Research Topics in Radar for Academics

PW van der Walt

Reutech Radar Systems

why?

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

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!

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

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

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

Two antenna arrays

Single stick, non squinting

2x12 stick arrays, squinting

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

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?

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?

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

L Band LDMOS

Low Z ports

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

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

Measured phase noise

Noise floor set bysystem architecture

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

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?

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

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

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

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

"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

Resolution and data

Ts

TP

1/TP

1/Ts

tt

f

d

Dx

u

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

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

"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

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

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