Research Topics in Radar for Academics

1. Research Topics in Radar for Academics PW van der Walt Reutech Radar Systems why? 2008 Ledger Conference

2. Introduction • After 70 years of development, radar is a mature system • Radar provides unique sensing capabilities • No other sensor can search volumes of comparable size continuously • New sensing requirements demand new radars • Radar requirements are becoming more demanding • Coupled with the rapid development of electronic technology, radar is still evolving rapidly • Radar architectures that designers could only dream about a mere 15 years ago have become implementable 2008 Ledger Conference

3. Introduction • In the 1950's, the appearance of the ICBM spurred the development of completely new radars. User requirements changed: • 1945: Locate a fighter aircraft at 100 km to • 1955: Locate the equivalent of a metal grapfruit at 1000 nm • In the 2000's, piracy on the high seas may again spur the development of completely new radars • Locate small craft approaching large ships in rough seas within an area of 360000 nm2 • Radar is the only single sensor that can provide the information • Francois Anderson has the answer! 2008 Ledger Conference

4. Introduction • Radar remains a dynamic and challenging system, not fully understood yet, offering many opportunities for research • In the signal path, processing and structural hardware • In new and improved processing algorithms to extract useful information from data • In designing algorithms to match specific hardware platforms • manage peak loads • optimise throughput • In this talk I will outline topics from the necessarily biased and limited perspective of someone involved mainly with hardware in the analogue domain which I think can provide useful research opportunities for academics, in • radar hardware • radar information problems 2008 Ledger Conference

5. Antennas • The antenna is a critical radar component • The ability of a radar to locate a target in 3D space is ultimately dependent upon the radiation pattern, bandwidth, impulse response and stability of the antenna • Radar has a unique combination of requirements for antennas, including • Stringent electrical performance requirements for the radiation pattern and losses • Exceptional mechanical stability in unfriendly environments • High mobility and spatial re-orientation • Long life expectancy 2008 Ledger Conference

6. Antennas (ctd) • Antennas are undergoing rapid evolution on two fronts • Our ability to meet increased performance requirements made possible by powerful computer-based design tools • The appearance of new (and not so new!) materials and manufacturing processes challenging the designer to apply these creatively to reduce manufacturing cost and mass • metallized plastics as opposed to metal • bonding as opposed to welding • The radar antenna is an interdisciplinary challenge to electronic and mechanical engineers requiring teamwork to an extraordinary degree 2008 Ledger Conference

7. Two antenna arrays Single stick, non squinting 2x12 stick arrays, squinting 2008 Ledger Conference

8. Antennas (ctd) • There are research opportunities in "rediscovering" known antenna configurations • Using modern tools to investigate the performance limits to which these can be pushed, including parameters such as • Bandwidth • Size • Beamshape • Mass • Using non traditional materials in their construction 2008 Ledger Conference

9. Antenna wishlist • An antenna "plank" • Bandwidth 20% • Azimuth Beamwidth 1° non-squinting • Elevation beamwidth 70° • Gain > 26 dB • at the price of a travelling-wave antenna • Can one perhaps make a centre-fed pill-box with f/D=0.2 do this? • or do it with left-handed materials in a travelling wave array? 2008 Ledger Conference

10. Passive components • Our ability to design and produce complex filters has increased in leaps and bounds with new EM analysis software • There are also interesting developments in materials and manufacturing technology • Can you use rapid prototyping techniques to produce components in small quantities? • What are the limitations on component performance with these techniques? • How far can you go with metal plated plastics? 2008 Ledger Conference

11. The Powertrain • Monostatic radar requires large average transmit power. This creates ongoing opportunities for research • Solid state power technology is advancing rapidly, currently with LDMOS and HVVFET, and GaN in the near future. Per device: • Last week: 350 W output power @ 10% duty in L band • This week: 500 W output power @ 25% duty in L-band • 17 dB gain per stage • 80 W output power reported in X band • Equally important is DC power conditioning for the amplifier • Pulsed loads of 20 A @ 50 V • Voltage must be stable to mV level from pulse to pulse • Must meet stringent EMC requirements 2008 Ledger Conference

12. L Band LDMOS 500 W Low Z ports 2008 Ledger Conference

13. The Powertrain • Control devices • Eg solid state electronic duplexers and limiters • X band • 8 kW peak • 500W average • 20% bandwidth • 60 dB isolation • Isolated combiners • L band • 10 – 20 kW peak • 1 – 2 kW average 2008 Ledger Conference

14. Low noise sources • With the increasing extraction of information from radar returns, there is a growing need for sources with low close-in noise • FMCW search radars require sources with very low far-out noise • e.g. -150 dBc/Hz @ 1 MHz offset in X-band • Research topics • phase noise mechanisms in non-linear circuits • architectures for low phase noise synthesizers • low phase noise power amplifiers 2008 Ledger Conference

15. Measured phase noise Noise floor set by system architecture 2008 Ledger Conference

16. Receivers • Modern MMIC's and new pcb materials are revolutionising the way we build receiver and transmit chains • They include niceties such as high IP3 diode mixers with on-chip LO amplifiers, requiring less than 0 dBm of LO drive power • Gain stable and cascadable wide band amplifiers • High performance downconverters • Power detectors 2008 Ledger Conference

17. Receivers (ctd) • A single conversion radar receiver with electronic image rejection better than 50 dB is now possible for frequencies in L-band • A receive chain can consist of a low noise amplifier and RF filter, a demodulator, an IF filter, amplifier and an analogue to digital converter • It is possible to build multi-channel radar receivers in academic laboratories on academic budgets • opening up a world of research possibilities into modern and experimental radar system approaches • bonus: an inexhaustible supply of signal processing problems! • Can these architectures migrate to practical systems in the field? 2008 Ledger Conference

18. Future receivers • Still over the horizon because of bandwidth requirements: • the software defined radar receiver • One sampler • several simultaneous receive channels formed digitally • Bandwidth > 500 MHz 2008 Ledger Conference

19. Mechanical & Mechatronic Technology • Radar presents the mechanical engineer with demanding structural requirements • Radar also requires tight integration of computerized control in mechanical systems • This is a problem that industry must manage • There is room for academic research on a sub system level, including • characterisation and evaluation of materialsconstruction technology • cooling technology for electronics • corrosion control measures 2008 Ledger Conference

20. System architectures • Radar architecture is driven by requirements and constrained by available technology • Often leading to compromises • The action is moving to the digital domain, where detection sensitivity is achieved by increasing processing gain rather than transmit power • Staring radars are interesting options for low-cost systems • Transmitter illuminates large search volume with a possibly stationary antenna • Multiple receivers are used for digital beamforming • Long integration times deliver processing gain 2008 Ledger Conference

21. Staring Radars • Questions: • How can radar help to change the cost equation in asymmetrical warfare? • with staring radar? • with passive radar? • with bistatic radar? • Once hardware problems are solved, you can start working on THE radar problem • How do you extract information from data? • e.g. how do you distinguish between small targets and sea clutter? • How long can you stare at a target? • What are the limits to processing gain? • We think there are interesting processing approaches out there still waiting to be discovered • These are problems for multidisciplinary teams, including engineers, computer scientists and mathematicians 2008 Ledger Conference

22. "Super Resolution" • Usually super-resolution refers to • means to increase effective bandwidth • special processing algorithms that do better than the discrete Fourier transform to measure the frequency of a sine wave, such as the MUSIC algorithm • This is not what I have in mind • I'm referring to resolution that is out of proportion to the volume of data • Often because of sub-Nyquist sampling 2008 Ledger Conference

23. Resolution and data d t Ts x t D TP f 1/TP u 1/Ts 2008 Ledger Conference

24. Sub-Nyquist Sampling • The best-known example is Doppler/MTI radar • In X-band, the Doppler shift for a target with a radial velocity of 300 m/s is about 20 kHz • An observation time of 16 ms will give a velocity resolution of about 1 m/s • At the Nyquist rate we require 640 samples @ 40 kHz • In MTI radar we would perhaps take only 32 samples at 2 kHz 2008 Ledger Conference

25. The Prize and the Price • Our prize is that we still have a Doppler resolution of 1 m/s • The price we pay for this is • Ambiguity • blind speeds, where we cannot see targets • measurements lost in clutter, where we cannot see targets • A countermeasure to reduce the price is stagger the PRF and/or use multiple frequencies 2008 Ledger Conference

26. "Super Resolution" and cost • We can apply the same principle to whenever we sample • e.g. by increasing spacing between radiators in an antenna array • Prize: large hardware savings • Price: spatial ambiguity • Countermeasure: stagger electrical spacing • e.g. sampling IF in FMCW system at sub-Nyquist rate • Prize: Increased range resolution • Price: range ambiguity • Countermeasure: staggered chirps or filtering 2008 Ledger Conference

27. Questions for Research • Subsampling schemes: • Quantify the Prize and the Price • Devise effective countermeasures • Quantify the final system performance • There are many more questions! • Polarization – what to use, multiple, how to switch? • Behaviour of clutter • RCS 2008 Ledger Conference

28. Conclusions • Technological advance and new user requirements continuously generate new radar questions • Radar continues to offer stimulating research topics • Few things in life com free • We hope soon to be able to provide data to academics who want to become involved in the exciting world of radar for free! 2008 Ledger Conference