1. Advanced Synthetic Aperture Radars

2. Objectives
Explain how synthetic aperture radar is able to provide an extremely high-resolution radar image.
Explain how inverse synthetic aperture radar works and how it is able to provide an actual “image” of a contact.

3. Angular Resolution of Radar Systems
Beamwidth is approximated by:
q = kl / L {radians}
If L  q 
At a given range, ability to resolve objects in cross range direction (azimuth) is known as cross range resolution.
D Rcross = R q (q = Arc length swept in radians swept at radius R)
If q  D Rcross  (gets better)
Better the D Rcross , better the detail of the image.
For 6o beamwidth (0.1 radians) at 2000 yds
D Rcross = 200 yds

4. Synthetic Aperture Radar (SAR)
Uses motion of transmitter/receiver to generate a large effective (synthetic) aperture.
Creates a very narrow beamwidth & better resolution
Often incorporated in satellite systems
High velocity
Large distance traveled
Exact position known
System stores several returns while SAR system moves.
Reconstructs the returns as if they were taken simultaneously, to create the synthetic aperture.
Used for imaging large stationary objects.
   uses motion of transmitter / receiver to generate a large effective aperture
ii)       this creates a very narrow beamwidth & better resolution
iii)     used with satellite radar systems since satellites travel at a high velocity; the accuracy of these systems can be made very high.
iv)     the system stores several returns taken while antenna (satellite) is moving (distance S) and then reconstructs them as if they came simultaneously, to create a synthetic aperture (also = S); the synthetic aperture is “substituted” for the physical antenna aperture.

5. Synthetic Aperture Radar (SAR)

6. SAR Grid
Consider an airborne SAR imaging perpendicular to the aircraft velocity as shown in the figure below. Typically, SARs produce a two-dimensional (2-D) image. One dimension in the image is called range (or along track) and is a measure of the "line-of-sight" distance from the radar to the target. Range measurement and resolution are achieved in synthetic aperture radar in the same manner as most other radars: Range is determined by precisely measuring the time from transmission of a pulse to receiving the echo from a target and, in the simplest SAR, range resolution is determined by the transmitted pulse width, i.e. narrow pulses yield fine range resolution.
The other dimension is called azimuth (or cross track) and is perpendicular to range. It is the ability of SAR to produce relatively fine azimuth resolution that differentiates it from other radars. To obtain fine azimuth resolution, a physically large antenna is needed to focus the transmitted and received energy into a sharp beam. The sharpness of the beam defines the azimuth resolution. Similarly, optical systems, such as telescopes, require large apertures (mirrors or lenses which are analogous to the radar antenna) to obtain fine imaging resolution. Since SARs are much lower in frequency than optical systems, even moderate SAR resolutions require an antenna physically larger than can be practically carried by an airborne platform: antenna lengths several hundred meters long are often required. However, airborne radar could collect data while flying this distance and then process the data as if it came from a physically long antenna. The distance the aircraft flies in synthesizing the antenna is known as the synthetic aperture. A narrow synthetic beamwidth results from the relatively long synthetic aperture, which yields finer resolution than is possible from a smaller physical antenna.
Achieving fine azimuth resolution may also be described from a doppler processing viewpoint. A target's position along the flight path determines the doppler frequency of its echoes: Targets ahead of the aircraft produce a positive doppler offset; targets behind the aircraft produce a negative offset. As the aircraft flies a distance (the synthetic aperture), echoes are resolved into a number of doppler frequencies. The target's doppler frequency determines its azimuth position.
While this section attempts to provide an intuitive understanding, SARs are not as simple as described above. Transmitting short pulses to provide range resolution is generally not practical. Typically, longer pulses with wide-bandwidth modulation are transmitted which complicates the range processing but decreases the peak power requirements on the transmitter. For even moderate azimuth resolutions, a target's range to each location on the synthetic aperture changes along the synthetic aperture. The energy reflected from the target must be "mathematically focused" to compensate for the range dependence across the aperture prior to image formation. Additionally, for fine-resolution systems, the range and azimuth processing is coupled (dependent on each other) which also greatly increases the computational processing.

7. SAR Data Collection
Data collected sequentially and then processed simultaneously.

8. SAR Advantages Disadvantages/Limitations
Extremely large effective aperture
Outstanding resolution.
Disadvantages/Limitations
Exact position of transmitter/receiver must be known.
Requires massive storage capacity to accumulate returns while radar (aircraft or satellite) moves.
Requires massive computing power to process returns as if they were received simultaneously.
Flight Path Restrictions

9. Flight Path Restrictions for SAR Mapping
Scan Limit
Mechanically Scanned Array Radars scan limit of ± 60°
AESA Radar scan limit of ± 70° - degraded past ~ ± 55 °
Forward-mounted and fixed
Modified by installation tilt and aircraft altitude
Some radars have side facing antennas (JSTARS) not limited.
INS/Autofocus Limit
INS supplies data for MOCOMP (motion compensation) to keep the image focused
“Autofocus” used to correct for acceleration errors
Acceleration errors in the INS build up over time, degrading resolution
Accurate INS required for Autofocus

10. SAR
With actual SAR processing, about a thousand calculations are performed for every single pixel.
This image of Washington, D.C., is made up of several million pixels.
Dark areas = low return
White areas = high return

11. SAR Applications Reconnaissance, Surveillance and Targeting
Treaty Verification and Nonproliferation
Interferometry (3-D SAR)
Navigation and Guidance
Foliage and Ground Penetration
Moving Target Indication
Change Detection
Environmental Monitoring
Reconnaissance, Surveillance, and Targeting.
 Many applications for synthetic aperture radar are for reconnaissance, surveillance, and targeting. These applications are driven by the military's need for all-weather, day-and-night imaging sensors. SAR can provide sufficiently high resolution to distinguish terrain features and to recognize and identify selected man made targets. (Example: SAR image of M-47 Tanks, 1-ft resolution) and an optical photo of the same tanks)
Treaty Verification and Nonproliferation.
 The ability to monitor other nations for treaty compliance and for the nonproliferation of nuclear, chemical, and biological weapons is increasingly critical. Often, monitoring is possible only at specific times, when overflights are allowed, or it is necessary to maintain a monitoring capability in inclement weather or at night, to ensure an adversary is not using these conditions to hide an activity. SAR provides the all-weather capability and complements information available from other airborne sensors, such as optical or thermal-infrared sensors.
Interferometry (3-D SAR).
 Interferometric synthetic aperture radar (IFSAR) data can be acquired using two antennas on one aircraft or by flying two slightly offset passes of an aircraft with a single antenna. (Example: Interferometric SAR image created by two imaging passes [two synthetic apertures]) Interferometric SAR can be used to generate very accurate surface profile maps of the terrain.
 Sandia has developed new mathematical techniques for relating the radar reflection from the terrain surface to the time delay between radar signals received at the two antenna locations. The techniques are directed at removing ambiguities in estimates of surface heights and are referred to as 2-D least squares phase unwrapping.
Navigation and Guidance.
Synthetic aperture radar provides the capability for all-weather, autonomous navigation and guidance. By forming SAR reflectivity images of the terrain and then "correlating" the SAR image with a stored reference (obtained from optical photography or a previous SAR image), a navigation update can be obtained. Position accuracies of less than a SAR resolution cell can be obtained. SAR may also be used to guidance applications by pointing or "squinting" the antenna beam in the direction of motion of the airborne platform. In this manner, the SAR may image a target and guide a munition with high precision.
Foliage and Ground Penetration.
 Synthetic aperture radars offer the capability for penetrating materials which are optically opaque, and thus not visible by optical or IR techniques. Low-frequency SARs may be used under certain conditions to penetrate foliage and even soil. This provides the capability for imaging targets normally hidden by trees, brush, and other ground cover. To obtain adequate foliage and soil penetration, SARs must operate at relatively low frequencies (10's of MHz to 1 GHz).
 Recent studies have shown that SAR may provide a limited capability for imaging selected underground targets, such as utility lines, arms caches, bunkers, mines, etc. Depth of penetration varies with soil conditions (moisture content, conductivity, etc.) and target size, but individual measurements have shown the capability for detecting 55-gallon drums and power lines at depths of several meters. In dry sand, penetration depths of 10's of meters are possible.
Moving Target Indication.
The motion of a ground-based moving target such as a car, truck, or military vehicle, causes the radar signature of the moving target to shift outside of the normal ground return of a radar image. Sandia has developed techniques to automatically detect ground-based moving targets and to extract other target information such as location, speed, size, and Radar Cross Section (RCS) from these target signatures. Please view our MTI / CCD imagery library.
Change Detection.
 A technique known as coherent change detection offers the capability for detecting changes between imaging passes. (Example: Coherent Change Detection image of vehicle tracks and an optical photo of the same area) To detect whether or not a change has occurred, two images are taken of the same scene, but at different times. These images are then geometrically registered so that the same target pixels in each image align. After the images are registered, they are cross correlated pixel by pixel. Where a change has not occurred between the imaging passes, the pixels remain correlated, whereas if a change has occurred, the pixels are uncorrelated. Of course, targets that are not fixed or rigid, such as trees blowing in the wind, will naturally decorrelate and show as having "changed." While this technique is useful for detecting change, it does not measure direction or the magnitude of change.
Environmental Monitoring.
 Synthetic aperture radar is used for a wide variety of environmental applications, such as monitoring crop characteristics, deforestation, ice flows, and oil spills. (Example: SAR image of a naturally occurring oil seepage) Oil spills can often be detected in SAR imagery because the oil changes the backscatter characteristics of the ocean. Radar backscatter from the ocean results primarily from capillary waves through what is known as Bragg scattering (constructive interference from the capillary waves being close to the same wavelength as the SAR). The presence of oil dampens the capillary waves, thereby decreasing the radar backscatter. Thus, oil slicks appear dark in SAR images relative to oil-free areas.

12. Satellite SAR good for large fixed targets (cities, military bases, TERCOM maps, etc.)

13. UAV’s

14. Lynx SAR
M47 Tanks

15. JSTAR
Joint STARS AN/APY-3 Joint Surveillance Target Attack Radar System
The Joint Surveillance Target Attack Radar System (Joint STARS) designated AN/APY-3, is a long-range air-to-ground surveillance and battle management system. It is capable of looking deep behind hostile borders to detect and track ground movements, in both forward and rear echelon areas and to detect helicopter and fixed-wing aircraft. Joint STARS provides air and ground commanders with the intelligence and targeting data for management of their war-fighting assets.     Joint STARS is a complex of systems. It comprises an airborne platform, four major subsystems and a ground station module that receives, in near-realtime, radar data processed in the aircraft. The four major subsystems, all integrated on the airborne platform, consist of an advanced radar; internal and external communications including UHF, VHF and HF voice links, the Joint Tactical Information Distribution System (JTIDS) and a newly developed surveillance and control datalink; operations and control including advanced computers and 18 operator display stations which perform the data processing and display functions for tens of thousands of targets and C3I operation; and a self-defence suite which is in the process of being specified. Joint STARS is a joint US Air Force and US Army development, with the air force being responsible for the airborne segment and the army for the ground segment.     Joint STARS detects, locates, classifies, tracks and targets potentially hostile ground movements in virtually all weather. It operates in near-realtime and in constant communication through a secure datalink with army mobile ground stations that, in turn, can use TACFIRE and the advanced field artillery tactical data system to talk to artillery for fire support or to the all-source analysis system using US message format. Joint STARS will also maintain constant communication with air force tactical command posts via JTIDS. The platform for Joint STARS is a modified and militarised version of the Boeing series aircraft.     The technology employed in the radar was initially demonstrated as part of the Air Force/DARPA Pave Mower programme in the late 1970s. Major technological achievements of Joint STARS include the software-intensive displaced phased-centre slotted array antenna radar with several concurrent operating modes; the unique 8 m antenna mounted under the fuselage of the aircraft developed by Norden Division of United Technologies; very high-speed processors, each capable of over 600 Mops; high-resolution colour graphic and touchscreen tabular displays; the wideband surveillance and control datalink and over one million lines of integrated software code. The radar operates as either a side-looking synthetic aperture radar for the detection of fixed or stationary targets, or a Doppler radar to track slow-moving targets such as tanks or troop platforms.     In September 1987, the air force, following the August Joint Requirements Oversight Committee review, increased the number of E-8 aircraft from 10 to 22. In April 1988, the Conventional Systems Committee, after reviewing the Joint STARS airborne segment, concluded that the airborne platform should be changed from used aircraft to new 707s. However, in October 1989, the air force returned to the plan to continue the airborne portion of the programme with used 707 aircraft, on the grounds of the higher cost of new aircraft, to meet an initial operational capability in     In September and October 1990, operational field demonstrations in a dense electromagnetic environment and poor weather were completed in Europe. After six weeks of flying demonstrations, consisting of 25 missions and 110 flying hours, Joint STARS concluded the demonstration of the system's capabilities and data gathering for the continuing development of the programme. US Air Force and Army officials assessed the demonstration as a complete success.     In November 1990, Northrop Grumman received a contract for the development of the third Joint STARS full-scale development aircraft and follow-on full-scale development work. Meanwhile, the two E-8A prototype aircraft (T-1 and T-2) achieved outstanding success in the 1991 Gulf War, and provided strong impetus for full development of the E-8C production configuration.
Specifications
Antenna: 7.3 m long, side-looking, phased-array, housed in canoe-shaped radome under forward fuselage aft of nose landing gear; scanned electronically in azimuth, steered mechanically in elevation from either side of aircraft. Operating modes: Wide area surveillance; fixed target indication; Synthetic Aperture Radar (SAR); moving target indicator; target classification. Workstations: 17 identical workstations for system operators; one navigation/self-defence workstation; each operator can perform: flight path planning and monitoring; generation and display of cartographic and hydrographic map data; radar management, surveillance and threat analysis; radar; radar data review; time of arrival calculation; jammer location; pairing of weapons and targets, and other functions. Communications: Surveillance and Control DataLink (SCDL) for transmission to ground stations; JTIDS; TActical Data Information Link-J (TADIL-J); Satcom; two encrypted HF radios; three encrypted VHF radios with provision for SINgle Channel Ground and Airborne Radio System (SINCGARS).
Taken from Janes Online

16. Synthetic Aperture Radar (SAR) Operation
Crisp Contrast
Between Roads
and Grass
Sharp Radar
Shadows from
the Trees

17. Very Long Range SAR Decoys Target Target Photo High Resolution SAR
Provides clear detail of cracks on runway, targets, & decoys

18. Zoom in on areas of interest. (TARGETS!!)
1 NM
Normal Size SAR Image

19. Inverse Synthetic Aperture Radar
Large synthetic aperture can be achieved without moving transmitter/receiver.
If target has motion yaw, pitch, roll, (read ‘ship’), then it has same effect as radar moving equal to corresponding arc length at range R.

20. ISAR y fD = 2(DY/Dt)L l l Rcross = 2(DY/Dt)tint
Rcross is independent of
range and totally dependent
on angular rotation rate
Rate of rotation
DY / Dt

21. ISAR Principle
Motion of the target causes a Doppler shift that depends on the angular velocity of the target motion
Need a moving (rocking) target
A plot of Doppler shift vs range will give an image
Image will slowly shrink and become inverted as the rocking motion goes from positive to negative producing an inverted image
i)        achieves the same large synthetic aperture without moving the transmitter / receiver
ii)       measures target Yaw Angle or slight rotation.
iii)     used for long range imaging and identification
iv)     best targets for ISAR are ships since they yaw periodically in the sea state
v)      Cross range resolution:

22. ISAR Resolution
ISAR is used for long-range target imaging and identification.
May not be able to identify target but can certainly identify warship/non-warship.
Can sometimes ID class depending on aspect angle
ISAR platforms can be fixed or moving.
Best targets are ships or submarine scopes.
Note: independent of range!

23. ISAR Example Problem 1 .089m Rcross = 2(.165 rad/s)10secs
Find the cross range resolution of an ISAR system operating at 3.35 GHz, that collects data over a yaw angle of 9.5o per sec, with an integration time of 10 secs
= c / f  x108 m/s / 3.35 GHz  m
DY/Dt = 9.5o /sec = .165 radians/sec
T = 10 secs)
= .027 m at any range.
Rcross =
2(.165 rad/s)10secs
.089m

24. ISAR 1

25. CRUISE SHIP ISAR IMAGES

26. SAILBOAT ISAR DETAILS Main Mast Main Mast Roller Furling Jib
Mizzenmast
Mizzenmast
Roller Furling Jib
Furled Sail
Mizzenmast
Furled Sail
47 Pixels (15 m)
Frame 32 “Flipped”

27. SAILBOAT ISAR IMAGES 340 Hz Doppler Range 032 050 84 m 060 088 104
near
84 m
far
060
088
104

28. TALL SHIP GUYAS ISAR IMAGES

29. Objectives
Explain how synthetic aperture radar is able to provide an extremely high-resolution radar image.
Explain how inverse synthetic aperture radar works and how it is able to provide an actual “image” of a contact.