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Joint Direct Attack Munition (JDAM) Operations

The JDAM program evolved to support Mission Need Statement (MNS) TAF 401-91 for an adverse weather, accurate strike capability. Adverse weather is defined as natural/man-made conditions such as rain, haze, dust, smoke, fog, snow, ice, wind, and/or clouds that preclude the use of current inventory weapons. This need is shared by both fighter/attack and bomber aircraft engaged in conventional warfare. The JDAM program satisfies this need by providing guidance sets for current inventory warheads, fuzes and associated components.

JDAM enables employment of accurate air-to-surface munitions from fighter/attack and bomber aircraft against high priority fixed and relocatable targets. Transfer alignment from the aircraft to JDAM provides GPS-quality position and velocity state vectors that initialize the JDAM navigation system. Once released from the aircraft, JDAM autonomously guides to the designated target coordinates using its GPS-aided INS. Navigation errors are used to generate guidance commands for the tail fins that maneuver the weapon along the optimum flight path. Target coordinates can be mission planned and loaded into the aircraft before takeoff, manually altered by the aircrew prior to weapon release via JPF, and/or automatically entered through target designation with onboard aircraft sensors. Multiple JDAM can be directed against a single target or multiple JDAM can be directed against multiple targets on a single pass.

JDAM is used worldwide against medium to high valued fixed targets in adverse weather conditions. JDAM is deployed from fighter, attack and bomber aircraft. It can be released at low to high altitudes and release maneuvers include dive, dive-toss, lateral toss, loft, or straight and level, within a release envelope that includes off-axis delivery options as well. An off-axis delivery is where the weapon glides toward its intended target on a flight path that curves away from the flight path of the releasing aircraft. This allows JDAM to reach a target without requiring the aircraft to overfly that target directly. The capability for off-axis attack provides increased aircraft and aircrew survivability. JDAM enables both single-pass multiple-target engagements with individual weapons, and single-pass single-target engagements with multiple weapons.

JDAM also provides flexible targeting and retargeting capability by accepting target coordinates that are mission planned and loaded before takeoff or by determining target coordinates and entering the data in-flight, prior to weapon release. Mission plans are loaded prior to takeoff and include release envelope, target coordinates, and weapon terminal parameters.

JDAM uses a GPS-aided INS to guide the weapon to preplanned precision target location coordinates achieving planned terminal impact parameters such as impact angle and azimuth. JDAM automatically begins its initialization process during captive carry when the aircraft applies power. It performs BIT, and aligns its INS with that of the aircraft. Targeting data is automatically down loaded to JDAM. When the aircraft reaches the release point within the LAR, JDAM can be released. The LAR depicts the area from which JDAM can be released and reach its target with the planned impact parameters. It is displayed to the aircrew while en route to the target. The aircraft onboard computers can handle JDAM release automatically or the aircrew can handle it manually. When released, JDAM begins its free flight operation. JDAM free flight operations involve separation from the aircraft, fuze arming, GPS satellite acquisition, guidance optimization, terminal trajectory adjustment, and target impact. JDAM free flight is further divided into three phases: Separation Phase, Optimal Guidance Phase, and Impact Phase.

The Separation Phase begins with weapon release. The weapon is released with the fins locked to prevent any control actions that could jeopardize safe separation from the aircraft. The fins remain locked for one second after release. After the one second delay, the fins are unlocked, electrical power from the initiator is applied to the fuze and the autopilot provides fin commands to damp angular rates and control the flight attitude.

The Optimal Guidance Phase takes place from the completion of the Separation Phase, when full guidance authority is achieved, until initiation of the Impact phase, which is the last second before weapon impact. During this phase there are two functions that happen simultaneously. These functions consist of GPS satellite acquisition and optimal guidance computation. The satellite acquisition process begins three seconds after release so JDAM is not shadowed by the aircraft and to minimize the possibility of receiving multipath GPS signals. The first satellite is acquired in approximately one second after the search begins; after two more seconds the second satellite is acquired and the third satellite in about four more seconds. JDAM then continues to acquire additional satellites, make position measurement corrections and achieve navigation accuracy. The time to first fix for the first valid navigation update is achieved in a maximum of 27 seconds after release. Simultaneous to this activity, JDAM employs an optimal guidance algorithm that adaptively computes, in real time, the minimal control maneuvers required to go from the present position and velocity state to impacting the target at the desired flight path and approach angle. These computations are continuously made throughout this phase and the resulting commands are executed by JDAM's autopilot. The optimal guidance algorithm is used for both horizontal and vertical targets with level, dive, loft and toss release conditions. The guidance algorithm continually computes the optimal trajectory from the current position to the target, to achieve an impact vector at the planned impact point, with the planned impact angle and impact azimuth. If all planned impact conditions are not achievable, the guidance law trades off impact velocity first, then impact angle/azimuth and finally impact point. By applying the algorithm in this manner, the weapon effectively optimizes the impact point. During the later portion of this phase, as JDAM nears its target, it will roll 180 degrees and pull down on the target to align its angle of attack with its velocity vector. For horizontal targets, this pull down results in a steep descent in order to maximize warhead penetration and to improve fuze and warhead reliability. For vertical targets, the weapon performs the same roll and pull down maneuver, but the resulting descent is not as steep. As previously indicated, the proper descent angle for both types of targets is continually computed by the guidance algorithm throughout JDAM's entire flight, until it enters the Impact Phase.

The Impact Phase is the last one second of flight, during which, the JDAM flight attitude is actively controlled, to zero the total angle of attack. This is done to align the warhead longitudinal axis to the velocity vector to prevent warhead breakup. The navigation system estimates the time to impact and the angle of attack. At one second prior to impact the guidance commands are zeroed and an attitude command equal to the velocity vector orientation is sent to the autopilot. This results in zeroing JDAM's angle of attack before impact. The resulting descent and minimum angle of attack results in maximum impact velocity for effective penetration of hardened targets.

In summary, the weapon's autonomous guidance system acquires GPS, which provides accurate position data to aid the INS and Mission Computer in computing the GPS optimum navigation solution to the target and guides the weapon to achieve the specified impact parameters.