By Ed Troy, President, AEROSPACE CONSULTING
Editor's Note:This article is reprinted from "Spread Spectrum Scene," December 1993
Introduction
This article represents the first of a new series of articles on the Global Positioning System
(GPS). GPS represents what is probably the first truly widespread use of spread spectrum
technology in a high volume product.
While GPS is certainly not new (my first involvement with GPS was back in the late 1970's),
it is only now getting widespread coverage in the general media. The purpose of this series is to
familiarize the reader with the operation, characteristics, and capabilities of the GPS service. In
addition, depending on how much equipment I can coerce vendors and manufacturers to loan to me
for evaluation, both for my upcoming book and for this column, I will be reviewing the offerings
of various manufacturers. Although the primary focus of this column will be strictly on GPS
equipment, I am looking for various types of spread spectrum devices to review in my upcoming
book, which will focus on GPS, but that will also cover spread spectrum technology in a broad
sense.
Personally, I am an engineering consultant with my own consulting firm, Aerospace
Consulting. Currently, I am writing a book on GPS and spread spectrum for Artech House. This
book should be available in the early summer of 1994. I am also currently looking to develop GPS
applications for clients.
History of Navigation
Ever since man first left his home in search of food, wealth, or whatever, he has needed a
way to find his way back home. He also needed a way to repeat his trip to a favored trading place,
or a prime hunting location. In the earliest days, this was done by simply following landmarks,
either along the road, or landmarks that consisted of prominent features on the coastline. This
method of navigation was obviously very limited when it comes to traveling across oceans, or even
large bodies of water. Navigation by reference to the sun, moon, and the stars was the next logical
progression. Techniques related to celestial navigation have improved over the last couple of
thousand years to the point where now it is possible to locate a position to an accuracy of better than
one hundred feet. However, the instruments required to measure latitude and longitude to an
accuracy of one hundred feet or so are definitely not suited to the rapid, dynamic measurements that
are necessary for navigation.
Navigation remained little changed from the late 1700's to the early 1900's. The primary
instrument was the sextant and a highly accurate chronometer, coupled with a compass and
something to measure the distance traveled since the last "fix". The problem with this technique,
obviously, is that you can only fix your position with any real degree of accuracy when the sun or
stars are visible. During cloudy weather, no reliable means of locating one's position existed.
Furthermore, although a compass can tell you in which direction you are headed, it can not tell you
in what direction you are actually traveling. These two directions could be somewhat different,
depending on conditions of wind, waves, and currents. Thus, dead reckoning is often not a very
satisfactory method of navigating for airplanes and ships.
In the early 1900's, electronic means of navigation were developed. These included such
systems as radio direction beacons. In these systems, the direction to the transmitter could be
determined with an antenna with a cardioid pattern. This type of pattern has a very sharp null in one
direction. By pointing this null at the transmitter, the direction to the transmitter could be
determined. By determining the direction to two or more transmitters, the user could determine his
position from the intersection of the lines pointing to the transmitters. Of course, for this to work
the user must know the precise location of the transmitter, and he must be within reception range.
Furthermore, since what is being measured is an angle, the precision of the observer's location
becomes progressively worse as the distance from the transmitters increases.
In a similar way, the introduction of radar in the 1940's allowed the user to measure both
direction and distance to a prominent landmark simultaneously. This simplified the navigation
problem somewhat, but radar had the limitations of generally working over a relatively short range
-- usually not much longer than over the horizon for the determination of landmarks -- which limited
its use to night navigation, or navigation during times when visible landmarks were obscured by fog
or heavy precipitation.
Better systems for long range navigation were based on time measurements. Since now the
characteristic that is being measured is the time of travel or, more specifically, the difference in
travel time, of radio waves, the errors tend to be independent of the distance between the observer
and the transmitting locations. This is a big advantage over angular based systems, where the error
increases rapidly as distance from the transmitting locations increases. Of course, there are some
distortions that get introduced into the time based systems that are based on distance -- such as
atmospheric and ionospheric disturbances -- but these are relatively minor compared to the
uncertainties introduced by angular measurement over large distances. These time based systems
include DECCA, LORAN, and OMEGA.
The first widespread use ofsatellites for navigation (and surveying) was the Transit system.
This is a dual frequency system that operates at 150 and 400 MHz. It determines position by
measuring the Doppler shift of radio signals from a satellite as it passes within range of the observer.
Since the location and orbital path of the satellite is known with a high degree of precision, it is
possible to determine the location of the observer by observing the time-varying Doppler shift of
the two transmitted frequencies. It is currently in widespread use to determine the location of ships
at sea and to determine the position of an observer where other precision navigation systems are not
available. This system can provide position fixes as good as about 30 to 100 meters. This offers
good precision in non-dynamic situations, but position fixes are only available when the satellites
are in view, which, at best, is perhaps once every hour.
GPS, unlike any of these other systems, offers a real-time position fix with an accuracy of
3 to 100 meters on a 24 hour per day basis. It also offers a way to determine time to a precision of
better than a few hundred nanoseconds almost anywhere on / around the surface of the earth. These
are capabilities that will completely revolutionize many areas of life in the near future.
Basics of GPS
The GPS system consists of a constellation of 24 satellites. While not officially declared
fully operational, for all practical purposes the system is now fully operational. These satellites orbit
the earth at an altitude of about 10,900 miles and at an inclination of 55 degrees. As I will
demonstrate in my next column, this orbit translates to an orbital period of 12 hours. The orbits are
distributed around the earth in such a way that at least 4 satellites are always visible from virtually
any point on the surface of the earth. This provides a means of precisely determining the position
of the user in longitude, latitude, and altitude. The satellites operate at two frequencies, known as
L1 and L2. These two frequencies are 1575.42 MHz and 1227.6 MHz, respectively.
The whole
system operates at a system clock frequency of 10.23 MHz, which is an exact submultiple of the L1
and L2 frequencies. The two transmission frequencies are modulated with a pseudo-random signal
to produce spread spectrum signals. The L1 channel is modulated with both a 1.023 Mbps
pseudo-random code known as the C/A (course/acquisition) code and a 10.23 Mbps PN code known
as the P (precision) code. The L2 channel is only modulated with the P code. The two codes are
considerably different in characteristics. The L1 code repeats every 1023 bits, or every 1
millisecond. The P code, on the other hand, only repeats itself every 267 days. Furthermore, the
P code can be encrypted by the Department of Defense, so as to make it unavailable to civilian
(or unauthorized) users. This limits the best accuracy obtainable by unauthorized users to about 30
meters, while allowing authorized users to achieve accuracies of up to 3 meters. Additionally, the
DOD, at its discretion, can disseminate slightly inaccurate information pertaining to the location of
the satellites, so as to further degrade the accuracy obtainable by unauthorized users to about 100
meters. These accuracy degradation capabilities are important, since hostile nations could use the
information against us in times of war, as the Iraqi government did for positioning SCUD missile
launchers during the Gulf War, according to a February 7, 1991 article in the Wall Street Journal.
As time has gone by, however, more potential applications have been developed for GPS and many
techniques have been developed to augment the accuracy available to unauthorized users.
Techniques like carrier phase tracking and differential GPS can allow users to obtain centimeter
level accuracy, especially in cases where measurements are being made at a fixed location.
However, it is well established that even aircraft positions can be determined to an accuracy of better
than several meters, even in real time. This technique is also being widely used to navigate ships
in narrow channels and along the coast of the U.S., by taking advantage of differential correction
signals that are being transmitted over various navigation beacons by the U.S. Coast Guard.
Other applications include moving map displays in cars and trucks. Attitudes of aircraft and
spacecraft can also be determined with GPS. GPS equipment is currently set-up in the San
Francisco area to allow researchers to measure the amount of shifting in the earth's surface during
the next earthquake. GPS was also recently used to measure the height of Mount Everest and K2.
Forest fire fighters use GPS to define the extent of fires and townships are using GPS equipped vans
to map roads in a small fraction of the time that would be required for conventional surveying tech-
niques.
In Closing
These and other applications, as well as more specifics on the operation of the system, will
be included in future columns. If you have a spread spectrum product that you would like to see
mentioned in my upcoming book on GPS and spread spectrum, please contact me.
References:
1. GPS World, March 1991, Newsfront, p. 18.
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