RADAR SURVEILLANCE SYSTEM
The invention relates to a radar surveillance system. More specifically but not exclusively it
relates to maritime surveillance radar designed to detect small targets.
In maritime surveillance radar, the key objective is to distinguish actual targets from apparent
targets. Apparent targets, or 'false alarms' are those that may be caused, by a radar
reflection of the sea surface being categorised as a potential target, when no real object is
present. This problem has been the subject of much research over the past 70 years. The
problem is essentially achieving a robust decision criterion, based on the mathematical
probability that a particular reflection from the area under surveillance is or is not a target.
In conventional maritime surveillance radars, the antenna produces a fixed beam shape,
which is scanned over the area of interest, up to and including complete 360° coverage, in
the azimuth plane.
Such systems then use non-coherent azimuth integration of amplitude information reflected
from this area of interest to determine the threshold for declaration of target or not. Typical
existing processing schemes to statistically address this question include, but are not limited
to, thresholding the data after area averaging, applying an M out of N detection criteria, or
thresholding post azimuth filtering. In such maritime radar detection, the key discriminant is
the amplitude correlation of the target, if present, during the dwell time, compared with the
amplitude correlation of the background sea clutter return. Hence if both target and sea
clutter returns are each highly correlated, a mathematically sound, automatic test hypothesis
is difficult, if not impossible to achieve. This results in either many false targets being
displayed or real targets being suppressed. Both conditions are unacceptable as operator
workload and search time can become excessive in trying to achieve the functional role of
target detection.
There are a number of known systems that will be described below. However, all suffer from
problems that the present invention aims to overcome.
The exploitation of the differences between target amplitude temporal de-correlation and sea
clutter amplitude temporal de-correlation is not a new feature in radar mode design.
However the existing modes that benefit from a delay in time between looks on a specific
dwell area all compromise the operation of the radar and hence target detection
performance. This is further described under each existing mode type, with key factors
relating to the specific impact on performance.
l
Fast Scan.
In this implementation the 360° (or limited scan) is achieved by an antenna, rotating around
an azimuth gimbal at a rate that enables a re-visit of an area rapidly, typically every 1 or less
seconds. The radar returns are then processed across two or more scans to achieve the
necessary clutter temporal de-correlation. There are two main drawbacks of this
implementation. Firstly, the physical aspect of rotating a large mass at high angular velocity
creates problems, from the difficulty in achieving reliability to the gyroscopic impact on an
airborne host platform (helicopter or fixed wing aircraft). Secondly, due to the high angular
rotation rate, the number of radar pulses or PRIs in each look or dwell can be as low as one.
Hence a large number of scans is needed to achieve target integration above a detection
threshold and whilst this may result in the required target/clutter discrimination, the time
taken to capture this number of scans, together with the very short exposure time per scan,
can result in a small fleeting target being missed, due to obscuration of sea swell or the short
exposure of targets such as a periscope. There are also other considerations relating to the
complexity of range walk correction due to the large number of scan periods required to
make a target decision.
Slow Scan
This can be implemented with an antenna azimuth angular rate that is slow enough to allow
the temporal de-correlation of sea clutter within the dwell time on target. Again this mode
compromises the opportunity to 'see' the fleeting target as the time to cover the 360° scene
is long (typically 10's of seconds), to allow de-correlation within the dwell period.
Scan to Scan
Another existing compromise approach involves, for example, scanning the maritime scene,
at an azimuth rate of say between 60°/s and 120°/s, integrating within each dwell but
achieving the necessary clutter de-correlation by comparing a batch of scans in a scan to
scan fashion. Again, this only offers performance on large targets which appear stationary
during the period of scan to scan integration. This is not a practical mode for operation
against the fleeting small target such as a periscope, small boat or a person in adverse sea
conditions.
The present invention relates to the use of novel scan strategy, with interleaved dwell
periods, which uses a technique to enhance the probability of detection of fleeting targets in
adverse sea clutter conditions.
According to the invention there is provided a maritime surveillance radar system for
detecting targets comprising an electronically scanned radar antenna, the antenna beam
having look back modes and interleaved dwell periods spaced over time, such that signal
returns indicative of clutter may be identified independently of signal returns generated by
targets thereby improving the detectability of the targets in sea clutter.
According to the invention there is further provided a method of improving the detectability of
targets in sea clutter comprising the steps of scanning an area under surveillance with an
electronically scanned radar antenna and re-scanning each area of interest after a short
period of time, by electronically reconfiguring the scanned beam to an offset position for an
interleaved sub-dwell, within a given scan period, such that signal returns indicative of clutter
may be identified independently of signal returns generated by targets, thereby improving
the detectability of the targets in sea clutter.
Preferably, this is achieved by employing an electronically scanned antenna. Preferably, the
system offers a mode design that can be implemented in a manner that does not degrade
either the look time or the search capability of the radar, as is the case with current
techniques as described above. Furthermore, the present invention offers additional
degrees of freedom to further enhance target detection and additional rejection of clutter
returns, as this mode is compatible with inclusion of pulse to pulse frequency agility or subdwell
frequency agility and also the implementation of sub-dwell fixed frequency coherent
processing.
The present invention preferably includes the use of an electronically scanned radar antenna
(E-Scan antenna) and associated processing in a maritime surveillance role. The invention
offers enhanced target discrimination against sea clutter, particularly for small or fleeting
targets such as small boat detection, man-overboard scenarios, life raft searches or
submarine periscopes.
Preferably, the implementation supports up to all round (or 360 °) coverage from a host
platform either through the use of multiple fixed E-Scan antennas or the use of one or more
E-Scan Antenna(s) mounted on an azimuth gimbal system, which allows the antenna(s) to
rotate physically up to 360 °.
Furthermore, the invention exploits specific natural characteristics of potential targets and of
sea clutter to offer an improved ability to discriminate one from the other, increasing the
probability of target detection in a short exposure time and reduced false alarms. In a
practical role, such as search & rescue, this reduces workload for radar operators. In a
military role, the enhanced performance reduces false searches, saving wasted time &
unnecessary reactions to apparent threats.
The present invention aims to overcome the problems of the prior art systems and provide a
maritime surveillance radar system capable of more accurately discriminating between false
alarms and real targets.
One particular implementation of the invention will now be described with reference to the
accompanying diagrammatic drawings, in which
Figure 1 shows the conventional application of a scanning antenna in a maritime
surveillance application,
Figure 2 shows a schematic of one form of maritime radar surveillance system in
accordance with the invention implemented with an E scan radar having interleaved dwells
implemented through a combination of electronic and mechanical scanning.
Figure 3 shows a schematic of the radar system of Figure 2 at a time T ; and
Figure 4 shows a schematic of the radar system of Figure 3 at a time of T 1 plus (typically) 1
second.
Figure 1 shows the conventional application of a scanning antenna in a maritime
surveillance application, where Q is the mechanical scan rate and y is the azimuth antenna
beamwidth. The dwell period is gated in the range to r2 in azimuth by f and number of
pulses (PRI). In this application the beam shape (determined by the physical antenna
attributes) and the range sampling clock can be used to determine an area of interest, for the
declaration of a target or not, within that search sub-area. Typically this is an area of a few
degrees in azimuth and a number of range gates down the length of an imaginary 'spoke'.
At any moment in the azimuth scan of the antenna beam, a number of radio frequency (RF)
pulses illuminate this dwell angle, dependent on the designed pulse repetition rate of the
radar and the azimuth scan rate.
In the first embodiment of the invention, the difference in approach over existing systems is
that whilst in a conventional surveillance radar the scanning beam sequentially covers the
area of interest, in terms of a dwell period and a range swath (see Fig 1), in the present
invention the radar beam re-visits each area of interest after a short period of time, by
electronically reconfiguring the scanned beam to an offset position for an interleaved subdwell
1s 2S, within the scan period, (see Fig 2). This 'look-back' capability, where the area
under test is re-visited after approximately 1 second, allows the natural de-correlation of the
sea clutter to take place between the initial and look-back samples of the surveillance area.
Of course those skilled in the art will note that the re-visit time can be adjusted to best exploit
the de-correlation characteristics of the sea clutter return. The enhanced de-correlation of
the sea clutter improves the ability of the radar to reject false targets whilst still maintaining
the ability to integrate target returns within the sub-dwell periods to achieve a satisfactory
probability of detection.
The approach described here, where a number of sub-dwell periods 1 , 2S may be used to
illuminate the same dwell patch, but spaced in time, allows further degrees of freedom to be
exploited.
With reference to Figures 2, 3 and 4, where A and B are the same geographical location,
and considering a maritime surveillance mode operating in unambiguous, low pulse
repetition frequency (PRF) mode. The antenna, which in the first embodiment is a rotating
antenna with the capability to be electronically steered in two dimensions, rotates (clockwise
from above) on a gimbal axis, through the Z plane, giving 360° coverage.
The azimuth mechanical scan rate is defined as 6°/s (nominally, say, 60°/s). The dwell time,
defined by the azimuth scan rate, radar PRF and beamwidth, contains n pulses. The E-scan
antenna has, for example, two pre-set beam angles 1e, 2e around the mechanical boresight
5 of +0° and -0° and this E-Scan boresight 5 can be switched from one to the other of these
angles in one pulse repetition interval (PRI). Hence the antenna can offer a look-back angle
6 of 20°, which equates to a nominal 1 second re-visit time if, for example, 0=30°.
So, in a typical design, consider one dwell time of the mechanically rotating antenna, at an
E-Scan offset of +30°, with a dwell of n/2 pulses. The antenna is then reconfigured, within
one PRI, to look back -60° and then illuminate the look-back dwell area for n/2 pulses or
PRIs. The antenna is then reset to the +30° E-Scan position and this sequence is
continuously cycled, whereby every dwell is eventually made up from two sub dwells of n/2
pulses 3, separated in time by (scan rate/ E-Scan offset)0 which are processed together,
having been time tagged, and a target decision made.
It will be appreciated by those skilled in the art that the sub-dwell, containing a number of
radar pulses or PRIs, can contain pulses of different frequencies (frequency agility), offering
maximum clutter amplitude de-correlation.
Moreover, it will be appreciated that the sub-dwells 1s and 2S can alternately be coherent
processing intervals, using a fixed frequency for sub-dwell 1 and a different fixed frequency
for sub-dwell 2. This offers some clutter de-correlation in amplitude terms, between dwell 1
and 2, and also enables any differential Doppler of the target compared with sea clutter to be
exploited in the decision process.
Furthermore, it will be appreciated that this coherent waveform plan allows target to clutter
discrimination using the relatively narrow target Doppler spectrum against the relatively wide
clutter spectrum.
It will be appreciated that the use of interleaved sampling and 'look-back' beam
management, spaced in the time domain, allows the temporal de-correlation of sea clutter
returns to be fully exploited which hence offers improved target detection and false alarm
regulation by exploiting this natural temporal clutter de-correlation.
Furthermore, the ability to use coherent or non-coherent dwells, spaced in time, maximises
the opportunity to discriminate the target from sea clutter, by processing both in the
amplitude & Doppler domain.
In this way, the Maritime Surveillance Radar of the present invention having small target
look-back mode with interleaved dwells, offers an optimised capability for the search of
fleeting objects in adverse sea conditions, using a mode which exceeds the capability of
currently implemented designs. The mode design enables full exploitation of the three
mechanisms of (1) the natural temporal de-correlation of sea clutter, (2) the de-correlation
endowed by frequency agility and (3) the use of Doppler discrimination in the target
detection decision of the target. This is a unique feature of the present invention.
In operational terms this mode offers optimised performance for the detection of small,
fleeting targets such as small boats, semi-submersible vessels, submarine periscopes,
people in the search & rescue role, and other similar targets of interest.
It will be appreciated that, whilst the invention relates to maritime surveillance it is intended
for the surveillance of maritime areas. The radar antenna itself may be mounted on any
suitable platform, such as, but not limited to, an aeroplane, ship, or other vehicle.
Furthermore, the radar antenna may be shore based.
CLAIMS
1. A maritime surveillance radar system for detecting targets comprising an
electronically scanned radar antenna, the antenna beam having look back modes
and interleaved dwell periods spaced over time, such that signal returns indicative of
clutter may be identified independently of signal returns generated by targets thereby
improving the detectability of the targets in sea clutter.
2. A system according to claim 1 in which the dwell periods may be coherent or non¬
coherent and spaced apart in time thereby enabling discrimination of target and sea
clutter returns.
3. A system according to claim 1 or 2 in which the returns are processed in amplitude
and Doppler domains.
4. A system according to any preceding claim in which the electronically scanned radar
antenna can additionally be steered mechanically.
5. A method of improving the detectability of targets in sea clutter comprising the steps
of scanning an area under surveillance with an electronically scanned radar antenna
and, re-scanning each area of interest after a short period of time, by electronically
reconfiguring the scanned beam to an offset position for an interleaved sub-dwell,
within a given scan period such that signal returns indicative of clutter may be
identified independently of signal returns generated by targets thereby improving the
detectability of the targets in sea clutter.
6. A system or method according to any preceding claims in which the antenna is
mounted on a platform, the platform being an aeroplane, ship or shore-based vehicle.
7. A system or method as hereinbefore described with reference to the accompanying
diagrammatic drawings.