How and Why Radars Use Spectrum

A. G. Stove

© Thales UK 2011

2 / Some Uses of Radar

DefenceSurveillance of our Frontiers

Ground – SpaceGround – AirAir – AirMaritime Surveillance

Naval Situational Awareness, self protection, protection of other shipsAirborne Surveillance, Navigation and CombatBattlefield Surveillance Weapon Location/Attack Warning

track flight of projectiles and extrapolate back to point of launch/landingMissile Guidance

e.g. ExocetBrimstone as used in Libya

© Thales UK 2011

3 / ‘Value’ of Defence Spectrum

Defence of the realm is the Government’s Top Priority

Monetary value may be estimated by assuming it enables (most of) what is done with the defence budget - £44billion

AIP value said to be £155M/year

Current needs for spectrum in the UKSurveillance and Security – a relatively small part of the ‘military’ spectrumTraining, Testing and R & D – all the spectrum, but only in limited places/times

Radar uses ≈ 10% of the spectrum below 10GHzSeems to be only 25% of military spectrum

© Thales UK 2011

4 / Some More Uses of Radar

TransportAir Traffic Management

Primary radars at 1.3GHz and 2.8GHzSecondary radars (interrogate and respond) 1.03GHz/1.09GHz

Marine NavigationTraffic Lights

GATSOsAutomotive

24GHz and 77GHz

Weather MonitoringIn other countries esp. tornadoes

Environmental Monitoringe.g. land use/disaster effects from space

Note: without radar for Air Traffic Management landings at Heathrow would have to be controlled visually, at a cost of £3M/hourCivil Aviation is worth more than £60B/yr to the economy

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5 / Varieties of Radar

Sensitivity power/range4; Long range high power

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6 / Why Use Radar?

Long RangeRange resolution independent of absolute range

All weatherOnly slightly affected by weatherEffects can easily be predicted and allowed for in the design

Day and Night

Robust discrimination of target from backgroundCan robustly detect moving targets independent of ‘clutter’ background

AutonomousCan work without cooperation of targetWithout any other data

e.g. ATC is independent of GPS

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Choice of Frequency Band

Rain attenuation limits long range (>100km) to ≈ 5GHz or below

Beamwidth ≈ /d 10GHz: 1º = 1.5m 1GHz : 1º = 15m

Compromise between these two factors

7 /

© Thales UK 2011

Why Radars Need Spectrum

‘Necessary Bandwidth’ is determined by range resolution Fine range resolution can be a substitute for limited angular resolution Air Traffic Management – say 50m resolution 3MHz bandwidth Maritime Surveillance – say 3m 50MHz bandwidth Multiple ‘slots’ needed for many radar applications

Target/clutter decorrelation by extra agility Anti jamming Avoid mutual interference

Not proven that there is no other way to get the range information, but no other method is known

In time domain: needs to ‘listen’ when it’s not transmitting Might be able to radiate into the ‘back’

Would require careful synchronization Sidelobe ‘gain’ ≈ -10 dBi

8 /

© Thales UK 2011

Release of radar spectrum?

Radars need a lot of spectrum

Small proportional reduction large proportion increase for other users

e.g. in 2.7GHz..2.9GHz band

Co-channel Sharing not so promising Radars cannot tolerate significant in-channel interference without noticeable

loss of coverage Or else we would have saved money by using a less-powerful transmitter

9 /

© Thales UK 2011

Out-of-Band Emissions10 /

Rectangular Pulse sine(x)/x spectrum Goes on ‘forever’ Not a problem when radars were the only users of the spectrum

Modulated pulses have much faster roll-off (but might have worse mutual interference properties) Might then need more spectrum again to avoid mutual interference

Magnetron spectra are also being tightened-up

© Thales UK 2011

11 / Future Trends

Passive ATM radars as part of ‘sensor mix’ to reduce primary radar needsBut would need ‘primary’ spectrum again if UHF broadcasting stops!

Wide band radars above 10GHz to get ‘photo quality’ synthetic aperture images

Desire to get synthetic aperture images at lower frequencies, for ‘foliage penetration’Would clearly need to share bands‘Interrupted’ in time and frequencyCompressive sensing to restore image quality?