Needles in the Haystack: hunting mobile electronic targets

Air & Space Power Journal, Spring, 2003 by Maj Michael Pietrucha

0300 Zulu, 26 June 2006, the Persian Gulf Four F-15E Strike Eagles fly through the Zagros mountain range in southern Iran, their terrain-following radars guiding the aircraft safely at 300 fret in pitch-black conditions. In addition to their normal self-defense AMRAAM/Sidewinder loadout, the aircraft carry a variety of munitions intended for use against a specific SAM array--the S-400 Triumf and SA-20 Gargoyle batteries guarding the naval base at Bandar Abbas--and incidentally covering much of Oman, the UAE, and all of the Straits of Hormuz.

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RADAR DEFENSES ARE very difficult targets. The addition of mobility to their arsenal has greatly complicated the problem of finding and killing the radars that serve as the backbone of both the surveillance and "shooter" portions of an integrated air defense system (LADS). The United States is highly reliant on its standoff sensors to find radar targets. Unfortunately, the picture provided by these sensors is incomplete and lags the radar transmission event by a significant time. (1) It is long past time to take advantage of our other, underutilized sensor array--the gear on board the strike aircraft. If we want to detect and target the threat in single-digit minutes, the shooters must also be the sensors.

The introduction of the S-75 Volkhov (NATO code name SA-2 Guideline) surface-to-air missile (SAM) into Vietnam came as an unpleasant surprise to American airpower, which initially had few defenses against the system. Eight years later, the ZRK-SD Kub (NATO code name SA-6 Gainful) wreaked havoc with the Israeli Defense Force's fighters over the Sinai. (2) Both the United States and Israel began crash programs to defend themselves against these threats, resulting in successful operations in the Bekaa Valley and the Gulf War years later. But as US capabilities evolved, so did the threat. While NATO losses in Operation Allied Force were low, the Serbs demonstrated to NATO (and anybody else watching) the advantage that mobility provides the defender. We can no longer assume that defense systems will be easy to find or easy to hit.

To establish air superiority we must have the ability to find and suppress or destroy air defense systems. With increasing mobility, modern SAM threats are extremely fleeting targets--targets that cannot be allowed to roam the battlefield unhampered. However, the ability to destroy these targets is predicated upon the ability to find them--a capability that must be greatly enhanced.

Off-Board Sensors

The Strike Eagles are running under emissions control (EMCON) with only low-power modes of the terrain-following radar and the radar altimeter to betray them. Given the terrain, detection by active or passive means is extremely unlikely. But the crews are not blind. A high-bandwidth receive-only data link, relayed by satellite, provides them with a partial picture from off board sensors far from the area. An onboard precision radar-warning receiver (RWR) is silently listening for nearby threats.

The use of off-board sensors and data links to pass high-fidelity data to strike aircraft is an established concept. It is valuable when considered as an adjunct to the striker's own sensor array but dangerous if considered as a substitute. An analogy can be drawn with the F-15C in its air-to-air role. That aircraft is capable of independent detection, identification (ID), and weapons employment. Data link from off-board sensors merely enhances those abilities. Any suggestion that an F-15 pilot could rely on data-linked information from airborne warning and control systems (AWACS) aircraft, to the exclusion of its own radar, would be inaccurate and unwelcome.

Similar limitations exist with other sensors. Electronic surveillance (ES) sensors removed from the immediate battlefield have serious physical limitations; they are not generally in the radar's main beam and are often unable to see weak signals. Air-breathing sensors may be blocked by terrain and the curvature of Earth. All of these factors combine to make a distant sensor's picture incomplete.

Low-power signals are particularly difficult for our intelligence, surveillance, and reconnaissance (ISR) sensors to pick out at long range. The distant collector often has to detect the low-signal-strength sidelobes or backlobes, rather than the main beam. Additionally, the strength of a signal is further attenuated by distance (3) and atmospheric (4) and weather effects. (5) Thus, a distant sensor has much more difficulty picking up any signal. For example, a radar signal detected at a tactical range of 20 nautical miles (nm) is 100 times stronger than it is at 200 nm. This becomes a critical detection issue for ingressing aircraft because low-power signals, such as missile guidance, are less likely to be detected by sensors at standoff ranges (i.e., Global Hawk, RC-135, or space-based systems).

In addition, radar signals travel in straight lines, and both terrain and the curvature of Earth may block a signal's line of sight (LOS). For example, a collector must be at 25,000 feet to be able to detect a signal source at 195 nm, even with no obstructing terrain, due to the effect that simple Earth curvature has on the radar horizon. (6) The higher the collector, the greater the advantage; at 65,000 feet a collector can "see" a sea-level emitter at 315 nm. Unfortunately, this relationship is true only for very flat terrain or over the ocean, since high terrain can also block signals. Obrva airfield is located in the Kragnjevac river valley in the center of Serbia with high ridgelines to the east, north, and west. It was very well defended, and its position made it difficult for off-board collectors to search and detect signals. Therefore, no air-breathing standoff collector outside the target area could reliably detect signals in the valley because their LOS to the source of those signals was blocked by the high ridgelines. In our scenario, if the strike aircraft were reliant solely on off-board sensors, they might arrive at the target without any threat warning.