Document created: 5 March 03
Air & Space Power Journal - Spring 2003

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Needles in the Haystack

Hunting Mobile Electronic Targets

Maj Michael Pietrucha, USAF*

*Maj Mike Pietrucha is a former F-4G/F-15E IEWO and currently assigned to HQ USAF/XOXS.

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 feet 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.

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 (IADS). 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 distance3 and atmospheric4 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 Kragujevac 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. 

Data Link

During the ingress of the F-15Es, the lead and the number three aircraft execute a preplanned pop to an altitude just above the ridgeline in a 20-second target-acquisition maneuver (fig. 1). The ES sensors on board their aircraft detect the SA-20’s Clam Shell radar, but their location in the high terrain is outside the Clam Shell’s ability to detect them. While their individual RWRs locate the threat, the two aircraft communicate via a low-power interflight link to improve their individual passive solution. Within seconds, all four aircraft have shared the new location for the Clam Shell- not good enough for weapons employment, but good enough to confirm that the previous coordinates were out of date and to provide a cue for other sensors (fig. 2).

Figure 1. An F-15E, equipped with a Precision ES Array and armed with four JDAMs, is busy locating nearby threat radars.

Figure 1. An F-15E, equipped with a Precision ES Array and armed with four JDAMs, is busy locating nearby threat radars.

The assumption that all the participants will have a functional data link is implicit in the idea of networked sensors in general and off-board sensors in particular. One cannot disregard the possible loss of data link due to equipment failure or operator error, and the considerable adverse effects that loss would create. We should, therefore, not design an architecture that is totally dependent on having a data link. Any such architecture would be an invitation to an enemy to make a considerable effort to deny us the use of our own data links- a single point of failure. For example, a scheme that requires a number of sensors on various aircraft to coordinate their actions over long distances can be neutralized if only the data link is denied.7 A more robust architecture would allow an individual aircraft to get its own solution and cooperate through data link to enhance and refine its single-ship solution. This approach would allow networks to degrade gracefully with the loss of data link, and individual aircraft could still locate threats- just less precisely.

Figure 2. A pair of F-15Es locating radars. Each sees a portion of the threat array (solid lines) and, using data link (dotted line), performs cooperative ranging against the threats that are seen by both aircraft (dashed lines).

Figure 2. A pair of F-15Es locating radars. Each sees a portion of the threat array (solid lines) and, using data link (dotted line), performs cooperative ranging against the threats that are seen by both aircraft (dashed lines).

Data links need not reach across the battlefield. A flight of four aircraft could exchange information between nearby strike aircraft via a low-power data link that need not even use a radio frequency. A link can be designed for jam resistance and low probability of intercept.

Strike Aircraft Sensors

The Strike Eagles were 90 seconds from the initial point (IP) when the trailing element launched a salvo of miniature air-launched decoys (MALD). The decoys flew up to an altitude where they could be detected by the enemy IADS and proceeded toward the target area. The MALDs provided a rather rude awakening to the Tin Shield acquisition-radar crew, who had been presented with a convincing imitation of a large strike package headed toward the naval base. The automatic features of the SAMs came into play against the decoys, and the first Triumf missiles cleared the canisters before the MALDs were a third of the way toward the target. Within seconds, all target engagement radars in both batteries were radiating.

Putting aside the fact that current RWRs on US strike aircraft were not designed with the modern threat in mind, a hypothetical ES sensor suite (think advanced RWR) in the target area has a much greater chance of detecting a radar signal in its vicinity than would an off-board sensor. After all, the strike aircraft is nearby; and if it is being targeted, it can be assumed that the sensor is in the main beam and has a direct line of sight to the radar. Thus, the onboard sensor detects concentrated energy from a radar beam pointed directly at it rather than a much weaker sidelobe or backlobe that is scattered in other directions.

The ability of an RWR to accurately locate a modern SAM system is critical to the survival of the aircraft. A pulse-Doppler (PD) radar operator detects an aircraft by noting a difference in the frequency of the transmitted and reflected energy. That frequency (Doppler) shift is caused by the component of the aircraft’s velocity that is directed toward or away from the radar. Pilots in a detected aircraft may try to break the enemy radar’s tracking by turning and placing the radar at 90 degrees to their own vector. That change in direction reduces the velocity component toward or away from the radar site to near zero which results in a near-zero-Doppler shift. A reduced Doppler shift also enhances the effectiveness of chaff and decoys, which should allow the aircraft to break lock and hide in ground clutter. Most Doppler radar systems use a filter to reduce clutter by eliminating all returns below a certain velocity. To make the aircraft appear to have a velocity less than the filter velocity, or stay “in the notch,” the pilot of a strike aircraft flying at 540 knots must hold a heading (plus or minus three degrees) that is perpendicular to the direction from the aircraft to the radar (fig. 3).8 To do that, pilots must know the location of the threat radar precisely if they are to survive and attack the target. 

Figure 3. Doppler-Notch Diagram

Figure 3. Doppler-Notch Diagram. The target aircraft must fly a curved line to maintain a constant distance from the radar and remain in the zero-Doppler region.

If the strike aircraft can locate the emitter to within a 2,000-foot-radius circle, it can cue other sensors. The F-15E, F-18, B-1, and B-2 can use high-resolution synthetic aperture radar (SAR) maps to precisely locate the target cues by onboard ES, thus bridging the gap from the circle provided by ES to Global Positioning System (GPS) quality coordinates provided by the SAR. Most importantly, this precise location is done rapidly, entirely within the cockpit of a strike aircraft capable of conducting an immediate attack.

Sensor/Shooter

Four miles from the IP, the F-15Es enter a valley and achieve a direct LOS to the very-active radar array that is engaging the MALDs. The F-15Es are immediately detected, but it is too late for the defenders. The F-15E radars are fully active now, mapping the target array that had been located by their onboard ES sensors. They pass target location data via data link for use by other assets in-theater. Within 10 seconds of unmasking, the trailing element launches a pair of antiradiation missiles at the enemy radars. The crews identify target coordinates from the SAR maps and the jets drop behind a ridgeline and resume terrain masking. Total exposure time: 20 seconds.

Rather than simply being a user of the ISR data collected by larger, standoff systems, the strike aircraft also become providers of critical sensor data to other assets. Their positioning in the battlespace makes them an ideal collector. They stimulate the air defenses, becoming the reason that the radars turn on in the first place. They are the closest to an air threat. An array of onboard sensors, infrared (IR), radar, and electro-optics can be used to gather information, record it, and download it after the mission. Only the most time-critical data is transmitted using data-link bandwidth (fig. 4).9 Electronic intelligence (ELINT) information, for example, can be used to update threat databases, characterize enemy radars, and analyze enemy tactics. The ability to bring back recorded data and conduct a postflight download will provide additional and essential intelligence, remembering that not everything of value is needed in real or near-real time.

Figure 4. The Final Link. Data from the two strike aircraft, working in cooperation, is burst data-linked to a “traditional” ISR platform, the RQ-4 Global Hawk, providing an extension to the UAV’s own sensor array.

Figure 4. The Final Link. Data from the two strike aircraft, working in cooperation, is burst data-linked to a “traditional” ISR platform, the RQ-4 Global Hawk, providing an extension to the UAV’s own sensor array.

An immediate benefit of using strike aircraft sensors is shortening the time required to engage mobile SAMs and other fleeting targets. Rather than have the targeting data pass from a sensor through a targeting cell to the Air Operations Center controllers, the information starts and ends where it can do the most good- in the cockpit. This is an important improvement because strike aircraft have a very small time window in which to engage between the time a threat emits radar energy and reveals its position and before it packs up and drives away in less than 10 minutes.

Our sensors and architecture should also take advantage of the human-in-the-loop benefits of manned combat aircraft. We can make much better use of the crew than we currently do. These individuals are well trained in target recognition, threat knowledge, tactics, and weapons employment. The combat aircrew is accustomed to making rapid decisions on complex problems for high stakes. Given a set of well-written rules of engagement to operate under, the shooter is in an excellent position to make the decision to employ weapons.

Unattended Sensors

Shortly after ducking back into the mountains, the Strike Eagles make the most of the information gathered by the radar and onboard ES. The lead pair fires their first shots of the day, launching a total of four stealthy Joint Air-to-Surface Standoff Missiles (JASSM) at the enemy radars. The trailing element drops the last of their ordnance- another, smaller group of MALDs with jamming packages and a handful of AGM-154A Joint Standoff Weapons (JSOW). The overworked SAM crews continue to engage the new threat, but the saturated computers allow three JASSMs through, and the target-engagement radars go down for good- victims of 1,000-pound unitary warheads. The JSOWs arrive later, scattering the target array with small submunitions, and destroying launcher vehicles and support equipment. Hidden among the submunitions scattered over the ground are small, covert sensors that will continue to pass data long after the fighters have gone.

Any sensor net can have its collection capabilities improved by the inclusion of remote, unattended sensors. In Vietnam, Igloo White sensors were dropped by aircraft along the Ho Chi Minh Trail to provide target-detection data to listening aircraft. While there are serious technical limitations on the sensing and communications capability of small sensors, even limited sensors can provide important information. Strike aircraft will often be the delivery platforms, although cruise missiles and rocket artillery can also be used to seed an area with sensors.

Unattended sensors can be seeded into preplanned areas to pick up specified types of data. But they may also be deployed on an ad hoc basis by strike aircraft. For example, a strike aircraft that had detected a radar threat, but not its precise location, could deploy sensors in the area and wait for the target to move. A beer-can-sized submunition similar to the BLU-97/B could be loaded in CBU-87 canisters or AGM-154A JSOW bodies for easy, predictable dispensing.10

There are other uses for cheap, expendable remote sensors. Small and micro unmanned aerial vehicles (UAV) are often considered as part of an airborne net, but their usefulness need not be as limited as their airborne endurance. If the sensors aboard these tiny aircraft survive the inevitable crash (as they could be designed to do) after the UAV ran out of fuel, they could provide an additional enhancement to a distributed sensor net. If one of the MALDs used in the illustrated scenario had a data link and an ELINT sensor, it could have popped up above the mountains and sampled the electronic environment for the F-15Es. A cheap, expendable MALD will not have the ability to locate the threat, but it could see which signals are “on the air.” Then, the Strike Eagles could have unmasked their ES sensors, knowing which threats to look for.

Conclusion

The scenario above is entirely notional. The F-15E does not currently have the RWR to make this vision a reality. In fact, no US combat aircraft has the sensor array described above; the MALD is not fielded; and the small, unattended sensors described do not exist. Having said that, they are not beyond our technical or financial reach- especially given the high stakes.

While this article concentrates on air defenses, there is a requirement to engage varying classes of time-sensitive targets (TST). Putting sensors and shooters as far forward as possible applies the air-to-air model to attacking certain surface targets. F-15s today are capable of individually detecting, identifying, and engaging hostile aircraft and cruise missiles- very fleeting targets.

There is a demonstrated need to be able to counter enemy air defenses rapidly in any air campaign. The core capability to detect and locate the threat must be based on the strike aircraft, with additional enhancements built upon that solid core capability. The increased proliferation of advanced, highly mobile, and lethal SAM and radar systems makes targeting these systems extremely problematic. SAMs are a very special subset of TST because they shoot back; they must be detected in a timely fashion, rapidly and precisely located, and targeted in the shortest possible time. Off-board sensors suffer from the disadvantages associated with their distance from the battlefield. The use of a distributed network of ES sensors that not only includes, but relies on, strike aircraft could extend the reach of a typical ISR constellation to the heart of the battlefield, where it is most needed and useful. The ability to detect, locate, and subsequently suppress and destroy enemy air defenses is vital to the US armed forces’ ability to conduct air operations in defended airspace, and we must make good use of all of our available assets.

Herndon, Virginia

Notes

1. Any radar transmission.

2. The SA-6 Gainful missile was guided by a continuous-wave illumination beam that the Israeli and US RWRs of the time period did not detect. Egyptian Gainfuls capable of engaging targets at very low altitudes wreaked havoc among the Israeli strike fighters, who up to then had little respect for Arab defenses.

3. Signal strength is an inverse square function- the strength attenuates with the square of the distance. Thus, at 10 miles, a 10 gigahertz (GHz) signal would suffer 137 dB of attenuation; 157 dB at 100 nm. In plain English, a signal detected at 100 miles is one hundredth the strength of the same signal detected at 10 miles.

4. Most radar signals suffer from little atmospheric attenuation. Radar signals weaken rapidly at a frequency of 21–29 GHz, which is a water vapor absorption band, and around 60–72 GHz, which is an oxygen absorption band.

5. Medium rainfall (one-half inch per hour) adds about 0.1 dBm of attenuation per kilometer in the I band (8–10 GHz). Thus, at a distance of a mere 90 kilometers (54 nm), the signal strength is one-eighth of what it would be in clear air.

6. Since most radar signals travel in a straight line, this means that a receiver beyond the radar horizon cannot detect a transmitter. Ducting is a phenomenon that “traps” a radar signal below an inversion and allows it to travel over the horizon, but sensors above the inversion will have great difficulty collecting signals.

7. At least one proposed architecture requires that two systems, 45 degrees apart, use data link to work in concert to locate a threat with GPS-quality precision. Aside from the tactical difficulty of arranging an adequate geometry against a mobile threat, this system is functionally disabled if the data link is interrupted.

8. If one pictures a string connecting the aircraft to the radar, the aircraft must put the string at 90 degrees to the nose (directly off the left or right wing), which results in a curved flight path with the radar at the center. This means that the aircraft is not changing its distance from the radar, has no apparent velocity to the radar, and so is much harder to break out of clutter.

9. As an added benefit, this arrangement conserves data-link assets. Information gathered by the strike aircraft is transmitted to an ISR platform on a simple, line-of-sight link. The UAV (in the above example) then transmits the data beyond line of sight, using its own dedicated data links and removing the need to have a complex (and expensive) communications array aboard the strike aircraft.

10. If the sensors also look exactly like undetonated BLU-97/B submunitions, enemy soldiers will have an understandable reluctance to disturb them.


Disclaimer

The conclusions and opinions expressed in this document are those of the author cultivated in the freedom of expression, academic environment of Air University. They do not reflect the official position of the U.S. Government, Department of Defense, the United States Air Force or the Air University.


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