Introduction to GPS

1
Introduction to GPS
2
Global Positioning System

• A military based system designed to provide the
  information necessary to determine locations
  accurately, anywhere in the world

3
U. S. GPS

• NAVigation System Timing And Ranging (NAVSTAR)

Russian GPS
GLObalnaya NAvigatsionnaya Sputnikovaya Sistema
(GLONASS)
4
GPS replaces LORANTRANSIT
LORAN Triangulation method with continuous
position fixes, accurate to 300
meters, Limited coverage,
Lat/Lon
TRANSIT Doppler shift, 16 or less fixes/day,
Sub-meter accuracy in about 3
days, World Wide coverage,
Lat/Lon/Height
5
Global PositioningSystem (GPS)

• Continuous Position Fixes
• Worldwide Coverage
• Lat/Lon/Height
• Centimeter to 5-meter accuracy in seconds

6
GPS System has 3 Segments

• Control Segment
• Space Segment
• User Segment

7
Navstar Control
8
Space Segment Description
24 Satellites 6 planes with 55o orbit Each plane
4 SVs 20,200 km orbit 1 revolution / 12 hrs
Very high orbit for Accuracy Survivability Coverag
e
9
Status of Space Segment

• In development since 1973
• Block I (1978-1986)
• No signal degredation
• Block II/IIA (1989-1995)
• Signal degredation until spring 2000
• SA turned off May 1, 2000
• Block II
• Launched from 1996 to 2008
• Block IIIA satellites expected to begin launch in
  2014

10
Space Segment Satellites
Operational Satellites
11
User Segment (nearly everyone you can think of)
12
Trilateration
A position is calculated by knowing The
location of, and calculating the distance to, a
group of satellites (SVs)
The SVs act as precise reference points
13
Trilateration
One measurement narrows down our position to the
surface of a sphere
14
Trilateration
A second SV narrows our location to the elipse
created by the intersection of two spheres
15
Trilateration
The third SV narrows our location to just two
possible points
The fourth SV will determine which of those two
points is correct
16
A train leaves the station at 9 a.m. traveling 5
miles per hour.
How far from the station is it at 11 a.m.?
10 miles
17
A signal leaves a satellite at 90000.000 a.m.
traveling at 186,000 miles per second.
How far is it from the satellite at 90000.065
12,090 miles.
18
How Do We Know when Signal Left SV?

• Receivers SVs use same Code
• Receiver SV syncronized
• Receiver uses code to determine time since
  transmission

19
GPS Position Solution
(X1-Ux)2(Y1-Uy)2(Z1-Uz)2(SL(T1-Cb))2 (X2-Ux)2(
Y2-Uy)2(Z2-Uz)2(SL(T2-Cb))2 (X3-Ux)2(Y3-Uy)2(Z
3-Uz)2(SL(T3-Cb))2 (X4-Ux)2(Y4-Uy)2(Z4-Uz)2(SL
(T4-Cb))2
Xi, Yi, Zi are coordinates for the ith
satellite Ux, Uy, Uz are rover coordinates to be
determined SL is the speed of light (186,000
miles per second) Ti is the time necessary to
receive a signal from the ith satellite Cb is
the clock bias of the rover
20
Three Methods of Positioning

• Autonomous
• Differential
• Phase Differential

21
Autonomous Positioning

• Receiver is used to collect real-time locations
• No corrections are applied to locations
• Locations may be off by as much as 100 meters if
  SA is active
• Depends on Selective Availability
• How much is Falcon AFB distorting the signals?

22
Differential Positioning

• Corrections are applied to positions after
  returning from field
• Uses data collected simultaneously by a base
  station
• Gives sub-meter to 5 meter accuracy

23
Phase-Differential

• Correction is applied while still in field
• Makes use of additional signals being broadcast
  by a known ground station
• 10-100 centimeters Phase Processing
• 5 millimeters Wave processing

24
Need to Know SV Location

• SV location is transmitted to SV by ground
  control
• SV then relays its location to your receiver
• If a SV fails to maintain proper orbit,
  adjustments are uploaded
• If problem exists, control segment deems the SV
  unhealthy

25
Almanac Messages

• Almanac is a set of parameters used to calculate
  the rough location of each SV
• Almanacs are used for
• Rapid SV acquisitioin
• Pre-mission planning

26
Ephemeris Messages

• Ephemeris is a set of parameters used to
  determine exact location of SV
• Ephemeris used for
• Calculating a GPS position

27
Selective Availability (SA)

• Errors introduced to reduce accuracy
• Discourages hostile use
• Largest source of error
• Sum of two effects
• Epsilon - ephemeris fibbing
• Dither - clock variations

28
How Accurate Is It?

• That Depends on some variables
• Time spent at a geographic Location
• Design of receiver
• Relative position of satellites
• Rover configuration settings
• Correction methods

29
More on Accuracy Later

• Will discuss
• DOPs Dilution of Precision measures
• UERE User Equivalent Ranging Error