How and Why Radars Use
Spectrum
A. G. Stove
Some Uses of Radar
2 /
Defence
Surveillance
of our Frontiers
– Space
Ground – Air
Air – Air
Maritime Surveillance
Ground
Naval
Situational Awareness, self protection, protection of other ships
Airborne Surveillance, Navigation and Combat
Battlefield Surveillance
Weapon Location/Attack Warning
track
Missile
e.g.
flight of projectiles and extrapolate back to point of launch/landing
Guidance
Exocet
Brimstone as used in Libya
© Thales UK 2011
‘Value’ of Defence Spectrum
3 /
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 UK
and Security – a relatively small part of the ‘military’ spectrum
Training, Testing and R & D – all the spectrum, but only in limited places/times
Surveillance
Radar uses ≈ 10% of the spectrum below 10GHz
Seems
© Thales UK 2011
to be only 25% of military spectrum
Some More Uses of Radar
4 /
Transport
Air
Traffic Management
Primary
radars at 1.3GHz and 2.8GHz
Secondary radars (interrogate and respond) 1.03GHz/1.09GHz
Marine
Navigation
Traffic Lights
GATSOs
Automotive
24GHz
and 77GHz
Weather Monitoring
In
other countries esp. tornadoes
Environmental Monitoring
e.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/hour
Civil Aviation is worth more than £60B/yr to the economy
© Thales UK 2011
Varieties of Radar
5 /
Sensitivity  power/range4; Long range  high power
© Thales UK 2011
Why Use Radar?
6 /
Long Range
Range
resolution independent of absolute range
All weather
Only
slightly affected by weather
Effects can easily be predicted and allowed for in the design
Day and Night
Robust discrimination of target from background
Can
robustly detect moving targets independent of ‘clutter’ background
Autonomous
Can
work without cooperation of target
Without any other data
e.g.
© Thales UK 2011
ATC is independent of GPS
Choice of Frequency Band
7 /
Rain attenuation limits long range (>100km) to ≈ 5GHz or
below
Beamwidth ≈ /d
 10GHz:
1º = 1.5m
 1GHz : 1º = 15m
Compromise between these two factors
© Thales UK 2011
Why Radars Need Spectrum
8 /
‘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
© Thales UK 2011
Release of radar spectrum?
9 /
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
© Thales UK 2011
Out-of-Band Emissions
10 /
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
© Thales UK 2011
spectra are also being tightened-up
Future Trends
11 /
Passive ATM radars as part of ‘sensor mix’ to reduce
primary radar needs
But
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 frequency
 Compressive sensing to restore image quality?
© Thales UK 2011