The SlimSAR

Theoretical and Practical Design Considerations for a Small, Multi-

Band SAR: The SlimSAR

26 July 2010

Evan Zaugg, Matthew Edwards, and Alex Margulis ARTEMIS Inc. Hauppauge, NY

The SlimSAR

• Introduction– Design Heritage

• SlimSAR System Design Details– Methodology– Multiple Operating Frequencies– Deramp Mode

• System Performance Trade-offs and Flexibility• Example SAR Imagery• Conclusion

The SlimSAR

Introduction: Using SAR on a Small UAS

• UAS are extremely useful in dangerous, remote, or long duration situations

• Provide intelligence, surveillance, and reconnaissance capabilities

• Electro-optic/infrared (EO/IR) instruments are useful and very common

– Limited by obstruction due to clouds, fog, dust, and smoke

• Synthetic aperture radar (SAR) can be used on larger platforms

– Provides high-resolution imagery

– Day and night

– All weather conditions

– Additional information available at different frequencies• Change detection• Shallow dry-ground penetration• Enhanced target detection

• Operating a SAR on a small UAS is beneficial

– Often precluded by size weight and power (SWAP) of SAR systems

The SlimSAR

Introduction: The SlimSAR Solution

• A new advancement in SAR– High-performance– Small– Low-cost– Suitable for operation on small

UAS• Design Heritage

– Exploits techniques and technologies developed for previous systems

– Multi-Frequency• Increased capability and flexibility,

all in a small package

Frequency Bands L-band / X-band / UHF / +

Bandwidth / Resolution

Variable 660 MHz / 23 cm

Transmit Power 25 W

Operational Altitude

1000-6000 ft (AGL)

Radar Weight < 10 lbs

Power Consumption

< 150 W

Table 1: SlimSAR Specifications

The SlimSAR

History

• ARTEMIS, Inc. has been supporting SAR programs for over a decade with development and manufacturing– Our receivers, exciters, and up-converters (REUs) are a part of

• Global Hawk

• U-2

• ASTOR.

– Recent experimental programs include• UAVSAR (with Jet Propulsion Laboratory)

• NuSAR (with the Naval Research Laboratory and Space Dynamics Laboratory)

• MicroASAR (with Brigham Young University)

The SlimSARThe NRL UAS SAR System (NuSAR)

• Part of NRL's DUSTER program

– Team effort with BYU, ARTEMIS, SDL, and NRL

– Designed for UAS flight.

Frequency Bands L-band / X-band

Bandwidth / Resolution Variable 500 MHz / 30 cm

Transmit Power 25 W

Operational Altitude 2500-6000 ft (AGL)

X-Band NuSAR Image: The Bear River and I-15 North of Brigham City, Utah

The NuSAR L-band RF

Table 2: NuSAR Specifications

The SlimSARMicroASAR

• A continuous wave (CW) SAR system

– High SNR transmitting much less peak power

– Analog de-chirp on receive reduces the sampling requirements to keep the data rate low

Frequency Band C-Band

Bandwidth / Resolution Variable 200 MHz / 75 cm

Transmit Power 1 W

Operational Altitude 1000-5000 ft (AGL)

Weight 5 lbsThe entire MicroASAR

Table 3: MicroASAR Specifications

MicroASAR image of Arctic sea ice from CASIE-09 – see paper 3562 in poster area H on Thursday

The SlimSARSystem Design Methodology

• Quickest path from system requirements specification to deployment of successful solution

– Design is based on functioning, tested SAR systems (MicroASAR / NuSAR-B)

– Modify existing designs to meet new requirements, preserving design heritage

– Risks associated with new, untested technologies are minimized

Antennas mounted under the belly of the test bed

SAR equipment installed in the test bed

NuSAR-B

MicroASAR

Computer

UPS

IMU

The SlimSARSystem Design Methodology

• Benefits in system testing and integration– MicroASAR has been operating during the SlimSAR development period on a

small, manned aircraft which is used as a UAS surrogate– Test and refine data collection, handling, and processing methods which are used

with very little modification for SlimSAR– Ready for initial flight testing as soon as the hardware is completed– Immediate flight testing on the test bed aircraft reveals any changes which may be

necessary in the processing algorithms or other supporting systems

A Cessna O-2 Skymaster, our test bed aircraft “Surf Angel” on the runway at Brigham City, Utah

The SlimSARSlimSAR System Design

• Design concept as a compact pod-mount unit consisting of :– Wide-beam L-band antenna– Gimbaled X-band antenna

– The radar (L-band and X-band)– Tactical CDL– Motion measurement system

and GPS

Example illustration of the SlimSAR and all the supporting subcomponents in a single pod-mount package

• All this weighs less than 20 lbs and consumes less than 200 W

The SlimSARMultiple Operational Frequency Bands Using Block-Conversion

• System core is L-Band, and L-Band operation is always possible• Block-conversion and switching allows operation at other bands• X-Band and UHF block converters currently operational and tested• Ku-Band converter in design stages• Other bands possible

The SlimSARVariable Downconversion LO Enables Flexibility in Operating

Frequency

• Two DDS chips generate separate signals

• Each is upconverted in the same manner

• For direct-sampling mode, DDS 2 generates an LO. This allows any subset of the possible frequency range to be used

• For deramp mode, DDS 2 generates a time-delayed copy of the transmitted chirp

The SlimSARDeramp Mode

The received signal is mixed with a time-delayed copy of the transmit signal. In the resulting signal, targets at a certain range are mapped to a single frequency.

Spectrogram of deramped signal

• Bandwidth of the deramped IF signal may be significantly smaller than that of the transmitted signal

• In this way, resolutions which are higher than that supported by directly sampling the RX signal can be achieved

• Imaged swath is reduced in order to keep deramped signal within sampling requirements

The SlimSARSystem Specifications

• Supports a contiguous signal bandwidth up to 660 MHz

– L-band• 1119.4 MHz to 1779.4 MHz, Center frequency 1449.4 MHz• Possible reduced configuration: 1257.5 MHz with 85 MHz bandwidth• Horizontally and vertically polarized antennas for polarimetric operation

– X-band and UHF Block Converters Operational• Two separate X-bands

– 8954.6 MHz to 9614.6 MHz, Center frequency 9284.6 MHz

– 9934 MHz to 10594 MHz, Center frequency 10264 MHz

• UHF– 350 MHz to 550 MHz

– Any number of notches can be placed in the waveform to keep out of restricted frequency bands

• Built in solid-state power amplifier outputs 25 W peak power for pulsed operation.

– Sufficient for operational altitudes of 5000-8000 feet AGL.

– Can add external power amplifier to obtain a better SNR at higher altitudes

The SlimSAR

System Performance Trade-offs and Flexibility

• Every radar system has inherent performance tradeoffs, and SlimSAR is no exception

• The unique design of the SlimSAR, however, makes it very flexible

• By simply adjusting some of its operational parameters, the SlimSAR can be made to operate in a wide variety of imaging situations

• Second DDS and upconversion chain enable a wide range of operating frequencies and bandwidths in either direct-sampled or deramp mode.

The SlimSAR

Sample Image Products• Images collected with using SlimSAR at both L and X-Band• Flights in ARTEMIS Surf Angel over various areas• Polarimetric Data Collected

The SlimSAR

• Images

L-Band HH L-Band VV

Snohomish River South of Everett, WA

C-Band MicroASAR

The SlimSAR

L-band Sample Image Products• Images

North of Spanish Fork, UT

The SlimSAR

• Images

North of Spanish Fork, UT – 85 MHz L-Band SlimSAR

The SlimSARNorth of Spanish Fork, UT – 240 MHz X-Band SlimSAR

The SlimSARL-Band vx. X-Band Imagery

The SlimSAR

Conclusion• SlimSAR Advantages :

– A strong design heritage – Rapid testing and integration

• Quick schedule from initial concept designs in October 2008 to flight testing an LFM-CW version the week of 15 June 2009. The pulsed version had it’s first flight tests the week of 11 January 2010.

– Flight tests aimed at proving the SlimSAR against an array of targets of interest and readying the system for integration onto a small UAS

– The flexible design facilitates future modifications• Alternative frequencies• Higher bandwidths• GMTI• Interferometry• Littoral and maritime modes• Polarimetry