Aerospace and Electronic Systems Magazine April 2017 - 42

Conversation with Paul Rosen
Riccardo: After coming back from Japan, you started your career at Jet Propulsion Laboratory (JPL) and your radar adventure
began, correct?
Paul: Yes, and no. As a JPL summer student in the early
1980s, I worked on Seasat simulations and data processing, so I
was already bitten by the radar bug then. Further, as I mentioned,
bistatic radio occultation of Saturn's rings is a Fresnel diffraction-limited data set, and the deconvolution filter is mathematically similar to the synthetic aperture radar azimuth compression
process, so I was well into radar from the early 1980s, with a small
diversion in Japan into plasmas. But it is true that the contributions to radar imaging science, applications, and technology for
which I am known commenced when I returned to JPL in 1992.

The two years I spent in Japan
were very important because
I established contacts for the
research I was to begin at JPL,
and I learned how to work and
succeed in the international
space research community.
Riccardo: Your research activities were initially focused on the
development of synthetic aperture radar (SAR) and interferometric
SAR (InSAR) methods. Can you explain to our readers the basic
rationale of the SAR and InSAR techniques?
Paul: Synthetic aperture radar is a means to improve the
resolution of a radar otherwise limited by the wavelength and
antenna size of the radar system. For microwave systems, the
wavelengths are in the 1-30 cm range and practical antennas are
in the 1-15 m size range. Radars can measure distance with good
resolution by using short or coded pulses, but the achievable
resolution in the angular extent of the radar footprint is dictated
by the beamwidth, given approximately by wavelength divided by
antenna diameter, so at a far distance (say, 1000 km, or from
space), the resolution can be as coarse as kilometers in scale.
For example, for L-band at 24-cm wavelength, using a 12-m diameter antenna gives 20-km resolution at 1000-km distance. Images constructed at this resolution do not show enough detail at
the scale of human activity-say 1-10 m. SAR is a technique to
exploit the motion of the radar in orbit to synthesize an effectively
longer antenna. This is done by exploiting the notion that any
given point on the ground is observed many times as the radar
flies by and the beam footprint sweeps over the point. We are able
to adjust the timing and phasing of each of the observations by a
hyperbolic function (related to the Fresnel scattering regime) to
align them as though they were acquired by a fixed and very long
antenna. With a large synthesized antenna, often kilometers in
size, we can resolve points to meter scale. SAR can generate im42

ages that are resolved in two dimensions, range and cross-range.
Readers may be confused because the angular extent of a radar
has two dimensions, and range is a third dimension, but we are
able to make two-dimensional images at fine resolution. This is
because we are imaging a two-dimensional surface. We use the
coarse resolution of the beam in the cross-track direction to our
advantage to illuminate a wide area and use range and crossrange to image the surface that cuts through this volume.
InSAR is interferometric SAR. In this case, we make two image observations from either two different nearby vantage points
or at two different times. SAR is a coherent imaging technique,
like a laser, and we create complex images with both reflectivity
and phase information. By "interfering" the two images pixel
by pixel, we are able to look at the phase difference in an image
format. When the two images are taken from a different vantage point, the phase difference is proportional to the topography
(height) of the surface. When the two images are taken at two
different times, the phase difference is proportional to how far
the surface moved from one time to the next.
Riccardo: Your achievements on differential SAR interferometry are incredible, particularly for what concerns crustal deformation
mapping. Which do you consider as your main result in this context?
Paul: I want to say that many of the fundamentals of differential SAR interferometry were investigated by others at JPL before
I entered the picture. I believe my main contributions to the field
were twofold: 1) to construct a processing methodology that was
efficient and accurate enough to do Earth science in a meaningful way and 2) to communicate the results to discipline scientists
and work with them to exploit the data. Put in this way, it doesn't
seem like much, but it is hard to stress enough the importance of
the availability of tools that could easily examine a large volume
of data in "discovery mode" at a time when it was necessary to
first discover if data was available, then discover if it was interferometrically usable, then discover if there was anything interesting in the data. Without efficiency and accuracy, it would be an
impossible task. The second point-communicating and working
with scientists-this is also extremely important and somewhat
of a hallmark of my style. Many engineers are too enamored of
their methods or impatient with nontechnical people to take the
time to explain the good, the bad, and the ugly about their system
or technique. With differential SAR interferometry, there are a lot
of bad and ugly things that would dissuade a scientist from using
it-the image processing itself is magic to most people, and many
things can go wrong in image formation, particularly with poor
metadata. Temporal changes over time often create areas of unusable noise in images. Radar images are naturally distorted and
don't look like optical images. The atmosphere adds biasing noise
even where the temporal noise is low. All these impediments need
careful explanation, and generally I had to work side by side with
the scientists to coax out the results we were looking for. My early
papers with Gilles Peltzer were important papers in my view, as
they demonstrated the power of the methods to really accomplish
new science. The paper published in the journal Science studying
postseismic deformation resulting in changes in fluid pressure in
the crust was really a new result and demonstrated to the community the potentials in these data (Figure 3).

IEEE A&E SYSTEMS MAGAZINE

APRIL 2017



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