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The Interchangeability Problem:
Signals, Coordinate Frames, and Time
RITA M. LOLLOCK and THOMAS D. POWELL
The Aerospace Corporation
THOMAS A. STANSELL
Stansell Consulting
INTRODUCTION
The Global Positioning System (GPS) and the GLObal NAvigation Satellite
System (GLONASS) were developed independently. Although similar in certain
respects (e.g., constellation design and dual L-band frequencies), there was no
intent by the developers to make the systems interoperable. The first known
receiver to use both GPS and GLONASS signals for navigation was a prototype
developed by Magnavox and delivered to Lincoln Laboratory in 1990. To achieve
satisfactory navigation results the receiver had to compensate for differences in
system time, coordinate frame, signal frequencies, spreading codes, message
formats, and separate filter delays within the receiver. Nevertheless, this and sub-
sequent commercial receivers were able to demonstrate that two rather dissimilar
systems could be used together to achieve better results than with either system
alone, especially in difficult reception environments, even though the systems
were far from “interchangeable.”
In recent years new systems have been proposed and are being developed,
including Compass, Galileo, and QZSS (Quasi-Zenith Satellite System). Also,
both GPS and GLONASS are being modernized with new signals. These systems
are now referred to collectively as Global Navigation Satellite Systems, or GNSS.
Fortunately, the international climate has changed substantially such that “compat-
ibility” and “interoperability” are important topics at multilateral meetings such
as the International Telecommunication Union (ITU) and the International Com -
mittee on GNSS (ICG) as well as at bilateral meetings between system providers.
Compatibility between systems is vital to ensure that signals from one system
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76 GLOBAL NAVIGATION SATELLITE SYSTEMS
do not unacceptably degrade the performance of other systems. Primarily this
means that compatible systems must provide signals with similar maximum and
minimum received power levels.
The internationally accepted definition of interoperability is the “ability of
open global and regional satellite navigation and timing services to be used together
to provide better capabilities at the user level than would be achieved by relying
solely on one service or signal.” As a result of international cooperation, new and
modernized GNSS signals will have characteristics that substantially improve
interoperability as compared with the original GPS and GLONASS signals.
Some have suggested that the goal should be not only interoperability but
also interchangeability, meaning that there should be no discernible differences
between the signals from different systems. This paper addresses the problems of
achieving full interchangeability of signals while also showing why the remaining
differences will not affect users.
IDEAL INTERCHANGEABILITY
It is instructive to realize that the current GPS constellation is populated with
four different types of satellites with four significantly different designs from differ-
ent manufacturers, as shown in Figure 1. These are the GPS Block IIA satellites built
by Rockwell International (now Boeing), the GPS Block IIR and the Block IIR-M
satellites built by Lockheed Martin, and the GPS Block IIF satellites built by Boeing.
Later in this decade a fifth type of satellite, the GPS Block III being designed by
FIGURE 1 GPS receivers track four different satellite types.
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THE INTERCHANGEABILITY PROBLEM
Lockheed Martin, will join the constellation. The Block IIR-M added three new
signals, the Block IIF added a fourth, and the Block III will add a fifth.
From a current user’s perspective, signals from different types of GPS satel-
lites are completely interchangeable. This is because “backward compatibility” is
required for each new type of satellite. Each legacy signal shares a common signal
structure and the same orbit message parameters, geodesy, tracking stations, and
GPS time. They are part of a unified system.
This type of interchangeability is not practical for other global navigation
satellite systems. Each of these systems is separately owned, developed, and
operated. They employ different signal structures, message formats, orbit charac -
teristics, geodesy, tracking networks, control system software, and system time.
Therefore, different GNSS signals are not inherently interchangeable. This paper
evaluates whether current international cooperation on interoperability will pro -
duce true interchangeability from the user perspective. In other words, will users
experience differences between systems?
INTEROPERABILITY PROGRESS AND STATUS
As a result of international bilateral and multilateral meetings, the advantages
of GNSS interoperability have been recognized and major strides made toward
achieving the goal. A particularly important achievement is that right hand cir-
cular polarization, code division multiple access (CDMA) modulation, and two
identical GNSS carrier frequencies—1176.45 MHz and 1575.42 MHz—have been
adopted by GPS, Compass, Galileo, and QZSS. Figure 2 illustrates each GNSS
FIGURE 2 GNSS L1 and L5 signal plans.
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78 GLOBAL NAVIGATION SATELLITE SYSTEMS
provider’s signals centered on these two frequencies. GLONASS is considering
these possibilities as well as other options that could provide the advantage of
frequency diversity.
Differences in signal structure do exist, however. Some differences are
because each constellation of satellites is different, and messages describing the
orbits must be appropriate for each system. Beyond that, for example, the data
rate on both the GPS L1C and L5 signals is 50 bits per second (bps) whereas
the Galileo E1 OS data rate is 125 bps and the E5a data rate is 25 bps. GPS L1C
employs a time multiplexed binary offset carrier (BOC) or TMBOC waveform,
whereas the Galileo E1 OS employs a composite BOC (CBOC) waveform. The
length of the GPS L1C spreading code is 10 ms or 10,230 chips, whereas the
Galileo E1 OS spreading code length is 4 ms or 4,092 chips. Each signal has been
developed to serve different objectives. In spite of these differences, interoperabil-
ity is enhanced because of identical center frequencies and spectral characteristics.
However, users are not likely to notice any of these differences regardless of
which combination of signals is being used at the moment. This is because user
equipment is being designed to combine all types of signals to give one position,
velocity, and time (PVT) solution. If different signals are known to have different
levels of accuracy, they will be appropriately weighted in the combined solution.
Users will not know or care about the “nationality” of the signals, but they will
enjoy the benefits of improved availability and accuracy due to the larger number
of signal sources, as shown by the “Multi-GNSS” receiver in Figure 3.
FIGURE 3 Transparent multi-GNSS interoperability is achievable through system and
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receiver design.
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THE INTERCHANGEABILITY PROBLEM
The key to understanding how user equipment will compensate for the sig -
nal differences is to recognize two classes of difference, i.e., analog and digital.
Because of Moore’s law, the cost of digital electronics has continued to plummet
while performance has greatly increased over previous decades, and the trend
is expected to continue. On the other hand, dimensions of analog components
such as antennas and filters are constrained by the fixed signal wavelength and
have not experienced as much improvement. Therefore, it is important that signal
characteristics that affect analog components be as common as possible. These
include antenna polarization, center frequency, and waveform or spectrum. For-
tunately, the standards for GPS, Compass, Galileo, and QZSS are right-hand
circular polarization, 1575.42 MHz and 1176.45 MHz center frequencies, and a
common signal spectrum at each center frequency. This allows use of common
analog receiver components for each of these signals.
All other signal differences, such as the spreading codes, data rates, message
structures, etc., are handled by digital electronics and software. It is not that these
differences are unimportant, because receiver designers must accommodate each
one, but the differences are invisible to the user because they are hidden within
a special-purpose digital chip and internal software. Because digital chips and
memory have become so cheap and so competent, a receiver using signals from
all GNSS providers will cost no more than a receiver using signals from only
one. In fact, because competition will drive every manufacturer to offer the best
possible performance and features, it is likely that Multi-GNSS receivers will
become the norm in the future.
SYSTEM TIME AND GEODETIC COORDINATE SYSTEMS
Concern has been expressed about how to achieve a common time standard
and common spatial coordinate systems. There are two very effective approaches
to resolving these issues.
First, time differences can be resolved by software in individual receivers.
This was demonstrated in 1990 by the first GPS/GLONASS navigation receiver.
System time differences are calibrated very precisely by including a time offset
parameter in the navigation solution matrix. In other words, instead of solving for
three position coordinates plus a receiver time offset by observing four or more
signals from one system, it is straightforward to additionally solve for a system
time offset by observing five or more signals from two systems. Some think this
effectively removes one satellite from the observation matrix. However, that is
an oversimplification. Because the time difference between two systems changes
very slowly, the fifth variable can be determined quickly and then highly filtered.
Once established and change is constrained, every satellite signal is able to
participate in the navigation solution. The system time difference calibration can
also be remembered and used immediately when a receiver is next switched on.
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80 GLOBAL NAVIGATION SATELLITE SYSTEMS
The difference between geodetic coordinate frames can be established very
precisely by a network of receivers tracking signals from each system. The Inter-
national GNSS Service (IGS) is an excellent example of this capability. Therefore,
the offset between geodetic coordinate systems can be determined precisely and
provided to user equipment in many ways. Some GNSS may choose to include
coordinate offsets in their broadcast messages. These messages also will be pro -
vided over the Internet, transmitted by cell phone networks, or included as part
of receiver software updates.
The other approach is for GNSS providers to operate with common time and
geodetic standards. This is easier to achieve with the geodetic reference systems,
because all providers consider the International Terrestrial Reference System
(ITRS), which is established and updated by the IGS, to be the best available
Earth representation. Therefore, every provider has moved their internal reference
coordinate system ever closer to ITRS. As a result, most systems already agree or
soon will agree with ITRS to within a few centimeters. Not only is this negligible
for the vast majority of users, but also high-precision users employ differential
corrections that eliminate the effect of any residual difference. Therefore, except
for scientific geodesists, we can consider this problem solved.
System time differences are harder to resolve but easy to observe. Just as each
user equipment can observe, solve for, and eliminate the effect of system time
differences, networks can do the same. This ability would be enhanced by the
interoperability agreements. With identical center frequencies and signal spectra,
reference stations can precisely measure the time offset between satellites of dif -
ferent systems. Although providers could use this information to drive their own
system clocks to agree with any one of the other systems, it would be preferable
to establish an international GNSS time standard against which each system could
measure its time and drive any difference toward zero.
Measured time differences can be communicated to user equipment in many
ways. They could be included as part of the satellite message or provided by the
Internet or over cell phone communications. Provisions already have been made
for some systems to provide time offsets relative to one or more other systems.
Multiple systems have announced initial plans to use GPS Time for this purpose.
Knowing the offset in advance will help users in locations with limited visibility
to obtain a position fix quickly with a few signals from several different systems.
However, it should be noted that once many signals can be tracked simultaneously,
the user equipment will measure the time differences more precisely on its own.
SPECIFICATIONS AND STANDARDS
As stated above, manufacturers already are designing, if not shipping, receiv -
ers capable of combining signals from many GNSS. However, including a system
is not possible until its signal characteristics are well understood. This requires
early publication by each GNSS provider of accurate signal specifications. Manu -
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THE INTERCHANGEABILITY PROBLEM
facturers will guess at the specifications and provide for future software updates,
but only with accurate signal specifications will use of a particular system be
assured. There also should be a way for manufacturers to ask questions and receive
prompt replies from the system providers.
System performance standards help manufacturers anticipate how best to
combine signals. Ultimately, of course, signal combination algorithms will be
optimized by practical measurements.
When signals are used for safety of life applications, it is particularly impor-
tant for providers not only to define the signals and the expected performance
standards but also to rapidly issue service notifications as is done by GPS Notice
Advisories to Navstar Users (NANUs). For example, the International Civil Avia -
tion Organization (ICAO) requires specific service commitments and ongoing
support before a GNSS can be accepted for use.
SUMMARY AND CONCLUSIONS
GPS was declared fully operational on April 27, 1995. More than 16 years
later its performance continues to improve and the number of applications and
users continues to expand exponentially. As phenomenal as this has been, GNSS
is on the verge of revolutionary improvements as GPS signals complete their
modernization, the GLONASS constellation completes its replenishment, and
new systems such as Compass, Galileo, and QZSS become operational. In the
future, when three GNSS frequencies become generally available, wide-area
10 cm phase-based navigation will proliferate.
Availability and accuracy for all forms of navigation will improve dramati -
cally because the number of visible satellites will more than double. Figure 4
illustrates this for an environment with significant visibility obstructions, such as
an urban canyon. Using only the five GPS satellites in view results in a position
dilution of precision (PDOP) of 4.84. Adding the four Compass satellites in view
improves the PDOP to 3.92. Adding the two Galileo satellites gives a PDOP of
2.37. With three GLONASS satellites, the PDOP becomes 2.16. The clear mes -
sage of Figure 4 is that having many interoperable satellites will provide PNT
availability even in difficult environments and with much better accuracy than
possible with only one system.
International GNSS cooperation has improved dramatically so that users will
experience the full benefits of interoperable signals. Because receivers will seam -
lessly combine signals with different digital characteristics, the user perception
will be that all GNSS signals are fully interchangeable.
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FIGURE 4 Better user accuracy is achievable via better position dilution of precision
Lollocketal_Fig4.eps
(PDOP) due to multi-GNSS interoperability.
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