen Maser Atomic Clocks

19. Hydrogen Maser Clock
These clocks lose one second every
2,739,000 million years

20. GPS Orbits

21. GPS Orbits

22. Control Segment
Master Control Station is located at the
Consolidated Space Operations Center
(CSOC) at Flacon Air Force Station near
Colorado Springs

23. The Global Positioning System.
GPS antenna
GPS receiver
and batteries!
Windows CE handheld computer
Paper Map
with targets
Matt Evans of Abe591a and
Aaron Pierce of the Biology
Department; mapping the
Purdue University Ross
Reserve.
Sample Location
Flag/Pin
ASM 215 April
2009

24. User component
 Individuals with GPS
receivers
Prakash Kumar Sekar

25. Basic GPS Surveying Techniques
Presented by:
Neil Gray, Teacher-in-Charge, Columboola EEC.
On behalf of ICT Innovators Centre, STiS Project.

27. Not sphere but spheroid
 Newton and others in the 17th and 18th century
proposed that the Earth is flattened due to rotational
forces.
 Complex, repeated, highly accurate measurements
established that the curvature of the Earth was greater
at the equator than the poles
Image from ESRI online course
Prakash Kumar Sekar

28. Push the “Page” button until you get the screen which
displays location.
They will be set to display degrees, minutes and decimal
seconds.

29. Combining range measurements

30. Range measurement from coded
signal

31. Topographic assessment at Davis Purdue
Ag Center

32. Control Segment

33. Differential
correction
• Post-processed
• Real-time

34. Differential Correction
 Two receivers are used to
greatly improve the
accuracy of GPS
positional measurements
 Base station at known
location
Prakash Kumar Sekar

35. CSOC
 Track the satellites for orbit and clock
determination
 Time synchronization
 Upload the Navigation Message
 Manage DOA

36. Operational Capabilities
Initial Operational Capability - December 8,
1993
Full Operational Capability declared by the
Secretary of Defense at 00:01 hours on
July 17, 1995

37. GPS Transmitted Signal
 Two signals are transmitted on carriers:
L1 = 1575.42 MHz
L2 = 1227.60 MHz
These are derived from the system clock of
10.23 MHz (phase quadrature)
 Modulation used is Direct Sequence Spread
Spectrum
(code division multiple access - CDMA)

38. GPS Signals

39. GPS Clock Signals
 Two types of clock signals are transmitted
 C/A Code - Coarse/Acquisition Code
available for civilian use on L1 provides
300 m resolution
 P Code - Precise Code on L1 and L2 used
by the military provides 3m resolution

40. Spread Spectrum
 Spread Spectrum is used because
- resistance to jamming
- masks the transmissions
- resist multipath effects
- multiple access
 All 24 GPS satellites transmit on the same
two frequencies BUT use a different ID
sequence

41. GPS Signals
 The satellites transmit as part of their
unique Spread Spectrum signal a clock or
timing signal
 The range or distance to the satellite is
obtained by measuring how long it takes for
the transmitted signal to reach the receiver
 This is not the “true” range due to clock
errors - what is obtained is know as the
“pseudo-range”

42. GPS Position
 By knowing how far one is from three
satellites one can ideally find their 3D
coordinates
 To correct for clock errors one needs to
receive four satellites

43. GPS: How does it work?
 Typical receiver: one channel C/A code on
L1
 During the “acquisition” time you are
receiving the navigation message also on L1
 The receiver then reads the timing
information and computes the “pseudo-
ranges”
 The pseudo-ranges are then corrected

44. GPS: How does it work?
 Corrected ranges are used to compute the
position
 This is a very complicated iterative
nonlinear equation

45. Navigation Message
 To compute your position one must know
the position of the satellite
 Navigation Message - transmitted on both
L1 and L2 at 50 bits/s for 30 s
 Navigation message consists of two parts:
- satellite almanac
- clock bias

46. Why Do I Need
To See 4 Satellites?
 The problem is that the clock signal from
the satellite is corrupted by atmospheric
refraction
 Another major problem is that the receiver’s
clock is not very accurate
 For a 2D fix <=> 3 satellites

47. Why Do I Need
To See 4 Satellites?

48. Denial of Accuracy (DOA)
 The US military uses two approaches to
prohibit use of the full resolution of the
system
 Selective Availability (SA) - noise is added
to the clock signal and the navigation
message has “lies” in it
 Anti-Spoofing (AS) - P-code is encrypted
 The military sometimes turns off both DOA
techniques

49. Differential GPS
 Used to improve accuracy
 Put a “satellite” on the ground at a precise
position
 Differential signal is not “transmitted” on
standard satellite frequencies

50. Uses of GPS
 Airplane and Boat Navigation
 Continental Drift
 Surveying
 Precise Timing
 Iceberg Tracking
 Archaeological Expeditions
 Mobile Multimedia

51. Applications of GPS

52. GPS Clock Rollover
 GPS System Time rolled over at midnight
21-22 August 1999, 132 days before the
Year 2000
 On 22 August 1999, unless repaired, many
GPS receivers claimed that it is 6 January
1980
 http://www.navcen.uscg.mil/gps/geninfo/
y2k/gpsweek.htm

53. Conclusion
 GPS will find more civilian uses
 DOD has promised to eliminate SA
 Russia has a system known as GLONASS
 The EU is discussing deploying its own
system

54. References
 B. Hofmann-Wellenhof, H. Lichtenegger,
and J. Collins, GPS: Theory and Practice,
Third Edition, Springer-Verlag, 1994.
 T. Logsdon, The Navstar Global
Positioning System, Van Nostrand, 1992.
 A. Leick, GPS Satellite Surveying, Second
edition, Wiley, 1995.

55. References
 T. A. Herring, "The Global Positioning
System," Scientific American, pp. 44-50,
February 1996.
 N. J. Hotchkiss, A Comprehensive Guide to
Land Navigation with GPS, Alexis, 1994.
 Special Edition on the Global Positioning
System, Satellite Times, March/April 1996.
 D. Sobel, Longitude, Walker, 1995.

56. Web Sites
 GPS Program Office:
http://www.laafb.af.mil/SMC/CZ/homepage/
 US Coast Guard Navaigation Center
http://www.navcen.uscg.mil/default.htm
 GPS Precise Orbits
http://www.ngs.noaa.gov/GPS/GPS.html
 GPS World Magazine
http://www.gpsworld.com/