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The next step is to determine if signals from these radars will be close to or exceed the
LNA saturation level. If any do, the assignment is not progressed further without
agreement from both radar operators that they are willing to accept the situation.
Assuming there are no problems with LNA saturation, the next check is to identify a
candidate frequency. For this centre frequency, it is established whether signals from
nearby radars will fall within the new radar’s IF band. If this is the case, the level of
received pulses are calculated and checked that they are not more than 40 dB above the
radar’s sensitivity. (This is considered to be an acceptable level for directly received
pulses, which will not overload or distort the radar’s performance). If the received signals
are above this level, then alternative frequencies are considered. In some cases, an
alternative option is to consider whether the radar-to-radar antenna coupling can be
reduced by applying some tilt to the new radar, thus benefiting from the antenna’s low
elevation angle cut-off characteristics.
The margin for directly received pulses (40 dB) assumes that both radars have similar
PRF’s and pulse widths. If nearby radars have higher PRF’s or longer pulse widths, the
margin must be reduced accordingly.
When this process has been completed, and suitable frequencies (and frequency spacing
if appropriate) have been located, the assignment is notified to MoD (Defence Spectrum
Management) and MCA (if above 2.9 GHz) for co-ordination, where they apply their own
compatibility process against their own radars. Assuming MoD and MCA confirm
compatibility, the assignment is completed.
3.2.8.3 UHF Primary Radar
There are two radars operating in the United Kingdom. It may be opportune to consider
whether it would be feasible to replace these systems in favour of S or L band equipment.
This would move the radar systems out of the UHF band. It would be necessary to review
the requirement for these systems and the replacement timescales with the relevant
operating authorities.
3.2.8.4 L Band and S Band Civil ATC Radars
In these bands, we consider ATC radars provided by civil organisations such as NATS
and airport operating companies. These bands are the cornerstone of ATC primary radar
based operations in the UK. As the study does not include the examination of the military
use of these bands, overall band utilisation cannot be accurately defined.
However, by way of an example, it is instructive to look at the current utilisation of S band
(see Table 3-2). S band (2.7GHz to 3.1GHz) is occupied by some 34 civil ATC radars.
These radars are a mix of pulse compression systems (19) and single pulse systems
(15). The number of frequency allocations varies according to main and standby
requirements; or short and long pulse requirements in the case of pulse compression.
The total occupied bandwidth (by civil radars alone) in terms of the minus 20 dB
bandwidth is some 460 MHz in an occupied spectrum of 375 MHz.
The limits on out of band transmissions are defined by the –40dB bandwidth which is
much greater than the necessary bandwidth particularly for magnetron radars. When
considering out of band transmissions there is a very high degree of overlap even when
taking into account the re- use of frequencies due to path length and other geographical
considerations. The fact that the services can operate on this basis is due to the PRF
discrimination incorporated in the radars but it should be noted that this process has
performance limitations.
Given the disparate nature of the current and future systems, the consequences of
improvements in spectral efficiency are also difficult to quantify. However, as outlined in
section 3.2.5, there are a number of technology improvements which should be
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