CHAPTER 12

WEAPON AIMING

Different weapons demand different methods of targeting. Targeting a weapon, be it an air-to-ground weapon or in an air-to-air combat, can be very complex and place great demands on the performance of the aircraft systems. For the accurate targeting of a smart weapon or an old fashion gun, it is essential that the aircraft and weapon axis reference systems are initialized to provide a common reference, thereby removing position and velocity errors.

This chapter will discuss the fundamentals of the aiming systems that is a part in this process, together with navigation systems, sensors, and integration of all to operational strategies.

12.1 – DEFINITIONS

Slant range: is the direct distance between the aircraft position at release and the target.

Aim off point: is the point on the ground that marks the extension of the line that contains the velocity vector of the aircraft.

Sight depression: is the angle that the aiming system shall be adjusted to take in consideration the ballistics of the weapon to hit the target. The reference line depends on the system structure, i.e., on a HUD can be the flight path mark, for fixed sight it’s the piper mark and so on. All the geometric corrections (parallax, corrections for wind, etc) should be incorporated in the specific tables for the tabled MIL setting.

Figure 12.1 Angle definitions

Bomb range: is the horizontal distance traveled by the bomb after release.

CEP: refers to the radius of a circle in which 50% of the values occur, i.e. if a CEP = 5m is quoted then 50% of horizontal point positions should be within 5 meters of the true position.

12.2 - FLIGHT ENVELOPE LIMITATIONS – EXTERNAL LOADS

Several limitations other than those of normal aircraft occur with external stores. The reasons for this extra limitation are caused by structural loads, or aerodynamics bad characteristics, that can result in problems to control the aircraft.

The main limitations are:

AOA or rolling limitations – The increase of moment of inertia around the X-axis

limits the maneuverability because of aerodynamics loads can pass the

maximum structural loading. Adjustments are necessary if there are

asymmetries (including fuel asymmetries).

Flutter limitations – The external stores alters the structural model of

aeroelastic effects (interaction of aerodynamics forces versus inertial that may

result in coupling with increase due to some resonance).

Control lost – due to asymmetries, the flight control systems may not be

enough to keep the aircraft under control or in the event of a spin will be more

difficult or even impossible to get back to controlled flight.

There are differences between limits for each external loading configuration for

carriage, employment or jettison. This happens because the flight conditions

are quite different. The limitations are in different speeds, load factor, or a

combination of them.

The employment and jettison shall consider the behavior of the load after

leaves the aircraft.

The maximum load factor is different if the maneuver is symmetrical or if it is

done together with rolling. In this case the load factor is less.

Other parameters that may produce changes in limitations: altitude,

temperature or total weight of the aircraft.

12.3 – AIR TO GROUND BOMBING MODES

12.3.1 Continuously Computed Impact Point (CCIP)

CCIP is visual air to ground attack mode. Pilot has to make a run in target direction and when the piper overflies the target, he has to press the weapon release button. CCIP uses radar or other means to determine the slant range to the target.

CCIP has the Projected Bomb Impact Line (PBIL) that represents the linear prediction of where the CCIP will track across the ground.

Figure 12.2 – CCIP HUD simbology

12.3.2 Continously Computed Release Point (CCRP)

CCRP is used during night or IMC conditions, although it is possible to use in VMC conditions.

CCRP requires target designation prior to attack. This can be made through inserting target coordinates, radar or HUD designation.

Once the target is designated, an Azimuth Steering Line appears in the HUD. The pilot has to put the velocity vector over the Azimuth Steering Line, press the consent to release button and wait until the bomb separates from the aircraft.

The Azimuth Steering Line can be thought of as a line extending up and out of the target itself. Placing the line in the center of the screen will have the aircraft fly over the target.

Figure 12.3 – CCRP HUD simbology

12.3.3 DIVE TOSS

Dive Toss requires target designation prior to attack. When flying in target direction, pilot initiate a pull up and when the computer predicts the best ballistic trajectory the bomb is dropped. This mode is used to avoiding overfly the target.

Figure 12.4 – Dive Toss HUD simbology

12.4 SIGLE AND RIPPLE RELEASE

Manual bombing using fixed sight was the basic weapon aiming until the 70’s.

At that time, pilot was using ballistics tables, based on freestream drag characteristics only and the effect of the aircraft separation (air flow and induced bomb oscillations) was not considered.The pilot dropped all bombs in one event (single release).ted

To improve aiming accuracy the pilot start to drop several bombs in a very small pre selected interval (ripple release) increasing the probability of hitting the target.

12.5 AIMING SOURCE ERRORS

12.5.1 Conventional sight

After leaving the aircraft, a conventional bomb will describe a ballistic trajectory based on its own aerodynamic derivatives. The SDA take account this flight path of the bomb and the adjustments of the visor can be accomplished. Since the trajectory of the bomb is function of the initial parameters, any difference of the planned condition for the release will induce an error of aim.

In summary, the conventional sight with the SDA adjusted for a pre-established release condition will show the predicted impact point. The main errors are:

- DIVE ANGLE: if the error is to a bigger angle the bomb will go long.

- SPEED: if the error is to a bigger velocity of the aircraft there will be two

effects. The initial speed of the bomb will be bigger; also the AOA of the

aircraft will be lower, that will cause another error in SDA, which will sum to

the other. Both errors will induce a longer bomb, i.e., the bomb will impact

after the target.

- ALTITUDE: if the error in the planned altitude is for a lower release, again

the bomb will hit a point after the target, or the bomb also will be long.

- SIDESLIP: if the airplane is flying with sideslip, then its flight path and its line

of sight are not coincident. A bomb released at this condition will fall to the

right or to the left of the target.

- LOAD FACTOR: for an aircraft in maneuvering under G loading means that

the AOA is increased. This error causes an error in aiming already discussed.

- WIND EFECTS: probably one of the most undesirable factors. It is

unpredictable. The corrections for this effect are found in the specific

manuals (-34).

12.5.2 HUD Systems

The employ of HUD for weapons delivery requires the use of complex system integration and sensors. The integration is accomplished by the “Operational Flight Program” (OFP) that gets the data from the Fire Control Radar (FCR) and communicate them to the Fire Control Computer (FCC). Then, the fire solution is displayed in the HUD.

Of all the external inputs to the FCC, errors associated with the FCR can have a profound effect on the air-to-ground accuracy. Essentially, the slant range and look-down angle of the radar provide the basic starting point to the FCC air-to-ground integration routine. The FCC uses these inputs to calculate the aircraft height above the target. Therefore, if errors in this data are not detected, the value of the FCC integration can be greatly degraded.

The effects of errors in radar ranging are fairly straightforward. If the FCR reports a value which is smaller than it should be, the resulting bomb range calculated will also

be shorter than it should be. This is true because the aircraft thinks it is closer to the ground than it actually is. Consequently, the aircraft will be allowed to travel closer to the target before release, and bombs will fall long of the aim point. Along the same line, a reported slant range which is larger than it should be results in the bombs impacting short of the target.

Since errors in the radar look-down angle are associated with the physical radar antenna mounts in the nose of the aircraft, the effects of these errors require study to understand. In all visual air-to-ground modes, the FCC commands the radar to look at a designated point on the ground. If the radar antenna is not aligned properly, the lookdown angle value reported to the FCC will not reflect the true look-down angle of the antenna. If, for instance, the antenna look-down angle reported is less than the actual angle, the radar will be slaved to a point further down range than it should be. In effect, it will give a higher value for the slant range. The resulting altitude calculations are, therefore, degraded not only as a function of the sine of the look-down angle error but also as a function of the aircraft speed and actual altitude as well. In addition to this, as the grazing angle (the angle at which the radar beam strikes the ground) decreases, the allowable tolerance in the radar slant range increases, adding further errors to the system.

In summary, errors associated with false antenna look-down angle values are compounded and unpredictable unless specific cases are investigated. The worst case of all radar problems is when the look-down angle is off and radar ranging is bad.

Since velocity and acceleration are important parts of the weapon delivery calculations, errors associated with the INS and the CADC (Central Air Data Computer, if is the case) can also have a significant impact on bombing accuracy. INS errors are not as specific as radar errors, and many are caused by erroneous pilot inputs rather than hardware problems. If the system is not initialized properly, it will be inaccurate for the duration of the flight.

Another error source, which is not due to external input but which does have a direct effect on bombing accuracy, is the alignment of the HUD Pilot Display Unit (PDU). For more complex systems a detailed analysis shall be done.

12.6 1TO - 34 BALISTIC TABLE

The manual 1TO – 34 deals with the operations of armed aircraft. The contents of it shall include:

Description of: aircraft weapon system, weapon employment, suspension equipment, aircraft weapon and sensors, combat support system, etc.

Normal and Emergency procedures related to use of the weapon system. Data (charts and tables) related to the qualified armament and planning

procedures.

The ballistic tables, fragmentation envelope, the mil settings and any other necessary parameters for the correct use of the weapon shall be included in this manual for the operational employment of the aircraft. The data incorporate all modes authorized for the aircraft, like ground attack, reconnaissance or air-to-air combat. Inside the tables are possible to observe the influence of the aircraft speed, dive angle, altitude, wind corrections, etc; also the procedures for the fire radar control or any other sensor present in that configuration.

12.7 CIRCULAR ERROR PROBABLE (CEP)

CEP refers to the radius of a circle in which 50% of the values occur, i.e. if a CEP of 25 meters is quoted then 50% of horizontal point positions should be within 25 meters of the true position.

Figure 12.5 CEP

Another way to analyze the performance of the weapons delivery or aiming or any other data set that requires statistics its necessary to define the difference between precision and accuracy.

Accuracy and precision are often used to describe how good any system performed. A distinction should be made between these two concepts.

Accuracy is the degree of closeness of an estimate to its true, but unknown value and the precision is the degree of closeness of observations to their means. Figure 12.6 illustrates various relationships between these two parameters. The true value is located at the intersection of the crosshairs, the center of the shaded area is the location of the mean estimate, and the radius of the shaded area is a measure of the uncertainty contained in the estimate.

Figure 12.6 Accuracy versus Precision

FONTE DAS FOTOS:

http://ehangar.com/videos/2012/12/16/falcon-4-bms-tutorials-fcr-ccip-ccrp-dtoss-lofting-bombs/

http://wiki.hoggit.us/view/HUD#Continuously_Computed_Impact_Point_.28CCIP.29

FIGURAS 12.1; 12.5; 12.6: Desenhadas pelo autor.

BIBLIGRAFIA:

AGARD

MANUAL DE TB DO CEV