Radar – Rotating vs. Panels

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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[1]https://www.radartutorial.eu/19.kartei/07.naval/karte008.en.html
 
[2]https://www.radartutorial.eu/19.kartei/07.naval/karte027.en.html