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?
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
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
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?
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
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
25. Basic GPS Surveying Techniques
Presented by:
Neil Gray, Teacher-in-Charge, Columboola EEC.
On behalf of ICT Innovators Centre, STiS Project.
26.
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.
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
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)
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
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
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/
