The Global Positioning System
(GPS) is a great technological success story. It was developed by
the Department of Defense (DoD) primarily for the U.S. military to
provide precise estimates of position, velocity, and time. Civil use
was a secondary objective. On the basis of national security
considerations, the civil users of GPS have been limited to a
purposefully degraded subset of the signals. Nevertheless, the civil
applications of GPS have grown at an astonishing rate. Applications
unforeseen by the designers of the system are thriving, and many
more are on the way. GPS has found applications in land
transportation, civil aviation, maritime commerce, surveying and
mapping, construction, mining, agriculture, earth sciences, electric
power systems, telecommunications, and outdoor recreational
activities. The civil community has found ways to get around the
purposeful degradation of the signal and to use the military
signals, encryption notwithstanding. The system is being used to
provide accuracy levels which would have been unthinkable 20 years
ago. The commerce in GPS-related products and services has grown
rapidly in the 1990's. The U.S. Department of Commerce estimates
that the annual worldwide sales will reach $8 billion in the year
2000, and could exceed $16 billion by 2003. GPS is on its way to
become a part of our daily lives as an essential element of the
commercial and public infrastructure.
This Special Issue of this PROCEEDINGS offers a survey of the GPS
technology and some of its civil applications. We begin this essay
with a short introduction to GPS: the system, signals, and
performance. The objective is to provide the necessary background
and to introduce the basic concepts and vocabulary needed for the
papers which follow.
GPS is not the first satellite navigation system. The first
operational system was fielded by the U.S. Navy in 1964, and was
named the Navy Navigation Satellite System [1]. This system, better
known as Transit, was based on a novel concept discovered in the
late 1950's following the launch of the Soviet Sputnik: measurements
of Doppler shift in the signals broadcast by a satellite in a known,
well-defined orbit could be used to estimate one's position. The
system was realized with five satellites in low-altitude (1100-km),
nearly circular, polar orbits. Each satellite broadcast narrowband
signals at 150 MHz and 400 MHz. Only one satellite was in view at a
time, and a user waited up to 100 min between successive satellite
passes to determine position. After a satellite came in view, it
took 10-15 min to compute two-dimensional position of a stationary
or slow-moving user. The system was used by the U.S. submarine fleet
to update a ship's position and reset the inertial navigation
system. Transit saw a limited civil use in the maritime industry and
geodesy starting in 1967, and it was decommissioned in 1996.
Unlike Transit, GPS is based on an idea that is both very simple
and quite ancient: one's position, e.g., coordinates (x, y,
z), can be determined given distances to objects whose
positions are known. The situation can be modeled using
high-school-level analytic geometry. Each measured distance can be
related to the three unknown position coordinates by an equation.
Given the distance measurements to three objects, there are three
such equations which can usually be solved for the three unknowns.
What is novel in GPS is the realization of this idea with the
technology of the late twentieth century in a global navigation
system with capability to provide estimates of position, velocity,
and time to an unlimited number of concurrent users instantaneously,
continuously, and inexpensively. The DoD approved the basic
architecture of the system in 1973, and the first satellite was
launched in 1978. The system was declared operational in 1995. The
cost of development of GPS has been reported to be about $10
billion; the annual operation and maintenance cost is estimated to
be between $250 million and $500 million. The history of development
of GPS is recounted in [1] and [2]. A comprehensive treatment of the
system, signals, performance, and applications can be found in [3].
In the realization of GPS based on the idea described above, the
satellites, which broadcast their positions, are the objects at
known locations. The distance between the user and a satellite is
measured in terms of transit time of the signal from the satellite
to the user. Indeed, the development of precise, ultrastable clocks
and stable space platforms in predictable orbits are the two key
technologies which have made GPS possible. The GPS satellites are
moving in space at a speed of about 4 km/s, but the position of each
at any instant can be estimated with an error no worse than a few
meters based on predictions made 24-48 h earlier. The clocks, or
more appropriately, frequency references, are carried aboard the
satellites and used to generate signals with precise and
synchronized timing marks. Each satellite typically carries a pair
each of cesium and rubidium atomic standards. The frequency
stability of these clocks over a day is about one part in
1014 and one part in 1013,
respectively. There are about 105 s in a day,
and an error of 1-10 ns can accumulate over a day, if left
uncorrected. Actually, the satellite clocks are maintained in
synchronism by monitoring the signals from a network of tracking
stations, estimating the correction parameters for each clock, and
uploading new parameter values to update the navigation message
broadcast by each satellite.
In order to measure the true transit time of a signal from a
satellite to a receiver, clearly, the clocks in the satellite and
the receiver must be maintained in synchronism. Fortunately, this
onerous requirement is easily sidestepped, allowing use of
inexpensive quartz oscillators in the receivers. The bias in the
receiver clock at the instant of the measurements affects the
observed transit times for all satellites equally. The corresponding
measured ranges are thus all too short, or too long, by a common
amount, and they are called pseudoranges. The receiver clock bias
thus becomes the fourth unknown to be estimated, in addition to the
three coordinates of position. A user, therefore, needs a minimum of
four satellites in view to estimate his four-dimensional position:
three coordinates of spatial position plus time. An idealized
geometrical view of the pseudorange measurements and the resulting
equations to be solved for the user position and receiver clock bias
are given in Fig. 1.
We should note that GPS is not the only system of its kind. A
similar system, called GLONASS, was developed by the Soviet Union.
GLONASS, like GPS, was designed primarily for the military, and a
subset of its signals was offered for civil use apparently as an
afterthought in the era of perestroika. Since the
dissolution of the Soviet Union, the responsibility for GLONASS has
been assumed by the Russian Federation. The system had a full
constellation of 24 prototype satellites broadcasting briefly in
early 1996, but it has since declined [4]. No satellites were
launched between January 1996 and October 1998, and the
constellation has been down to 14 working satellites since August
1997. The civil user community sees great benefits in having access
to signals from two autonomous systems, and the absence of
purposeful signal degradation in GLONASS is particularly appealing.
However, general uncertainty about the future of the system appears
to have limited the demand and discouraged the receiver
manufacturers from producing GLONASS receivers. There is no basic
challenge in design and manufacture of combined GPS-GLONASS
receivers, and some models are now available.
GPS is the first passive, one-way ranging satellite system to
become operational. GLONASS may follow in a few years. Other systems
and satellite-based augmentations to be fielded by governments,
international consortia, and commercial interests are expected to
follow. The generic name given to these systems is Global Navigation
Satellite System (GNSS). The European Commission, European Space
Agency, and Eurocontrol are collaborating on studies to develop a
system, named GNSS-2, which would be entirely under civil control.
Commercial ventures would choose to exploit the full potential of a
satellite system to provide communication, navigation, and
surveillance services. Indeed, Boeing recently proposed a system of
16 satellites in medium-Earth-orbit to provide such service. There
are other proposals for systems with low-Earth orbit (LEO) and
geosynchronous satellites. For the next five years, however, GPS is
expected to remain the only operational system of its kind. Changes,
however, are coming to GPS. In about ten years, GPS is expected to
have expanded considerably capabilities for both military and civil
users, as discussed in Section VI.
I. GPS OBJECTIVES AND POLICIES
The principal objective of the DoD in developing GPS was to offer
the U.S. military accurate estimates of position, velocity, and time
(PVT). In quantitative terms, this statement was interpreted broadly
as providing estimates with position error of 10 m, velocity error
of 0.1 m/s, and time error of 100 ns, all in the root-mean-square
(rms) sense. These estimates were to be available to an unlimited
number of users all over the globe continuously and nearly
instantaneously. The planned military use also required the system
to be usable on high-dynamics platforms, and the signals to have a
measure of resistance to jamming and interference. Finally, the
adversaries of the United States were to be denied the full benefits
of the system.
The civil users of GPS were to be provided with a ``reasonable''
accuracy consistent with the national security considerations. The
initial view in the 1970's was to limit the accuracy of the position
estimates to 500 m. Subsequent considerations allowed greater
accuracy, with horizontal position error generally below 100 m.
These considerations were formalized in a policy to offer two kinds
of services: Standard Positioning Service (SPS) for open,
unrestricted civil use, and Precision Positioning Service (PPS) for
the DoD-authorized users.
Access to the full capability of the system (i.e., PPS) is
restricted by encryption in two ways. First, the signals available
for unrestricted use are purposefully degraded under a policy called
Selective Availability (SA). The signals are generally degraded by
``dithering'' the satellite clock and, therefore, the timing marks
on the ranging signals. Another mechanism to degrade the
performance, though apparently it is not used often, is to broadcast
erroneous or imprecise values of the satellite ephemeris parameters
in the navigation message. Such degradation can be undone by the
DoD-authorized users. Actually, SA can be undone by the civil users,
too, but it requires additional expense, as discussed below. As
might have been expected, SA made few friends among the civil users,
and its scheduled departure in the next few years would be welcome.
The second feature to limit access to signals is via encryption of
the ranging code, referred to as anti-spoofing (A-S). The aim of A-S
is to deny an adversary opportunity to generate and broadcast a GPS
signal with the aim of spoofing the receiver. SA has been active
nearly continuously since 1990, and A-S since 1994.
The policy for civil use of GPS was first announced by the DoD in
the late 1970's with an assurance that the SPS signals will remain
freely available. In 1983, Korean Airline Flight 007 went off course
into the Soviet airspace, apparently due to navigation problems, and
was shot down. This disaster drew attention to the potential
benefits of GPS to civil aviation, and the U.S. policy on civil use
of GPS was reaffirmed by President Reagan. At the time, the system
was ten years away from being declared operational. Subsequently,
the U.S. made a formal commitment in 1991 to the International Civil
Aviation Organization (ICAO), a specialized agency of the United
Nations, to make ``GPS-SPS available for the foreseeable future on a
continuous, worldwide basis, and free of direct user fees.'' While
free signals and growing markets for GPS products and services
created general enthusiasm in the civil sector worldwide, the
governments, particularly in Europe, remained less sanguine about
becoming dependent upon a system controlled by the military of a
foreign government. There has been general reluctance to dismantle
the existing terrestrial navaids and invest in the GPS-based
infrastructure. There is a widely held view that technology as basic
and vital as GPS requires an international institutional framework
for development of policies, regulations, and standards.
The U.S. policy on GPS is based on balancing the basic
requirement of retaining the military advantage of this technology
with considerations of commercial and international policy. An
interesting account of the issues and considerations is found in the
report of a study conducted in 1994-1995 at the direction of the
U.S. Congress by the National Academy of Public Administration
(NAPA) and the National Research Council (NRC) of the National
Academy of Sciences and Engineering [5]. This report has been
influential in shaping the evolution of both GPS and the U.S. policy
on its use. The report concluded that SA was not serving its
intended purpose and recommended that it be discontinued
immediately. The report also recommended several measures on
governance of GPS to achieve the national goals and to promote its
international acceptance. The Presidential Decision Directive (PDD)
of 1996 on GPS appears to have adopted most of the recommendations
of the NAPA-NRC report. According to the PDD, one of the U.S. policy
objectives is to promote integration of GPS into peaceful civil,
commercial, and scientific applications worldwide, and to advocate
acceptance of GPS as an international standard. Accordingly, the
Directive assigned to the DoD a softer role as ``stewards'' of the
system, and included civil agencies in policy-making and management
roles. The evolution of the U.S. policy on GPS is discussed by Shaw
et al. in ``The DoD: Stewards of a Global Information
Resource, the Navstar Global Positioning System.''
II. SYSTEM ARCHITECTURE
GPS consists of three segments: the space segment; the control
segment; and the user segment. The space segment comprises the
satellites and the control segment deals with the management of
their operations. The DoD is responsible for both the space and
control segments. The user segment covers activities related to the
development of GPS user equipment and services. The development of
the receivers and services in the civil sector is essentially left
to the market forces. The civil sector, however, clearly benefited
from the investments by the DoD in the 1970's and 1980's in
development of military receivers. This favor is now being returned
as the design of a new generation of GPS military user equipment
benefits from the extraordinary developments over the last ten years
in civil receiver design and manufacturing.
The GPS baseline satellite constellation comprises 24 satellites
fielded in nearly circular orbits with a radius of 26 560 km, period
of nearly 12 h, and stationary ground tracks. The constellation is
shown in Fig. 2. The satellites are arranged in six orbital
planes inclined at 55
relative to the
equatorial plane, with four satellites distributed somewhat unevenly
in each orbit. With this constellation, almost all users with a
clear view of the sky have a minimum of four satellites in view. It
is more likely that a user would see six to eight satellites. The
satellites broadcast ranging signals and a navigation message
allowing the users to measure their pseudoranges and to estimate
their positions in passive, listen-only mode.
An initial batch of ten prototype or developmental satellites,
called Block I satellites, was launched between 1978 and 1985 and
used to demonstrate the feasibility of GPS. The prototypes were
followed by production models named Block II, Block IIA, and Block
IIR, each successive batch designed with higher capabilities, longer
service lives, and, it is worth noting, lower price tags. Fisher and
Ghassemi survey the capabilities of these satellites and discuss
Block IIF satellites, now being designed for launch starting in
2002, in their paper ``GPS IIF--The Next Generation.''
The U.S. Air Force (USAF) Space Command is responsible for
planning, acquisition, and launch of the satellites, as well as
general maintenance of the system. The USAF also manages the
tracking of the satellites from five monitoring sites spread around
the globe in longitude (Ascension Island, Diego Garcia, Kwajalein,
Hawaii, and Colorado Springs) for orbital prediction and health
indicators. Three of these monitoring stations, Ascension Island,
Diego Garcia, and Kwajalein, also have the communication capability
to upload (via S-band radio links) data to be broadcast by the
satellites. The operations are managed by the Master Control Station
located at the Schriever (formerly named Falcon) Air Force Base near
Colorado Springs, CO.
GPS requires a precisely defined global terrestrial reference
frame in which to express the positions. Clearly, this reference
frame would have to be fixed to Earth so that coordinates of a
stationary point would remain fixed. GPS uses an Earth-centered,
Earth-fixed Cartesian coordinate frame defined as a part of the
DoD's World Geodetic System 1984 (WGS 84), developed by the former
Defense Mapping Agency (now a part of the National Imagery and
Mapping Agency). The satellite positions and, therefore, the user
positions are expressed in this coordinate frame. The Cartesian
coordinates, though convenient for calculations, are not practical
for representations on maps, which historically have used
curvilinear coordinates: latitude; longitude; and height above a
reference surface. Such coordinates are defined in WGS 84 relative
to a reference ellipsoid. Before the advent of satellite navigation,
many national and regional coordinate frames, called geodetic
datums, had been defined using terrestrial surveying techniques.
Maps based on these datums and various map projections remain in use
worldwide, occasionally creating problems for an unsuspecting GPS
user--the coordinates of a point in WGS 84 and a local datum can
differ by hundreds of meters. Positions obtained from GPS can be
converted into a local datum with an appropriate transformation, and
the military receivers provide for over 100 such transformations. In
time, these diverse national and regional datums would be abandoned
in favor of WGS 84 and its refinements, and that in itself would be
a great service of GPS.
Precise measurement of time and time interval is at the heart of
GPS. GPS time is an atomic time, uniform and continuous, defined on
the basis of a set of cesium atomic clocks included in the control
segment. GPS time is similar to Coordinated Universal Time (UTC),
but the discontinuities in the UTC time scale introduced by leap
seconds are sidestepped for simplicity of operation. Between 1980
and 1998, 12 such leap seconds were introduced in UTC. The GPS time
is specified to be maintained within 1 µs of UTC(USNO), the
UTC as maintained by the U.S. Naval Observatory, not counting the
leap seconds. In 1998, the GPS time differed from the UTC(USNO) by
less than 10 ns. The clocks aboard the satellites are kept
synchronized to GPS time. The synchronization is accomplished by
estimating the time offset, drift, and drift rate of each satellite
clock relative to GPS time, and transmitting the parameters of a
model of this bias as a part of the satellite's navigation message.
It is to the credit of the DoD that GPS has performed extremely
well since it was declared operational, and glitches due to
equipment malfunction or operational lapses have been rare.
III. SIGNALS AND MEASUREMENTS
Each GPS satellite transmits continuously at two frequencies in
the L band: 1575.42 MHz (L1) and 1227.6 MHz (L2). The signal
intended for unrestricted use is broadcast by each satellite at L1,
and it is modulated by a pseudorandom noise (PRN) code called
coarse/acquisition (C/A) code. Each C/A-code is a length-1.023 Gold
code with a chipping rate of 1023 Mchips/s (Mcps), and null-to-null
bandwidth of 2.046 MHz. Each satellite also broadcasts a pair of
signals for the DoD-authorized users, one at L1 in phase quadrature
with the civil signal, and the other at L2, as shown schematically
in Fig. 3. Access to these signals is controlled by
encrypting the PRN codes. When not encrypted (now rare), these
signals are referred to as P codes; the encrypted codes are called
Y-codes. In either case, the chipping rate is 10.23 Mcps, ten times
that of the C/A-codes, and a period of one week. The null-to-null
bandwidth of this signal is 20.46 MHz, ten times that for a C/A
code. Altogether there are three signals: C/A code at L1 intended
for the SPS users, and P(Y) codes at both L1 and L2 primarily for
the PPS users.
Each signal is made up of three elements: a carrier; a unique PRN
spread spectrum code; and a binary data message. Each element of the
signal is derived coherently from a single clock aboard the
satellite. The structure of the signal available for civil use is
shown in Fig. 4. It consists of the L1 carrier; a unique
1023-bit-long C/A code, which repeats each millisecond; and a 50
bits/s navigation message organized in frames and subframes and
containing data on the satellite orbit, clock, health, and other
parameters. The carrier is modulated by the code and the navigation
message using binary phase shift keying (BPSK).
The codes (C/A and P) transmitted by the GPS satellites were
chosen for their auto- and cross-correlation properties. The
autocorrelation function in each case has a sharp peak for zero
shift. For a C/A code, for example, the autocorrelation function is
more than 24-dB lower for all shifts greater than one chip width. A
GPS receiver can measure the code phase with submeter precision by
tracking the peak of the autocorrelation function. The various codes
used are also nearly uncorrelated: cross correlation between two
C/A-codes for any time shift is at least 24 dB lower than the peak
of the autocorrelation function of each. This near-orthogonality of
the codes allows all satellites to broadcast at the same two
frequencies via code division multiple access (CDMA) without
interfering with each other. This signal design also offers a
measure of protection against multipath for the reflected signals
which are delayed by more than 1.5 chip width. Finally, the
spread-spectrum nature of the signals provides a processing gain
against interference of about 43 dB for a C/A code and 53 dB for a
P/Y-code.
The GPS signals received on Earth are extremely weak. The RF
power at the antenna input port of a satellite is about 50 W, of
which about half is allocated to the civil signal. The satellite
antenna is designed to spread the RF signal roughly evenly over the
surface of Earth below. The specifications on the minimum received
power level for the users on Earth are -160 dBW for the
C/A-code, -163 dBW for P(Y)-code at L1, and -166
dBW for P(Y)-code at L2. These signal powers are well below the
receiver noise level. The low signal power is widely seen as the
Achilles' heel of GPS, especially in military use. Even in civil
use, there is concern about the vulnerability of the signals as the
national commercial and public infrastructure comes to rely more and
more on GPS. Frequency diversity and increased signal power are
being considered under modernization plans to make the GPS service
more robust.
The signal acquisition process essentially consists of ``tuning''
to each satellite in view. The receiver attempts to acquire the
known C/A-code with an initial time uncertainty of 1023 code chips
and frequency uncertainty of up to 5 KHz due to Doppler shift.
Acquisition of P(Y)-code, if the user is authorized, is based on the
coherence of the C/A- and P(Y)-codes at the satellite in two steps:
the receiver first acquires the C/A-code and then acquires the
P(Y)-code with the aid of the timing information in the data
message. Direct acquisition of a P(Y)-code is difficult by design
due to the length of the code and would require an ultra-precise
clock and/or thousands of parallel correlators in a receiver. A
receiver makes the following measurements: pseudorange based on code
phase measurements; Doppler shift in carrier frequency
characterizing the rate of change in pseudorange; and carrier phase
measured relative to the phase of the signal generated by the
receiver clock, also used to measure change in pseudorange.
The measurement of a pseudorange is conceptually quite simple, as
shown in Fig. 4, and it is based on tracking the sharp peak of the
autocorrelation function. The PRN code transmitted by each satellite
is known to the receiver, which generates a replica of it. The delay
between this code replica and the signal received from the satellite
is the apparent transit time of the signal. Basically, the receiver
slides the code replica in time until it matches the code received
from the satellite. This process of correlating the received signal
with the receiver-generated replica gives the apparent transit time
of the signal modulo 1 ms. Multiplying the apparent transit time by
the speed of light gives pseudorange. Pseudoranges measured from
four (or preferably more) satellites are used to compute position. A
GPS receiver does this automatically, continuously, and virtually
instantaneously.
Doppler shift and carrier phase measurements are formed in the
carrier tracking loop. The Doppler shift, caused by the relative
motion of a satellite and the user, is the projection of the
relative velocity on the line of sight and can be converted into
pseudorange rate. Given the pseudorange rates corresponding to four
satellites and the satellite velocity vectors (transmitted in the
navigation message), a user can compute his velocity. Braasch and
Van Dierendonck discuss the formation of these measurements in their
paper ``GPS Receiver Architectures and Measurements.'' How these
measurements are used in estimation of position and velocity is
discussed by Misra et al. in ``GPS Performance in
Navigation.''
IV. ESTIMATES OF PVT
The quality of PVT estimates obtained by a user from GPS depends
basically upon two factors. The first is the number of satellites in
view and their spatial distribution relative to the user, referred
to as satellite geometry. The satellite geometry changes with time
as the satellites rise, move across the sky, and set. Roughly
speaking, the geometry is good if the satellites are on all sides of
the user and offer good coverage in azimuth and elevation. If a
significant part of the sky is somehow blocked, the user may still
be able to compute PVT estimates if four or more satellites are in
view, but there would be an accuracy penalty for the poor geometry.
The second factor determining the quality of the PVT estimates is
the quality of the measurements obtained from GPS. There are several
sources of random and systematic errors which affect the
measurements from GPS: uncompensated error in the clocks in the
satellites; accuracy of the predicted satellite positions; unmodeled
propagation delays in the ionosphere and the troposphere; multipath;
and receiver noise. For the SPS users, currently the largest source
of measurement error is due to the purposeful degradation of the
signal in accordance with SA. The rms error in the pseudorange
measurements due to SA is estimated to be about 25 m. Ionospheric
propagation delay can be large too, ranging from several meters to
several tens of meters, depending upon the state of the ionosphere
and the elevation of the satellite. This error, however, can be
removed substantially by a user equipped with a dual-frequency
receiver.
The performance achievable from GPS is dynamic, changing both
with time and place as the satellite geometry and measurement errors
change. A global characterization of the performance is given in
statistical terms, e.g., as rms error or various percentiles of the
error distribution. The GPS performance specifications for
positioning and time are given in Table 1 as ninety-fifth percentiles of errors for users
worldwide [5], [6]. There are no specifications for velocity
estimates. The PPS performance specifications essentially show the
limits of the current system due to the size of the GPS satellite
constellation and its deployment, and the error sources inherent to
the measurements. The difference between the PPS and SPS
specifications is mostly due to SA. The actual SPS performance in
recent years has consistently exceeded these specifications.
Accuracy is the simplest and most commonly used performance
measure of a navigation system, but not the only one. In
safety-critical applications, it is essential that a user be assured
that the system is operating within design tolerances and that the
position estimates derived from it can be trusted to be within
specifications. This is the so-called integrity requirement. Civil
aviation, for example, requires that a pilot be warned in a timely
manner of a navigation system anomaly which may create a hazard.
Such ``time to alarm'' varies with the phase of flight, and ranges
from 30 s for an aircraft in en route phase to 2 s for precision
landing. At present GPS does not have the capability to detect a
system anomaly and reset the appropriate parameters in the
navigation message of a satellite quickly enough to meet such
requirements, and the responsibility for integrity monitoring is
left to the user. Such integrity monitoring is usually based on
redundant measurements, or system augmentations, which are discussed
later.
Availability and continuity of service are also important
performance measures of a GPS-based service. Consider, for example,
a hypothetical application with accuracy and integrity requirements
which are met only when eight or more satellites are in view. Such
service would be available from GPS less than one-third of the time.
The continuity of service is vital in civil aviation which requires
that once an airplane embarks upon a precision approach, the service
must remain available for the duration of the approach, typically
about 1-min long.
V. GPS AUGMENTATIONS AND DIFFERENTIAL GPS
As noted previously, the quality of the position estimates from
GPS depends upon the geometry of the satellite constellation and the
error sources inherent to the measurements. It turns out that the
errors can be mitigated simply. It was recognized early that the
errors associated with the GPS measurements are similar for users
located ``not far'' from each other, and they change ``slowly'' in
time. In other words, the errors are correlated both spatially and
temporally. The errors in the measurements of two users separated by
tens of kilometers are generally similar. The errors also change
slowly over time: the error due to SA, the largest source of error
in SPS, is highly correlated over 5-10 s; the error due to
atmospheric propagation delays changes more slowly over minutes and
tens of minutes. Clearly, error in a measurement can be estimated if
the receiver location is known. These error estimates computed at a
reference receiver, if made available to other GPS users in the
area, would allow them to mitigate errors in their measurements. Of
course, to be usable for navigation, such ``differential
corrections'' have to be transmitted in real time over a radio link.
That is differential GPS, generally abbreviated as dGPS or DGPS.
DGPS can provide meter-level and even submeter-level position
estimates depending upon the closeness of the user to a reference
station and the latency of the corrections transmitted over the
radio link. Such performance can meet the requirements of much of
land transportation and maritime traffic; dGPS services, both
commercial and federally provided, are now widely available. The
commercial services provide differential corrections via
communication satellites and FM subcarrier, and use of such services
is now common for offshore oil exploration and fleet management. The
U.S. Coast Guard (USCG) provides the differential corrections for
free on marine radiobeacon frequencies (285-325 KHz) from about 50
broadcast sites in the coastal areas and around Great Lakes and
inland waterways in the conterminous United States (CONUS), and in
parts of Hawaii, Alaska, and Puerto Rico. This service, called
Maritime dGPS, provides accuracy of several meters at a distance of
100 km or more from a reference station. Several countries are now
implementing similar systems to enhance maritime safety in their
waterways. The success of Maritime dGPS service has led to a plan in
the United States for Nationwide dGPS to be completed in the next
few years. This service would essentially extend the network of
maritime radiobeacons nationwide, benefiting operations of
railroads, agriculture, environment, forestry, and emergency
response.
The Federal Aviation Administration (FAA) is developing GPS
augmentations to meet the needs of civil aviation. The first such
initiative, the Wide Area Augmentation System (WAAS), currently is
being implemented and would be used operationally starting in 1999.
WAAS would augment GPS with GPS-like signals transmitted from
geostationary satellites. These signals would be modulated with data
messages so that a user in the U.S. airspace would have additional
ranging signals and would also receive satellite integrity data and
differential corrections to the measurements. WAAS is planned to
meet the accuracy, integrity, and continuity requirements of en
route and terminal phases of flight and nonprecision and Category I
precision approaches. Europe and Japan are developing similar
geostationary augmentations of GPS signals. Another FAA initiative
called the Local Area Augmentation System (LAAS) would soon offer
capability for Category I precision approaches at airports not
served by WAAS. LAAS is being designed to support Category III
precision approaches, including autolandings.
DGPS compensates for errors that are common to the measurements
at the reference and user stations. One source of error that is
unique to each antenna/receiver and its environment is multipath:
interference due to reception at the antenna of a direct signal from
the satellite, and one or more reflections from the ground or
structures in the area. Multipath cannot be mitigated in
differential mode and can be a significant source of error in
applications requiring precise positioning. An approach being
examined is to use antennas that exhibit a measure of immunity
against multipath. Counselman discusses the design of such an
antenna in his paper ``Multipath-Rejecting GPS Antennas.''
VI. CIVIL APPLICATIONS
The success of GPS in large-scale civil use is attributable
almost entirely to the revolution in integrated circuits, which has
made the receivers compact, light, and an order of magnitude less
expensive than thought possible 20 years ago. The first
geodetic-quality receivers, introduced in the mid-1980's, provided
code and carrier phase measurements from four to six satellites and
were priced at over $100 000. Receivers with much higher
capabilities are now available for less than $10 000. As late as
1980, it had only been hoped that the receiver manufacturers would
be able to produce a basic GPS receiver for the mass market for
about $2000. Price barriers, however, fell quickly. An important
industry milestone was reached in 1992 with the introduction of the
first hand-held receiver priced below $1000. In 1997, the industry
breached the $100 barrier with a pocket-size receiver running on two
AA batteries. There are now hundreds of GPS receiver models on the
market.
Knowledge of one's precise three-dimensional position by itself
is only of academic interest for most users. But the position
information can be invaluable if given in relation to the intended
path, showing points of interest and potential hazards: a hiker's
position in relation to a trail; a car on a moving street map; or a
boat in relation to islands and obstacles. Combined with
communication technology, say, a cellular phone, the knowledge of
position can be life saving, reducing search and rescue to simply
rescue. The full power of GPS would be realized in civil
applications in combination with other technologies, especially
communication systems and geographic data bases.
The civil applications of GPS may be divided roughly into: 1)
high-precision (millimeter-to-centimeter level) positioning; 2)
specialized applications such as aviation and space; 3) land
transportation and maritime uses; and 4) consumer products. While
the positioning capability of GPS receives most attention among the
civil users, GPS is also a global source of precise time. The use of
GPS as a timing service is growing rapidly in the electric power and
telecommunication industries. We review these applications briefly
below and introduce the remaining papers in this issue, which offer
detailed treatments of five application areas.
The community of geodesists and geophysicists appears to have
been the most surprising and unintended beneficiary of GPS. The
geodesists have adopted techniques developed previously for radio
astronomy to achieve millimeter-level accuracy in relative
positioning with GPS carrier phase measurements. These techniques
are now being used widely to study tectonic plate motion and crustal
deformation, earthquakes, volcanic processes, ice sheet processes
and postglacial rebound, and variations in Earth's rotation. In
these studies, the answers generally are not needed in real time,
and data collected over hours and days at different locations are
postprocessed to obtain estimates of relative position vectors good
to millimeters. The value of GPS in studies of the dynamics of Earth
has been enhanced greatly by the establishment of the International
GPS Service for Geodynamics (IGS), which coordinates collection and
analysis of GPS measurements recorded continuously at sites around
the world. Herring surveys the role of GPS high-precision
positioning in ``Geodetic Applications of GPS.''
The high-precision positioning capability of GPS is also being
used to monitor deformations of large engineering structures in real
time under actual loads, e.g., bridges and towers under actual
traffic and wind-loading conditions. Such real-time estimates
typically are good to centimeter level. GPS-based precise estimates
of position and attitude of an aircraft are proving invaluable in
airborne surveys and photogrammetry. Atmospheric scientists are
using GPS measurements for precise, real-time characterization of
electron densities in the ionosphere and water vapor content of the
troposphere.
Actually, the market for high-precision GPS receivers for
scientific studies is rather small. The techniques developed in
these studies, however, have become indispensable tools of the
surveyors and mapmakers, creating a large market for high-end GPS
receivers and services. Indeed, the field of surveying has been
revolutionized by GPS with vast improvements in accuracy, speed, and
economy. The convergence of GPS and personal computer technologies
has made it possible to collect vast amounts of positional data and
to organize them into geographic databases. A number of positioning
and attribute collection systems are on the market. At its simplest,
such a system typically consists of a backpack containing a
battery-powered personal computer with a GPS card (and perhaps radio
modem to receive differential corrections for real-time dGPS) and a
hand-held keyboard and display unit. A user thus equipped can walk
around gathering information and entering it through the keyboard to
create or update a spatial database showing, e.g., the location and
status of every utility pole or manhole in a town, or a map showing
concentrations of toxic or radioactive wastes. Such geographic
databases are becoming invaluable tools for monitoring and
management of forests and coastlines.
Until a few years ago, civil aviation relied entirely on
ground-based radionavigation aids. Such aids are expensive to
operate and maintain, and large parts of the world lack even the
basic radionavigation infrastructure. GPS is widely seen as the most
important advance in civil aviation since the advent of
radionavigation, with a potential to enhance greatly both the
economy and safety of air operations. Civil aviation is a demanding
application with stringent requirements on accuracy, integrity, and
continuity of navigation service. In order to ensure that these
requirements would be met by a navigation system, the regulatory
authorities adopt and promulgate avionics standards and
certification criteria. GPS has been certified for use in the U.S.
airspace as a supplemental system for en route, terminal, and
nonprecision approach phases of flight, and as a primary system for
oceanic and remote area operations. A GPS augmentation to support
instrument approach and landing under poor visibility conditions is
described by Enge in ``Local Area Augmentation of GPS for the
Precision Approach of Aircraft.''
GPS receivers aboard satellites in LEO's would become the primary
source for position, velocity, attitude, attitude rate, and time,
replacing an array of sensors and reducing the cost and complexity
of spacecraft. A number of successful experiments and demonstrations
have been carried out in various space missions in the last ten
years, and a few spacecraft have actually integrated GPS
measurements into their operational control systems. The
International Space Station is designed to use GPS for navigation,
attitude determination, tracking of vehicles approaching the
station, and as a source of time for scheduling vehicle operations.
GPS measurements have been used extensively for orbit determination,
both in real time and postprocessing mode, and GPS has been shown to
perform much better than any ground-based tracking system in use
today. On-orbit attitude determination based on GPS has been
demonstrated but not used routinely yet. This would change with the
introduction of a new generation of GPS receivers designed with
hardware and software capabilities for orbital applications. One
such application is considered by Axelrad and Behre, who focus on a
technique for using GPS for coarse attitude determination in
``Satellite Attitude Determination Based on GPS Signal-to-Noise
Ratio.''
The largest current application of GPS is in land transportation,
especially vehicle navigation and tracking. GPS-based systems and
services for motorists, commercial fleets, public transit, and
emergency response agencies are in great demand, and a vast majority
of the GPS receivers manufactured in 1998 were intended for this
market. Rental-car companies now routinely offer GPS-based
navigation and route-guidance systems. High-end cars have
incorporated GPS-based roadside assistance systems and automatic
vehicle location systems as standard features. Railroad companies
are considering GPS for positive train control. In time, GPS-based
services would offer the motorists the best route to office, given
the traffic and road conditions of that day.
At least four satellites are required to be in view for
three-dimensional navigation with GPS, as noted previously. Terrain,
foliage, and buildings, however, can obstruct parts of the sky, and
maintaining four or more satellites in track continuously is often
impractical for land vehicles. A land-vehicle navigation system
must, therefore, supplement GPS with dead-reckoning sensors, e.g.,
gyroscopes, compasses, odometer, inclinometer, and accelerometers.
These sensors cannot provide the absolute position, but they can
measure change in position accurately over a short period. DGPS can
provide absolute position, but only intermittently. The
complementary nature of GPS and the dead-reckoning sensors is
analyzed by Abbott and Powell in ``Land-Vehicle Navigation Using
GPS.'' The authors discuss the impact of the individual navigation
sensors on the performance of a land-vehicle navigation system.
There are millions of pleasure boats, fishing boats, ferries,
cruise lines, cargo lines, and oil tankers in the world. All would
benefit from GPS. Since the oil spill caused by the grounding of
Exxon Valdez, the harbors are exploring active monitoring
of the oil tankers. A promising approach is to transmit dGPS-derived
tanker positions via radio link to a control station monitored by
the harbor authorities. Such automatic dependent surveillance (ADS)
would make the harbor operations safer and more efficient. A similar
concept has been proposed for the surveillance of air traffic.
The consumer market for GPS products is believed to be vast and
is expected to see an explosive growth in the next five to ten
years. At present, this market consists basically of inexpensive
hand-held receivers for hikers, backpackers, and fishermen. The
convergence of wireless communications, Internet, and GPS
technologies is seen as the key to the consumer market. Two-way
messaging devices incorporating GPS technology are on the market now
with which a user can navigate and communicate his or her position,
course, or any other information, to anyone on Earth who has an
e-mail address. When combined with access to databases for services,
this would lead to development and growth of ``location-aware''
services directing a user in an unfamiliar place to a gas station,
Indian restaurant, or a tourist attraction. The consumer market
would be fueled by single-chip GPS receivers costing $10 or less.
Such receivers would be integrated into an array of consumer
products: personal digital assistants, personal communicators, and
security devices for personal possessions ranging from cars to
computers.
GPS is a worldwide resource of unprecedented accuracy and
precision for time and frequency. The national and international
laboratories which serve as standards for time and frequency rely on
GPS as a source of time and for time transfer. An inexpensive GPS
receiver can provide estimates of time with an accuracy heretofore
offered only by atomic standards, and GPS-derived time, therefore,
is a logical choice for recording times of events for scientific
purposes and correlating events recorded at different geographic
locations. GPS has also become an essential element of the
commercial and industrial infrastructure as a source of precise
time. The electric utilities are using GPS to analyze the state of a
power grid via precisely synchronized measurements of the voltage
phasor at different substations. In the future, GPS-synchronized
measurements are expected to become an essential element of power
system control. GPS is also being used increasingly to synchronize
the elements of telecommunications networks, both wireline and
wireless. In fact, GPS is now used to set the clocks at the major
Internet nodes, and the time is passed down to the local-area
networks. Lewandowski et al. survey the revolution in time
transfer technology in ``GPS: Primary Tool of Time Transfer.''
Novel applications of GPS include automatic tracking of oil
spills and flooding with especially fitted buoys, sounding of the
upper atmosphere with radiosondes equipped with GPS receivers, and
tracking of wild animals with special GPS collars for research on
habits and habitats. GPS-guided construction, agriculture, and
mining machinery are on their way. Indeed, the applications appear
limitless.
VII. GPS MODERNIZATION
The world has changed since the time GPS was designed and the
policies on its use by the military and civil users were adopted by
the U.S. Government. Changes in the world political order and
emergence of commerce as a national priority has given the civil
users and the GPS industry in the United States a new clout, which
they have used to push for changes in GPS design and policies. The
discontinuation of SA (the policy of purposeful degradation of the
signals available for civil use) which is scheduled to occur before
2006 in accordance with the PDD of 1996 would mark an important
change in direction for the U.S. policy. With our growing dependence
on GPS, some other concerns have emerged, too. Several
well-publicized cases recently of inadvertent signal interference
and loss of GPS service have brought attention to the vulnerability
of the signals. Indeed, the signals are extremely weak, and the
spread-spectrum processing gain against interference is modest. RF
interference (RFI) constitutes a single point of failure for the
civil users and is a hurdle in the use of GPS for safety-of-life
applications. In recent years, pressures have also grown for
commercial exploitation of the electromagnetic spectrum, especially
for mobile satellite communication. The GPS community was jolted
recently when a satellite communication provider nearly succeeded in
winning approval at the ITU World Radio Conference (WRC) of a
proposal to share a portion of the frequency allocation for GPS at
L1.
It is a measure of success of GPS and its importance to national
security and domestic and foreign policy issues that an ambitious
program of modernization is underway for this system which has been
operational for barely three years. All through 1997 and 1998, the
DoD, DOT (and other civil agencies of the United States), and the
civil user community have conducted intensive discussions and
negotiations on additional capabilities to be built into the future
GPS satellites to serve the civil community better. Of course, the
military has also learned from their experiences with GPS and would
make some changes in the military signals as well. The challenge is
to accommodate the expanding civil use, and its demands for greater
accuracy and robustness, with the military mission of GPS.
Before discussing the planned changes and their implications for
accuracy and robustness of service, let us review the current
capabilities of GPS. As discussed earlier, and as shown in Fig. 5(a), the accuracy of GPS position estimates can
range from tens of meters to centimeters depending upon the
augmentation. With code measurements, real-time position estimates
within tens of meters of the true location are obtained from SPS.
The military users, unencumbered with SA and capable of estimating
the ionospheric effect with dual-frequency measurements, obtain
substantially improved accuracy. To obtain even better performance,
additional investment is required in the form of augmentation to the
GPS signals. With about 30 reference stations distributed over
CONUS, GPS/WAAS would provide meter-level accuracy. Submeter
accuracy would be available locally from GPS/LAAS with a reference
station at an airport. The position of a user relative to a local
reference station may be determined with centimeter-level accuracy
in real time using carrier phase measurements. Using the density of
the required network of reference stations as a simple, qualitative
measure of the cost of service, the diagonal line across Fig. 5(a)
can be interpreted as the cost-performance curve for today's GPS.
Befitting its importance, the announcement on the plans for GPS
modernization was made by Vice President Gore on 30 March 1998. It
is planned to provide two new signals for civil use in addition to
the one available today. The structure of the additional signals is
yet to be decided. The future GPS satellites will transmit a civil
code at L2, in addition to the C/A-code at L1. A third civil signal
will also be added at as yet unspecified frequency. A decision on
the placement and structure of the third civil signal will be made
late in 1998, or early 1999. The frequency diversity would alleviate
concerns over accidental interference and would constitute the key
step toward achieving robustness of service. The civil signal at L1
will remain unchanged, thereby ensuring that all of the fielded GPS
receivers will continue to operate. Two-frequency signaling would
allow civil users to estimate the ionospheric propagation effect
and, in the post-SA era, would offer positioning accuracy of better
than 10 m without any augmentation.
While two frequencies would improve accuracy for the civil users
significantly, the third frequency would bring about an improvement
of an order of magnitude. With three frequencies, the user equipment
can multiply and filter the individual signals to create two beat
frequency signals. The L1 and L2 measurements can be processed to
create a beat frequency signal with a wavelength of approximately 86
cm. Since this wavelength is significantly greater than the
wavelength for either L1 or L2, the corresponding measurement is
called a wide-lane observable. A second wide lane would be generated
by multiplying the third signal with either the L1 or L2 signal.
This pair of wide-lane measurements can be used to solve for the
pseudorange and the ionospheric delay from user to satellite. Unlike
the two-frequency case, these estimates would be accurate to within
a fraction of the wide-lane wavelength, or a few centimeters. Taken
together with the planned improvements in the GPS ephemeris, this
three-frequency technique would yield submeter positioning accuracy
for GPS in real time with a relatively sparse ground infrastructure.
The benefits to the civil users from these changes in the form of
improved accuracy and robustness of service would come slowly over
the next 10-20 years as new satellites with expanded capabilities
are produced and launched to replace the current satellites. A
simple, qualitative view of the benefits is provided in Fig. 5(b) as
cost-performance curves akin to Fig. 5(a) for GPS of today. Fig.
5(b) shows a dramatic reduction in cost for the nominal service with
three-frequency signals: submeter accuracy from the basic satellite
system without any augmentation. Secondly, it envisions a system
which degrades gracefully in stages when faced with RFI. The service
degrades when access to one of the signals is lost. When two of the
signals become unavailable, the cost-performance curve reduces to
that for GPS of today in Fig. 5(a).
The civil users, especially the civil aviation community, would
prefer the third signal in the 960-1215-MHz band currently allocated
for Aeronautical Radio Navigation Service (ARNS) on a primary basis
and protected for safety-of-life applications. Given the pressures
on the electromagnetic spectrum, agreement on frequency allocation
for the new signal would not be easy. The hardest of all, given the
budgetary pressures, would be an agreement on sharing the costs for
these additional capabilities to be built into the new satellites.
VIII. SUMMARY
GPS is widely seen as the most important gift of the DoD to the
civil world, perhaps with the exception of the Internet. (A GPS
system summary is included in Table 2.) Civil applications unforeseen by the
developers of the system are thriving and many more are on the away.
Commerce in GPS equipment and services continues to grow rapidly.
This success has also created expectations, indeed demands, which
the system was not designed to meet. It is expected that the planned
GPS modernization, when complete, would make determining position as
easy as determining precise time is today. This knowledge of
position would come to occupy the same important place in our daily
lives as time does today.
We have assembled ten papers in this Special Issue to introduce
the various aspects of GPS: the satellites; receivers; positioning
algorithms; and several important civil applications. We hope that
the reader will find them useful for understanding the technology
and applications of this new global resource.
Acknowledgement
The Guest Editors are grateful to
their colleagues who served as peer reviewers for the papers in this
issue: Prof. M. Braasch; R. Braff; Dr. C. Comp; Prof. C. C.
Counselman, III; R. L. Ferranti; R. French; G. J. Geier; G. Green;
Dr. C. Hegarty; Dr. E. Krakiwsky; Prof. G. Lachapelle; Prof. K.
Larson; Prof. P. Levin; Dr. G. Lightsey; Dr. P. Loomis; K. D.
McDonald; Dr. C. M. Meertens; Prof. W. Michalson; Dr. A. Niell;
Prof. J. D. Powell; Dr. S. Pullen; B. Schupler; C. Shively; Dr. J.
Studenny; Dr. F. van Diggelen; Prof. F. van Graas; and Dr. T. Yunck.
The Guest Editors are also grateful to J. Calder, N. Romanosky, and
M. Scanlon of this PROCEEDINGS' staff for their support.
References
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About the Authors
Per Enge
received the B.S. degree in electrical engineering from the
University of Massachusetts, Amherst, in 1975. He received the M.S.
and Ph.D. degrees from the University of Illinois, Urbana-Champaign,
in 1979 and 1983, both in electrical engineering. His Ph.D.
dissertation was in the area of spread spectrum multiple-access
communications.
Since 1992, he has been at Stanford University, Stanford, CA,
where he currently is an Associate Professor in the Department of
Aeronautics and Astronautics. At Stanford University he teaches
courses in satellite navigation and control theory. He is also
responsible for the Federal Aviation Administration's program to use
differential operation of the Global Positioning System (GPS) for
navigation during aircraft approach and landing. Specifically, he
investigates and flight tests wide-area differential GPS for
Category I precision approach, and local area differential GPS for
Category I, II, and III precision approach. Currently, both systems
are being deployed for operational use. From 1986 to 1992, he was
with Worcester Polytechnic Institute (WPI), Worcester, MA, where he
achieved the rank of Associate Professor of Electrical Engineering.
At WPI, he taught and directed research for the U.S. Coast Guard.
The research included the design of a medium-frequency (MF) radio
system to broadcast differential GPS corrections to marine users.
Today, this system covers much of the world's coastline and provides
positioning accuracy of one or two meters to hundreds of thousands
of marine users.
Dr. Enge is the recipient of the 1996 Thurlow Award from the
Insitute of Navigation, and he is Executive Vice President of the
same institute.
Pratap
Misra (Senior Member, IEEE) received the B.S. degree from
Indian Institute of Technology, Kanpur, in 1965 and the M.S. degree
from Lehigh University, Bethlehem, PA, in 1967, both in mechanical
engineering. He received the Ph.D. degree in engineering sciences in
1973 from the University of California, San Diego.
Since 1983, he has been with Lincoln Laboratory, Massachusetts
Institute of Technology, Lexington, MA, where he is now a Senior
Staff Member. Since 1989, he has been Program Manager for
FAA-sponsored studies related to application of global navigation
satellite systems to civil aviation. During 1983-1989, he was
engaged in developing algorithms for adaptive array processing and
high-resolution spectral analysis. From 1974 to 1983, he was with
IBM Federal Systems Division (FSD), where he worked on
NASA-sponsored studies of pattern recognition techniques to identify
crops in Landsat images and performance modeling of computer
systems.
Dr. Misra is a member of the Institute of Navigation. In 1980 he
received an IBM/FSD Outstanding Achievement Award for developing a
emematical model of the IBM 3850 Mass Storage System.
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