Re: What is the difference between SAR, RADAR Interferometry, and InSAR

Greetings Todd:

References:

1. JPL Imaging Radar Home Page, What is imaging radar?:
http://southport.jpl.nasa.g ov/index.html

2. The Shuttle Radar Topography Mission (SRTM) Home Page
http://www.jpl.nasa.gov/srtm/

3. NASA DC-8 aircraft with AIRSAR/TOPSAR
Washington 090-2A
http://southport.jpl.na sa.gov/airsar/topsar/

4. Navy Research Laboratory (NRL) Radar Division ISR
http://radar- www.nrl.navy.mil/Areas/ISAR/

RADAR

RADAR (Radio Ranging And Detection) was developed during the late 1930s and became a
crash project early in World War II (1939-1945). The key component during the
development of RADAR was the invention of the magnetron transmitter tube in
the United Kingdom and later manufactured by the U.S. Magnetrons can produce
high power microwave pulses with peak powers up to several megawatts; however, their
poor frequency stability (incoherence) during the pulse was a limitation for processing
the received echo signals. During the1950s the Traveling Wave Tube Amplifier
(TWTA) was invented at the Bell Telephone Laboratories in New Jersey and it
was first manufactured and used in RADAR at the Hughes Aircraft Company during
the late 1950s.

The TWTA is able to amplify the frequency and amplitude profile of low level coherent
and modulated signals inserted into it and it has the ability to generate many
thousands of watts of coherent microwave power. Unlike the magnetron, the TWTA can
produce short or long pulses and can generate continuous wave (CW) microwave power
for communications and RADAR applications. To this day TWTAs are used in most
sophisticated RADARS and in most communications satellites. Using the advanced
properties of TWTAs during the 1960s and continuing today has enabled coherent signal
processing techniques to be developed starting from moving target indicators (MTI)
using Doppler processing to pulse compression techniques, and finally the development
of synthetic aperture RADAR (SAR) techniques. SAR prossing is essentially a Doppler
processing technique.

The References present very well illustrated and animated answers to your questions and
so I will start with a top level overview of your questions and then you can find the
details in the references.

SAR Techniques

The excellent coherent properties of TWTAs over many seconds of time have enabled
SAR techniques to be developed during the 1960s. Using a side looking antenna with a
wide beam an aircraft RADAR could record the interference (Doppler) pattern from echo signals
over several hundred to several thousand feet along the flight path. This information
was originally recorded on film for processing on the ground into a synthetic antenna
images with angular resolution equivalent to antennas several hundred to several
thousand feet in length. These photographs of radar signals off of a cathode ray tube
(CRT) are microwave holograms and by illuminating the RADAR exposed film with a laser,
which had been invented in 1960 at the Hughes Research Laboratories in California,
the result was a very high resolution RADAR map of
the illuminated terrain. Interestingly, although the concept of a hologram was first
proposed by D. Gabor in 1948 as a possible technique for improved electron microscope
imaging, it was SAR researchers at the University of Michigan that made the first
optical holograms in the mid-1960s using the first Helium Neon red lasers!

As digital processing speeds and computer processing power increased, SAR
optical processing was replaced by digital processing in the 1970s. The digital
SAR receiver data had to be processed on the ground into images after the aircraft
returned from flight. By the 1980s, U-2 and SR- 71 spy planes were sending the
received SAR signals by microwave links to world wide mobile processing centers in
trailers on the ground so that the radar maps were more quickly available to the
users. Finally by the 1990s, airborne signal processors with enough capability to
process the SAR signals in near real time, while in the air, became possible. Today
the SAR signals are being processed in the air and on the ground through satellite
links in near real time from radars in space, aircraft and pilotless vehicles and they
are being used for military applications, land resource management, and planetary
exploration.

Interferometric SAR

Inteferometric SAR was developed to add a third dimension (elevation) to SAR maps.
NASA has made extensive use of this technique for space borne imaging of the earth
and planets and for airborne imaging of the earth (See References 2 and 3). By
having two antennas side by side these systems can make stereo pairs of SAR images
of the earth providing maps with elevation information or actual 3D presentations
requiring special glasses for viewing. These viewing techniques are the same as those
used to view 3D motion pictures and References 1 and 2 present 3 D SAR images that
require red and blue glasses for viewing on your PC screen. These SAR maps often use
color to show elevation profiles on a 2 D SAR contour map. Reference 3 has several
examples of this technique.

Inverse SAR (ISAR)

During the late 1980s and continuing today, Inverse SAR (ISAR) techniques have been
developed primarily for imaging ships and spaceborne objects. However, One of the first
applications of ISAR was performed by radio astronomers when they made a radar map of
the planet Venus from earth. Until recently these techniques have been mostly clothed
in secrecy; however, today commercial shipboard and airborne RADAR systems with ISAR
capability are now becoming available.

To make a conventional SAR image as an aircraft moves it views and records signals from
fixed targets at many different angles. A fixed ground based or slow shipboard radar could
not make use of SAR techniques until ISAR was developed. In ISAR the radar records
echo signals from moving targets such as ships, spacecraft or rotating planets.
These recordings are made over many viewing angles forming a microwave hologram of the moving
or rotating target. The resolution of early ISAR images have not been very good as
the Naval Research Labs (NRL) image of a ship in reference 4
demonstrates. However, these images are continuing to improve. As the NRL web site
indicates, ISAR is now becoming airborne and by having an aircraft orbit a moving
target an ISAR image can be also developed. The image in Reference 4 is a very early
image; however, the animation on the web site demonstrates the ISAR technique.

Comments on the References

NASA and JPL have several excellent, animated websites describing SAR and
Interferometic SAR. Reference 1 describes SAR in detail. Reference 2. describes
the Shuttle Topography Mission which mapped the earth using interferometric SAR and
has numerous radar images. The Shuttle Radar Topography Mission (SRTM) obtained
elevation data on a near-global scale to generate the most complete high-resolution
digital topographic data base of Earth. SRTM consisted of a specially modified radar
system that flew onboard the Space Shuttle Endeavour during an 11-day mission in
February of 2000. Reference 3. describes NASA's DC-8 airborne interferometric SAR.
The image of Washington DC on the web site clearly shows the Pentagon buildings
elevation in green raising above the blue parking lot in the upper left corner of
the image (Image Washington 090-2A). Reference 4. the U. S. Naval Research Labs
website presents an ISAR image of a ship and discusses the technique.

Best regards, Your Mad Scientist
Adrian Popa