1. Global Positioning System Overview
Author: Peter H. Dana
Herman Li
Oct 18, 2004

2. NAVSTAR-2 GPS satellite
What is GPS
GPS = Global Positioning System
Position, velocity, time
Network of min. 24 (29) satellites, orbiting the earth every ~12 hours
Funded and controlled by the US DOD
Cost: 13 billion US
Significant advantage in used in middle east
29 in sky (Saturday), 24 minimum
Pass by same area twice a day
Unofficial: on Dec 8, 1993
Fully operational: on Apr 27, 1995
Outdoor use only
NAVSTAR-2 GPS satellite

3. Space Segment Altitude of 20,000km
Positioned such that 5 – 8 satellites are in range at any time
Coded radio signals from 4 satellites can pinpoint location on earth
6 orbital planes
Used for position, velocity, time
6 orbital planes, 4 SV each, equally spaced (60 deg apart), inclined at ~55 deg wrt equatorial plane
4 satellites locate 3D position

4. Trilateration 1 satellite – a sphere 2 satellites – a circle
3 satellites – 2 points
ECEF XYZ coordinates
Distance = Travel Time x Speed of Light
Trilateration (based on known position + distance) vs triangulation (based on angle + distance)
1,2,3 satellites
Distance formula
Assumed location known
CLICK!! Where are the satellites
Gravatational pull…etc changes orbit
Time inaccurate
Where are the satellites
Time is different in the sky

5. Control Segment 1ns drift / 3 hours 1ns = ~30cm error
5 monitor stations
3 ground antennas
Radars located around the world
GPS broadcasts received ephemeris and clock correction data to receivers
Practically few ns error = ~ 1m
10cm/h, 2.4m/day
Radar uploads ephemeris + clock corrections to GPS, GPS broadcasts

6. Positioning Services Precise Positioning Service (PPS)
22m horizontal, 27.7m vertical accuracy
200ns accuracy
Standard Positioning Service (SPS)
100m horizontal, 156m vertical accuracy (SA included)
340ns accuracy
SA turned off as of May 2, 2000
Have accurate clocks, know where satellites are, need to find the distance
PPS – more accurate (explain later)
SPS – less accurate
SA – degration of SPS in time varying bias (30m – 100m)

7. GPS Satellite Signals Two microwave carrier signals
L1 ( MHz)
L2 ( MHz)
L5 band coming
Speed of light varies
Ionosphere slows down lower frequencies more
Use L1 & L2 to remove ionosphere effects
Difference in arrival time removes error
Moisture in Troposphere also slows down signal
Reason of L1 + L2
L1 + L2 offset ionosphere
Only special receivers has L2

8. GPS Satellite Signals C/A (Coarse/Acquisition) Code
a.k.a Civilian Code
Unique pseudo-random-noise (PRN) code modulated on L1 and repeated every 1ms
P(Y)-Code (Precise Code)
PRN code modulated on both L1 and L2 and repeated every 7 days
Codes used for downloading ephemeris every 30 sec and almanac every 12.5 min
PRN code unique, 32
Know which satellite signal was from
P encrypted with Y = P(Y)
Need special hardware to decrypt
Anti-spoofing = avoid fake transmission
Anti-jamming by using code
Ephemeris = short sections of satellite orbits

9. GPS Data 1.5 kbit frame, 5 subframe, 30 sec
25 pages subframe = almanac + other data
Almanac, predict where any satellite appears (orbital data for all satellites)
Used when starting up unit
Complete SV data set includes ionospheric model
Send offset of GPS time / UTC time
Parity bits provide error detection / limited correction

10. GPS Signals

11. Code Phase Tracking (Navigation)
Uses C/A or P(Y) code
Gets pseudo-range, includes eroror and receiver clock error
Know: prefect clock, distance = travel time X speed of light

12. Clock synchronization
Satellites have 4 atomic clocks
Receivers are cheap, 1us drift / sec
Assuming distance from satellites already known, 4th satellite solves the extra variable
4 atomic clock (2x cesium, and 2x rubidium)
Satellite = accurate clock
Receiver = inaccurate clock

13. Differential Code Phase GPS
Base station (known position) computes corrections for each satellite signal
Corrections sent to rover receivers
Removes errors except multipath and receiver errors
US Coast Guard combats SA

14. Carrier Phase Tracking (Survey)
Requires specially equipped receivers
Track L1 and/or L2 carrier signals
No time of transmission info
Requires differential calculations of receivers within km
Sub-cm accuracy possible

15. Differential GPS DGPS – Differential Code Phase GPS
Instantaneous results, less accurate
Real-time or post-processed
CPD – Carrier Phase Differential
Increased accuracy due to increased frequency
RTK – Real-time Kinematic
ie. Real-time Carrier Phase
Time to determine initial full cycles, accurate
Time + position already discussed
Velocity = change in position over time
Or Doppler frequencies.

16. GPS Error Sources PRN code noise (1m), receiver noise (1m)
Selective availability (no longer the case)
Uncorrected satellite clock error (1m)
Ephemeris data errors (1m)
Tropospheric delays (10m)
Unmodeled ionosphere delays
Multipath (0.5m)
Geometric Dilution of Precision (GDOP)
Bad when angles between receiver and satellites are similar
Bias, Noise, Blunders
Uncorrected satellite clock: 10cm/hr
Human error, hardware failure

17. GDOP Example

18. User Equipment Segment
GPS receivers and user community
Cheap outdoor GPS ~ $180US
Outdoor GPS with map ~ $375US
Personal GPS with street map ~ $590US
Avionics GPS ~ $??
Ability to locate yourself anywhere on the globe: Priceless

19. Now and Beyond WAAS (Wide Area Augmentation System) GLONASS Galileo
FAA + DOT for precision flight approaches
Corrected differential messages broadcast by 2 geostationary satellites
Glonass: horizontal 57 – 70m vertical 70m, 24 satelliltes, 19km, 3 orbits
Galileo: Civil system, 30 satellites, 24km orbit, 3 orbits
GLONASS
Russian Federation’s satellite navigation system (2006)
Galileo
European Union and European Space Agency (2008)

20. References Questions: Position, Time & GPS – confusing
A GPS Tutorial
FAA GPS FAQ
GPS Overview
How GPS Receivers Work
The Fundamentals of GPS
DGPS Explained
USNO GPS Timing Operations
Garmin
Questions:
Position, Time & GPS – confusing
Why time in 3 different position location methods

21. Time Dilation “Net secular relativistic effect is 38.6s per day
Nominal clock rate is MHz
Satellite clocks are offset by – parts in 1010 to compensate effect
Resulting (proper) frequency in orbit is Hz
Observed average rate of satellite clock is same as clock on the geoid” 
“Relativity has become an important practical engineering consideration for modern precise timekeeping systems. These relativistic effects are well understood and have been applied successfully in the GPS.”
summaryrpts/41stmeeting/18%20Nelson%20.PPT