The BYU Interferometric SAR, A revolutionary
SAR for use in topographical mapping.
What is SAR?
A Synthetic Aperture Radar (SAR) is an imaging radar which
uses signal processing to improve the resolution beyond the
limitation of the physical antenna aperture. A real aperture
radar requires large antennas to create a small antenna beam
width in the along track, azimuth, direction. This small beam
width facilitates the azimuth discrimination. The SAR uses the
Doppler shift of the signal to improve azimuth discrimination
thus decreasing the needed antenna length.
The BYU SAR
Synthetic Aperture Radar (SAR) systems are typically very
complex and expensive. They generate enormous quantities of
data, requiring very high capacity data storage, transmission,
and processing systems. We have developed an experimental SAR
system known as YSAR which has a very simple design which includes
near-real-time onboard processing. This system is based on recent
developments in low-cost, high-rate analog-to-digital (A/D)
and digital-to-analog (D/A) data conversion systems. Most of
the system is based on off-the-shelf components. A very simple
RF subsystem is used. The system has been successfully operated
from a moving surface vehicle and exhibits a range resolution
of 2.5 m though this could be improved to 1.5 m at the expense
of higher sidelobes. The four look azimuth resolution is 0.4
m.
The BYU SAR (YSAR) is a relatively inexpensive, lightweight
system. The system is designed to be flown in a four or six
passenger aircraft at altitudes up to 2000 feet.
The system cost and complexity are kept low by using commercially
available parts for most of the components. A standard PC system
is used, with plug-in cards for the analog-to-digital conversion
and digital signal processing. The chirp is generated by a low-cost
200 MHz Arbitrary Waveform Generator (AWG). A simple RF subsystem
up-converts the transmitted chirp using double-sideband modulation
and down-converts the received signal. Figure
1 (111K) is a photo of the system on board the airplane.
The PC is in the middle on the right, the AWG is at the top
on the right, and the RF system is on the bottom at the right.
The black components of the left are the AC power supply and
batteries.
The YSAR system has been successfully tested from a truck
and an aircraft (67K).
The system has a range resolution of 1.5 m and an azimuth resolution
of 0.5 m.
The following describes the YSAR system and presents results
obtained from system tests. The first section
shows the block diagram and describes each component. The
next section describes the deployment of the system. The
third section presents test results.
System Description
The YSAR system is composed of an RF subsystem, a chirp generation
subsystem, a digital subsystem, and an antenna subsystem. A
block diagram of the system is shown in Figure
3 (3K). The entire system weighs approximately 360 lbs,
with over half that coming from the battery-power supply. Each
of the subsystems is described below.
RF Subsystem
The RF subsystem consists of a transmitter, receiver, and
offset local oscillator and weighs approximately 70 lbs. The
transmitter mixes the 100 MHz bandwidth chirp up to 2.1 GHz
for transmission. The receiver and local oscillator are used
to mix the RF radar return from the antenna to an offset baseband
and amplify it so it can be sampled by the digital subsystem.
Chirp Generation
To reduce cost, the chirp is transmitted and received with double-sideband
(DSB) modulation, as shown in Figure
4 (2K). This avoids the cost associated with single-sideband
chirp generation and increases the effective bandwidth of the
chirp.
The baseband chirp signal is generated by a commercial Arbitrary
Waveform Generator (AWG). The chirp is first calculated by the
PC and then down loaded with timing information to the AWG's
memory over an RS-232 channel. The AWG is synchronized to the
local oscillator in the RF unit and is used to control the timing
for the entire system. The chirp bandwidth, the delay before
triggering the digital sampling, and the pulse repetition frequency
(PRF) are all software selectable. The LFM chirp may be windowed
with 6 different windows to allow tradeoffs between range sidelobes
and resolution. The AWG is the smallest system component at
about 25 lbs.
Digital Subsystem
The digital subsystem consists of a 486-based Personal Computer
system which has a total weight of 55 lbs. A high performance
analog-to-digital converter operates at a sampling rate of 500
MHz. The software can be configured to do the range compression
and display in real-time or to simply collect and store the raw
data. In order to meet timing constraints, the data is collected
into memory and dumped to the disk after a maximum sample length
of about 100 seconds. The data can be compressed onboard or downloaded
to high-end workstations for further processing.
Antenna Subsystem
The antenna subsystem consists of two custom microstrip patch
arrays. Each antenna array is approximately 3 by 1.5 feet and
is connected to the RF subsystem by standard SMA cables. Two such
arrays are used to improved isolation between the transmitter
and receiver portions of the RF subsystem. The two antenna arrays
are identical and are mounted end to end.
The Sonnet Software electromagnetic analysis package was used
in the design of the microstrip patch array. The patches in
the array were designed to resonate at three different frequencies
to improve the bandwidth of the antenna. The feed lines were
matched to the port of the antenna using transmission line methods.
The patches are fed in phase and with equal power. The arrays
were fabricated on an inexpensive substrate, resulting in a
somewhat lossy though well-matched antenna. The standing wave
ratio (SWR) of the array is below 2 over the entire 200~MHz
bandwidth and is 1.27 at the center frequency. The beam width
is 8.8 degrees in azimuth and 35.0 degrees in elevation at the
center frequency. The center fed antenna array layout is shown
in Figure 5 (10K). A
photo of the antennas mounted to the fuselage of the airplane
is shown in Figure 6 (69K).
Deployment
The initial system tests were made with the system mounted on
a truck in a nearby canyon. Corner reflectors were placed at strategic
locations to aid in identifying items in the image. The images
obtained from these tests are lower quality because of the grazing
incidence. The speed and direction of travel were also not as
constant in the truck as in an airplane.
In a recent series of test flights, the antennas were mounted
below the airplane fuselage, and the rest of the hardware occupied
the seat directly behind the pilot. The operator sat in the
rear seat. The initial test was in a rural area with corner
reflectors placed in the primary target areas. Several passes
were made to try different parameters and altitudes.
Results
A representative image from the truck tests is shown in Figure
7 (66K). This image was taken at approximately 22 m/s (50~mph)
with an azimuth sample rate of 200 Hz and a chirp length of 1us.
Several of the identified features are labeled in the figure.
The radar was on the road at the top of the image (not seen),
moving to the left and looking down the page. There is a short
section of guardrail along the road to the right of the figure.
Just behind that and a little further along the road are some
parked cars. Near the left of the picture and close to the road
is a set of small hills with a corner reflector on top of one
of them. In the center of the image there are several tree-covered
hills, with a corner reflector identified on one of them. In other
images further up the canyon a pipeline can clearly be seen at
about 200 m up the hillside.
Figure 8 (155K) shows
an image taken from the initial airplane test. An air photograph
of the same area is shown in Figure
9 (38K). The image was taken at approximately 52 m/s (100
knots) with an azimuth sample rate of 200 Hz and a chirp length
of 1.5 us. The altitude is 1000 ft. Important features are labeled
in both figures. The airplane was flown parallel to the road
seen near the bottom of the image (just below the edge of the
photograph). A church building can be seen near the road at
the right of the image. A parking lot surrounds the building,
with a fence and concrete-lined ditch behind the parking lot.
Just behind the fence and between the houses to the left are
unplowed fields. Further left is a road perpendicular to the
line of flight, with houses and other buildings along it. Further
away from the flight path near the center of the image is a
plowed field with a corner reflector pattern in it. The corner
reflectors were arranged in the form of a line, with a large
(1m) reflector in the center and smaller (0.6 m) reflectors
at the ends. This reflector pattern can be seen more clearly
in Figure 10 (19K), which
is a closer view of that portion of the image.
Early YSAR Truck test results
These early results were obtained with a sub-optimal antenna system
operating at 10 GHz.
Raw data Image
(89K)
Compressed, Annotated
Image (69K)
Site Photograph
(250K)
Plot of 100 MHz
bandwidth chirp
The BYU Interferometric SAR (IFSAR)
An interferometric SAR uses two antennas to recieve the returned
echo signal. The phase difference between the signals recieved
by each antenna can be converted into topographical information.
We are currently developing an IFSAR system. The BYU interferometric
SAR should be operational in about a year.
