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Introduction / History
Trying to figure out where you are
and where you're going is probably one of man's oldest pastimes. Navigation
and positioning are crucial to so many activities and yet the process has
always been quite cumbersome. Over the years all kinds of technologies
have tried to simplify the task but every one has had some disadvantage.
Finally, the U.S. Department of Defense decided that the military had to
have a very precise form of worldwide positioning, and fortunately they
had the money ($12 Billion) to fund such a project. The result is the Global
Positioning System, a worldwide radio-navigation system formed from a constellation
of 24 satellites and their ground stations.
Global Positioning Systems (GPS) provide 24 hour three-dimensional position,
velocity and time information to suitably equipped users anywhere on or
near the surface of the Earth (and sometimes off the earth). Global Navigation
Satellite Systems (GNSS) are extended GPS systems, providing users with
sufficient accuracy and integrity information to be useable for critical
navigation applications. The
NAVSTAR system, operated by the U.S. Department of Defense, is the
first GPS system widely available to civilian users. The Russian GPS system,
GLONASS, is similar in operation
and may prove complimentary to the NAVSTAR system. Neither system is quite
up to the task of providing users world wide with navigation capabilities
that are both accurate and reliable to serve all it's potential users.
Nowadays, GPS receivers have been miniaturized to just a few integrated
circuits and so are becoming very economical and that makes the technology
accessible to virtually everyone. The GPS is finding its way into cars,
boats, planes, construction equipment, movie making gear, farm machinery,
even laptop computers.
Since the early 1980's, GPS receivers have begun finding utility in space
as well. Although the system was designed for near Earth navigation, it
can be employed in the highly dynamic environment in Low Earth Orbit (LEO).
Since the early 1980's, the development of this application of Spaceborne
GPS (SGPS) began slowly, but the number of space missions utilizing GPS
receivers has steadily increased. A spaceborne GPS receiver differs from
a terrestrial receiver, as it has to cope with higher Doppler shifts and
Doppler shift rates, which can be of the order of ±100kHz in LEO.
The Landsat satellite was the first civil spacecraft to carry a GPS receiver
into orbit, but the receiver technology has advanced to a stage where SGPS
receivers routinely can be carried on smaller platforms.
General information
The Global Positioning System consists
of, as mentioned before, of 24 satellites and their ground stations. GPS
uses these "man-made stars" as reference points to calculate
positions accurate to a matter of meters. In fact, with advanced forms
of GPS you can make measurements to better than a centimeter! You could
say that in a sense it's like giving every square meter on the planet a
unique address.
GPS devices operate by receiving spread-spectrum RF signals from a minimum
of three satellites in orbit. Once the receiver aquires three satellites
and calculates a distance to each, triangular techniques (more information
later on) then provide the exact position, time and velocity, but it needs
four, rather than three, satellites to eliminate clock-synchronization
errors in the receiver and to calculate the altitude. The satellites continuously
broadcast time, instantaneous position, almanac and ephemeris (describes
the satellite orbit) data using a code-division multiple-access (CDMA),
spread-spectrum communication scheme. Each satellite broadcasts on the
same frequency but uses a different pseudorandom-noise (PRN) code (more
about that later on).
The actual accuracy specification/given value of the receiver is not completely
true because of the fact that the U.S. Department of Defense developed
the GPS and therefore reserves use of the system in order to give precise
accuracy for U.S. military applications only. This is done by an encrypted
precision code (P code), which allows a receiver to calculate a position
with accuracy less than a meter in any direction. Meanwhile, the Department
of Defense (DoD) developed a technique called selective availability (SA)
to degrade the resolution of transmitted data that is available to anyone.
The resulting Coarse/Acquisition (C/A) code effectively limits accuracy
to 100m horizontally (latitude and longitude) and 156m vertically (altitude).Commercial
GPS equipment suppliers, however, have developed the GPS to an more advanced
system called differential GPS (DGPS), which combines the satellite data
with another reference source (mainly a beacon on the FM-band), such as
an coast beacon, to pinpoint the location within 3m horizontally and 5m
vertically. DGPS eliminates, because of mentioned fact, any reason of the
U.S. government to maintain an encrypt P code. (Rumors say that the P code
will be dropped as soon the departments engineers develop a way to jam
the GPS-signals over strategic military terrain.) That's about all to mention
about overall GPS performance (more precise information will follow) and
the DGPS, since the differential system won't work in the environment we
are interested in, Space!
GPS works in five basic steps steps
- The basis of GPS is "triangulation"
from satellites.
- To "triangulate," a GPS
receiver measures distance using the travel time of radio signals.
- To measure travel time, GPS needs
very accurate timing which it achieves with some tricks.
- Along with distance, you need to know
exactly where the satellites are in space. High orbits and careful monitoring
are the secret.
- Finally you must correct for any delays
the signal experiences as it travels through the atmosphere.
What can be achieved with an SGPS receiver
onboard a Satellite/Spacecraft
Technical possibilities
SGPS ATTITUDE DETERMINATION
Active attitude control actuators (e.g.
control momentum gyros, reaction wheels, offset thrusters, magnetic torque
rods, etc.) are the mechanisms that keep spacecraft properly oriented.
Measurements from closed-loop attitude control feedback sensors (e.g. magnetometers,
horizon sensors, sun sensors, gyroscopes, star trackers, etc.) determine
the orientation of the spacecraft and through computer controls order attitude
corrections to be performed by the actuator. A GPS receiver can provide
attitude and attitude rate data to the actuator for real-time, autonomous
attitude determination and control.
Benefits of GPS
The benefit of using a GPS sensor is
the elimination of different sensors and their interfaces. This in turn
can reduce costs, power requirements, weight, complexity, and increase
system reliability.
Methodology
An array of GPS antennas (at least
three, but four preferable) oriented in the same direction are placed on
the rigid structure of a spacecraft. Software controlled multiplexing allows
for signals being received at all antennas to reach a single receiver.
The carrier phase measurements are used to determine differential range
between the "master" antenna and each of the other antennas in
the direction of the line of sight from a GPS satellite to the master antenna.
The components of baseline vectors between antennas in the spacecraft body-fixed
frame and integer ambiguity resolution are required. Additional on-board
code computes attitude from successive carrier phase measurements for real-time
determinations. Given data about the dynamics of the spacecraft, a Kalman
filter can be used to improve attitude estimates.
Main performance issues are
- Update rate - how often is attitude
data being feed into the loop.
- Antenna placement - the further apart
the antennas are, the better the result and the more antennas that are
receiving signals from the same satellites the better the result.
- Ambiguity resolution - since carrier
phase observations are begin collected the correct integer ambiguities
must be resolved.
- Multipath - the largest contributor
to the noise in the measurements.
SGPS RELATIVE POSITIONING
The main application of relative spaceborne
GPS positioning is for real-time spacecraft rendezvous and docking. Automated
GPS-based systems are seen as a promising technique to be used for this
evolving and expanding application.
Benefits of GPS
GPS can provide highly accurate post-processed
or real-time relative positioning between space vehicles.
Methodology
The methodologies are analogues to
terrestrial differenced and relative GPS positioning. The former involves
orbit determination being performed on both spacecraft, and the "target"
spacecraft (e.g. a space station) transmitting its solutions to the "chaser"
spacecraft (e.g. a space shuttle) for differencing. The latter involves
processing the single-difference observations (observations differenced
between pairs of pseudorange measurements of a common GPS satellite) or
double-difference observations (observations differenced between pairs
of single differences for the same epoch) and transmitting the raw measurements
from the target to the chaser for computation of relative position and
velocity.
Mission examples
In a trial, orbits were determined
independently with two receivers, one on the Wake Shield Facility-02 and
one on the Space Shuttle. These one hour arcs were formed from double difference
observations between the on-orbit receivers and International GPS Service
for geodynamics (IGS) network receivers. Pseudoranges were observed with
one receiver and carrier phase with the other. The distance between the
two receivers was computed by Shuttle instruments. 10 meters relative positioning
accuracy was attained in this post-processed experiment.
SGPS SPACECRAFT SYSTEM CLOCK SYNCHRONISATION
A GPS receiver onboard a satellite
can provide precise time for all of the satellite subsystems and can provide
precise time interval measurement, all in an autonomous mode.
Benefits of GPS
Inexpensive and simple precise spacecraft
timing.
Methodology
At the time of GPS signal acquisition
by a GPS receiver, the crystal oscillator (usually a TXCO-based receiver
clock) is synchronized with GPS time by means of pseudorange measurements
and the GPS navigation message. A spacecraft bus architecture can be used
to supply GPS-derived time to synchronize all spacecraft subsystem clocks.
In this manner, daily clock updates from ground stations will not be required
and precise time-tagging of downloaded satellite data will be possible.
Once receiving GPS signals, a GPS receiver is capable of a conservative
GPS time clock synchronization accuracy of approximately 300 nanoseconds
with SA engaged. This accuracy is quite sufficient for many satellite missions.
GPS time transfer to local clock
Synchronization is usually accomplished
through the use of a feedback loop and an appropriate filter. A computer
tracks the difference between GPS time (or UTC) and the local clock, and
uses these data to steer the local clock by means of e.g., a phase microstepper.
Mission examples
- The GADFLY mission to be launched
on the SSTI-Lewis spacecraft requires a precise timing reference of 1 millisecond.
- The planned Gravity Probe B mission
will have a clock synchronization with GPS time in the order of 100 nanoseconds.
SGPS REAL-TIME ORBIT DETERMINATION
On-orbit, orbit determination (OD)
is invariably performed in real-time or near real-time in the GPS receiver.
Therefore to perform real-time orbit determination with GPS, it is most
efficient to incorporate the OD software into the receiver. This involves
absolute position determination with a GPS receiver.
Benefits of GPS
The ability to determine space vehicle
orbits quickly, with comparable or superior accuracy's than can be achieved
via ground station tracking.
Methodology
Absolute position determinations can
be performed in real-time with GPS receivers that have software capability
to track GPS signals from fast moving space platforms. The range of the
received Doppler frequency signals are about 100 kHz in a LEO spacecraft,
as compared to a receiver on the ground for which this received frequency
width is approximately one tenth as wide. By utilizing its pseudorange
and carrier phase observations, the receiver can determine its position
and velocity in real-time as does a receiver operating on the earth's surface.
This is its navigation solution or state vector. It must be observed that
the accuracy's of these solutions are difficult to assess due to the lack
of known position information. Software code in the receiver or in another
system on-board the satellite must convert the receiver-derived state vector
to an orbit determination. The simplest case is an osculating ellipse for
a single epoch or a more comprehensive orbit determination would involve
the use of a satellite force model with the state vectors and a Kalman
filter.
Scientific possibilities
Various
GPS can be used in various scientific
ways like ionospheric research. This is achieved by monitoring GPS signals
as the satellite-to-satellite path skims the upper atmosphere layers (Radiowave-occultation).
Since there will be further development in GPS during the forthcoming years,
new techniques and scientific usability will be covered using this system.
Some examples on scientific use are as below.
- Earth gravity field modeling (e.g.
TOPEX/Poseidon)
- Atmospheric occultation (e.g. MicroLab-1)
- Ionospheric imaging
- Interferometric SAR remote sensing
(e.g. TOPSAT)
What's GPS-receivers/Chip Sets are on
the market and what differs?
Because of this being a sub-report
for the nanosatellite study-report by the Swedish Space Corporation, I
deliberately will not mention any of the SGPS receivers known/built today
by Trimble, ESA, NASA JPL aso., as they are to big for a n-Sat and weight
about 2 kg or more. The only way to get a GPS to fly onboard such a small
S/C is to evaluate and build a own receiver of either GPS chip sets available
ore by modifying a OEM receiver. By doing this, the SGPS receiver will
be able to weight well below 50g and the power requirements will be less
than 1.4 W (some avaliable OEM products require as low as 0.6 watts).
The amount of parts in an GPS-receiver
has today drastically decreased. All you need is about two or three GPS-specific
ICs and two or three memory components. In some cases, you can even use
the microcontroller that's inherently found in the chip sets (some different
will be mentioned later) to handle the GPS task and your own application
code. The newest GPS chip sets are approaching the level of integration
that can make GPS cost-feasible in, for example, a watch. In addition to
low prices, the chipset vendors are nowadays making the GPS technology
available so that non-GPS experts can design a receiver into any kind of
system.
Evaluating GPS chip sets
At present time, there are only six
vendors with a complete chip-set to offer for application development (more
are on the way). Generally, each chip set includes an RF front-end IC that
is essentially a down-converter from the GPS satellite transmission frequency
of 1.575 GHz to a rate of around 4 MHz.
Rockwell currently holds a leadership
position in market share. That position, however, can be attributed both
to the fact that Rockwell was first to market with an integrated chip set
and to the quality of the implementation. At first glance, it's tough to
differentiate the different RF chips. GEC Plessey Marketing Manager David
Richardson claims that the company's GP-2010/2015 offers several advantages.
For example, Richardson says that the three-stage down-converter offers
superior rejection of out-of-band signals, and it allowed the designers
to adopt a frequency plan that minimizes interference from other wireless
devices. He points out that the frequency plan is optimized to allow the
GPS RF device to operate alongside a cellular phone with no signal-jamming
problems. Not everyone agrees that three IF stages provide an advantage.
SiRF Marketing Manager Greg Turezky points out that his company's single-stage
converter, which produces a single IF, is ultimately less likely to cause
jamming problems. The other vendors implementations use two-stage converters,
but all of the companies claim no problems operating around cellular devices.
Another major difference in the RF
chips is in the A/D-converter implementation. With the exception of Motorola's
products, the chip sets all integrate the A/D converter in the RF chip.
GEC Plessey and SiRF include 2-bit quantization, which yields a 2-dB advantage
in S/N ratio during operation. The other vendors either rely on 1-bit quantizationas
simple as a comparator--or don't specify that detail. The accuracy demands
of some applications, such as aviation navigation, require 2-bit quantization
(which our space applications also would need).
DSP-based correlators
All of the chip sets include CMOS chips
with a signal-processing block that accepts the digitized bit stream. Some
of the vendors label the block "DSP," but, in reality, the so-called
correlator functions use hard-wired state machines to implement the signal-processing
functions. The implementations vary in how the companies integrate the
DSP block. Philips, Rockwell, and SGS-Thomson offer ICs that combine the
DSP function with a processor when the other three vendors use an external
processor. All the chip sets require external ROM and RAM. For the most
part, the vendors have chosen different processors, although Motorola and
SiRF both use 68K-compatible µPs.
The choice of processor, DSP partitioning,
and development support for the chip set could be important factors in
a choice of purchase. These three factors significantly affect development
time and costs. First, consider both chip-set and system cost. The chip
sets with integrated processors would presumably offer lower chip counts
and potentially lower costs. In the case of Philips SC1575, the $30 (10,000)
price includes turnkey binary GPS software. Moreover, the company claims
that complete OEM system costs can be as low as $45 (250,000). Philips,
however, hasn't fully characterized its design and isn't sure how much
spare processing power might be available on the XA microcontroller for
user code. Moreover, the company is not currently planning to sell a development
system. Should you need more functionality than the Philips reference design
offers, you may have to add a second processor. Philips partnered with
GPS vendor Ashtech (Sunnyvale, CA) in developing Philips' chip set and
refers OEMs that need to modify or enhance the reference software to Ashtech.
Rockwell doesn't quantify how much spare processing power can be harnessed
for a user application in their Zodiac chip set either. The company does
point out that existing customers have built handheld, automotive, and
marine GPS receivers using no resource other than the on-chip AAMP2 microcontroller.
SGS-Thomson, meanwhile, boldly states that GPS-specific code requires less
than 50% of the computation cycles available in the company's ST20 RISC
core, thus leaving computer power for almost any user application. The
company also offers a complete, low-cost development kit, although its
GPS binaries and code libraries are prices, starting at $7000.
Spare MIPS for user applications
Motorola, SiRF, and GEC Plessey all
have more than enough processing power for GPS and user code with their
external processors. Moreover, should designers using these products require
more or less performance, they can choose a faster or slower processor.
Motorola and SiRF, in fact, may have a significant advantage because of
their 68000-centric designs, and many designers would prefer to work with
the well-known processor architecture and instruction set. Motorola offers
binary GPS software and a complete reference design with its $1200 Oncore
evaluation kit. SiRF currently offers an evaluation kit but is still in
the planning stages of a development kit and reference design.
Having processing power to spare can
also reduce costs in other parts of the system. For example, Motorola uses
a temperature sensor, a low-cost crystal, and temperature-compensation
software to generate the precise clocks that GPS requires. The software
scheme saves costs and PC-board real estate compared with designs that
require an external temperature-compensated oscillator.
An external processor can also pay
off in cost savings when you might not expect it. SiRF's Turezky points
out that a typical GPS receiver might require an LCD, a keypad, and other
peripheral functions. The immense library of Motorola microcontrollers
allows designers to choose a processor with functions such as LCD and keyboard
controllers integrated on chip. GEC Plessey may develop the same sort of
advantage over time as the ARM processor family proliferates.
Evaluating performance
The final evaluation criterion for
the GPS ICs centers on performance, and performance is highly subjective.
None of the chip sets offers significantly better positioning accuracy
than any other because the lack of P code. Primarily, however, performance
differences come down to how fast the receivers can acquire a satellite
and begin tracking position, the time to first fix (TTFF), measured in
seconds. TTFF is typically specified in four ways:
Hot start (HS)
TTFF when a GPS receiver has been tracking
a satellite previously and has stored the following information about the
satellite in battery-backed memory: time, location, almanac, and ephemeris
(parameters defining the satellite's orbit). For example, a receiver in
a car can perform a hot start after the car is parked for an hour or two,
but the ephemeris data is good for only two to three hours.
Warm start (WS)
TTFF when the receiver has no current
ephemeris data but has time, location, and almanac data either supplied
by the user or stored from previous operation. A receiver in a car parked
overnight could likely perform a warm start. Note that the difference between
hot- and warmstart specs is at least 30 sec, because it takes that long
to receive updated ephemeris data.
Cold start (CS)
TTFF when the GPS receiver has no data
from previous operation in battery-backed memory and no user data.
Reacquisition
TTFF after a momentary blockage of
a satellite due to circumstances such as when a car with GPS receiver passes
under a bridge.
The chip sets that feature faster TTFF
specs achieve those results in one of several ways. Motorola's chip set,
for example, features the second fastest cold-start time. The company claims
that its architecture is heavily software-intensive during initial satellite
acquisition, and the spare power in the 68331 µP results in the 90-sec
cold-start performance.
Wide pseudorandom-noise (PRN) windows speed
TTFF
SiRF's chip set is even faster, and
the company's designers added more signal-processing hardware to achieve
the 60-sec result. The design uses 10-bit-wide windows in the correlator
channels when trying to acquire a satellite signal. Moreover, you can cascade
all 12 channels to make a
120-bit window. The PRN sequence is
only 1024 bits long, so the SiRF window quickly searches through the received
bit stream. SGS-Thomson, meanwhile, offers the fastest hot-start performance
and matches Motorola's cold-start performance. SGS-Thomson also takes a
hardware approach to accelerating TTFF. During acquisition mode, the receiver
operates with a sampling rate four times faster than the rate used during
routine tracking operations.
The number of channels is another differentiating
factor among the GPS chip sets, although all but Philips' and Motorola's
support 12 channels. Strictly, no more than eight satellites are ever visible
to a GPS receiver at any time, and it's a rare occasion in a flat area
when eight are visible. Some of the vendors claim 12 channels can help
accelerate TTFF, but the eight-channel Motorola Oncore chip set sports
some of the fastest specs.
Products & Vendors
GEC
Plessey
http://www.gpsemi.com
Wiltshire, UK
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WS:45 s |
HS:15 s |
Reacq:2 s |
PC-based GPS Builder development kit
is available for $4995 and $7500 for standalone GPS Architect dev.kit,
each with binary and source GPS code. The chipset requires approximately
0.5 W.
Motorola
www.mot.com/SPS/General/rf/applications/gps/gps.html
Phoenix, Az, USA
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WS:45 s |
HS:15 s |
Reacq:1 s |
Motorola supplies the Oncore evaluation
kit with 68331 mircoprocessor, Windows-based software binary GPS code and
reference design for $1200.
Philips
Semiconductor
http://www.philips.com
Sunnyvale, Ca, USA
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WS:? s |
HS:<30 s |
Reacq:2.5 s |
Binary GPS code is provided free with
their coming product. No prices available yet.
Rockwell
Semiconductor
http://www.nb.rockwell.com
Newport Beach, Ca, USA
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WS:45 s |
HS:15 s |
Reacq:2 s |
Rockwell manufactures the Zodiac development
kit with binary GPS code and set of AAMP development tools for $1000.
SGS-Thomson
http://www.st.com
Lincoln, Ma, USAST -20 evaluation and
development kit including compiler and tools can be purchased for $995.
For another $7000, the binary GPS code can be bought .
SiRF
Technology
http://www.sirf.com
Sunnyvale, Ca, USA
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WS:42 s |
HS:18 s |
Reacq:0.1 s |
Available is a 68340 based evaluation.kit
for $995. A Development kit with source software and reference design planned.
Trimble
http://www.trimble.com
Sunnyvale, Ca, USA
SVeeSix-CM3
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WS:50 s |
HS:30 s |
Reacq:<2 s |
SVeeSix-104
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WS:50 s |
HS:30 s |
Reacq:<2 s |
Lassen-SK8
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WS:<45 s |
HS:<20 s |
Reacq:<2 s |
The Trimble Corporation has a series
of GPS modules for embedded OEM like the SVeeSix-CM3 (1.15/1.20 W) and
the Lassen-SK8 board (0.75/0.88 W), which may be modified for our purpose.
Canadian
Marconi Company
http://www.marconi.ca
Quebec, Canada
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WS:45 s |
HS:15 s |
Reacq:<3 s |
The CMC has no dev.kits ready yet but
has a series of OEM products like the Allstar GPS receiver OEM Module (1.4
W) based on the GEC Plessy chipset.
Summary
Advantages
The main advantages in using an SGPS
receiver onboard a Spacecraft, since the amount of external components
is reduced and thereby minimizing weight, are:
- Attitude determination
- Relative positioning
- Spacecraft system clock synchronization
- Real-time orbit determination
The scientific advantages/scientific
usage (so far) are
- Earth gravity field modeling (e.g.
TOPEX/Poseidon)
- Atmospheric occultation (e.g. MicroLab-1)
- Ionospheric imaging
- Interferometric SAR remote sensing
(e.g. TOPSAT)
Disadvantages
The major disadvantages on the other
hand are, since there are no commercial available SGPS receivers suited
for our purpose (size, weight, dopplerskift capability,...), is that we
have to construct/test/evaluate a own new product (with help of chip sets).
Another disadvantage is that the evaluating kits are rather expensive and
that the GPS source code not always is included in the package. Also some
of the points mentioned under advantages may be disadvantages because of,
for example, a special purpose needs some antennas of a specific length
in several specific direction according to the S/C which are shadowing
the limited amount of solarpanels powering the platform.
For more information, please go to
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More information about GPS (the most detailed
page I have found) |
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GPSWORLD newsmagazine |
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Information about: Missions, SGPS, Satellites
with SGPS launched, aso. |
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List of other GPS Manufactures/Vendors |
References
Trimble
corporation.
University of Texas.
Candian Marconi Company.
Gallant, John, "GPS receivers: System revolutionizes surveying and
navigation," EDN, Jan 7, 1993.
Quinnell, Richard A, "Directionally dyslexic? Don't worry: The car
knows the way," EDN, Dec 21,
1995.
Wright, Maury, "Time, Position and Velocity? Just ask your GPS ChipSet",
EDN, March 3, 1997.
IRF -
RYP -
SwRI -
Luth
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