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
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."
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
"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.
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
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
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
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.
Copyright 2002 eDefenseonline.com & Horizon House Publications