Information Sheet Number: 1.15
Pre-Action Calibrations (PAC) firings are an integral part of gunnery exercises and operational preparedness. They prepare the gun and crew for actual shoots as well as ensuring the proper operation of the mount. Peacetime operations require PAC as part of pre-fire maintenance requirements. This lesson discusses the theory, periodicity, and requirements for PAC.
(a) SW300-BD-ORD-010 Preparation, Analysis and Predicted Accuracy for Naval Gunfire
(b) COMNAVSURFLANT/PACINST 3502.2 Surface Force Training Manual Bulletins
1. The Pre-Action Calibration (PAC) is fired prior to an exercise to determine the magnitude and direction of the Mean Point of Impact (MPI), as measured relative to the Gun Target Line (GTL), in order that a spot can be applied to move the MPI to the target for subsequent rounds.
a. Dispersion - The dispersion of a shot is the distance of the point of impact of that shot from the MPI. Dispersion range is measured parallel to the line of fire, and deflection is measured perpendicular to the line of fire.
b. Wild Shot - A wild shot is a shot with an abnormally large dispersion in range or deflection.
c. Pattern - The pattern of a salvo is the area covered by the points of impact of the shots, excluding wild shots. The pattern in range is distance measured parallel to the line of fire, between the closest and farthest points of impact. Deflection is measured perpendicular to line of fire.
d. Mean Point of Impact is the geographical center or summation of dispersions of the points of impact of the various shots to which corrections are added to account for errors.
2. In addition to ballistic correction information, PAC firings can expose inoperable equipment prior to any particular exercise or hostile confrontation.
1. PAC in its simplest form consists of firing a small group of rounds at an imaginary target that is normally on an arbitrarily assigned safe bearing and range.
a. A target is established by the fire control radar, normally a test target.
b. Fall of shot is observed in range by the radar and deflection by optics or radar.
(1) The Weapons Control Console Operator (WCC) or Gun Control Officer Console (GCO MK 34) can take note of the fall of the shot on the WCC B-Scan and apply offsets referred to as "spots" before actual firing at a target takes place.
(2) Once spots have been entered into the Gunfire Control System (GFCS), subsequent firings are automatically adjusted, which increases the probability of a first round hit on an actual target.
c. Estimate and record the MPI location relative to the imaginary target. This information is used in the ballistic correction calculation worksheet.
(1) PAC should be conducted at a range and bearing that corresponds to the expected target range and bearing for the actual exercise or engagement (PAC may be used for exercises of different ranges.)
(2) Use MPI range and deflection error as an initial spot.
C. GUIDANCE FOR PAC RANGE SELECTION
1. The most important criteron in choosing range and bearing for the conduct of PAC is the true bearing of the target you will actually be firing at.
a. Fire the PAC within 20 degrees of the expected true bearing for the engagement or exercise.
b. Synchro and gyro errors for any particular bearing can vary based on ordered true bearing.
2. Relative bearing to the actual target is the second most important criteria because the ship can be maneuvered to compensate for errors.
3. Range is the third most important criteria.
a. This order of priority may seem backwards because of the fact that range errors are potentially much larger than deflection errors. But if range errors cause a fall of shot to be long at 12,000 yds, it is likely that they will be long at 8,000 yds or at 16,000 yds.
b. This predictable trend is not true in deflection. If rounds impact to the right of the target at a relative bearing of 090, they may not impact to the right at a relative bearing of 000 degrees also.
c. Select a range in the upper part of the expected range band for the exercise or engagement.
4. Each PAC firing should consist of four rounds fired one round at a time. The cumulative effect of four splashes closely spaced in time allows better estimates of MPI.
a. Whether observed by radar, optically, or both, four rounds are the minimum number of rounds required to remove most of the random error effects from the PAC pattern.
b. No more than six rounds should be fired for a PAC per gun exercise.
NOTE: Presently, severe restrictions have been levied on Non Combat Expenditures Allocations (NCEA).
5. As in actual firings, PMS pre-fire checks are required prior to firing Pre-Action Calibrations.