SYSTEM DESIGN & ENGINEERING – Navigation, Spectrum & Surveillance
National Air Traffic Services Ltd
Spectrum House, Gatwick, West Sussex, RH6 0LG
Direct Tel:
+44 (0)1293 576556
Direct Fax: +44 (0)1293 576431
Switchboard: +44 (0)1293 576000
E-Mail:
steve.mitchell@natsnav.co.uk
www.nats.co.uk
28th September 2001
Mr Daniel Storey
Secretary
Independent Review of Radio Spectrum Management
c/o Radiocommunications Agency
Wyndam House
189 Marsh Wall
London E14 9SX
Dear Mr Storey,
RADIO SPECTRUM MANAGEMENT REVIEW – COMMENT ON COMMENTS
Please find attached NATS comments on the responses that have been received under
the Radio Spectrum Management Review. The NATS response to this review did not go
into any technical detail however having seen other responses, we feel that it is important
to provide some detail to supplement our response from a technical perspective.
Yours sincerely,
Steve Mitchell
Spectrum Manager
Spectrum Management & Safeguarding
National Air Traffic Services Ltd. Registered in England 3155567 Registered Office: One Kemble Street, London WC2B 4AP
comments on commentsV2
There appears to be a perception in some of the responses that radars differ in terms of
their "spectrum efficiency". We take this to mean that some respondents think that some
radar systems need a smaller bandwidth for a given throughput, or enable more
information to be transmitted in the same bandwidth, than other radar systems. This is
not the case. The concept of spectrum efficiency is redundant for radars. As explained in
this note, the bandwidth requirements of radar systems are determined by the laws of
physics and not by functions of the generation of technology being used. This
fundamental difference between radar and communications systems needs to be
recognised.
We also note comments in the MoD response about the differences between
communications and radiolocation (radar) systems. In both national and international
radio regulatory fora, approaches to spectrum management have been discussed that
may be meaningful for communications systems but which are not automatically valid in
the case of primary radar systems. NATS’ view that the technical differences between
radar and communications systems must be taken into account by the spectrum review
when considering proposals for economic incentives to the use of spectrum by primary
radar systems.
A radar system is a measuring instrument and not a communications device and the two
classes of system operate differently at a fundamental level. Perhaps the most obvious
difference is that a radar system is looking for a very low level signal that is reflected by
the target as a result of a comparatively high power transmitted signal. By contrast a
communications transmitter is only required to transmit a power level that is sufficient for a
single path to a receiver at an ‘acceptable’ signal error rate. Less obvious is that the
overall bandwidth required by a radar system is a direct function of the resolution of the
measurement to be made - in the surveillance radar case, the distance between aircraft or
between traffic on the surface at an airport - and the level of confidence that is required in
the measurement being taken. The bandwidth requirements of radar systems are not
functions of the generation of technology being used.
In the Air Traffic Control case, the ability of a radar system to resolve closer targets
enables a reduction in the separation between aircraft and hence increases ATC capacity.
Greater target resolution can be achieved through either the use of shorter transmit
pulses, which increase the signal bandwidth directly or the use of wider non-linear
modulation on the transmitted pulse shape. This non-linear modulation is removed on
reception to compress the transmitted pulse into a small time window, hence increasing
the target resolution. Therefore, as the non-linear modulation frequency excursion
increases to improve the resolution, so the required transmission bandwidth increases.
For any radar system, improvement in the system's ability to resolve more closely spaced
targets, i.e. to improve its operational efficiency, requires that the bandwidth used must be
increased.
"Newer" radar systems may use solid state transmitter technology, which produces longer
modulated pulses that can be compressed to a greater degree to increase the
measurement resolution and hence increase the overall capabilities of the radar system.
These longer modulated pulses require more bandwidth. If the occupied bandwidth was to
be constrained then this would have the effect of reducing ATC system capacity as
aircraft would have to be kept further apart to maintain safe operations.
The bandwidth requirements for radar systems and the number of bands that are used
stem from the fundamental laws of physics, as identified in the response to the Review
from the CAA. An analogy with the broadcasting service may be of use: sound
broadcasting uses bands at the low end of the spectrum to provide long range services
and higher bands for comparatively short range services (with better audio quality). The
different bands have differing physical characteristics that make them suitable for these
different broadcasting purposes. In the same way, long range ATC surveillance radar
systems use lower frequency bands than do systems used for airport approach or surface
movement control. Systems at airports are also required (under internationally agreed
standards) to update the radar "picture" more often than is the case with long range
systems. This places limits on the physical size of the antenna that can be used due to
the need to rotate the antenna more quickly, which then also has a bearing on the
frequency band that can be used, as does the operation range that is required. It is also
the case that different airports may have approach radar systems operating in different
frequency bands according to the varying requirements as to the coverage and
performance of their approach radar facilities.
Ground based primary air traffic radar systems are used to provide controllers with a
picture of all of the aircraft in the airspace in which the controller has an interest, to a level
of detail and confidence that is appropriate to the airspace type. Moving from long range
surveillance, through medium range, to the final approach to an airport and then to the
control of aircraft and vehicles on the airport itself, the distances between successive
targets reduce and the radar systems used to support each stage of a flight must be
capable of resolving closer (and smaller) targets.