- Easton, Richard D. and Eric F. Frazier.
GPS Declassified.
Lincoln, NE: Potomac Books, 2013.
ISBN 978-1-61234-408-9.
-
At the dawn of the space age, as the United States planned to launch its
Vanguard
satellites during the
International
Geophysical Year (1957–1958), the need to track the orbit of
the satellites became apparent. Optical and radar tracking were considered
(and eventually used for various applications), but for the first very
small satellites would have been difficult. The Naval Research Laboratory
proposed a system,
Minitrack,
which would use the radio beacon of the satellite, received
by multiple ground stations on the Earth, which by
interferometry would
determine the position and velocity of a satellite with great precision. For
the scheme to work, a “fence” of receiving stations would
have to be laid out which the satellite would regularly cross in its orbit,
the positions of each of the receiving stations would have to be known
very accurately, and clocks at all of the receiving stations would have to
be precisely synchronised with a master clock at the control station which
calculated the satellite's orbit.
The technical challenges were overcome, and Minitrack stations were placed
into operation at locations within the United States and as far flung
as Cuba, Panama, Ecuador, Peru, Chile, Australia, and in the Caribbean.
Although designed to track the U.S. Vanguard satellites, after the unexpected
launch of Sputnik, receivers were hastily modified to receive the frequency
on which it transmitted its beeps, and the system successfully proved itself
tracking the first Earth satellite. Minitrack was used to track subsequent
U.S. and Soviet satellites until it was supplanted in 1962 by the more
capable
Spacecraft
Tracking and Data Acquisition Network.
An important part of creative engineering is discovering that once you've
solved one problem, you may now have the tools at hand to address
other tasks, sometimes more important that the one which motivated the
development of the enabling technologies in the first place. It didn't
take long for a group of engineers at the Naval Research Laboratory (NRL) to
realise that if you could determine the precise position and velocity of a satellite
in orbit by receiving signals simultaneously at multiple stations on the
ground with precisely-synchronised clocks, you could invert the problem and,
by receiving signals from multiple satellites in known orbits, each with an
accurate and synchronised clock on board, it would be possible to determine
the position, altitude, and velocity of the receiver on or above the Earth
(and, in addition, provide a precise time signal). With a sufficiently
extensive constellation of satellites, precision navigation and time signals
could be extended to the entire planet. This was the genesis of the
Global
Positioning System (GPS) which has become a ubiquitous part of our
lives today.
At the start, this concept was “exploratory engineering”: envisioning
what could be done (violating no known law of physics) if and when
technology advanced to a stage which permitted it. The timing
accuracy required for precision navigation could be achieved by
atomic clocks
(quartz frequency standards were insufficiently stable and subject to
drift due to temperature, pressure, and age of the crystal), but
in the 1950s and early '60s, atomic clocks were large, heavy, and
delicate laboratory apparatus which nobody imagined could be put
on top of a rocket and shot into Earth orbit. Just launching
single satellites into low Earth orbit was a challenge, with
dramatic launch failures and in-orbit malfunctions all too common.
The thought of operating a constellation of dozens of satellites in
precisely-specified high orbits seemed like science fiction. And
even if the satellites with atomic clocks could somehow be launched,
the radio technology to receive the faint signals from space and
computation required to extract position and velocity information
from the signal was something which might take a room full of
equipment: hardly practical for a large aircraft or even a small
ship.
But the funny thing about an exponentially growing technology is
if something seems completely infeasible today, just wait a few
years. Often, it will move from impossible to difficult to
practical for limited applications to something in everybody's
pocket. So it has been with GPS, as this excellent book recounts.
In 1964, engineers at NRL (including author Easton's father,
Roger L. Easton)
proposed a system called
Timation,
in which miniaturised and ruggedised atomic clocks on board
satellites would provide time signals which could be used
for navigation on land, sea, and air. After ground
based tests and using aircraft to simulate the satellite signal,
in 1967 the Timation I satellite was launched to demonstrate
the operation of an atomic clock in orbit and use of its signals
on the ground. With a single satellite in a relatively low orbit,
the satellite would only be visible from a given location for
thirteen minutes at a time, but this was sufficient to demonstrate
the feasibility of the concept.
As the Timation concept was evolving (a second satellite test was
launched in 1969, demonstrating improved accuracy), it was not
without competition. The U.S. had long been operating the
LORAN system for
coarse-grained marine and aircraft navigation, and had beacons
marking airways across the country. Starting in 1964, the U.S. Navy's
Transit
satellite navigation system (which used a Doppler measurement system
and did not require a precise clock on the satellites) provided
periodic position fixes for Navy submarines and surface ships, but
was inadequate for aircraft navigation. In the search for a more capable
system, Timation competed with an Air Force proposal for regional
satellite constellations including geosynchronous and inclined
elliptical orbit satellites.
The development of GPS began in earnest in 1973, with the Air Force
designated as the lead service. This project launch occurred in the
midst of an inter-service rivalry over navigation systems which did not
abate with the official launch of the project. Indeed, even in retrospect,
participants in the program dispute how much the eventually deployed
system owes to its various precursors. Throughout the 1970s the design
of the system was refined and pathfinder technology development missions
launched, with the first launch of an experimental satellite in February
1978. One satellite is a stunt, but by 1985 a constellation of 10
experimental satellites were in orbit, allowing the performance of the
system to be evaluated, constellation management tools to be
developed and tested, and receiver hardware to be checked out. Starting
in 1989 operational satellites began to be launched, but it was not until
1993 that worldwide, round-the clock coverage was available, and the
high-precision military signal was not declared operational until 1995.
Even though GPS coverage was spotty and not continuous, GPS played an
important part in the first Gulf War of 1990–1991. Because the
military had lagged in procuring GPS receivers for the troops, large numbers
of commercial GPS units were purchased and pressed into service for
navigating in the desert. A few GPS-guided weapons were used in the
conflict, but their importance was insignificant compared to other
precision-guided munitions.
Prior to May 2000 the civilian GPS signal was deliberately degraded in
accuracy (can't allow the taxpayers who paid for it to have the same
quality of navigation as costumed minions of the state!) This
so-called “selective availability” was finally discontinued, making
GPS practical for vehicle and non-precision air navigation. GPS units
began to appear on the consumer market, and like other electronic
gadgets got smaller, lighter, less expensive, and more capable with
every passing year. Adoption of GPS for tracking of fleets of trucks,
marine navigation, and aircraft use became widespread.
Now that GPS is commonplace and hundreds of millions of people are
walking around with GPS receivers in their smartphones, there is a great
deal of misunderstanding about precisely what GPS entails. GPS—the Global
Positioning System—is precisely that: a system which allows anybody
with a compatible receiver and a view of the sky which allows them to see
four or more satellites to determine their
state vector
(latitude, longitude, and altitude, plus velocity in each of those three
directions) in a specified co-ordinate system (where much additional complexity
lurks, which I'll gloss over here), along with the precise time of the measurement.
That's all it does. GPS is entirely passive: the GPS receiver sends
nothing back to the satellite, and hence the satellite system is able to
accommodate an unlimited number of GPS receivers simultaneously. There is
no such thing as a “GPS tracker” which can monitor the position
of something via satellite. Trackers use GPS to determine their position,
but then report the position by other means (for example, the mobile phone
network). When people speak of “their GPS” giving directions,
GPS is only telling them where they are and where they're going at each
instant. All the rest: map display, turn-by-turn directions, etc. is a
“big data” application running either locally on the GPS
receiver or using resources in the “cloud”: GPS itself plays
no part in this (and shouldn't be blamed when “your GPS” sends
you the wrong way down a one-way street).
So successful has GPS been, and so deeply has it become embedded in our
technological society and economy, that there are legitimate worries about
such a system being under the sole control of the U.S. Air Force which
could, if ordered, shut down the civilian GPS signals worldwide or
regionally (because of the altitude of the satellites, fine-grained
denial of GPS availability would not be possible). Also, the U.S. does
not have the best record of maintaining vital infrastructure and has
often depended upon weather satellites well beyond their expected lifetimes
due to budget crunches. Consequently, other players have entered the
global positioning market, with the
Soviet/Russian GLONASS,
European Galileo,
and Chinese BeiDou
systems operational or under construction. Other countries, including
Japan, India, and Iran, are said to be developing their own regional
navigation systems. So far, cooperation among these operators has been
relatively smooth, reducing the likelihood of interference and making
it possible for future receivers to use multiple constellations for better
coverage and precision.
This is a comprehensive history of navigation systems and GPS from inception
to the present day, with a look into the future. Extensive source
citations are given (almost 40% of the book is end notes), and in the
Kindle edition the notes, Web documents cited within
them, and the index are all properly linked. There are abundant technical
details about the design and operation of the system, but the book is
entirely accessible to the intelligent layman. In the lifetimes of all but
the youngest people on Earth, GPS has transformed our world into a place
where nobody need ever be lost. We are just beginning to see the ramifications
of this technology on the economy and how we live our day-to-day lives (for
example, the emerging technology of self-driving cars would be impossible
without GPS). This book is an essential history of how this technology
came to be, how it works, and where it may be going in the future.
July 2015 