TADS Acquisition And Ranging¶

In similar fashion to using the HMD as the selected sight, detecting, acquiring, and engaging targets with the TADS is fairly straightforward, in that the TADS simply needs to be directed toward and stabilized on the target to generate an appropriate weapon solution. TADS targeting modes such as LMC and IAT assist the Copilot/Gunner (CPG) in target tracking, and the LST assists the CPG in acquiring an offboard laser designation, but autonomously and efficiently acquiring targets using the TADS and performing an accurate ranging of the target location is critical in achieving a successful target engagement. Generally speaking, TADS sensors and fields-of-view (FOV) are best used in the following manner.

Initial detection of targets on the battlefield: FLIR Wide/Medium FOV.
Individual target acquisition: FLIR Wide/Medium FOV or DTV Wide FOV.
Target identification: FLIR Narrow FOV or DTV Narrow FOV.
Target tracking: FLIR Narrow/Zoom FOV; DTV Narrow/Zoom FOV.

As with most tactical situations, many variables may determine the ideal sensor and field-of-view. Factors such as target size, range to target, time of day, and movement of the aircraft or target will play significant roles in determining which sensor or field-of-view should be. For example, a company of tanks will be much easier to detect and acquire in Wide FOV, whereas a squad of infantry at the same range may not be detected until Narrow FOV is utilized. In addition, the reduced slew rates and narrow fields-of-view of Narrow or Zoom FOV levels may make them unsuitable for engaging targets at close range, depending on whether the intended targets (or the aircraft) are moving at a high angular velocity.

If a target is lost or inadvertently exits the TADS field-of-view, it is normally wise to “back out” to the previous field-of-view to re-acquire the target within the center of the TADS video, stabilize the TADS movement as necessary, and then “step in” to the next field-of-view to resume tracking as desired. It is important to note that the Zoom FOV setting is simply an electronic magnification of the video received from the TADS sensors and will not provide an increase in target resolution or detail. However, the larger video image may aid in precision tracking against long-range targets in which precise placement of the TADS laser beam is critical in accurate ranging or designation of the target.

Acquisition Sources¶

The use of acquisition sources can reduce the time necessary to bring the TADS onto an intended target. When a target is detected using another sensor onboard the aircraft (to include visual detection by either crewmember), setting that sensor as the acquisition source and enabling the SLAVE function increases the efficiency of target acquisition within the TADS.

Acquisition sources also reduce the amount of verbal communications and coordination of sensors that must occur between the crewmembers, which also increases combat efficiency. By replacing the verbal description of a target (or threat) using a verbose “description, direction, distance” with a concise “sight to source” command, target handovers between crewstations may become near instantaneous with a proficient aircrew. In addition, slaving the selected sight directly to the location of the acquisition source reduces the effects of inaccurate range estimations and removes the need for verbal descriptions of the intended target, or visual reference points to aid in locating the correct target amongst many.

An example of an inefficient target handover from the Pilot’s HMD to the CPG’s TADS is shown below:

“Gunner, target tank at 11 o’clock, approximately 4 kilometers, dug-in along the north-south running treeline.”

The CPG must manually slew the TADS to the approximate direction, visually identify the correct treeline along that general direction (assuming the Pilot’s range estimate was accurate) and then locate the correct tank in that vicinity.

Examples of efficient target handovers utilizing acquisition sources are shown below:

“Gunner, target, Pilot helmet sight. Tank in the treeline.”

“Gunner, target, FCR. Tracked vehicle, range 4.6.”

In either case, the CPG simply selects the announced source of target information as the acquisition source and enables SLAVE, slewing the TADS line-of-sight directly to the location of the intended target, removing most of any targeting ambiguity that exists.

Acquisition sources that will command the TADS to a specific azimuth and elevation relative to the aircraft nose are listed below.

PHS. Pilot Helmet Sight; commands the TADS to the line-of-sight of the Pilot’s helmet. May be used to direct the TADS to the location designated by the Pilot’s HMD LOS Reticle.
GHS. Gunner Helmet Sight; commands the TADS to the line-of-sight of the Copilot/Gunner’s helmet. May be used to direct the TADS to the location designated by his/her own HMD LOS Reticle.
SKR. Seeker; commands the TADS to the line-of-sight of the next-to-shoot AGM-114 missile seeker. May be used to direct the TADS to the target location that is currently being tracked by the next-to-shoot AGM- 114 missile, possibly to confirm target identification prior to weapons release.
FXD. Fixed forward; commands the TADS to align with the Armament Datum Line (ADL) at 0° in azimuth and -4.9° in elevation.

Acquisition sources that will command the TADS to a 3-dimensional location relative to the ownship position are listed below.

FCR. Fire Control Radar; commands the TADS to the location of the next-to-shoot target designated on the FCR page. May be used to direct the TADS to the location of the designated target to perform visual identification prior to weapons release, or to engage the designated target while using TADS as the sight.
W##, H##, C##, T##. Waypoint, Hazard, Control Measure, or Target/Threat point; commands the TADS to the coordinates of the selected point within the navigation database. May be used to direct the TADS to a pre-planned, stored, or transmitted location for reconnaissance, visual identification, weapons engagement, or re-acquisition if line-of-sight was lost.
TRN. Terrain point; commands the TADS to the coordinates of the selected terrain point within the navigation database. May be used to direct the TADS to a cursor-selected location on the TSD that is not associated with an existing Waypoint, Hazard, Control Measure, or Target/Threat point for reconnaissance, visual identification, or weapons engagement.

Range Sources¶

Once a target has been successfully acquired and is being tracked within the TADS field-of-view, the next and perhaps most critical step in ensuring a successful weapon engagement is determining an accurate range to the intended target. As the TADS line-of-sight can only determine a target’s relative azimuth and elevation with respect to the aircraft’s attitude, measuring the range to the target along the TADS line-of-sight provides the third dimension needed to determine the target’s 3-dimensional position relative to the ownship. Among the three sights (HMD, TADS, and FCR) that may be employed by the AH-64D aircrew for targeting, the TADS facilitates the most options when determining what method of ranging may be employed. When a method of ranging is employed, this is called a “range source”, and is displayed within the High Action Display in both Flight Symbology and Weapon Symbology.

The range sources that are available to the Copilot/Gunner (CPG) when using the TADS as the selected sight are shown below, from the most accurate to the least accurate.

Laser range. Laser range is automatically entered as the range source any time the CPG fires the TADS LRFD to measure the slant range between the ownship and the target using reflected laser energy.
Navigation range. Navigation range is automatically entered as the range source any time the TADS is slaved to a Waypoint, Hazard, Control Measure, Target/Threat, or a Terrain point. The slant range between the ownship and the corresponding point’s navigational coordinates are calculated and is subject to the position confidence of the aircraft.
Automatic range. Automatic range is selected from the WPN page by pressing MANRNG> (VAB B6) and entering “A” on the Keyboard Unit (KU) in place of a numerical value. Auto-range is determined by the radar altitude of the aircraft over the ground and the elevation “look-down” angle of the TADS turret.
Manual range. Manual range is selected from the WPN page by pressing MANRNG> (VAB B6) and entering a numeric value on the Keyboard Unit.
Default range. Default range is selected upon initial aircraft power-on, or any time the crewmember’s current range source is no longer valid.

When manually tracking a target while utilizing Linear Motion Compensation (LMC), it is best to use a dynamic range source to minimize “range jumps”, which increases the CPG’s workload in maintaining the TADS crosshairs on the intended target. As LMC will automatically convert TADS angular rates based on the current range value to compensate for motion parallax, the greater the difference between the previous range value and the next will cause a greater jump in TADS slew rates.

Dynamic range sources include a laser range when a continuous laser designation is employed (2nd detent of the laser trigger), a navigation range to a point stored in the vicinity of the target, or an automatic range when operating over relatively flat terrain. As these range sources may not always be practical depending on the specific tactical situation or the terrain over which the aircrew is operating, the range source that is best suited for the situation should be chosen to reduce the CPG workload during targeting operations.

Static range sources include a laser range when a single range-finding pulse is employed (1st detent of the laser trigger), a manual range entered on the WPN page, or the default range which is displayed upon initial power- up of the aircraft or if the current range source is rendered invalid. Such range sources are not dynamic and represent fixed points in space to which weapon solutions are calculated. As such, the TADS elevation relative to the target will need to be manually adjusted to account for rounds that impact prior to or beyond the target (also referred to as landing “short” or “long”).

Laser Range¶

The TADS laser rangefinder/designator (LRFD) provides the most precise range source available when using the TADS as the selected sight. Based on the TADS line-of-sight (LOS) and the slant range measured by the LRFD, a weapon solution is generated based on the selected weapon system (gun or rockets) and, in the specific case of the rockets, the selected warhead type.

If the aircraft and/or the target is moving, a continuous laser designation using the 2nd detent of the laser trigger should be employed. This will continuously update the precise slant range to the target and will also engage the Target State Estimator in calculating lead angle compensation against targets that are moving across the battlefield.

It is important to note that lasing accuracy is more critical when employed by attack helicopters due to the shallow targeting angles compared to fixed-wing strike aircraft. Laser designations should be focused precisely onto the target to prevent the laser beam from impacting the ground prior to the target (laser underspill) or beyond the target (laser overspill). Due to the greater difference in slant range calculations when such events occur, weapons accuracy is significantly affected when lasing at shallow angles.

Laser overspill during target designation

When the LRFD is fired using the 1st detent of the laser trigger, or 14 seconds has elapsed since the LRFD stopped firing using the 2nd detent of the laser trigger, the laser range will become “stale”, and will remain at a fixed range value. When this occurs, the weapon solution will continue to update based on the TADS line-of-sight (LOS), but the range value will be to a fixed point in space along the LOS and may not represent the true slant range to the target.

If the selected weapon is employed while using a stale laser range, the weapons will likely impact prior to or beyond the target (also referred to as landing “short” or “long”). If necessary, manual aiming adjustments may be made by adjusting the TADS LOS higher above the target to correct for rounds landing short, or adjusting the TADS LOS further below the target to correct for rounds landing long.

Navigation range is calculated by simply measuring the slant range between the ownship position and the coordinates of a point stored within the navigational database. Although a Nav range is more accurate than using Auto-ranging or Man-ranging, in that it is dynamically updated with aircraft movement and does not rely on flat terrain as is the case with Automatic range, Nav range is not as accurate as using a laser range.

Any time the SLAVE button is pressed when the Copilot/Gunner’s acquisition source is set to a Waypoint, Hazard, Control Measure, Target/Threat, or Terrain point, the slant range to the corresponding point is entered as a Nav range. This may be particularly useful when using “indirect fire” techniques, such as firing unguided rockets from behind cover against a set of coordinates or when employing laser-guided Hellfire missiles in a “Remote Fire” engagement method. However, when employing weapons against targets within the TADS line-of-sight, a Navigation range is only preferred if the intended target is within close proximity to the coordinates of the point being referenced by the Nav range, or if employing area effect weapons against the general area surrounding the point.

In the figure below, the intended target is at a range to the ownship that is substantially closer than the point being referenced (T01) for the Nav range. In such a situation, rockets would impact beyond the target due to the equivalent slant range being applied along the TADS line-of-sight, placing the weapon solution beyond the target and below the surface plane. The TADS LOS Reticle would need to be displaced below the target within the TADS field-of-view to adjust the weapon solution in such a manner so that the weapon trajectory would intersect with the target location.

Aside from long-range, indirect fire techniques, the most common use cases for employing a Nav range are when the CPG desires a smooth transition to a laser range source when firing the LRFD while LMC is enabled, or when the Pilot is employing unguided rockets with variable time delay fuzes independently of the CPG. Rocket types such as the M255A1 or M261 are dependent on an accurate range to target for their warheads to be effective. Unless fired in Cooperative mode with the CPG’s TADS supplying an accurate laser range, the most accurate range source that can be used by the Pilot to achieve the desired warhead effects is a Nav range.

Automatic Range¶

Automatic range is calculated using trigonometric ratios of right triangles, where the TADS look-down angle relative to the vertical axis between the aircraft and the surface below it is the acute angle; and the altitude above ground level (as measured by the radar altimeter) is the adjacent edge of the right triangle.

Using these two variables, along with an assumption the target is at an elevation equal to the that of the surface directly below the aircraft along a lateral axis that represents the opposite edge of the right triangle, the hypotenuse is calculated to determine the slant range to target. This calculation is performed continuously based on the TADS turret elevation and the radar altitude of the aircraft.

As the TADS line-of-sight (LOS) approaches the horizon and the look-down angles become quite shallow, the Auto-range calculations will become increasingly less precise due to the trigonometric ratios becoming quite large which is exacerbated if the aircraft is operating at very low altitudes over the surface.

Automatic range relies upon a functioning radar altimeter and will only be usable when the aircraft’s altitude above ground level (AGL) is less than 1,428 feet (the maximum altitude that is capable of being measured by the radar altimeter).

It is important to note that the accuracy of Automatic range is predicated on the assumption that the target is at the same elevation as the surface directly below the aircraft. As this may not always be the case, using Auto- range for weapon engagements should only be performed in areas with minimal terrain relief, such as open plains, non-mountainous deserts, large plateaus, or basins. Auto-range should not be used when operating over mountains, rolling hills, or complex urban areas.

In situations where the terrain directly below the aircraft is at a significantly higher elevation than that of the intended target, it should be expected that any weapon solution generated using Auto-range will result in unguided, ballistic munitions such as rockets or gun rounds landing short of the target location. Likewise, in situations where the terrain directly below the aircraft is at a significantly lower elevation than that of the intended target, it should be expected that rockets or gun rounds will land long beyond the target location.

Manual/Default Range¶

Manual range is the most inaccurate form of ranging when using the TADS. When a manual range value is entered, the weapon solution will continue to update based on the TADS line-of-sight (LOS), but the range value will be to a fixed point in space along the LOS at the range entered on the WPN page.

A Man-range is analogous to zeroing a rifle at a specific distance, and manually compensating for bullet drop by manually elevating the weapon sight higher or lower depending on the estimated range to the target relative to the range to which the rifle was zeroed.

If the intended target is estimated to be at a range greater than the manual range value, the TADS LOS Reticle must be aimed above the target to compensate for the additional bullet (or rocket) drop.
If the intended target is estimated to be at a range less than the manual range value, the TADS LOS Reticle must be aimed below the target to compensate for the reduced bullet (or rocket) drop.

Note

Default range for the Copilot/Gunner (CPG) is 3,000 meters (displayed as “3.0”), but the Default range source is essentially the same as using a corresponding manual range (displayed as “M3.0”).