(I have absolutely zero motivation to search and include some nice pictures for a more pleasant reading today. This is a long, not exactly easy text. Brace yourself. ;-) )
Maybe it's a good time for a general article about protection against air power on the battlefield. The topic is quite encompassing, and the approaches vary a lot even among NATO allies.
First, let's have a look at the priorities as I see them. Some readers might disagree with this prioritisation, having the assumption of NATO or U.S. air supremacy in mind. Well, that's not cast in stone and even if it was; this article is a bit more encompassing than about classic aircraft (manned or unmanned).
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Protection against air threats is first and foremost about measures that reduce the vulnerability to attempts of detection and attack.
This is mostly about passive measures - the basics of camouflage, concealment, deception and radio discipline as practised by all competent ground forces. There's nevertheless also the possibility that active jammers could be employed. Such equipment tends to be rather centralized and ranges from radar jammers (the Russians have a model to counter the E-8 J-STARS, for example) to satellite-blinding lasers.
Next comes the necessity to reduce the air threat's repertoire.
Force the hostiles to fly high, to fly in less efficient strike packages, to fly with partially defensive payload, to minimize the number of attack runs and to attack from a long distance. The desired effect is a reduction of the hostile air power's effect on our remaining vulnerabilities.
Third come the actually destructive responses.
This is about the damaging and destruction of both platforms and munitions.
Laymen often overemphasize the third aspect.
Navies emphasize the very last aspect (intercept of munitions) while air forces and armies neglect it. They had few really high value targets (HQs, pivotal bridges) to protect while navies had to protect expensive and difficult to replace warships and had little hope of hiding on the open, flat sea.
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Now let's look about topics of importance:
There was the general trend of miniaturization since the invention of transistors. We're now at a point where two pound flying drones might be enabled to seek and kill (with EFP warhead, for example) individual soldiers. Other loitering killer drones can be sent against vehicles (Germany researched this for thirty years and would have fielded such a drone years ago if the Cold War hadn't ended).
Non-lethal drones are still more important, though; reconnaissance and electronic warfare drones are especially interesting.
Such miniaturized, quantity produced drones can be useful and cheap at once. A drone (target) can reach a critical threshold where it costs the same as the munition meant to destroy it. The defence with said munition becomes unaffordable in all but a few extreme situations (even before that threshold was reached).
Modern battlefield air defences are primarily if not exclusively meant to destroy platforms, not munitions. Critical parameters such as sensor capabilities, minimum firing range and cost per kill (both in weight and money) are acceptable for the defence against helicopters and low-flying combat aircraft, but a lightweight aerial drone could slip by many battlefield air defences without being identified as something different than a bird.
The problem begins with their low speed (radars use the Doppler effect to ignore everything that doesn't move quickly enough in order to minimize false alarms) and extends through their small size to their infra-red signature (different temperatures than combat aircraft).
The problem of smallish aerial drones is a problem all-troops air defence; no centralised defence system will be able to handle tiny hostile drones. I repeat myself: Bird-like drones require bird hunting ammunition; shotgun ammunition.
Larger drones can fly and be useful beyond machine gun range and require a form of countermeasure that is affordable and offers enough coverage. Today's battlefield air defences are quite unlikely to succeed in this role, save for a few autocannon designs with timed frag or shrapnel projectiles. Guns of 35-76mm calibre seem to be a promising choice; the anti-air artillery (AAA) may experience a revival on an unexpected scale.
Such a revival might in turn diminish the relevance of the drones, or push them on a path of development towards more sophisticated, expensive and survivable designs.
AAA has proved its multi-role capabilities in WW2 when AA weapons from 20 to 88mm calibre proved their worth in ground combat. We might become enticed to consider this for future AAA as a feature. Heavy (armoured) forces might use medium calibre tank guns and infantry fighting vehicle autocannons as AAA (with the necessary ground/air sensor technology).
"Light" formations with a focus on the dismounted fight such as infantry units might become interested in multi-role guns for both indirect artillery fire and air defence. A quick-firing 76mm gun not much unlike WW2 AAA designs might be worth a look.
Some machine guns had dedicated flip sights for ground/air fires (such as the MG3 "Fliegervisier"). These were known to almost useless against modern combat aircraft and even against attack helicopters. They might become almost self-evident in the future. A possible alternative is the use of tracer cartridges.
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OK, that was about the long-since emerged challenge of aerial drones. Drones are borderline between platforms and munitions. Another challenge for modern air defence forces is beyond this border; munitions as targets.
The dedicated, classic battlefield air defences are in need of a reform. We need to look more into the interception of ammunitions instead of primarily the interception of platforms.
The advances in sensors and miniaturization have enabled stand-off precision attack capabilities. The best reason for buying such stand-off equipment is of course the desire to avoid the kill zone of air defences. It's all quite tricky, but the widespread readiness to invest in such stand-off capabilities points strongly towards the conclusion that this stuff is effective. That is bad news for classic battlefield air defences, of course.
Battlefield air defences can hardly be numerous and capable enough at once to defeat platforms beyond their attack range. Well, unless we consider semi-mobile air defence units such as Patriot or Aster batteries as "battlefield" air defences.
This is indeed a possible answer to the stand-off munitions challenge; set up air defences with a greater range than stand-off missile-equipped aerial attackers can have.
This might indeed work - at least partially. We will not have air defences that can out-range a 250 km air launched missile. Such missiles are still a threat to stationary targets; critical infrastructure such as bridges. Tank crews do not really need to fear such long-range missiles.
Is it feasible to protect every army brigade with a full-blown air defence battery of 20+ km effective radius?
The existing force structures point out that no army has allocated such heavy air defence assets to a brigade or division yet (as far as I know). The classic battlefield air defences fit into the short and very short range air defence bracket (ShorAD, VShorAD) instead. Missiles with ranges such as 5 to 15 km are typical.
Maybe we could pull it off technologically. maybe we could have de facto mobile air defence batteries with protected 8x8 trucks. They might even be dispersed, connected only by radio and power by APUs. A swarm-like cloud of air defence trucks (C4, sensor & launcher models) might maintain a permanent protective umbrella of medium range surface-to-air missiles - even during a brigade march (few 8x8 trucks moving at once).
An optimistic army might expect that such a setup could survive. Less optimistic air forces might be plagued by the idea that a competent opponent usually finds a way to hit such a basket full of eggs.
This leads back to the necessity of killing munitions instead of platforms. Few battlefield air defence systems have an officially claimed and useful capability against missiles. Some types of air defence munitions and fuses are even unsuitable for the intercept of missiles by design. This affects especially the hit-to-kill munitions (both shell and missiles); incoming munitions tend to be too small for a reliable direct hit.
The widespread interest in 35-40mm guns with shrapnel or air burst shells can be explained with this defence problem.
The intercept of munitions also knows a high end; the rise of precision guided artillery projectiles and missiles demands for an effective answer on part of the defence. Radio controlled missiles tend to be among the very cheapest missiles capable of hitting moving targets. Radio control partially fell out of favour for the defence against platforms because those platforms are expensive and often equipped with emitters capable of countering such a guidance. Incoming missiles are not equipped with such emitters, though. The Swedish RBS-23 system is an example for a ShorAD system with a claimed capability to intercept even supersonic anti-radar missiles (one of the most difficult targets).
The ability to intercept Mach 3 missiles is close to the ability to intercept guided artillery munitions. Again, the defender's ammunition should not be more expensive than the attacker's ammunition.
The critical threshold is complicated, though. The whole affair is close to the counter-artillery business of the artillery (again, air defence and artillery meet!). The artillery's radars can detect and track mortar, artillery and rocket munitions in flight. This helps friendly firing units because they get feedback about the drift of their dumb munitions. It does also enable the detection of hostile firing units (by calculating the trajectory of dumb munitions back to their origin - this doesn't work as well for guided ones).
Finally, it enables a quick assessment whether the incoming munitions will hit anything of relevance or miss. This could even lead to GPS/radio-based early warning systems for troops. Many troops and vehicles already carry a lot of electronic gadgets with them - why not give them a software-based acoustic early warning if they're about to be hit by artillery in fifteen seconds?
At this point it should be visible that you do not need to intercept all incoming munitions - you could ignore those which are going to miss. This in turn influences the affordability threshold for defensive vs. offensive munitions.
Counter artillery rocket mortar (C-RAM) systems have so far mostly been based on existing hardware. It began probably with a 114mm cannon shell being hit by a naval Sea Wolf SAM sometime around '80. Today's systems are rather short-ranged; one system is based on a six-barrelled 20mm Gatling gun and another one is based on 35mm autocannons with shrapnel munition. There were also tests with self-propelled howitzers attempting to intercept other howitzer's shells in flight.
The efforts of Israel are quite outstanding. their objective is more political than military in nature and they developed several missile types for the intercept of dumb rockets.
Numerous other projects surely exist without striving for as much publicity.
Very short-range C-RAM systems seem to dominate in NATO today because today's mission profile is about the defence of fortified camps in guerrilla warfare against the weapons of guerrillas (mostly short-ranged mortars and very compact rocket launchers). This hardware won't help us much in a possible great war when we might face pulsing saturation attacks from competent "shoot & scoot" artillery forces.
The technical problems are certainly formidable; how could we develop a really cheap munition capable of hitting a supersonic manoeuvring munition in flight? It seems that the necessary answer is that we must not in any case launch a development project to meet this challenge. That would be the worst possible move because of the embarrassing inefficiency of NATO members' military hardware procurement agencies. The industry might develop such a system on its own initiative, on order by an export customer or maybe the Israelis, or Swedes end up developing an adequate hardware solution.
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This is a great moment to recall the prioritisation:
Protection against air threat is first and foremost about measures that reduce the vulnerability to attempts of detection and attack.[...]Next comes the necessity to reduce the air threat's repertoire.[...]Third come the actually destructive responses.
It makes sense to keep the active defence priority in regard to rocket, artillery and mortar threats low because this kind of response is likely the least cost-efficient one. A low budget for R&D as well as procurement does not exclude a good effort at tactics and theory about hard kill defences, though. Navies had defend themselves against munitions since the 70's (and should better have done so since the 40's!). Air forces should have a close look at the topic as well. Land forces should at the very least recognize its relevance to the artillery fight and the protection of key infrastructure (the famous 'critical bridge').