Transcript Evolution of GPS - Princeton University

Evolution of Military GPS
 Deployment of Sputnik and its Doppler Shift
 Doppler Data and the Birth of the Transit
System and Timation System
 Early GPS
 Today GPS
Richard Stewart
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Sputnik: Learning From Our
Rivals
 In 1957, the Soviets successfully
launched Sputnik into space. It was
the size of a basketball and weighed
about 180 pounds.
 The Soviets’ success embarrassed the
United States who thought they were
the world’s most advanced industrial
nation. As a result of their humiliation,
the country took an active role into
researching military precision.
 The Department of Defenses primary
purposes in developing a Global
Position System was to use it in
precision weapon delivery
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Sputnik: Learning From Our
Rivals
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Studying the Doppler Shift of the Sputnik, the Johns Hopkins
Applied Physics Lab recognized a variance of the Sputnik signal
in comparison of its position. As the fast-moving Sputnik
approached, the scientists noted that signal frequency was
higher than the transmitted frequency. They also observed that
the frequency lowered until the satellite reached its closet point
of approach (CPA), when the observed frequency equaled the
transmitted frequency. The frequency decreased as Sputnik
moved farther away, although the rate of decrease slowed with
increasing distance from the observers. This Doppler shift
phenomenon that the scientists observed was the same one you
experience when an express train rushes by you at a local
station and you hear its roaring sound drop sharply in pitch.
At the end of their experiments, the scientists reasoned that they
could accurately locate their position on earth by carefully
measuring the satellite's Doppler shift and knowing the satellite's
position in orbit at the exact instant of each measurement.
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Doppler Data and the Birth of the Transit
System and Timation System
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Under the direction of Dr. Richard Kirschner, the John
Hopkins Applied Physics Laboratory created the Transit.
Transit was conceived in the late 1950s and deployed in
the mid-1960s
Transit was the first operational satellite-based navigation
system which included seven low-altitude (1075 km) polarorbiting satellites
In Transit
1. a satellite broadcasted radio signals
2.the ground stations tracked the satellite
3. facilities updated satellite orbital parameters
The result- transit users determined their position on earth
by measuring the Doppler shift signals transmitted by the
satellites
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Transit
 Developed to provide accurate navigation data
for Polaris missile submarines and other ships
at the ocean surface
 The way it worked
– Doppler-shifted signals enabled a determination
of the orbit using data recorded at one site
during a single pass of the satellite. So if a
satellite’s orbit was known, a radio receiver’s
position could be determined from the same
Doppler measurements.
– Unlike the Doppler data used to track the
Sputnik satellite, Transit satellites could be
precisely determined by tracking them at widely
spaced fixed sites.
 A satellite had a 107 min. period of revolutionunder favorable conditions accuracy was
between 35 and 100m.
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Positives and Negatives of Transit
 Positives
– Proof that a space system could offer excellent
positioning reliability
– Was available to civilian users in 1967 including
commercial marine navigators and personal crafts
– Created technologies
 Negatives
– Large gaps in coverage
 unavailable coverage measuring up to several hours
 To compensate, users had to interpolate (to estimate a value of a function
between two known values) their positions between passes.
 Users had to compensate for their velocity
 Weather conditions could disrupt signal
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Negatives cont.
– The process was slow and required long
observation time
– Only provided two-dimensional positioning
– Transit was not efficient for the more high
velocity technology (aircrafts, missiles, etc.)
being developed by the military
 Nevertheless, the Transit system continued
to be used until 1996 after about 33 years of
service.
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Timation
 Timation was a spacedbased navigation system
technology program that
was developed in 1964 by
the Navy.
 Advanced the
development of the
stability of clocks (time
precision), time-transfer,
and two-dimensional
navigation.
 1967- First Timation
satellite launched
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Timation Models
 The first satellite used a very stable quartzcrystal oscillators
 Later models incorporated the first atomic
frequency stability
– Atomic clocks have better frequency stability
than earlier clocks=better prediction of satellite
orbits
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Timation Contribution
 Development in the atomic clock for satellite
navigation
 The last two Timation satellites were used as
GPS prototypes
Navigation Technology
Satellite 1 (3rd satellite)
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System 621B
 Around the same time the Navy was developing
Timation, the Air Force worked with a design
concept using a similar technology program called
System 621B.
 Biggest Improvement- 3 dimensional navigation!!!
(latitude, longitude, and altitude)
 By 1972, the system was using a certain type of
satellite ranging signal called pseudorandom noise
(PRN)
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System 621B
 System originated at the Aerospace
Corporation in 1963 developed for precise
positioning of aircrafts
 By Oct. 1963, System 621B was successful
enough to get Air Force supporting and
funding
 Around 1972, ran a series or aircraft tests in
New Mexico using a ground and air
transmitters to simulate satellites…result…
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White Sands Proving Ground in New
Mexico
 Pseudorandom noise pinpointed
the positions of aircrafts to within
a hundredth of a mile= 16 meters
 PRN technique had the capability
to reject interference noise
including jamming or deliberate
interference. PRN techniques
allow all satellites to transit on
same frequency, therefore a
communication channel could be
added permitting user to receiver
data (satellite location and clock
information.
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Military Division
 By the late 1960s, the U.S. Navy, Air Force, and
Army worked independently on radio navigation
systems that would provide all-weather, 24-hour
coverage, and accuracies that would enhance the
military capabilities of their respective forces.
 John Hopkins Applied Physics Laboratory-Transit
 Naval Research Laboratory-Timation
 Army-SECOR (Sequential Correlation of Range)
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Military Unity
 1968-Department of Defense steps in creates a
joint tri-service committee called the NAVSEG
(Navigation Satellite Executive Group)
 What NAVSEG had to determine?
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–
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How many satellites?
What altitude?
What would be the signal codes
What would be the modulation techniques
Cost
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Early GPS
 September 1973, a system was developed
combining early Navy and Air Force
programs called NAVSTAR GPS
– System merged System 621B and Timation
technology
 Satellites orbits were based on Timation but would
have a higher altitude.
 Signal structure and frequencies were based on
System 621B
 Satellites would use atomic clocks
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3-Phase Development of the
NAVSTAR GPS
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(1974-1979)- $100 million program
– First two NAVSTAR satellites were
refurbished Timation satellites
carrying the first atomic clocks ever
launched into space
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(1978-1985)- Rockwell International
– Built a total of eleven Block I
satellites launched on the Atlas-F
booster.
– Design life was only 3 years but
many last 10+ years
 6th Block I satellite carried a nuclear
explosion detection sensor which
was launched on April 36, 1980
– Block I satellites were being tested
for on aircraft, helicopter, ships,
trucks, jeeps, and even by men
using 25-pound backpacks.
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2nd Stage of GPS
 Funding cut!!!- Secretary of Defense cut
program by 30% ($500 million)
 Result– Number of satellites were cut down: 24-18 (plus
3 on-orbit spares)
– Development of Block II satellites were dropped
 Increased the interest in the program by
stressing that GPS could increase bombing
accuracy
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Block II Satellites
 With a renew interest by
the Secretary of Defense,
Block II satellites were
developed and launched in
Feb. 1989
 Operational by April
1989/created Rockwell
International
– Improvements
 Full selective availability/
advanced system securities
 Improved reliability and
survivability
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3rd Phase
 Launch of the Block II/IIA/IIR and finally IIR
combining up to 33 satellites (including spares) to
consist of our new GPS. GPS today uses 24
satellites and numerous ground stations
LAND
SEA
AIR
Able to get quick accurate readings no matter the weather
condition, time of day, or velocity
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Bibliography
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Steven R. Strom Charting a Course Toward Global Navigation
http://www.aero.org/publications/crosslink/summer2002/01.html
History of GPS http://www.bugclub.org/beginners/history/gps.html
Mary Bellis Global Positioning System - GPS
http://inventors.about.com/library/inventors/blgps.htm
Robert A. Nelson The Global Positioning System Via Satellite, November
1999 http://www.aticourses.com/global_positioning_system.htm
Sam Wormley’s DGPS Resources Differential GPS (DGPS) http://www.eduobservatory.org/gps/dgps.html
J.A. Murray and R.L. Beard Space Navigation and Time 1985 http://ncstwww.nrl.navy.mil/NCSTOrigin/Timation.html
Bradford Parkinson and Ronald Beard A History of Satellite Navigation 1995
http://ncst-www.nrl.navy.mil/NCSTOrigin/Timation.html
Navigation Satellites & GPS http://www.vectorsite.net/ttgps.html
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