1. A Basic Introduction to The Global Positioning System (GPS)
~~~~~~~~~~
Rev. Ronald J. Wasowski, C.S.C.
Associate Professor of Environmental Science
University of Portland
Portland, Oregon
13 October 2015

2. An Overview of GPS: Outline
Space segment
Multiple satellites in multiple orbits
Ground segment
Mobile receivers
Base stations
Degraded accuracy
Selective availability SA
Satellite geometry
Receiver vicinity
Atmospheric conditions
Receiver quality
Differential GPS
Marine beacon stations

3. The GPS Space Segment Nominal characteristics
Operated by & for the U.S. Department of Defense
≥ 24 operational spacecraft 95% of the time
≥ 1 spare satellites always in orbit 7 spares now
6 orbital planes + 30° latitude
Pole-to-pole line-of-sight coverage
18,000 km orbital altitude
~ 2 orbits per day
Prolonged line-of-sight visibility
Broadcasts at 3 operational frequencies
< 50 Watts total transmission power
12 year design life
Replacements funded through at least 2016

4. The GPS Satellites: 17 August 2015

5. The GPS Signal Content Three components Send only
Pseudo-random number PRN
Satellite ID numbers
Maximum of 32 ID numbers
Related to launch sequence, not to orbital characteristics
Ephemeris
Satellite status Health
Date & time System-synchronized
Used to determine straight-line distance from satellites
Almanac Updated daily
Satellite location as a function of time
Data for all satellites in the system

6. GPS Signal Processing Dynamic triangulation
Acquire & lock onto multiple spacecraft signals
Minimum of 3 & maximum of 12 satellites at any one time
20 to 200 seconds to successful lock-on
Positional possibilities
> 3 satellites Calculation of latitude + longitude
> 4 satellites Calculation of latitude + longitude + altitude
Latitude/longitude accuracy ~10x better than altitude accuracy
Derivative possibilities
Travel velocity: Speed + direction
Travel distance: Already covered + Remaining to be covered
Waypoints: Pre-set or dynamically set
Always a best-fit calculation

7. Degraded GPS Accuracy Basic possibilities Selective availability SA
Satellite geometry
Receiver vicinity
Atmospheric conditions
Receiver quality

8. GPS Selective Availability
Intentional degradation by the DoD
Latitude/longitude accuracy degradation
Maximum combined degradation < 100 m
Typical combined degradation < 30 m
Altitude accuracy degradation
About 10x latitude/longitude degradation
Degradation amount is randomly changed
Time of degradation change
Amount of degradation change
Purpose of degradation
Reduce locational accuracy for potential enemies
Present status of SA
Removed at Pres. Clinton’s request 1 May 2000

9. GPS Satellite Geometry
Dynamic relationship between satellites
All line-of-sight satellites in same part of the sky
Small satellite-to-satellite distances
Narrow, elongated triangles
Large potential calculation errors
All line-of-sight satellites in different parts of the sky
Large satellite-to-satellite distances
Small potential calculation errors

10. The GPS Satellite Orbits

11. GPS Receiver Vicinity Enclosures Tree cover Obstructions
Vehicles
Auxiliary antenna in an upward facing window or outside
Buildings
Few options for hand-held GIS receivers
Tree cover
Reduce/eliminate satellite fix & position calculation
Increased time to achieve a satellite fix
Obstructions
High-rise buildings
Canyons
Steep mountain slopes with lots of sky obscured
Multipath settings
Reflections from building or rock surfaces
Shortest path is the direct path

12. GIS and Atmospheric Conditions
Satellite signal paths
Space No effects
Essentially a vacuum
Ionosphere Primarily electromagnetic effects
Rapidly changing due to solar wind variations, flares…
Troposphere Primarily density effects
Pressure variability
Humidity variability

13. GPS Receiver Quality Clock Software
Set on the basis of satellite signals
Satellite clocks accurate to sec
Runs on a quartz mechanism
Seldom perfectly accurate
Software
GPS receivers are highly specialized computers
Inherent capabilities of the software
Inherent speed of the processor chip

14. Differential GPS Underlying theory of DGPS Basic possibilities
SA is uniform at any one time in any one region
Error is identical for all locations in that region
Determine precise location of a base station
Correct degraded positions accordingly
Basic possibilities
Establish base station networks
Hundreds of kilometers apart, depending on topography
Satellite or FM radio transmissions
Often involves a user fee
Marine Beacon Stations
U.S. & many other governments
Free of charge

15. Three GPS Misconceptions
GPS units transmit data to the satellites
GPS satellites have no ability to receive signals
GPS receivers have no ability to transmit signals
GPS units all do exactly the same thing
Specialized for vehicles, boats, airplanes, hiking …
You get what you pay for
Antenna sensitivity
Computing power
The more satellites, the better, but …
Four satellites provide all necessary data
GPS receiver picks “best of four” satellites
GPS receiver solves a dynamic least-squares equation

16. Russia’s GLONASS System
Russian alternative to the U.S. GPS
Development began in the Soviet Union in 1976
Orbital segment
First launch on 12 October 1982
Constellation completed in 1996
Geopolitical issues
Fell into disrepair following Soviet Union collapse
Fully operational “constellation” of 24 on 2011/10/03
GLONASS available worldwide
Promoted for civilian use
August 2008 South Ossetia war
U.S. GPS shut down regionally

17. GLONASS Details The satellites Design accuracy Orbits
GLONASS Since 1982
GLONASS-M Since 2003
GLONASS-K Since 2011
Design accuracy
Standard precision (SP) signal
Open to all users
5–10 meters horizontal & 15 meters vertical precision
High precision (HP) signal
Restricted to authorized users
Better horizontal & vertical precision
Orbits
19,100 km altitude 11,842 mi
3 orbital planes inclined 64.8° to the equator
Optimized for Northern hemisphere locations

18. GLONASS Satellite

19. GLONASS M Launch: 26 April 2013

20. GLONASS Orbit Constellation

21. China’s Compass Navigation System
Chinese alternative to the U.S. GPS
Orbital segment
Initial constellation of satellites
Medium Earth orbit 30 satellites
Geostationary Earth orbit 5 satellites
Eventual constellation of ≥ 75 satellites
Optimized for “urban canyons”
Compass–M1 launched 14 April 2007
Geopolitical issues
No dependence on the U.S. GPS system

22. Compass System Launches

23. Compass Coverage Area

24. Europe’s Galileo Positioning System
European alternative to the U.S. GPS
Cooperative project of EU & ESA
1999 Germany, France, Italy & United Kingdom
26 May 2003 Agreed upon officially
Current status
First launch 20 October 2011
Initial services ~2014
Completion by ~2019
Orbital segment
Constellation of 30 satellites
23,222 km orbital altitude
3 orbital planes inclined 56° to the equator
Geopolitical issues
Independence from GPS & GLONASS
Potential radio frequency interference with GPS
Decision to use different radio frequencies

25. Galileo Positioning System Tests
Galileo In-Orbit Validation Elements GIOVE
GIOVE-A 28 December 2005
Meets frequency allocation & reservation requirements
GIOVE-B April
Real data used for risk mitigation
GIOVE is an indirect tribute to Galileo Galilei
Discovered Giove’s (Jupiter’s) four largest moons
Used those moons to determine longitude on Earth
Galilean satellites became essentially a universal clock

26. Galileo GIOVE-A Launch

27. Indian Regional Navigation Sat. Sys.
Seven satellites
3 in geostationary orbits
Fixed positions on the equator
34° East, 83° East & 132° East longitude
4 in inclined geosynchronous orbits
Crossings of the equator
Two at 55° East & two at 111° East
Two modes
Special Positioning Service
Precision Service
IRNSS-1D launched on 28 March 2015

28. IRNSS System Architecture

29. IRNSS Geographic Coverage

30. Positioning Satellite Orbits