The first thing everyone should understand is -- Radar Horizon.
Horizon calculator - radar and visual
Basically...The higher the radar antenna, the further the distance it can see, but the principle is also applicable to the target, meaning the higher the target is off the ground, the more visible it will be for the seeking radar. This is why we have low altitude attacks by manned aircrafts, to get as below the defender's radar horizon as possible. The terrain following (TF) radar system for the F-111 is one example. The TF equipped autonomous cruise missile that can be launched by several means is another example of radar horizon avoidance.
The moment both attacker and defender see each other, visually or electronically, is unofficially called 'horizon break' and begins the coundown for the threat response time. The goal for the attacker is to compress that response time to as short as possible. The defender's radar mast could be at ten meters off the ground or in an AWACS who would have the furthest radar horizon. The horizon break relationship also involve the attacker's altitude. If the defender's radar is an AWACS, then it really does not matter the attacker's altitude. The radar horizon here is several hundreds km. The attacker would have to rely on hiding in the ground clutter, but that is another radar issue. Suffice to say that an AWACS offer the defender the greatest amount of response time. If the defender does not have an AWACS, then it falls upon radar antenna mast height and target altitude.
So in the radar horizon calculator link above...If we insert 10 meters for both radar mast height and attacker's altitude we have the visual horizon of 22 km and radar horizon of 26 km. If we change the attacker's altitude (h1), which would be the aircraft depicted, to 5 meters, then we have a visual horizon of barely 20 km and a radar horizon of 22 km. If we change the defender's radar mast (h2) to ten thousand meters, which would make it an AWACS, then we have a visual horizon of 365 km and a radar horizon of 421 km. Notice no change to the attacker's altitude of 5 meters. Now if we insert the closing speed of which the defender and attacker approaches each other, and it does not matter if the defender is stationary, moving like a ship or like an aircraft, the response time changes. But if the attacker is moving at 600 km/h and the defender is moving away at 601 km/h, then we really do not have closing speed issue and therefore no threat, the defender is escaping at a very slow 1 km/h advantage. This is why the Exocet type missile is so deadly to ships that are not covered by airborne radars regardless of the attacker's speed. If the radar horizon is only 22 km, 600 or 800 km/h attack speed really does not make much difference except for kinetic energy at impact.
The Brahmos' supersonic speed does not offer much tactical advantage
IF the defender is covered by airborne radar. In less than five seconds the defender can deploy a distraction/seduction chaff/flare defense that will effectively cover several thousands square km in radar and infrared area view for the missile's sensors. The missile can maneuver around the target area and try to reacquire the target but this would increase the response time for the defenders who can track the Brahmos with their own radars and bring it down with active defense such as guns or missiles. Chaff/flares are considered passive measures. It does not matter from a radar horizon perspective if the Brahmos is air delivered or not. The launch aircraft is still considered an attacker, its flight altitude is still subject to the radar horizon equation and naturally the horizon break versus response time relationship. If the AWACS can detect the launch aircraft at several hundreds km out, that begins the response time countdown for the defenders.
Compressed response time is a double edged sword. At supersonic closing speed, if there are any changes in the defender's position from the moment of horizon break the Brahmos may not be able to compensate. But against ships that are slow moving compared to an aircraft, and the Brahmos is an aircraft, then unless the targeted fleet is covered by airborne radar, the Brahmos is a very good weapon. Still...What the Brahmos must do on horizon break is to take a radar snapshot of the area, as much as its radar is able to scan, select the largest radar return, lock the entire image in memory, and home in on that largest radar return. Does the Brahmos have a mechanically swept radar? If yes, then there must be a delicate balance between its supersonic speed and its radar mechanical scan rate. The Brahmos may miss an aircraft carrier and home in on a supply ship because its supersonic speed effectively outran its own radar scan rate.
Or how about this scenario...The Brahmos broke horizon on a fleet and an aircraft carrier, the largest radar return in the snapshot, is right at the edge of its radar sweep and the furthest detected distance by the missile's radar.
IF the system is programmed to home in on the largest radar return the missile must make a correction. Depending on the distance between the aircraft carrier and the missile at the point of that course correction, and remember that the aircraft carrier is at the very edge of the missile's radar sweep limits, there may be sufficient time for the aircraft carrier to deploy passive electronic distraction/seduction defense, make drastic maneuvers and align its active defense towards the direction of the threat. Supersonic speed is not a negative but it does demand the system designers to compromise in other areas, such as decision making process for the missile. Limited fuel and aerodynamic stresses at supersonic maneuvers are other factors to consider in this decision making process. The designers may decide to forgo the largest radar return criteria and program the missile to limit its targets within a radar view that is less than its scan limits and distance less than its maximum radar range.
This is to point out that weapons design and testing are not as simple as media releases often portrayed. Subsonic attacks are still potent when balanced with sophisticated avionics such as small AESA radars and highly responsive flight control systems. Against operationally sophisticated defenders who can deploy airborne radar coverage 24/7, the Brahmos would be treated no differently than subsonic missiles. Radar horizon equals to awareness and that will drastically change every scenario.