Received 8 October 2021; accepted 15 November 2021. Date of publication 23 December 2021; date of current version 7 January 2022.
Digital Object Identifier 10.1109/JMW.2021.3129111
Microwave Pioneers: Charles Elachi,
“Dare Mighty Things”
PETER H. SIEGEL
1,2,3
(Life Fellow, IEEE)
(Special Series Paper)
1
THz Global, La Canada, CA 91011 USA
2
Department of Electrical Engineering, California Institute of Technology, Pasadena, CA 91125 USA
3
NASA Jet Propulsion Laboratory, Pasadena, CA 91109 USA (e-mail: phs@caltech.edu)
ABSTRACT
This is the fifth article in our continuing series of biographical pieces with a technical lean (two
in vol. 1, issue 1, and one each in vol. 1 issue 2 and issue 3). The subject of this paper is noted microwave
radar remote sensing pioneer and National Aeronautics and Space Administration Jet Propulsion Laboratory
(NASA/JPL) Director, Dr. Charles Elachi. Without a doubt, Dr. Elachi almost single-handedly pioneered
the field of active microwave remote sensing from space. His early work on aircraft imaging radar and
the deployment of microwave synthetic aperture techniques for ocean and land surveying led to a series of
groundbreaking Space Shuttle missions and opened up whole new fields of research for oceanographers,
geologists, planetary scientists, geophysicists, and even archeologists. Elachi’s scientific and engineering
accomplishments quickly led him up a long ladder within the NASA/JPL community, eventually landing
him in the Director’s chair. The title of the article reflects his mantra for success, both personally and for the
institutions he played such a large role in.
INDEX TERMS
Charles Elachi, Caltech, earth remote sensing, microwaves in space, microwave pioneers,
NASA JPL, shuttle imaging radar.
I. INTRODUCTION
Dare Mighty Things
1
is a favorite phrase from a favorite his-
toric figure admired by Charles C. Elachi, the subject of this
Microwave Pioneer article.
2
It not only occupied a prominent
1
The phrase is from a portion of an 1899 speech by U.S. President
Theodore Roosevelt which reads, “Far better it is to dare mighty things, to
win glorious triumphs, even though checkered by failure, then to take rank
with those poor spirits who neither enjoy much nor suffer much, because they
live in the gray twilight that knows not victory nor defeat.” The phrase was
a favorite of Charles Elachi in his role as Director of NASA JPL and it was
encoded in binary on the inside of the Perseverance Rover’s parachute as it
dropped down to the surface of Mars on Feb. 18, 2021
2
This article was composed after an in-person interview on Sep. 22, 2021
with Dr. Elachi at one of his two offices at Caltech – the smaller one is in
EE, the larger one in Geophysics – perhaps appropriate to the scale of his
contributions as he sees them! Although we focused mostly on Dr. Elachi’s
pioneering work on microwave remote sensing, the subject of his very well-
known college text – which has just been released in its 3
rd
Edition, we
also had a chance to also talk about his role as JPL director and the many
challenges and success stories he has had leading a major NASA center and
responding to the demands of several different NASA administrations. This
article explores only his microwave work, but this is by far the less dramatic
of his many career successes!
place on his office wall but
was also broadcast to the world
– although playfully in binary
code - on the underside of the
parachute that glided the Per-
severance rover safely down to
the surface of Mars on Febru-
ary 18
th
, 2021 (Fig. 1). As you
will read, it is also a very appro-
priate description and testament
to an individual who, when the
opportunity presented itself, did
not hesitate to take up the chal-
lenge, no matter the risk of failure. In his case, it seems always
to have resulted in resounding success!
Charles Elachi was one of four siblings growing up in a
small town of around 1000 people in Rayak (also Riyaq or
Rayaq), Lebanon, approximately 60 km east of Beirut. Rayak
was known as a hub of rail traffic in the Middle East and
Charles’ father, Rokos, was a director of the rail station. There
This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/
VOLUME 2, NO. 1, JANUARY 2022
13
SIEGEL: MICROWAVE PIONEERS: CHARLES ELACHI, “DARE MIGHTY THINGS”
FIGURE 1.
View of parachute underside on the Perseverance rover with
overlayed coded message text. Each of the three inner rings of white and
red lines represents a 7-bit binary code spelling out “Dare Mighty Things”
going clockwise with A
=
000001, and with the outer ring spelling out the
Lat. and Long. of JPL’s Visitor Center. Image NASA/JPL, 2021: https://mars.
nasa.gov/imgs/mars2020/spacecraft/markings/25646_PIA24431-
parachute-decoder-ring-1200.jpg.
was also a large French military airbase near the town that
supplied significant dreams about flying and perhaps having
a career working in engineering and technology. Although
neither of his parents completed high school, and his only
links to the west were a magazine called “Life in America,”
distributed by the American embassy in Lebanon and a local
movie theater that showed American films, the influence of the
trains and planes was strong, and Charles gravitated towards
math and science. At age 11, in a prescient happenstance, he
recalls reading an article on the Jet Propulsion Laboratory
which had just launched America’s first satellite – Explorer
1, in January 1958.
What would become a very independent mindset, was en-
hanced by his attendance at a boarding school (Collège des
Apôtres in Jounieh, near Beirut) starting from age 11 and
extending through high school at École Orientale, in Zahlé
(closer to home). Elachi graduated in 1964 with the presti-
gious score of number one in Lebanon on the Mathématiques
Élémentaires – a national college entry exam. This distinction
earned him a fellowship which could be applied to a university
outside of Lebanon. Elachi was recommended to apply for a
slot at the Joseph Fourier University in Grenoble, France, to
which he was admitted. After a year he was able to transfer
into an Ecole Nationale Supérieure program in what would
become the National Polytechnical Institute (INPG), where
he received a master’s degree in electrical engineering in 1968
and where he studied radio electricity and gas discharge spec-
troscopy. One of his professors and mentors, and the President
of INPG, was 1970 Nobel Laureate in Physics, Louis Néel,
best known for his work on ferrimagnetism and superparam-
agnetism.
During a summer research program, Charles happened to
meet a couple of US transplants studying French at Grenoble,
who suggested he might like continuing his studies in the
United States. One of his friend’s father happened to be a
Dean at University of Illinois and he sent Elachi a set of
application forms for a bunch of top science schools in the
US. In a typically haphazard manner, Charles applied to, and
received acceptances from schools such as MIT, Stanford, Illi-
nois and other US institutions, but he happened to notice that
one of these, California Institute of Technology, a college he
had never heard of, was very close to Hollywood (memories
of American movies). In fairness, Caltech was also highly
recommended by his professors at Grenoble. He accepted
Caltech’s full Ford Foundation Fellowship and arrived at the
campus in Pasadena, California, in mid-August 1968.
In California, Elachi was housed with a family in which the
husband worked at Hughes Aircraft, and in which the wife’s
family had Lebanese roots. JPL was occasionally a part of the
dinner conversation, and Elachi came to understand that it was
connected to the University – flash back to the article he had
read as a child on the developers of Explorer I. Meanwhile,
on the Caltech campus, Elachi hooked up with notable Radio-
physics and Electrical Engineering Professor, Charles Papas,
and started working on electromagnetic wave interactions in
periodic media. He earned his Master’s degree in 1969 and his
Ph.D. in 1971 [1]–[4]. By 1970, he had also managed to land
a part-time paying job at JPL, which he gladly substituted for
his university fellowship.
After completing his doctorate in the summer of 1971,
the JPL position became full time, but Elachi maintained a
research fellowship at Caltech. He focused heavily on his
specialty area – optical wave interactions in periodic media
– publishing a considerable number of papers on the topic
[5]–[8], many with nearby UCLA colleague and former Cal-
tech student himself, Cavour Yeh [9]–[15]. He also received
several patents on distributed feedback lasers and oscillators
[16]–[19]. Elachi summarized much of his wave interaction
research in a nice review article for P
ROCEEDINGS OF THE
IEEE in 1976 [20].
At the same time, Elachi was working for JPL’s Walter
E. Brown – a pioneer in microwave radar for airborne
applications [21]. He began, at Brown’s request, to learn about
and then help to put together a planetary imaging radar study
– PIRS [22]. He was also taking advantage of his waveform
expertise to look at electromagnetic waves interacting with
both the atmosphere and the ground [23] – which would later
lead to a strong and persistent interest in geophysics. Although
the PIRS study never went forward as a NASA mission, it led
to a proposal and fledging flight program to use synthetic aper-
ture radar techniques to penetrate the Venusian atmosphere
and to map the surface features, VOIR – Venus Orbital Imag-
ing Radar [24]. VOIR was cancelled in 1982 due to budget
overruns but was soon replaced with the Venus Radar Mapper
in 1983. VRM was itself renamed Magellan in 1985 and
launched in 1989 from the Space Shuttle Atlantis (STS-30) on
a very successful four-year surface mapping mission at Venus
– and remains a highlight of JPL’s major planetary programs.
Tacking back to the mid 1970s and Elachi’s early work
on radar imaging, and before there were any satellite pro-
grams, there was an active aircraft campaign in Walter
Brown’s group. Brown was doing microwave backscatter
14
VOLUME 2, NO. 1, JANUARY 2022
FIGURE 2.
Seasat image of Boston harbor, Cape Cod and Nantucket Island
where shallow shoals produce surfaces wave patterns contrasted by
differences in the SAR scattering amplitude. Image credit NASA/JPL-Caltech
/Alaska Satellite Facility, 2014: https://photojournal.jpl.nasa.gov/catalog/
PIA18137.
measurements and synthetic aperture radar at L-band (25 cm;
1.2 GHz) and at 2 meters (150 MHz, VHF) and flying over ice
fields (mostly Alaska and Greenland). The microwave signals
could penetrate down to depths of 100 meters or more through
snow and ice and retrieve layering information [25]. Elachi
became the team leader, and on one flight in 1974, coming
back from a flight over Alaska with their instrument aboard
a NASA CV990, the team decided to turn on the radar as
they crossed the open ocean to see if any surface features were
visible. At the time, most SAR techniques depended on having
a stationary target – and of course the ocean was covered with
moving waves. What they observed was to open up a whole
new field of geophysics – they were able to see, and thus track
waves and features moving across the water surface [26], [27].
The resolution came from differences in the radar signature
when reflected off the wave tops – which were very rough
and scattered strongly, versus the wave troughs, which were
much smoother in feature size at the scale of the microwave
signal. Brown showed the results to John Apel [28] at the
National Oceanic and Atmospheric Administration, Seattle,
Washington who was extremely excited, and suggested that
the L-band SAR instrument might be used to map wave pat-
terns through cloud cover or bad weather, and might even be
able to visualize the waves under a hurricane – something that
was extremely interesting to oceanographers and geoscien-
tists. Elachi was to experience such a flight himself – only
once – when he later took the aircraft instrument through to
the eye of Atlantic hurricane Gloria in September 1976, on a
quest that went well beyond the call of duty in the interests of
science [29]!
At this point there was still considerable skepticism about
how well a SAR instrument might perform in space, versus a
low flying aircraft. First, the resolution would be a major issue
(an enormous antenna was required), but other problems also
seemed significant – like being able to collect and process all
the required SAR data, which at the time was captured on film
using optical interferograms. Also, it was not clear that the
same contrast in the wave crests and troughs that was observed
on the aircraft instrument would still show up on a space view.
At the time NASA was getting ready to populate the Seasat
platform, the first civilian satellite to focus on the oceans,
and they already had an altimeter, a microwave scatterometer,
and radiometers for height, wind speed and direction, and
ocean temperature, respectively. Brown, with the help of Apel,
Elachi, and the JPL radar team, were able to convince NASA
to add an L-band SAR instrument to the payload under the
conditions that it would be an engineering demonstration and
that it could be built for a very limited cost.
Seasat, along with the new 1275 MHz SAR system which
boasted a 10.7
×
2.16-meter deployable antenna – almost the
size of the full solar panel array, was successfully launched
from Vandenburg Air Force base in California on June 27,
1978 and operated for a little over 100 days in a circular orbit.
The radar imaging instrument had a resolution of 25 meters
and was able to see contrast both over land and water, validat-
ing the use of the SAR technique on a satellite platform [30]
(Figure 2). It even generated some unanticipated controversy
when a claim surfaced that the system could detect the wakes
of submerged submarines, but this was never confirmed. What
is certain, is that Seasat set the stage and the technology for an
enormous number of subsequent radar-based satellite instru-
ments, both for Earth remote sensing and planetary explorers,
that continues to this day, and Elachi’s JPL radar team was at
the center of it all [31]–[35].
Even before Seasat launched, the JPL radar team was
continually flying its instruments on aircraft platforms
[36]–[44], honing its algorithms [45], and looking for
additional opportunities. It so happened that in the late 1970s,
the new NASA Space Shuttle program was getting ready for
its first flights which were designated to be for engineering
purposes only – no science instruments were to be deployed
from the Shuttle bay for at least the first four launches.
However, rather than flying the expensive new spacecraft
empty, NASA released an announcement of opportunity for
potential payloads that could fit entirely within the envelope
of the cargo hold – nothing extending – and do something
of purpose. Elachi’s team was completing work on the
Seasat instrument, and there were lots of spare parts and a
complete working engineering model of the SAR system. He
proposed that these be reworked and fitted into the Shuttle
cargo bay and be deployed to observe the Earth as the shuttle
passed over (top side looking down) with the bay doors fully
open. The proposal was selected towards the end of 1977,
and Elachi became the Principal Investigator of a major
multimillion dollar space instrument at the tender age of 30.
This prompted a quick trip back to school to get a Master’s
degree in Business at USC, which Elachi received in 1979.
SIR-A, for Shuttle Imaging Radar A, was launched on
Columbia during STS-2, the second space shuttle mission
[46]. It lifted off from Kennedy Space Center, Florida, on Nov.
12, 1981, and landed 54 hours later (having completed 37
orbits) at Edwards Air Force Base in California. SIR-A was
by far the largest part of the five instrument payload aboard
Columbia. The fixed 1275 MHz antenna was 9.4
×
2.16 m,
weighed 181 kg, and stared at a 47-degree angle ahead of the
VOLUME 2, NO. 1, JANUARY 2022
15
SIEGEL: MICROWAVE PIONEERS: CHARLES ELACHI, “DARE MIGHTY THINGS”
FIGURE 3.
View of STS-2 cargo bay with 5 instruments: SIR-A, OCE (Ocean
Color experiment), SMIRR (Shuttle Multispectral Infrared Radiometer), FILE
(Feature Identification Location Experiment) and MAPS (Measurement of
Air Pollution from Satellites. Image Credit: NASA [46].
orbital track (side-looking SAR). The ground track pixel size
was 40
×
40 meters, and the radar pulse power was 1 kW. The
backscatter radar interferograms were recorded onto an opti-
cal film through a cathode ray tube using an instrument left
over from the Apollo 17 Lunar Sounder Experiment and was
14 years old when it was flown on SIR-A [46]. A photo of the
payload on STS-2 with labels for the five instruments flown
is shown in Figure 3. Approximately eight hours of observa-
tions were recorded, which demonstrated many of the SAR
capabilities already realized from Seasat [47]–[52] and which
led to a remarkable and unexpected geological discovery plac-
ing space-based radar observations at the heart of a revolution
in geologic observations of the Earth as well as a significant
new tool for archeologists!
When the images from SIR-A were processed, like the
Seasat data, there were contrast features that were not im-
mediately recognized as being due to expected backscatter
patterns. Several such occurrences were spotted over imaging
tracks that took SIR-A across very dry regions, especially in
the sand covered desert areas of northern Africa. Geologists
on the team noticed meandering patterns and coalescing tracks
that appeared a lot like water pathways, but where overlay-
ing NASA Landsat imagery showed nothing but sand dunes.
Although the team was aware of the penetrating capabilities
of microwaves in ice, they were not expecting significant soil
penetration. However, the loss tangent of dry sand is in the
range of 10
−
2
and the permittivity at 1 GHz is in the range of
3–4. The SIR-A pulses were penetrating an estimated 1.5–6
meters below the surface in places like the Sahara Desert
[52]. What looked like riverbeds in the SIR-A images were
very likely buried features with density differences sufficient
to show backscatter contrast against the loose overlying sand
(Figure 4).
FIGURE 4.
Comparison of Landsat optical image (top) with SIR-A SAR
image (bottom) of the region in the Arbain desert where Elachi
etal.
demonstrated that SIR-A was able to penetrate the overlaying sand dunes
and down to hidden riverbeds. The center region in the SIR-A image shows
two riverbeds coming together. The bright diagonal streaks in the Landsat
photo are sand dunes that hide the underlying alluvium. Image reproduced
from [59], Fig. 2. © IEEE, 1984, with permission (note image in [59] has (a)
& (b) labelled incorrectly in the caption – top is Landsat, bottom SIR-A).
In late September of 1982, a team from the US Geological
Survey office in Flagstaff, Arizona put together a ground sur-
veying expedition to a “buried river” feature highlighted in the
SIR-A images in the Arbain desert – an extremely arid, dune
covered area in southern Egypt and crossing over into northern
Sudan. Elachi joined the expedition which included geologist
and instrument scientist Ron Blom from JPL, long-time col-
leagues Gerald Schaber, J.F. McCauley and others from the
USGS, a geo-archeologist, C.V. Haynes from University of
Arizona, and a large local team from the Egyptian Geological
Survey and Mining Authority. After flying to Cairo, they all
met up in Kharga Oasis 200 km west of the Nile and at the
boundary of the Arbain Desert. Another 10-hour drive across
the sands brought them to the SIR-A track where, with a series
of test pits, they were able to confirm the riverbed alluvium
under the sand as well as uncovering artifacts from the human
inhabitants of the region when it had been wet and habitable.
Their findings, along with an overview article on SIR-A by
Elachi [53], were published in the Dec. 3, 1982 issue of
Sci-
ence
[54], which included a cover photo, and drew immediate
global attention [55]–[57].
Coming back to the flight of SIR-A, literally within weeks
of the STS-2 landing in Nov. 1981, Elachi had secured a
second shuttle flight for what would be called SIR-B. SIR-B
would field an improved instrument using most of the same
16
VOLUME 2, NO. 1, JANUARY 2022
hardware as flown on SIR-A, but with the ability to change
the antenna angle and imaging track to allow for stereographic
measurements and to reveal topological features. The JPL
team began preparing for their next deployment, which was
still a couple of years away.
Perhaps spurred on by this ever-increasing application of
and attention to his research, and his association with geology
and geologists, Elachi managed to set aside some time to go
back to school and get a master’s degree in geology at UCLA
in 1983. Apropos, and during this period, JPL geologist, Ron
Blom received an unsolicited phone call from documentary
film maker Nicolas Clapp, who was putting together a story
about an ancient and well rumored but lost city known as
Ubar. It was thought to be somewhere in modern Oman and
had served as a notable desert oasis and crossroads for the
Frankincense trade more than 2000 years ago [58]. Clapp
believed that NASA’s penetrating radar imagery could help
locate the lost city. Blom spoke to Elachi about the possibility
of using SIR-B to look for Ubar and arranged a meeting with
Clapp. Clapp is quoted [58] as stating that Elachi’s response
was: “Okay, let me get this straight: You want to use my space-
ship to find your lost city?” It was too good an opportunity
for Elachi – a newly degreed geologist with a strong Middle
Eastern heritage – to pass up. It was agreed that so long as it
did not interfere with the main mission goals, Ubar would be
a target of opportunity for SIR-B. Meanwhile, more rigorous
algorithms for pulling out the subsurface L-band SAR contrast
in dry sands were being worked out [59].
SIR-B was slated for STS-41G which would launch on Oct.
5, 1984 and stay in orbit for a tad over a week. It would test
the SAR instrument at three different altitudes (varying reso-
lutions on the ground) and a number of different track angles
(inclinations). This time, all the SAR data was digitized and
saved on a high-density data rate recorder (HDDR) on board
the shuttle and immediately relayed through TDRS (tracking
and data relay satellites) to various ground stations. There
was also a back-up optical system as on SIR-A. This was
fortunate, as several technical problems, including a major
fault in the positioning of the TDRS data transmit antenna,
reduced the HDDR dataset to only around 7.5 hours, but the
optical recorder on board brought in an additional 8 hours of
imagery [60]. Despite the problems, the major engineering
goals and a good portion of the science were all realized [61]–
[64], including image tracks that would later be used to hunt
for (and most likely uncover) the lost city (oasis at least) of
Ubar
3
[65], [66], [67]! The benefits of performing microwave
SAR measurements from space were now securely confirmed
3
Nicholas Clapp did indeed put together both a documentary film and
a book highlighting his expeditions to Oman to search for the lost city of
Ubar. Both Ron Blom, and for a short time, Charles Elachi, accompanied the
expedition, which took place in 1993. As it turned out, the team did uncover
an ancient oasis, and with the use of ground penetrating radar, a fort around it,
using the SIR-B data to get to the general location where Ubar was expected to
lie buried. Many subsequent archaeological investigations followed, and Ubar
is now a major tourist attraction in Oman (https://omanpocketguide.com/tale-
of-a-lost-city-ubar/).
and the data was now being sought after by oceanographers,
geologists, and archeologists [68]. Elachi also continued work
on SAR data capabilities and analysis [69]–[71].
In 1984, Elachi took on the role of Manager of the entire
Earth and Space Science Division at JPL, a major respon-
sibility change from supervising a technical group. He was
now overseeing hundreds of scientists and engineers, dozens
of space flight instruments and missions, and many more pro-
posals for potential JPL space science investigations.
By 1987, the radar group was ready for the next phase of
the SIR series. This time the goal was multispectral imaging:
using three microwave channels in L-, C-, and X-band (1.25,
5.3 and 9.6 GHz) to simultaneously acquire SAR data that
could be used for more nuanced topological and geophysi-
cal signatures (for example, higher frequencies scattered off
treetops, while longer wavelengths penetrated to the forest
floors). It was also time for another major promotion. At JPL
Director Lew Allen’s urging, Elachi became the head of a new
Directorate: Space and Earth Science Instruments, and at age
41, took a seat on the prestigious JPL Executive Council. As
if he wasn’t busy enough, he also turned his popular Caltech
class notes from EE/GE 157, “Introduction to the Physics of
Remote Sensing,” which he had been teaching since 1982, into
a textbook [72], providing a comprehensive resource for the
next generation of space-geologists and geophysicists. Elachi
was also rewarded by election to the National Academy of
Engineering in 1989.
SIR-C was a much more complex instrument than anything
the JPL radar team had built to date [73]. The three frequency
bands required development of an extremely complex antenna
and transmit/receive arrangement. The L and C band channels
were implemented with dual-polarized microstrip patch an-
tennas (18 in each band) and this time fed by multiple transmit
receive (T/R) modules: 14 per antenna panel at L band, and
28 per antenna panel at C band. The arrangement created a
phased array that could implement electronic beam steering
from 38 to 123 degrees off the track direction. This was a
major improvement over SIR-B, as the very large antenna
structure (12
×
3.7 meters) and now weighing over 3300 kg,
no longer had to be mechanically moved for data collection.
The trade-off however, was that there were over 750 T/R
modules needed to operate the two lower frequency channels.
The L and C bands also had complete quadrature polarization
sensing for the first time (HH, HV, VV, and VH) so that
differences in backscatter signals due to target characteristics
that had preferential polarization signatures could be used to
enhance contrast.
The new X-band channel was contributed by DARA/DLR
(the German Space Agency) and the ISA (Italian Space
Agency) who were now, after seeing what JPL had done,
heavily involved in developing their own SAR systems for
space. The DARA/ISI X-band channel used a long slotted
waveguide antenna with only one polarization and was me-
chanically tilted (it weighed only 49 kg) to align with the
lower frequency L- and C-band channels during observations.
A traveling wave tube amplifier provided the 3.35 kW pulsed
VOLUME 2, NO. 1, JANUARY 2022
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SIEGEL: MICROWAVE PIONEERS: CHARLES ELACHI, “DARE MIGHTY THINGS”
source (up to 1700 pulses/sec) for the 9.6 GHz signal. The
ground resolution for the L and C band channels was approx-
imately 30 meters, and the X-band instrument had a 25-meter
azimuth footprint. The corresponding range resolutions were
25, 25 and 20 m respectively for the three frequencies. The
overall data rate was 90 Mbit/sec for the L and C channels,
and 45 Mbit/sec for the X-band channel. All data was col-
lected and stored on HDDR tape on board the shuttle for later
processing – 32 terabits of it [73]!
SIR-C flew on two separate shuttle missions with near-
identical overpass tracks but separated by six months: STS-59
from Apr. 9–20, 1994, and STS-68 from Sep. 30-Oct. 11 of
the same year. This allowed the SAR instrument to collect and
compare repeat passes over the same landscape and visualize
both seasonal variations and potential movement within the
scene – for example changes in snow or ice pack coverage,
earthquake fault lines, etc. On both flights the SIR-C instru-
ment performed beyond expectations, providing the first com-
plete polarization/multispectral datasets for space-microwave
SAR and imaging more than 400 key ground sites. The NASA
portal describing the SIR-C instrument perhaps summarizes it
best [73]: “The SIR-C/X-SAR science team, consisting of 52
investigator teams from more than a dozen countries, were
using the SIR-C/X-SAR data in studies of ecology, hydrol-
ogy, geology, and oceanography. Interferometric data were
used for topographic mapping and surface change monitoring.
In addition, observations of rainstorms demonstrated for the
first time the capability of a multifrequency, multipolarization
spaceborne radar system to quantify precipitation rates and to
classify rain type ...
Radar imaging of Earth has never been
the same since SIR-C/X-SAR’s demonstration of what’s known
as simultaneous multifrequency, fully polarized, repeat-pass
interferometric SAR
.” Quoting Charles Elachi directly, “SIR-
C/X-SAR was the path opener for multiple U.S. and interna-
tional missions that followed.” A favorite Elachi image from
SIR-C appears in Figure 5.
Soon after the extremely successful SIR-C flights, Elachi
became Director for Space and Earth Science programs
at JPL, a precursor to his becoming the Director of the
Laboratory in 2001. The SIR-C legacy was continued under
the SRTM (Shuttle Radar Topography Misson) which flew
on STS-99 in February 2000 and orbited for 11 days. SRTM
carried C-band and X-band SAR imagers in the shuttle
bay, similar to SIR-C, but with an externally deployed
telescoping mast to hold additional phase-linked receive
antennas in both bands at a distance of 60 meters from
the primary transmit antennas. This gave the instrument
instantaneous interferometric capability and allowed detailed
topographical measurements (30
×
30 m horizontal track
with 6 -10 m vertical resolution) to be made over 80 percent
of the Earth’s surface during the length of the mission [74],
directly in digital format, thus replacing the traditional human
labor-intensive stereographic approach (Figure 6).
Meanwhile, the planetary study that began Elachi’s JPL ca-
reer way back in 1971 [22], had finally become a mission. The
Magellan spacecraft lifted off from Kennedy Space Center on
FIGURE 5.
SIR-C Image (bottom) compared to Landsat Image (top) of a
region in Egypt where the Nile River has changed course in recent times.
The SAR image (bottom) clearly shows the buried riverbed hidden under
the sand – which appears in the Landsat (top) photo. Note the much
higher contrast and detail of the SIR-C multispectral radar image
compared to the SIR-A image shown in Fig. 2. Figure from Charles Elachi
(NASA/JPL), 1994, with permission.
FIGURE 6.
SRTM Topological map of the Los Angeles Basin and
surrounding mountains. Red dot
=
JPL. Image credit NASA/JPL/ NIMA,
2002: https://photojournal.jpl.nasa.gov/catalog/pia04967.
May 4, 1989, with the major objective of topologically map-
ping the surface of Venus using microwave SAR. Magellan
also carried radiometers for temperature and spectroscopic
sounding of the upper atmosphere and an altimeter. The radar
operated at 2.4 GHz using a 3.7-meter diameter antenna and
had 150-meter range resolution. By the end of its roughly
four- and half-year mission, Magellan had mapped 98% of the
Venusian surface with an accuracy of around 100 meters [75].
Elachi was a science team member and was overseeing many
aspects of the mission in his Directorate’s role at JPL. He also
worked with science team members on publishing some of the
key initial SAR measurements, which came out in a special
issue of
Science
in 1991 [76]–[79].
In the interim between major missions, Elachi continued to
work, lecture, and publish [80]–[83]. Next up on the space-
craft queue was the major ESA/NASA/ISA (Italian Space
Agency) Cassini - Huygens mission, which was going to Sat-
urn with several planetary encounters along the way, and for
our first detailed look at Saturn’s moon, Titan. Elachi had been
the team leader for the Cassini SAR instrument since 1990,
so this was a particularly important mission for him. Cassini
was the most ambitious planetary space system ever launched.
18
VOLUME 2, NO. 1, JANUARY 2022
FIGURE 7.
Photo of RainCube, a JPL/NASA CubeSat instrument deployed
from the International Space Station in March 2018 and demonstrating a
Ka band (35.75 GHz) radar for Earth precipitation mapping. The instrument
was only 12 kg and used 35 W of power – compare to SRTM at 13,600 kg
and 900 kWh! Image credit NASA/JPL ESTO 2018: https://www.jpl.nasa.
gov/cubesat/missions/raincube.php.
Weighing in at over 5700 kg and carrying 12 instruments plus
the Huygens probe that was to descend to Titan, it launched
(appropriately) on a Titan IV rocket from Cape Canaveral on
Oct. 15, 1997, on a 6-year trip to Saturn and a mission that
lasted just a few weeks shy of 20 years! The original concept
dated back to a European Science Foundation proposal from
1982 and became a competing US effort in 1983. Budget
and teaming considerations turned the mission into a joint
ESA/NASA collaboration in 1988 [84].
The Cassini imaging radar system operated at 13.78
GHz generating five separately switched and targeted beams
through the satellite’s main high gain antenna (3 m diameter).
Weighing in at only 41.4 kg and consuming only 108 W it
produced a beam with 0.35–1.7 km resolution and could pen-
etrate through the atmosphere of Titan [85]. The ubiquitous
science data from Cassini-Huygens classifies the program as
one of the most productive and inspiring of any other plan-
etary mission other than perhaps Voyager. Some of Elachi’s
most important papers from the SAR instrument are listed
in [86]–[101], with the most exciting being the discovery of
Titan’s methane seas [90], [98]. Cassini appeared as the cover
article in the May 13, 2005 issue of
Science
[89].
In 2001, Elachi became the 10th Director of JPL, taking
the reins from Voyager Principal Investigator, Edward Stone.
During his 16-year tenure as JPL Director, Elachi oversaw the
launch of 24 space missions including the spectacularly suc-
cessful landing of three rovers on Mars: Spirit, Opportunity,
and Curiosity [102]. His many innovations in both science
and mission proposals and instruments, as well as his dynamic
and progressive management style earned him a prominant
place within the historical leadership of NASA during what
can only be termed a Golden Age for US space exploration. In
2006, David Gergen, writing for
US News and World Report
named him as one of 20 of “America’s Best Leaders” on a
most enviable list [103]. After retiring from the Directorship
in 2016, the JPL Mission Control Center was renamed “The
Charles Elachi Mission Control Center,” a fitting honor.
Elachi continues to work at Caltech, to do research and
to publish [104]. He just released the third edition of his
Remote Sensing tome [105], with long-time friend and col-
league at JPL and Caltech, Jacob van Zyl, who sadly passed
away just a couple of weeks before the textbook was to be
released. Perhaps the best moniker to describe Charles Elachi
and his long and fruitful career was expressed by the title of
an article written about him in in
Aerospace America
[106],
Charles Elachi, “Space Explorer.” In a fitting tribute to the
technical progress this
Space Explorer
stimulated and spun
off in the field of microwave radar from space is the image
shown in Figure 7 of a recent JPL/NASA CubeSat instrument
that was delivered to, and launched from, the International
Space Station in 2018. Compare this to the SRTM! Last year
Elachi received the National Air and Space Museum Lifetime
Achievement Award.
We close this short biographical piece by looping back to
our opening, and state that Charles Elachi has clearly lived
up to his own mantra and
Dared Mighty Things
.Wehaveall
benefitted!
SUBJECT BIO
CHARLES ELACHI
(Fellow, IEEE) received the B.Sc. degree in physics from
the University of Grenoble, Grenoble, France, in 1968, the Dipl. Ing. degree
in engineering from the Polytechnic Institute, Grenoble, France, in 1968, the
M.Sc. and Ph.D. degrees in electrical sciences from the California Institute of
Technology, Pasadena, CA, USA, in 1969 and 1971, respectively, the M.B.A.
degree from the University of Southern California, Los Angeles, CA, USA,
in 1979, and the M.Sc. degree in geology from the University of California,
Los Angeles, Los Angeles, CA, USA, in 1983.
In 1970, he joined the Jet Propulsion Laboratory (JPL). From 2001 to 2016,
he was the Director of JPL and the Vice President of the California Institute of
Technology. He is currently a Professor (Emeritus) of electrical engineering
and planetary science with the California Institute of Technology. During his
46-year career with JPL, he was a leader with developing the field of space-
borne imaging radar. He is currently a Principal Investigator on numerous
research and development studies and flight projects sponsored by NASA,
including the Shuttle Imaging Radar series (Science Team Leader), the Mag-
ellan Imaging Radar (Team Member), and the Cassini Titan Radar (Team
Leader), a Co-Investigator of Rosetta Comet Nucleus Sounder Experiment,
and a member of Science Team on NASA’s mission to Europa. During his 15
years as the Director of NASA’s JPL, he has completed 23 successful deep
space missions, including landing three rovers on Mars (Spirit, Opportunity,
and Curiosity) and two missions that returned extraterrestrial samples to Earth
containing solar wind molecules and comet particles that are more than 4.5
billion years old. He is the author of more than 230 publications and the holder
of several patents.
Dr. Elachi is a Fellow of the American Institute of Aeronautics and As-
tronautics and a member of the National Academy of Engineering and the
International Academy of Astronautics. He was the recipient of the von
Karman Award from the International Academy of Astronautics (IAA), the
highest honor, in 2019. He was also the recipient of numerous other awards,
VOLUME 2, NO. 1, JANUARY 2022
19
SIEGEL: MICROWAVE PIONEERS: CHARLES ELACHI, “DARE MIGHTY THINGS”
including the ASP Autometric Award in 1980 and 1982, the NASA Excep-
tional Scientific Medal in 1982, the IEEE Geoscience and Remote Sensing
Distinguished Achievement Award in 1987, the IEEE Medal of Engineering
Excellence in 1992, the Wernher Von Braun Award in 2002, the NASA
Distinguished Service Medal in 1999, the NASA Outstanding Leadership
Medal in 1994, 2002, and 2004, the American Astronautical Society Space
Flight Award in 2005, the NASA Exceptional Service Medal in 2005, the
America’s Best Leaders by
U.S. News & World Report
and the Center for Pub-
lic Leadership at the Harvard University’s Kennedy School of Government in
2006, the International von Ka
́
rma
́
n Wings Award in 2007, the International
von Ka
́
rma
́
n Wings Award in 2007, the Sigma Xi William Procter Prize
for Scientific Achievement in 2008, the National Academy of Engineering
Arthur M. Bueche Award in 2011, the Space Foundation J. E. Hill Lifetime
Space Achievement Award in 2011, the AIAA Carl Sagan Award in 2011, the
2016 National Space Trophy, the 2016 IAF Allen D. Emil Memorial Award,
the Association of Space Explorers (ASE) Congress Crystal Helmet Award
in 2012, the Aviation Week Lifetime Achievement Award in 2016, and the
Enigma Magazine
Award in 2019. He received two very special honors: the
“Chevalier de la Legion d’Honneur” from France in 2011 and the “National
Order of the Cedar” from Lebanon in 2006. He has also an asteroid named
after him in recognition of his contributions to planetary exploration, and the
NASA/JPL Mission Control Center was renamed the “The Charles Elachi
Mission Control Center” in his honor in 2016.
ACKNOWLEGMENTS
The author would like to thank Nora Mainland, long time JPL
assistant to Dr. Elachi, for help with background material and
images used in the text. The author also thanks staff librarians
at both JPL and Caltech for assistance with some of the more
difficult to acquire references as well as at the Los Angeles
Public Library for a copy of the US News and World Report
article in reference [103].
REFERENCES
[1] C. Elachi, “Electromagnetic wave-propagation and wave-vector dia-
gram in space-time periodic media,”
IEEE Trans. Antennas Propag.
,
vol. 20, no. 4, pp. 534–536, Jul. 1972.
[2] C. Elachi, “Dipole antenna in space-time periodic media,”
IEEE Trans.
Antennas Propag.
, vol. 20, no. 3, pp. 280–287, May 1972.
[3] C. Elachi, “Parametric interactions between Alfven waves and sonic
waves,”
IEEE Trans. Antennas Propag.
, vol. 21, no. 6, pp. 907–909,
Nov. 1973.
[4] C. Elachi, “Cerenkov and transition radiation in space-time periodic
media,”
J. Appl. Phys.,
vol. 43, no. 9, pp. 3719–3723, 1972.
[5] C. Elachi, “Parametric interactions between Alfven waves and
sonic waves,”
IEEE Trans. Antennas Propag.
, vol. AP-21, no. 6,
pp. 907–909, 1973.
[6] C. Elachi, “Acoustic microwave generation in a periodic piezoelec-
tric medium with drifting charges,
Appl. Phys. A
, vol. 5, no. 2,
pp. 159–164, 1974.
[7] C. Elachi, “Distributed feedback acoustic surface-wave oscilla-
tor,”
IEEE Trans. Microw. Theory Techn.
, vol. MTT-22, no. 10,
pp. 907–908, Oct. 1974.
[8] C. Elachi, “Magnetic wave propagation in a periodic medium,”
IEEE
Trans. Magn.
, vol. MAG-11, no. 1, pp. 36–39, Jan. 1975.
[9] C. Elachi and C. Yeh, “Periodic structures in integrated optics,”
J.
Appl. Phys.
, vol. 44, no. 7, pp. 3146–3152, Jul. 1973.
[10] C. Elachi and C. Yeh, “Stop bands for optical wave-propagation
in cholesteric liquid-crystals,”
J. Opt. Soc. Amer.
, vol. 63, no. 7,
pp. 840–842, 1973.
[11] C. Elachi and C. Yeh, “Distribution networks and electrically con-
trollable couplers for integrated optics,”
Appl. Opt.
, vol. 13, no. 6,
pp. 1372–1375, Jun. 1974.
[12] C. Elachi and C. Yeh, “Mode conversion in periodically disturbed thin-
film guides,”
J. Appl. Phys.
, vol. 43, no. 8, pp. 3494–3499, 1974.
[13] C. Elachi, G. Evans, and C. Yeh, “Fiber and diffused waveguide struc-
tures for distributed-feedback lasers,”
IEEE Trans. Microw. Theory
Techn.
, vol. MTT-23, no. 6, pp. 532–536, Jun. 1975.
[14] C. Elachi, C. Jaggard, and C. Yeh, “Transients in a periodic slab -
coupled waves approach,”
IEEE Trans. Antennas Propag.
, vol. AP-23,
no. 3, pp. 352–358, Mar. 1975.
[15] C. Elachi, G. Evans, and C. Yeh, “Transversely bounded DFB lasers,”
J. Opt. Soc. Amer.
, vol. 65, no. 4, pp. 404–412, 1975.
[16] C. Elachi, “Acoustically controlled distributed feedback laser,” U.S.
Patent No. 3,906,393, 1976.
[17] C. Elachi, G. Evans, and C. Yeh, “Fiber distributed feedback laser,”
U.S. Patent No. 3,958,188, 1976.
[18] C. Elachi, “Diffused waveguide capillary tube with distributed feed-
back for a gas laser,” U.S. Patent No. 3,939,439, 1977.
[19] C. Elachi, “Distributed feedback acoustic surface wave oscillator,”
U.S. Patent No. 4,025,876, 1977.
[20] C. Elachi, “Waves in active and passive periodic structures: A review,”
Proc. IEEE
, vol. 64, no. 12, pp. 1666–1698, Dec. 1976.
[21] W.E. Brown, Jr., “Radar studies of the earth,”
Proc. IRE
, vol. 57, no. 4,
pp. 612–620, 1969.
[22] W. E. Brown, Jr., C. Elachi, R. Jordan, A. Laderman, and T. Thompson,
“Planetary imaging radar study,” JPL Int. Rep., no. 701-145, vol. 1-2,
Jun. 1972.
[23] L. Roth and C. Elachi, “Coherent electromagnetic losses by scatter-
ing from volume inhomogeneities,”
IEEE Trans. Antennas Propag.
,
vol. AP-23, no. 5, pp. 674–675, May 1975.
[24] J. R. Rose and L. D. Friedman, “Design for a venus orbiting imaging
radar mission,”
J. Spacecraft Rockets
, vol. 12, no. 2, pp. 106–112,
1975.
[25] C. Elachi and W. E. Brown, “Imaging and sounding of ice fields
with airborne coherent radars,”
J. Geophys. Res.
, vol. 80, no. 8,
pp. 1113–1119, 1975.
[26] W. E. Brown, C. Elachi, and T. W. Thompson, “Radar imaging of
ocean surface patterns,”
J. Geophys. Res., Oceans Atmos.
, vol. 81,
no. 15, pp. 2657–2667, 1976.
[27] C. Elachi, “Wave patterns across North-Atlantic on September 28,
1974, from airborne radar imagery,”
J. Geophys. Res., Oceans Atmos.
,
vol. 81, no. 15, pp. 2655–2656, 1976.
[28] C. Elachi and J. R. Apel, “Wave observations made with an airborne
synthetic aperture imaging radar,”
Geophys. Res. Lett.
, vol. 3, no. 11,
pp. 647–650, 1976.
[29] C. Elachi, T. W. Thompson, and D. King, “Ocean wave patterns un-
der hurricane Gloria: Observation with an airborne synthetic-aperture
radar,”
Science
, vol. 198, no. 4317, pp. 609–610, 1977.
[30] R. L. Jordan, “The Seasat–A synthetic aperture radar system,”
IEEE J.
Ocean. Eng.
, vol. OE-5, no. 2, pp. 154–164, Apr. 1980.
[31] F. F. Sabins, R. Blom, and C. Elachi, “Seasat radar image of San-
Andreas Fault, California,”
AAPG Bull., Amer. Assoc. Petroleum Ge-
ologists
, vol. 64, no. 5, pp. 619–628, 1980.
[32] C. Elachi, “Spaceborne imaging radar: Geologic and oceanographic
applications,”
Science
, vol. 209, no. 4461, pp. 1073–1082, 1980.
[33] F. Leberl, J. Raggam, C. Elachi, and W. J. Campbell, “Sea ice motion
measurements from seasat sar images,”
J. Geophys. Res., Oceans
,
vol. 88, no. NC3, pp. 1915–1928, 1983.
[34] R. G. Blom, R. E. Crippen, and C. Elachi, “Detection of subsurface
features in Seasat radar images of Means Valley, Mojave Desert, Cali-
fornia,”
Geology
, vol. 12, no. 6, pp. 346–349, 1984.
[35] D. L. Evans
et al.
, “Seasat–A 25-year legacy of success,”
Remote Sens.
Environ.
, vol. 94, no. 3, pp. 384–404, Feb. 2005.
[36] D. Atlas, C. Elachi, and W. E. Brown, “Precipitation mapping with an
airborne synthetic aperture imaging radar,”
J. Geophys. Res., Oceans
Atmos.
, vol. 82, no. 24, pp. 3445–3451, 1977.
[37] C. Elachi and W. E. Brown, “Models of radar imaging of ocean
surface-waves,”
IEEE Trans. Antennas Propag.
, vol. 25, no. 1,
pp. 84–95, Jan. 1977.
[38] C. Elachi and D. D. Evans, “Effects of random phase-changes on
formation of synthetic aperture radar imagery,”
IEEE Trans. Antennas
Propag.
, vol. 25, no. 1, pp. 149–153, Jan. 1977.
[39] M. Daily, C. Elachi, T. Farr, and G. Schaber, “Discrimination of
geologic units in Death Valley using dual frequency and polarization
imaging radar data,”
Geophys. Res. Lett.
, vol. 5, no. 10, pp. 889–892,
1978.
[40] M. C. Malin, D. Evans, and C. Elachi, “Imaging radar observa-
tions of Askja Caldera, Iceland,”
Geophys. Res. Lett.
, vol. 5, no. 11,
pp. 931–934, 1978.
[41] W. F. Weeks, A. G. Fountain, M. L. Bryan, and C. Elachi, “Differences
in radar return from ice-covered North Slope Lakes,”
J. Geophys. Res.,
Oceans
, vol. 83, no. NC8, pp. 4069–4073, 1978.
[42] F. Leberl, M. L. Bryan, C. Elachi, T. Farr, and W. Campbell, “Map-
ping of sea ice and measurement of its drift using aircraft synthetic
20
VOLUME 2, NO. 1, JANUARY 2022
aperture radar images,”
J. Geophys. Res., Oceans
, vol. 84, no. C4,
pp. 1827–1835, 1979.
[43] W. J. Campbell, P. Gloersen, H. J. Zwally, R. O. Ramseier, and C.
Elachi, “Simultaneous passive and active microwave observations of
near-shore Beaufort Sea ice,”
J. Petroleum Technol.
, vol. 32, no. 6,
pp. 1105–1112, 1980.
[44] C. Elachi and T. G. Farr, “Observation of the Grand-Canyon wall
structure with an airborne imaging radar,”
Remote Sens. Environ.
,
vol. 9, no. 2, pp. 171–174, 1980.
[45] L. Warne, D. L. Jaggard, and C. Elachi, “Wave tilt sounding of multi-
layered structures,”
Radio Sci.
, vol. 14, no. 6, pp. 1069–1076, 1979.
[46] H. J. Kramer, “SIR-A (Shuttle imaging radar)/OSTA-1 payload
on STS-2 mission,” NASA, eoPortal, Airborne Sensors. Accessed:
Sep. 29, 2021. [Online]. Available: https://directory.eoportal.org/web/
eoportal/satellite-missions/s/sir-a
[47] C. Elachi, “Earth resources observation with the shuttle imaging
radar,”
Proc. Soc. Photo-Opt. Instrum. Eng.
, vol. 278, pp. 73–78, 1981.
[48] C. Elachi and A. Fontanel, “Radar observation of the earth,”
Recherche
, vol. 12, no. 128, pp. 1366–1375, 1981.
[49] C. Elachi, T. Bicknell, R. L. Jordan, and C. Wu, “Spaceborne
synthetic-aperture imaging radars: Applications, techniques, and tech-
nology,”
Proc. IEEE
, vol. 70, no. 10, pp. 1174–1209, Oct. 1982.
[50] C. Elachi and J. Granger, “Spaceborne imaging radars probe in depth,”
IEEE Spectr.
, vol. 19, no. 11, pp. 24–29, Nov. 1982.
[51] N. Engheta and C. Elachi, “Radar scattering from a diffuse vegetation
layer over a smooth surface,”
IEEE Trans. Geosci. Remote Sens.
,
vol. GE-20, no. 2, pp. 212–216, Apr. 1982.
[52] R. Blom and C. Elachi, “Spaceborne and airborne imaging radar
observations of sand dunes,”
J. Geophys. Res.
, vol. 86, no. NB4,
pp. 3061–3073, 1981.
[53] C. Elachi
et al.
, “Shuttle imaging radar experiment,”
Science
, vol. 218,
no. 4576, pp. 996–1003, 1982.
[54] J. F. McCauley
et al.
, “Subsurface valleys and geoarchaeology of the
eastern Sahara revealed by shuttle radar,”
Science
, vol. 218, no. 4576,
pp. 1004–1020, 1982.
[55] A. Toufexis and J. Kane, “The Sahara’s buried rivers,”
Time Mag.
,
vol. 120, no. 23, p. 66, Dec. 1982.
[56] J. Eberhart, “SIR-A: A radar look from space at the rocky earth,”
Sci.
News
, vol. 120, no. 22, p. 341, Nov. 28, 1981.
[57] C. Elachi, “Radar images of the earth from space,”
Sci. Amer.
, vol. 247,
no. 6, pp. 54–61, Dec. 1982.
[58] P. Lem, “Peering through the sands of time: Searching for the origins
of space archeology,” NASA Earth Observatory, p. 25, Aug. 7, 2017,
Accessed: Sep. 30, 2021. [Online]. Available: https://earthobservatory.
nasa.gov/features/SpaceArchaeology
[59] C. Elachi, L. E. Roth, and G. G. Schaber, “Spaceborne radar subsur-
face imaging in hyperarid regions,”
IEEE Trans. Geosci. Remote Sens.
,
vol. GE-22, no. 4, pp. 383–388, Jul. 1984.
[60] H. J. Kramer, “SIR-B (Shuttle imaging radar B)/OSTA-3 payload
on STS-41G mission,” NASA, eoPortal, airborne sensors. Accessed:
Sep. 29, 2021. [Online]. Available: https://directory.eoportal.org/web/
eoportal/satellite-missions/s/sir-b
[61] J. Cimino, C. Elachi, and M. Settle, “Sir-B-the second shuttle imaging
radar experiment,”
IEEE Trans. Geosci. Remote Sens.
, vol. GE-24,
no. 4, pp. 445–452, Jul. 1986.
[62] C. Elachi, “Special issue on the shuttle imaging radar (Sir-B) -
Foreword,”
IEEE Trans. Geosci. Remote Sens.
, vol. GE-24, no. 4,
pp. 443–444, Jul. 1986.
[63] C. Elachi, J. Cimino, and M. Settle, “Overview of the shuttle imaging
Radar-B preliminary scientific results,”
Science
, vol. 232, no. 4757,
pp. 1511–1516, Jun. 1986.
[64] T. G. Farr, C. Elachi, P. Hartl, and K. Chowdhury, “Microwave pene-
tration and attenuation in desert soil: A field experiment with the Shut-
tle Imaging Radar,”
IEEE Trans. Geosci. Remote Sens.
, vol. GE-24,
no. 4, pp. 590–594, Jul. 1986.
[65] Nicholas Clapp,
The Road to Ubar: Finding the Atlantis of the Sands.
A Mariner Book
. New York, NY, USA: Houghton Mifflin, 1998, 366
pp.
[66] N. Clapp, “The road to Ubar,” A documentary film released in 1996,
with appearances by Charles Elachi and Ron Blom. Accessed: Sep.
30, 2021. [Online]. Available: https://www.imdb.com/title/tt6566010/
[67] R. Blom
et al.
, “Space technology and the discovery of the lost city
of Ubar,” in
Proc. IEEE Aerosp. Conf.
, Aspen, CO, USA, Feb. 1997,
pp. 19–28.
[68] K. R. Carver, C. Elachi, and F. T. Ulaby, “Microwave remote-sensing
from space,”
Proc. IEEE
, vol. 73, no. 6, pp. 970–996, Jun. 1985.
[69] R. Blom and C. Elachi, “Multifrequency and multipolarization radar
scatterometry of sand dunes and comparison with spaceborne and
airborne radar images,”
J. Geophys. Res., Solid Earth Planets
, vol. 92,
no. B8, pp. 7877–7889, Jul. 1987.
[70] J. J. van Zyl, H. A. Zebker, and C. Elachi, “Imaging radar polariza-
tion signatures: Theory and observation,”
Radio Sci.
, vol. 22, no. 4,
pp. 529–543, Jul.–Aug. 1987.
[71] C. Elachi, K. E. Im, F. Li, and E. Rodriguez, “Global digital topogra-
phy mapping with a synthetic aperture scanning radar altimeter,”
Int.
J. Remote Sens.
, vol. 11, no. 4, pp. 585–601, Apr. 1990.
[72] C. Elachi,
Introduction to the Physics and Techniques of Remote
Sensing
. New York, NY, USA: Wiley, 1987.
[73] H. J. Kramer, “SIR-C/X-SAR payload on STS-59 and STS-
68 missions,” NASA, eoPortal, Airborne Sensors. Accessed: Sep.
29, 2021. [Online]. Available: https://directory.eoportal.org/web/
eoportal/satellite-missions/s/sir-c
[74] H. J. Kramer, “SRTM (Shuttle radar topography mission),” NASA, eo-
Portal, Airborne Sensors. Accessed: Oct. 1, 2021. [Online]. Available:
https://directory.eoportal.org/web/eoportal/satellite-missions/s/srtm
[75] JPL, “Magellan mission to Venus.” Accessed: Oct. 3, 2021. [Online].
Available: https://www2.jpl.nasa.gov/magellan/
[76] G. L. Tyler, P. G. Ford, D. B. Campbell, C. Elachi, G. H. Pettengill,
and R. A. Simpson, “Magellan: Electrical and physical-properties of
Venus, surface,”
Science
, vol. 252, no. 5003, pp. 265–270, Apr. 1991.
[77] R. E. Arvidson, V. R. Baker, C. Elachi, R. S. Saunders, and J. A. Wood,
“Magellan: Initial analysis of Venus surface modification,”
Science
,
vol. 252, no. 5003, pp. 270–275, Apr. 1991.
[78] J. W. Head
et al.
, “Venus volcanism: Initial analysis from Magellan
data,”
Science
, vol. 252, no. 5003, pp. 276–288, Apr. 1991.
[79] R. Greeley
et al.
, “Aeolian features on venus: Preliminary Magellan
results,”
J. Geophys. Res., Planets
, vol. 97, no. E8, pp. 13319–13345,
Aug. 1992.
[80] J. van Zyl, C. Elachi, and Y. Kim, “Recent advances in radar technol-
ogy and techniques for affordable planetary remote sensing,” in
Proc.
IEEE Radar Conf.
, May 2000, pp. 12–16.
[81] E. K. Huckins, C. Elachi, and D. V. Woods, “Exploring the solar
system - A current overview,”
Acta Astronautica
, vol. 47, no. 2-9,
pp. 523–533, Jul.–Nov. 2000.
[82] C. Elachi, “Space imaging radar in planetary exploration and earth
observation,”
AIAA J.
, vol. 39, no. 4, pp. 553–563, Apr. 2001.
[83] S. J. Bolton
et al.
, “Ultra-relativistic electrons in Jupiter’s radiation
belts,”
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