Introduction to GPS

1
Introduction to GPS
1957 Russian Sputnik 1 viability of artificial
satellites
April 1960 1st US Doppler Prototype launched -
Transit
Early 1960s Russian Cicada similar to Transit
Early 1970s US DOD developed GPS (Global
Positioning System)
1988 Transit Launch Program Ended
1996 Transit System was retired
Intro
2
Overview of GPS
- December 8, 1993 Official IOC (Initial
Operation Capability)
- 24 Operational Satellites - 4 SV in 6 Orbital
Planes
- 4 to10 GPS Satellites visible anywhere in the
world (Elevation Mask 10)
- SV Orbits are Elliptical - inclination of 55
degrees to the equator
- 20,000km above the Earths service - Orbital
period 11hrs 58min
- July 17, 1995 was declared FOC (full operation
capability)
1.1
3
GPS Segments
- Space Segment (see sections 1.3 - 1.4)
24-Satellite constellation (atomic clocks) Signal
transmits 2 sine waves known as carrier
frequencies (modulations) 2 digital codes and a
navigation message (SV coords)
- Control Segment (see section 1.5)
World Wide Network of Tracking Stations (S-band
uploads) MCS (master control station) US
Schriever Air Force Base (Colorado Springs,
CO) Monitor predict SV locations, system
integrity, behavior of SV atomic clocks,
atmospheric data, SV almanac, and other
conditions
- User Segment
All Military and Civilian users can determine
their position GPS signal receiver from anywhere
in the world World wide system with no direct
charge
1.2
4
GPS Satellite Generations
- Block I Satellites 11sv - none (Feb. 1978 -
Oct. 1985)
Mainly for Experimental purposes Inclination
plane 63 degrees - 5 yr life plan (some 10 years)
- Block II/IIA Satellites 28sv-16 (Feb. 1989 -
Nov. 1997)
II-IIA increase from 14 180 days navigation
message storage Inclination plane 55 degrees
7.5 yr life plan (exceeded by most) Selective
Availability (SA) and Anti-spoofing added for
national security
- Block IIR/IIR-M 12R 1 of 8M (Jan. 17, 1997
Present)
Backward compatible with II/IIA transparent to
end user new codes 7.5 yr life plan Higher
accuracy expectation improved antenna/power 180
day autonomous operation inter-satellite
ranging capabilities
- Block IIF 0 of 33sv 12yr life- plan increase
accuracy (launch 2007)
- Block III ??sv sub-meter level (Launch
2013-2030)
1.3
5
Current GPS Satellite Constellation
- 29 SV Current constellation (as of January 2006)
0 - Block I 1 - Block II 15 - Block IIA 12 -
Block IIR 1 - Block IIR-M
- 6 Orbital planes labeled A through F (4-5 SV
per plane)
- Ground Control can change any orbital position
- Identification system (SVN) or (PRN)
- 4 onboard atomic clocks ( 2 cesium and 2
rubidium)
- PRN05 and PRN06 equipped with corner cube
reflectors
1.4
6
Control Sites
- Master Control Station
Schriever Air Force Base near Colorado Spring,
Colorado Central Processing facility of the
control segment Manned at all times to monitor
system integrity, health, outage
- World wide network of tracking (monitor)
Stations
Original 5 (MCS, Hawaii, Kwajalein, Diego Garcia,
and Ascension Islands) Uploads from (Kwajalein,
Diego Garcia, Ascension Islands, and Cape
Canaveral) New National Geo-spatial Intelligence
Agency Monitor Stations All tracking stations are
unmanned and operated remotely from MCS
- Ground Antennas (S-Band link)
Satellite navigation data, SV positions, SV Clock
Parameters, atmospheric data, SV almanac, other
1.5
7
GPS The Basic Idea
- Microwave Radio Signal
2 carriers 2 codes (digital codes help calculate
initial pseudoranges) A navigation message that
contains the SV coordinates
- The intersection of 3 spheres (3 distances)
A Fourth SV is needed to account for receiver
clock errors
- Accuracy Selective Availability (SA)
With SA 100m H 156m V 340 ns Without
SA increased accuracy of 7-10 times
- Doppler Shift for tracking Velocity Atitude
1.6
8
GPS Levels of Service
Precise Positioning Service (PPS)
Most precise autonomous positioning and Timing
services Transmitted GPS P(Y) code Authorized
users only (US military government agencies 22m
H 27.7m V 200ns at 95 probability Actual Field
measurements are more accurate Offers higher
jamming resistance and signal anti-spoofing
Standard Positioning Service (SPS)
Less precise than PPS - Civil GPS code
C/A-code 100m H 156m V at 95 probability with
SA active 13m H 22m V 40ns at 95 probability
with SA deactivated
1.7
9
Why Use GPS?
The GPS revolution of Surveying Mapping
Limited only by ones imagination
Surveying Mapping
GPS has been found to be cost effective
process 50 Cost Reduction GPS has been found to
be time saving 75 Time Savings Does not require
intervisibility Alternate methods in Urban canyons
Many applications of GPS increase with
Modernization
1.8
10
References
MCS Tracking Station (Air Force) - Hawaii Track
ing Station (Air Force) - Ascension
Island Tracking Station (Air Force) Ground
Antenna Diego Garcia Tracking Station (Air
Force) Ground Antenna Kwajalein Tracking Station
(Air Force) Ground Antenna Washington,
D.C. Tracking Station (NGS) - Cape
Canaveral - Ground Antenna Ecuador Tracking
Station (Air Force) - Argentina Tracking
Station (Air Force) - England Tracking Station
(Air Force) - Bahrain Tracking Station (NGS)
- Australia Tracking Station (NGS) -
Ref