Physics of SAR

1
Physics of SAR

• Summer 2003

2
SAR Radar
SAR
Synthetic-Aperture Radar
RAdio Detection And Ranging
Radar - Transmits its own illumination a
"Microwave flashlight"
3
Radar Imaging
Form a terrain image using a radar in a
Problem
moving airborne/orbital vehicle
Simplest Approach - Real-Beam Imaging Radar
Example
P
lan
P
osition
I
ndicator (PPI)
0
Range
Azimuth
Azimuth
90
270
Individual image points (pixels) must
be discriminated in two dimensions,
range and azimuth
180
PPI Display
4
Range Discrimination
2D
d
t
t
t
D
d
D
d
The transmitted pulse travels at the speed of
light 109 feet/second Þ 1 nanosecond/foot Round
trip "radar time" Þ 2 nanoseconds/foot (Dd 2
feet Þ Dt 4 nanoseconds) But target returns
overlap if targets are separated by less than t/2


5
Shorter Pulses
So for better range resolution, just make
SHORTER
the transmitted pulse
However , the shorter pulses must somehow
SAME ENERGY
transmit the
to the target



SHORTER
HIGHER
As the pulse gets
, the peak power gets
Peak power gets MUCH too high beforepulse length
even approaches high resolution
Problem
6
Coded Pulses
Transmit a long coded pulse that can be decoded
(compressed) afterreception into a much shorter
pulse
Solution
f
f
1
2
t
Linear F.M. (Frequency Modulation)
Linear Swept Frequency
"Chirp"
Note A typical 200 microsecond pulse extends
over more than 16 nautical miles in radar space
7
Pulse Compression
Frequency
f
2
D
f
Transmitted/Received Pulse
f
1
t
Time
t
t
1
2
Frequency
f
2
Variable Delay Line"Compression" Filter
D
f
f
1
Delay
Time
t
0
Frequency
f
2
1
Decoded / "CompressedOutput
D
f
D
f
f
1
Time
Pulse compression ratio pulse "time-bandwidth
product"
8
Pulse CompressionAdvantages

• Range resolution independent of transmit pulse
  length
• Transmit long pulses
• Keep peak power comfortably low
• Set range resolution with transmitted bandwidth
• Resolution inversely proportional to bandwidth
• 300 MHz ñ 2-foot resolution
• 600 MHz ñ 1-foot resolution
• Resolution independent of slant range

9
Azimuth Considerations
SAR
Synthetic-Aperture Radar
Antenna beamwidth is inversely proportional to
the number of wavelengths in its length (aperture)
l
L
q

radians
L
c
l

f
10
Azimuth Discrimination
Flight Path
L
D
d
L
R
Real-beam imaging radar

• As the collection vehicle moves along the flight
  path, targets are detected as they move in and
  out of the antenna pattern
• But target returns overlap if the targets are
  separated in azimuth by less than the antenna
  beamwidth
• So Achievable azimuth resolution decreases with
  range

11
Narrower Beamwidth

• So for better azimuth resolution, just make the
  antenna beam NARROWER!
• Generate more wavelengths in the antenna aperture
  by lengthening the antenna or by shorting the
  wavelength (increasing the frequency)
• However, very LONG antennas are difficult to
  carry and position, and very HIGH frequencies
  limit performance in weather and at long ranges

Antennas get MUCH too long and frequencies MUCH
too high before the beamwidth even approaches
high resolution
Problem
12
Synthetic-Aperture
Synthesize a long antenna apertureusing a
physically short antenna
Solution
SAR
Synthetic-Aperture Radar
Store the data collected sequentially and
coherently across a long aperture and then
process the data to synthesize a full aperture
collection
13
Azimuth Considerations
Flight Path
SyntheticallyProcessedAperture(LS)
Synthetically
Processed
Beam
l
LP
PhysicalAntenna(LP)
l/LS
Real Beam
14
Synthetic-ApertureAdvantages

• Increased angle of collection on a target allows
  increased resolution
• High resolution capability with short physical
  antenna
• Processed aperture size is easily increased as
  imaging distance increases
• Azimuth resolution independent of slant range