Which scenario can be detect earlier by an ADS? First one or second one?
It was on open question, I don't think we have to chose one of them. Much of the public view on ballistic missiles come from associating them with dictators or nuclear threat and WMDs so people do not like them. But superpowers use them too. In recent years they have been increasing range of conventional missiles, or adding different methods of delivery...
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For your question, I guess it depends on capabilities of the defender. USA or equivalent would detect missiles just after the launch via satellite for warning or engagement, in ideal conditions.
Cruise missile : slow flight, low altitude, terrain coverage, stealth can be applied, mobile launchers including aircraft, can follow waypoints, smart sensors
Ballistic missile: fast flight, ballistic trajectory, stealth may be applied to warhead/reentry vehicle, mobile launchers, aircraft may carry small missiles
Modern (quasi)ballistic missile: fast flight, depressed trajectory if needed, maneuvering, may be a carrier for gliding reentry vehicles
The end result is the same (deliver warhead to the target) but the means is different and each has its advantages. Because of this, defense methods are somewhat different.
What is the biggest advantage of ballistic missiles over cruise missiles (I did not try to list all of them):
- Time to target. Let's say for a target at ~2000km, flight time for the ballistic missile is around 10-12 minutes, maybe 7-8 minutes with a depressed trajectory. For the cruise missile with high subsonic speed, it's around 2 hours.
- Speed. Because of the speed, it's harder to intercept. If intercepted late, can still damage the target.
- Deception. Several low cost decoy missiles could be launched along with real ones, defending force might not differentiate between them. This is more important for nuclear weapons though, but can be applied to conventional missiles for surprise/saturation.
Cruise missile advantages:
- Low cost, off-the-shelf components make them accessible now.
- Can use terrain cover and radar horizon by flying low
- Able to use waypoints
Let's consider Turkey... what can we do against both:
- For CM threat we can use AEW&C aircraft, low altitude radars, fighters with look-down radar, sensor network, SAMs blocking possible CM routes. Turkey can do it now.
- For ballistic missiles we can use AEW&C, UAVs near enemy territory for launch detection, stand-off munition to target launchers. For interception we don't have much at the moment, SAMs like S400 would only protect a limited area. If the launchers are away from our reach we would have difficulties with those methods.
As a side note, unlike some people who view/want them as a cover for potential long range ballistic missiles, I believe the space rockets of Roketsan and Delta-V are more important than SRBM/IRBM/ICBM work. Independent access to space is way more important than a weapon.