Hammers of the Gods

by Michael Puttré
Sep. 1, 2002

There was a time in the distant past of military technological history – about 12 years ago – where the US husbanded precision-guided weapons (a.k.a. "smart bombs") for particularly sensitive or inconveniently located high-value targets. While the Persian Gulf War of 1990-91 is recalled in the collective mind's eye as a CNN video montage of individual bridges, buildings, and tanks blowing up as if by divine wrath, in reality only about six percent of the weapons dropped by US and allied pilots had some form of precision guidance. The rest were good old-fashioned iron bombs, very much similar to the sort the pilots' grandfathers dropped prodigiously on the Axis in WW II, killing millions of civilians.

Targeting has been the hobgoblin of bomber crews and mission planners since somebody first got the idea of tossing a grenade out of an open-cockpit biplane. Precision-aided weapons began as an attempt to improve on the use of the Mk I eyeball as a way to hit what is supposed to be hit without having to fly the same mission again and again. The Vietnam War saw the introduction of TV-guided and, later, laser-guided weapons. With TV guidance, the pilot or operator steers a weapon that is equipped with a TV camera by means of radio-frequency commands, which required the shooter to remain exposed to enemy counter-fire. Laser-guidance, where a seeker in the nose of the weapon senses reflected light of a particular wavelength from a laser designator, enabled a third party to "paint" the target so the attacking aircraft could shoot and scoot. The disadvantage here is that somebody – either another aircraft or a forward-observer team – has to be close enough to the target and have a clear line of sight to it. This is dangerous, possibly intolerably so in situations where the enemy commands the ground. Moreover, cloud cover can make the use of laser-guided bombs impossible. The laser spot tracker of the bomb has to be able to pick up that laser reflection, which it cannot do through cloud cover, smoke, dust, or other obscuring conditions.

This last shortcoming, in fact, is what prompted then US Air Force Chief of Staff Lt. General Merrill McPeak to handwrite a requirement on May 1, 1991, for an autonomous weapon that his pilots could drop through clouds and not have to wait for blue skies to go fight a war. Analysis of the air war over Iraq and Kuwait revealed that many missions by so-called all-weather fighters were in fact scrubbed due to bad weather, because pilots could not see their targets. Moreover, aircraft armed with laser-guided bombs were even more susceptible to cloud cover than those using iron bombs.

"The one variable we can't control is weather," said Captain Robert Wirt, Program Manager, Conventional Strike Weapons, at the US Naval Air Systems Command (NAVAIR) (Patuxent River, MD). "All it takes is a cloud getting between the path of the designator and the target, and a normal laser-guided weapon would lose track and go ballistic at that point."

Spirit in the Sky

Enter the Global Positioning System (GPS), a constellation of US Department of Defense (DoD) satellites broadcasting low-power signals for the purpose of aiding navigation that achieved full operational capability in 1995, but had been operating at reduced capability since before the Persian Gulf War. GPS satellites transmit on two L-Band frequencies: 1.6 GHz (L1) and 1.2 GHz (L2). The L1 signal has a sequence encoded on the carrier frequency by a spread-spectrum modulation technique that contains two codes, a Precision (P) Code and a Coarse/Acquisition (C/A) code. The GPS satellite signal contains information used to identify the satellite, as well as provide position, timing, ranging data, satellite status, and the updated orbital parameters (called, ephemeris). The L2 carrier contains only P Code that is encrypted mainly for military users. Most commercially available GPS receivers use the L1 signal and the C/A Code. The ramifications for precision-guided weapons are that an aircraft, ship, or artillery unit equipped with a P Code GPS receiver are capable of delivering weapons that also have GPS receivers plus an inertial-measuring unit (IMU) on target with great accuracy, within about 10 meters or less. With GPS guidance, the coordinates are already entered into the system, as part of the preflight mission planning or on the fly, enabling it to guide the weapon regardless of weather.

Most of the expensive, cruise-type missiles in the US inventory such as the Tomahawk, Conventional Air-Launched Cruise Missile (CALCM), and some land-attack versions of the Harpoon missile employ GPS and IMU in combination for navigation purposes. As important as these systems are to US warfighting capabilities, their cost relegates them to specialist missions against high-value targets. What the Pentagon wants to do is put GPS and INS on the vast majority of conventional bombs it sends against the enemy.

General McPeak's scribbled note led to joint Air Force and Navy requirements for a GPS-aided munition that would be common to both services. The requirements were simple: It had to be cheap, and it had to be accurate. It had to be compatible with existing weapons, aircraft platforms, and infrastructure. But cost was the key requirement, because the technical requirements were relatively straightforward. A competition – brief by Pentagon standards – between 12 contractors was won by the Boeing Co. (St Louis, MO) in 1995. The result of this competition was the Joint Direct Attack Munition (JDAM), the first family of an order of "J-weapons" that is doing more to change the US approach to the air-ground battle than any technology since World War II.

This is a bold statement for what seems, at least in the case of JDAM, a fairly humble rig. The JDAM is not constructed as a complete weapon, but rather as a "tail kit" assembly that converts existing "dumb bomb" warheads into precision-guided weapons. The kit consists of a GPS receiver, supplied by Rockwell Collins, that was developed from an existing commercial model; a single antenna; a Honeywell IMU; a Boeing mission computer; three mechanical actuators for moving the fins; a power supply; and cabling harness and connectors. The full system also includes mission-planning software, plus software for the carrying aircraft. Boeing supplies all software, and performs systems integration. Once the kit is in place, a person could be forgiven for not rushing to take pictures: It looks like a dumb bomb.

This was the whole point. "The Air Force and the Navy have a lot of 500-, 1,000-, and 2,000-lb. warheads in inventory, originally to be used as general- purpose [GP] bombs," said Carl C. Miller, Boeing's JDAM product-improvement program manager, who has been involved with the system since its inception. "In a GP bomb, you put an iron tail kit on the back without any kind of computer on board, as they did in Vietnam and quite a bit in Desert Storm. Our approach at Boeing was to say, we want to leave that configuration as close to the original general-purpose bomb as possible in order to minimize the work required to re-qualify the weapon on each airplane. We decided not to deviate a whole lot from the original design of the general-purpose bomb's outer mold line so it would hang and come off the plane much as it did before."

Except that in action JDAM behaves much differently than a GP bomb. While both are free-fall ordnance, the GP bomb follows a ballistic trajectory once free of the airplane with no course correction possible. The iron tail fins are for aerodynamic stability. The aircrew sends the bomb on its way generally using the aircraft's electro-optical sights for targeting. With JDAM, the weapon is provided with target coordinates before the flight as part of mission planning or entered by the in-flight aircrew using the aircraft's onboard computer. Prior to weapon release, the JDAM receives updated position data from the aircraft's GPS receiver. Once in free-fall, it spends the first 25-30 seconds of its flight acquiring the GPS satellite signals. These signals will then update the IMU. Data from the IMU is provided to the mission computer, which corrects the flight path of the JDAM as required using the actuated tail fins. If the GPS signals are lost due to a receiver failure or jamming, the IMU will continue to guide the weapon toward the target. The required accuracy for the JDAM with GPS and IMU functioning is 13 meters, and 30 meters with IMU alone after 100 seconds of flight.

The JDAM has been integrated on B-1, B-2, and B-52 bombers, as well as F/A-18, F-16, and F-15 tactical aircraft. Boeing is under contract for JDAM kits in the GBU-31(V)1B/3B (2,000-lb. Mk 84 GP), GBU-32 (1,000-lb. Mk 83), and GBU-35 (1,000-lb. BLU-110 GP) configurations. A GBU-38 (500-lb. Mk 82 GP) version is in development and undergoing flight testing.

The JDAM first was used in the "Allied Force" NATO campaign over the former Yugoslavia in 1999. B-2 Spirit bombers, then the only aircraft equipped to employ the weapon, dropped over 650 JDAMs when bad weather grounded other strikes. It wasn't long before the combination of intercontinental range, payload, and precision capability turned the frighteningly expensive (and previously much maligned) bomber from a novelty into a celebrity. One Pentagon analyst, in a post-war assessment, wrote that the B-2/JDAM combination "demonstrated the highest rate of target destruction of any aircraft/weapon combination used in the war."

Stand Back

Weather remained an important consideration behind the development, fielding, and use of weapons employing GPS receivers. However, the fencing match between Saddam Hussein's air defenses and the US and British jets patrolling the so-called "No-Fly Zones" of northern and southern Iraq had revealed another key feature of these weapons. They enable all aircraft – not just long-range bombers – to engage targets at effectively stand-off ranges with great accuracy, although, February 2001 saw a mass employment of the AGM-154A Joint Standoff Weapon (JSOW) in which fewer than half hit their targets. At the time, the US Navy and Pentagon officials were insisting the problems were related to the failure to incorporate wind conditions into the mission planning for the weapons, but there were questions raised about whether Iraq had employed GPS jammers to foil targeting accuracy. Subsequent raids against Iraq and in Afghanistan – where the JSOW was used when the Taliban's air defenses were a tangible if minor concern – have been more effective.

The JSOW is a more complex J-weapon built by Raytheon (El Segundo, CA). In essence, it is a glider bomb with a 500-lb. warhead and pronounced wings that deploy after release to give the weapon a stand-off range of 60 miles or more at altitude. It is a totally new weapon system, as opposed to a kit for existing ordnance. As its name suggests, it is used against targets from ranges outside short-range air-defense systems. Its use of GPS and IMU for initial guidance is similar to JDAM, but the JSOW C variant with a unitary warhead possesses an imaging-infrared seeker for terminal guidance to the target. The seeker enables the onboard computer to perform a terminal targeting function called scene matching, where an infrared image is compared with a stored image of the target that had been acquired from reconnaissance assets, such as aircraft, satellites, or unmanned aerial vehicles (UAVs). (The JSOW B variant with anti-tank submunitions has been put on hold for lack of need.)

The last of the initial batch of J-weapons is the Joint Air-to-Surface Standoff Missile (JASSM) from Lockheed Martin (Orlando, FL). The JASSM is a cruise missile with a jet engine and a range of about 200 miles. It has a 1,000-lb. penetrator warhead and would be used to engage high-value, well- defended targets. Like JSOW C, it has an imaging-infrared terminal seeker in addition to its GPS/IMU midcourse guidance. JASSM completed a test program late last year and is entering low-rate initial production.

"JDAM, JSOW, and JASSM have a common capability in that all are eventually going to be able to kill with precision," said NAVAIR's Captain Wirt. "How each is used will be driven by the threat. How far do I have to stand back from the target to shoot and not go into harm's way?"

In Afghanistan, US aircraft – including heavy bombers – loitering over the battlefield equipped with JDAMs were called on by US Special Forces to provide tactical close-air support. GPS-aided precision munitions allowed forces on the ground in the heat of battle to call in air strikes close to their own positions. This capability was not always flawlessly executed. In October 2001, the Pentagon said that a US Navy F/A-18 Hornet missed its intended target and inadvertently dropped a 2,000-lb. JDAM in a residential area a mile away from the Kabul Airport, where the aim point was located, killing at least four people and injuring others. The stated cause of the accident was that it occurred from a "targeting process error." The Washington Post reported that a friendly-fire episode in Afghanistan that killed US soldiers in December occurred due to a design feature in the GPS receiver used by the forward air controllers. The feature reset the receiver to its current location instead of the calculated target coordinates when the receiver's battery was changed while a bombing run was in progress. The reset coordinates were transmitted to the attacking aircraft, and the JDAM landed on the forward air controllers' position.

Mistakes such as these are indications that the users in the field are not entirely familiar with their new equipment. This is not an uncommon problem. "When you bring a new technology on board with any system, the CONOPS [concept of operations] has to go with it," said Captain Wirt. "Once they get them out into the field, CONOPS are adjusted on a continuing basis. There are employment considerations being thought up right now by the fleet that we never took into account in the design phases of these things." The plus side of this is that new capabilities are discovered and new ways of employment are developed. The dark side is that some new features and functions are occasionally found to be fatal to the wrong people.

Nevertheless, the US has jumped into GPS-aided, precision-guided warfare with both feet. According to the DoD, of the 12,000 bombs the US had dropped in the first months of Operation Enduring Freedom, 7,200 (60 percent) were precision guided. Of these, 4,600 were JDAMs. As the war entered its cave-clearing, Geraldo Rivera phase after the turn of the year, JDAMs had accounted for more than half of all tonnage dropped in Afghanistan. Not only are a much greater percentage of weapons being used with precision guidance – and GPS guidance in particular – but these are now the standard systems for attacking most battlefield targets. Massed iron bombs dropped by heavy bombers are saved for special targets, such as troop concentrations and trench systems.

Total Commitment

To understand just how revolutionary the advent of GPS-aided systems are, it is important to see bombing from the standpoint of people who fight wars. The conventional wisdom for tactical strikes prior to the Persian Gulf War was that it required six iron bombs to kill a particular "aim point." This is about the entire load of a typical strike fighter. And a given target might have more than one aim points, say an air base, depot, or missile site. So multiple aircraft would be tasked with striking the target. A percentage of these aircraft might be expected not to reach their aim points because of mechanical trouble or enemy action. Therefore, particularly valuable targets might have backup aircraft assigned for such eventualities. Compound this situation by dozens or scores of individual targets that need to be hit within a period of hours – hundreds within days, thousands within weeks – and the complexities and logistics required to mount and sustain an air campaign become clear.

The six-bomb rule was not because targets were so well protected, but because iron bombs tend to miss by such a margin that you need a six pack on average to get one good hit. The exception to this rule was for those few, special targets that rated a precision-guided weapon. But these were relatively few and far between, as were the numbers of such weapons in inventory. For a long time, since the Vietnam War at any rate, this was the accepted accounting: Drop six, kill one. Except that over time air defenses improved to the point where overflying most any defended target was risky business. And then people started to become more concerned about where the other five bombs went.

So being able to hit a target, or even multiple targets, with a single aircraft at stand-off ranges satisfied three key concerns: First, fewer sorties needed to be flown to achieve mission goals, saving wear and tear on equipment and personnel. Second, aircraft could attack targets from outside the effective range of air-defense systems, saving lives and reducing the possibility of crew capture. Third, collateral damage to civilian lives, property, and infrastructure could be reduced.

It is clear that neither the contractors at Boeing nor even the planners in the US Air Force and Navy fully realized the implications of the GPS revolution. Prior to the Afghanistan campaign, the US had a stock of approximately 10,000 JDAMs, and Boeing was turning out 700-750 per month at its St. Charles, MO, plant to fulfill a combined order for 87,000 tail kits for both services. This sounds like a lot, until it is realized that Boeing is now making about 1,500 kits per month, and the services have increased their joint requirements to 230,000 JDAMs. The Associated Press reported that Boeing has already built 9,000 new JDAM tail kits as of July 2002, compared to 10,000 in all of 2001. Demand has caused Boeing to add a second shift of workers to its St. Charles production facility. Raytheon, which makes laser-guided bombs and upgrades Tomahawk cruise missiles at its factory in Tucson, AZ, has added part of a third shift and is delivering some weapons up to six months ahead of schedule. Analysts say the buildup is to increase stocks in preparation for a campaign against Iraq, as well as replacing stocks depleted by the war in Afghanistan.

"We are moving away from dumb bombs as much as possible," Captain Wirt said. "The systems that are on our F-18s right now are very accurate for dropping dumb bombs. But given the nature of concerns over collateral damage, the need for close-air support, and the tight quarters in which we operate, there are real good reasons why we want the lion's share of our shipped builds to be precision-guided munitions."

Static on the Line

One of the recognized weaknesses of GPS is its susceptibility to jamming. The GPS signals from the satellite arrive at the user's PGS receiver at a very low level, typically -158 to -163 dBW, and are easily overpowered using moderate countermeasures resources. The satellites are 10,000 miles away, whereas the jammer has the advantage of being, say, half a mile away. So, it doesn't take much power to completely swamp the GPS signal in the receiver. Another, even more significant problem is that the receiver technology required to process the signal is known throughout the world. It's not the kind of thing where you can change the kind of processing that GPS does. You're stuck with it.

"GPS operates in L band, and GPS-jammer effectiveness tends to be limited to line-of-sight propagation," said Mario Casabona, president of Electro-Radiation, Inc. (ERI) (Fairfield, NJ), a maker of electronic-warfare and anti-jam GPS systems. "As such, ground-based GPS jammers tend to be short-range localized systems constrained by the horizon and terrain blockage/masking. Ground systems can be tailored to defend specific targets or attack routes. For this reason, it is best to situate ground jammers at points of high elevation, such as cliffs or mountains, or on the tops of buildings. Airborne GPS jammers, on the other hand, can be much more effective in denying GPS over a large region of the battlespace, but become high-value targets with limited usefulness and life expectancy. Airborne jammers can be extremely effective when you have air superiority over the battlespace."

GPS-aided weapon systems are vulnerable to GPS jamming at different stages of the mission. The launch platform can be jammed at any point in its mission, but generally maintains a stand-off range and can carry high- performance anti-jam capability to support weapons delivery. The weapon can be jammed after separation, during initial GPS acquisition, in route, or in the terminal stage of the trajectory to the target. Generally, an IMU capability is integrated with GPS to help ensure navigation continuity and provide inertial orientation to the munition.

"The assumption that goes with using a GPS-aided weapon like JDAM is that the weapon has a GPS-quality hand-off from the airplane," said Boeing's Miller. "This means that the aircraft still has GPS available to it for some time just before release, so that when JDAM separates and starts looking for GPS signals, the weapon locates itself based on the airplane location at the point of separation, plus input from the IMU. The unaided INS error rate grows as a second-order function, so in the first 30 seconds or so, you don't see much difference between GPS aided and unaided. After that, it starts getting larger than 13-meters accuracy figure and on up to the total IMU-only 30- meters accuracy."

When alternate sensor or seeker systems are available, such as imaging-infrared, laser, millimeter-wave radar, or acoustic, they can supplement guidance. Additional sensors tend only to make sense for specialized weapons, such as JSOW, JASSM, and cruise missiles, although the Navy said it is exploring the concept of adding seekers to JDAM. Boeing has a contract to develop a seeker that could be fitted to JDAM, although such a requirement has not yet been written. The program is called the Hornet Autonomous Reactive Targeting (HART). If applied to JDAM, the seeker would acquire targets without requiring GPS-assistance between the time the weapon is released and when the seeker finds the target, basically cutting GPS out of the loop. Of course, putting sophisticated seekers on JDAM will significantly increase the per-unit cost, rather undermining one of the weapon's initial requirements.

A potentially more cost-effective method of countering GPS jamming is to incorporate anti-jam features into the receiver design. GPS receivers with many channels can acquire some satellites even if others are jammed from a given direction. Al Story, a navigation and guidance expert at Boeing, said that since the GPS jammer would tend to jam from a particular direction, the computer on the weapon could have an algorithm to blank out seeing a satellite in that same general direction. So, with more satellites and with more channels to track those satellites, you might drop one or two of the satellites because of a particular jammer, but the other would continue to provide you a good solution. "We don't have blanking in an electronic-warfare sense, where we have the capability to steer a null," Story said. "With our 12-channel system, we can, though, operate without having all satellites available to us."

Nevertheless, in a war against a capable and resourceful opponent, such as Iraq, the US might expect to encounter GPS jamming from multiple sources. In 1998, Boeing participated in a program called Anti-Jam GPS Technology Flight Test (AGTFT) to show that the JDAM would benefit from anti-jam capability. The anti-jam technology for AGTFT was developed by Harris Corp. and took the form of a discriminator unit that was inserted between the antenna and the receiver.

With thoughts of sophisticated GPS jamming in mind, the US Air Force conducted the first test of a JASSM with a Selective Availability Anti-Spoofing Module (SAASM) technology in July 2002. This component will be included in JASSM production vehicles made in Lot 2 in 2004. The upgraded anti-jam capability improves JASSM's performance in acquiring and tracking GPS signals in a jamming environment. Nulling enhances the missile's ability to ignore signal noise from multiple directions that might interfere with target navigation. Beam steering provides the additional benefit of focusing the reception pattern only in the direction of the GPS satellites. Lockheed Martin Systems Integration (Owego, NY) produces the digital, anti-jam adaptive nulling and beam-steering GPS receiver.

Another possibility is to use multiple antenna elements so that the jammer signal arrives at a combiner port out of phase, essentially nulling the signal. "You don't cancel the GPS signal; you only cancel the jammer," said Lawrence Wells, an engineer with Interstate Electronics Corp. (IEC) (Anaheim, CA), a division of L-3, which produces anti-jam GPS technologies. "It's a cancellation technique, and those are getting very, very sophisticated nowadays, with various numbers of antenna elements and some very elaborate nulling algorithms being used to best take advantage of the information available regarding both the desired signal and the jammers to best suppress the jammers. There are a lot of hiddden 'gotchas' in that. But that's the basic technique. On a cruise missile, for instance, they usually make use of an antenna that is referred to as a controlled-radiation-pattern antenna [CRPA], which is just a fancy name for an antenna with more than one element, usually five to seven." IEC developed the CRPA that is the basis of the anti-jam system on the CALCM.

Wells indicated that the technique would be easier to implement on a missile than on a smaller bomb or an artillery round, on which there is not much real estate for multi-element antennas. IEC's efforts to add GPS assistance to artillery rounds, such as the Army's Excalibur program for 155mm artillery guns and the Navy's Extended Range Guided Munition (ERGM) program for 5-in. deck guns, have the additional challenge of developing guidance and anti-jam systems that will survive the 15,000-20,000-g forces of a round being shot from an artillery cannon.

Artillery presents other challenges for designers of GPS-guidance systems. As indicated previously, it currently takes up to 30 seconds to download the precision-location-data signals from the GPS satellites. For an artillery shot, the guidance system can't wait 30 seconds to get things moving. IEC addresses this with a receiver located with the gun that collects the current broadcast data. That information is downloaded into the projectile right before it is fired, so it doesn't have to do that in flight. The artilleryman has to put the round into an initialization stage because it has to be targeted; however, Wells said that this process is not especially demanding for the user. "All he has to do is put the round into the initialization system and wait for the green light before he puts the round into the gun," he said. "That takes a couple of seconds. It isn't a major thing, but it is essential to the success of the mission that all that happens and that it happens correctly."

At the same time, the receiver itself, while it is collecting this information, is doing a self-test to see if it can find any failures in its own operation. If it does, it will come up with a no-go, and the round will be treated as a dud and set aside and not fired.

Higher, Further, Smaller

Despite the potential Achilles' heel of GPS jamming, the US military remains very enthusiastic about GPS-aided precision weaponry. They point out that industry and the government labs are working very hard on anti-jam GPS. It is a priority.

Much of the enthusiasm for GPS weapons comes from the near-effortless way they were integrated into existing US systems, and even warfighting doctrines. The trend is for aircraft to engage more targets from stand-off ranges, and along comes the perfect weapon to enable them to do that. Another attractive aspect of GPS is that it ties in very nicely with considerable investments the US has made in battlefield sensors and reconnaissance systems, such as JSTARS and Global Hawk, that collect prodigious amounts of information, plus targeting systems on the shooters themselves, such as the Advanced Targeting Forward-Looking Infrared (ATFLIR) and in the future the Active Electronically Scanning Array (AESA) radar. With a little software, this information becomes target quality.

"There are many sources that can provide that information," said Captain Wirt. "The intelligence centers, the mission-planning centers, and the mission-planning tools can take advantage of many, many, many databases that are out there to pull that information in. The tools are set up such that the targeting systems know they are working for JDAM- or JSOW- or JASSM-quality information, and they reach out and go get the right information to allow them to plan a mission. The weapons are designed to be interoperable across any number of sensors or other information sources. We haven't even scratched the surface yet on all the information that could be brought to bear, and eventually we talk in terms of not sensor-to-shooter, but sensor-to-weapon."

This is network-centric warfare in its purest, where the platform dropping the weapon does not necessarily know where the targeting information came from, nor even necessarily where the weapon is going. The pilot is a bus driver, less even. He's an operator. Or maybe he isn't even there at all. Boeing's X-45 unmanned combat aerial vehicle (UCAV), which had its first flight in May 2002, is an autonomous attack aircraft that will carry the GPS-aided Small Diameter Bomb (SDB), which Boeing is also developing. The 250-lb SDB owes a lot to the JDAM program, and its small size is an indication of the precision that the developers are striving for. Small bombs delivered accurately by robots – like it or not, that's the future of strike warfare.