We previously compared vertical launch systems (VLS) to arm
launchers (see, “VLS Versus Arm Launchers”) and
concluded that VLS was not quite the unquestioned advantage that it was claimed
and assumed to be. Similarly, we’re now
going to compare rotating radars against fixed, flat panel arrays which are
assumed to be infinitely superior.
One of the major developments in naval sensors has been the
advent of flat panel radar arrays. The
panels are mounted on the sides of the superstructure with, typically, 3-4
spaced around so as to provide 360 degree coverage, each panel covering 90-120
degrees. This architecture is assumed to
be hugely more beneficial than conventional, rotating radars, presumably due to
the elimination of moving parts as well as the simultaneous improvement in
radar technology, generally. Is this
assumption of superiority valid? Let’s
see.
Let’s start by understanding the three basic types of radar
configurations:
Conventional Lattice – These are typified by the SPS-48/49
which are modern versions of the classic, mechanically steered, rotating radars
with a lattice framework.
 |
Wasp class with SPS-48 on the right and SPS-49 on the left |
Hybrid Panel – These place flat panels on a rotating
assembly to produce a hybrid rotating panel.
Examples include the TRS-3D which rotates at 10, 17, 20 or 60 revolutions
per minute (rpm) [1] or the TRS-4D which rotates at 15, 30 rpm [2]. Both are quite capable.
TRS-4D
is a G-Band three-dimensional, multi-function naval radar for surveillance,
target acquisition, self-defense, gunfire support, and aircraft control. It is
a software-defined radar using a rotating version of the active electronically
scanned array (AESA) with multiple digitally formed beams. …
The
TRS-4D radar simultaneously conducts a three dimensional search of the air
space volume and sea surface area around the ship. … The transmitter modules in
the active antenna are solid-state modules in Gallium Nitride technology. The
radar allows a graceful degradation of the transmitted power depending on the
required maximum range.
The
MRESR version of the TRS-4D was installed on US LCS ships of the U.S. Navy’s
Freedom class. It was designated by US Navy as AN/SPS-80.[2]
 |
TRS-3D |
Another example of a hybrid panel radar is the SPY-6(V)2
which is intended to be installed on amphibious ships and Nimitz class
carriers.
 |
SPY-6(v)2 |
Panel – These are the ubiquitous flat panels found on
US ships and include the various Aegis SPY-1 variants, SPY-6 (Air and Missile
Defense Radar, AMDR), SPY-6(V)3 Enterprise Air Surveillance Radar (EASR), and
whatever other names they’re known by.
 |
Flat Panel |
Advantages and
Disadvantages
Simplicity.
Flat panels are mechanically simpler in that they have fewer moving
parts although, to be fair, a motor and some bearings to rotate on are not
exactly rocket science in terms of complexity.
Still, no movement is undeniably simpler than rotating.
Of course, rotation is not the end of the simplicity
story.
Both types require sophisticated, complex computers/software
to control and process the signals so that’s a wash.
What isn’t a wash is the extent of electronic and utility
support that a panel requires. Each
element in a panel requires its own power, computer connections, data and
computer control connections, and cooling support. A rotating radar requires much the same but
only a single instance of each, as opposed to an instance for each element of
the array. Notably, rotating radars do
not require cooling which is a major requirement.
Further, the individual modules that make up a panel are
quite complicated and there is no hope of repairing one aboard ship. On the plus side, they can be swapped out
without too much difficulty, as I understand it. Similarly, the ‘guts’ of a hybrid panel are
similarly complex. The conventional
lattice is, of course, as simple as it gets.
Volume.
Rotating radars are essentially external to the ship whereas panels
require significant amounts of internal ship’s volume to house the array
elements and support equipment. Further,
panels typically exist as 3-4 repeated installations, each of which requires
its own, equal, large amount of ship’s volume to house it. Thus, rotating units require only, perhaps, a
tenth of panel’s volume. This is a
significant consideration in ship cost and design.
Weight. I do
not have data on unit weights but I assume that panels, with 3-4 duplicates and
large elements, have significantly higher total weight than rotating units.
Damage Resiliency.
Older, lattice type rotating radars have a degree of inherent damage
resiliency in that their lattice structure is mostly space. Shrapnel sprayed in their direction will
largely pass through with little resulting damage. The denser the lattice or, in the case of
rotating panels, the greater the degree of damage susceptibility. Rotating panels, while solid as opposed to a
lattice, are smaller than a rotating lattice and significantly smaller than
flat panels. Thus, their size confers a
degree of damage resilience.
Fixed panels, on the other hand, are absolutely certain to
sustain damage from shrapnel. One
hundred percent of shrapnel from nearby explosions will impact the panel with
every piece producing damage. Manufacturer’s
claim that panels are resistant to damage because the undamaged elements can
continue to function, albeit at a lower overall efficiency and
effectiveness. However, this claim is
unproven by any realistic testing. For
example, while a single damaged element may not significantly impact the
overall radar performance, what is ignored is the cabling, communications,
cooling, and power ‘behind’ the elements and those are extremely vulnerable to
damage and would, when damaged, likely affect large portions, or all, of the
panel. The manufacturer’s claims do not
consider this type of damage, at all.
When damage does occur, if you lose a panel, you lose that
coverage sector (90-120 degrees) permanently.
There is no alternative mechanism to compensate. You have a permanent hole in your coverage. Not good in combat! In contrast, a rotating radar provides full
coverage until it is completely incapacitated.
In addition, the typical radar arrangement of -48 and -49 allows either
radar to take over the other’s coverage in the event of damage.
Coverage.
Rotating radars, by their nature, provide only intermittent coverage as
the active (transmitting and receiving) portion of the radar is always
moving. In many cases, such as tracking
at long ranges, this is an insignificant issue since the target is not changing
location fast enough to matter. At
closer ranges and higher target speeds, such as supersonic missiles inside the
horizon, this can be a significant problem.
The problem of intermittent coverage can be mitigated by
using higher rotational speed or using double sided radars which have active
portions front and back thus providing near 360 degree coverage.
Alignment.
While I can’t speak to every panel radar that exists, the Aegis SPY
variants apparently require a very precise alignment as evidenced by the
impaired performance and required repairs of radars of ships that have grounded
or been in a collision. Whether this
alignment sensitivity is true of modern panels, I have no idea.
Protection. Flat
panels are likely easier to protect with armor.
A simple armored cover can slide over the panel, as needed. Rotating radars would require either a rotating,
box-like arrangement or a retractable mechanism – doable, of course, but a bit
more complicated.
Performance. How
effective is each radar type? Panels and
hybrids both use the same general technology so, ignoring size, there is no
difference. Of course, size does affect
performance under certain circumstances (long range detection of small or
stealthy targets, for example) and, in those cases, large panels would be
preferred.
Detection range (against some theoretical target), alone, is
not the measure of performance.
Performance is dependent on the circumstances of use. If one is attempting to detect very long
range, small targets, one would want the largest, most powerful panel possible. Alternatively, if one is attempting to
conduct a horizon range anti-air engagement, large panels are a waste and a
small, rotating or hybrid radar would be preferred.
However, performance cannot be divorced from other aspects
such as survivability, maintainability, repairability, size, weight, etc. Performance must be appropriately weighted in
balance with the other factors.
Bear in mind that manufacturers focus on extreme detection
range against ideal targets as the measure of performance. In reality, that is an unlikely use case in
combat (EMCON being the default state!) where horizon range engagements are the
far more likely scenario. Being able to
detect a stealth mosquito a continent away is of no use when engaging missiles
from the horizon in.
The interesting aspect of performance is the question, to
what degree can a conventional lattice radar be improved? There seems to be no end to the degree of
improvement that panels can undergo but what about lattice radars? Can they be improved? How much?
A closely related question is, to what degree do lattice
radars need to be improved. Given that
we’ve stated that horizon range engagements are the most likely use case, and
knowing that lattice radars have theoretical detection ranges of hundreds of
miles (against suitable, theoretical targets), how much better do they need to
be? Perhaps they’re more than
sufficient, right now? If a lattice
radar can provide, say, 90% of the required performance at a miniscule fraction
of the cost, is that not good enough? I
can’t answer that. I merely pose the
questions but they are important questions.
Conclusion
It is clear that each type of radar configuration has
advantages and disadvantages and that modern flat panels are not the unquestioned
superior choice that most assume. The
choice of radar configuration depends on the balance between all the
factors. It would seem that hybrid
rotating panels represent the best balance, overall. They have good performance, less weight,
consume little internal volume, provide adequate coverage, and have a
reasonable cost. Of course, much depends
on the use case. For example, a
dedicated AAW ship might well justify multiple, large panels.
For general purpose surface ships, a hybrid rotating panel
is the best choice.
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