#### Gunnery Officer

INFORMATION SHEET

BALLISTIC WIND AND DENSITY/INITIAL BALLISTIC CORRECTIONS

Information Sheet Number: 1.9

INTRODUCTION

A projectile moving through the air is effected by a number of forces which vary from one geographical location to another. Practical compensations for these forces can be found by the use of range tables and ballistic wind and density information. This lesson describes the procedures for requesting meteorological messages and other sources of ballistic wind data.

REFERENCES

(a) SW226-AB-MMO-010 Ballistic Wind and Density For Naval Gunfire

(b) SW300-BD-ORD-010 Preparation, Analysis and Predicted Accuracy for Naval Gunnery

(c) SW323-AF-ORD-010 Range Tables for 5"/54 guns (Full Service Charge)

(d) SW225-AO-MMO-010 Ballistic Wind and Density for Naval Guns

INFORMATION

A. BALLISTIC WIND

1. Definition - Atmospheric wind that will affect the projectile during its time of flight to the target. It is important that we calculate these forces to understand everything that will affect our ability to place ordnance on target. At a target range of 25,000 yards, a 1 knot wind can induce a 125 yard error.

2. Sources of wind direction and speed

a. Surface observation

(1) Local weather conditions are observed and recorded hourly by own ships Quartermasters (QMOW).

(1) The ship launches a helium-filled balloon with a radar reflective target suspended from it. This allows the ship's sensors to track the balloon through each layer as it rises.

(1) Usually found in CIC, it is a summary of weather conditions within the area in which the ship is operating. When working in a Battlegroup, this information is provided by the Meterological Detachment (OA Division) found aboard most aircraft carriers.

(2) The Technical Atmospheric Summary (TAS) section of the OPTASK METOC contains Upper Air Data which can be used to obtain ballistic wind information.

d. JMCIS/TWCS OPNOTES

(1) Ships equipped with JMCIS and/or theTomahawk Weapons Control System can request Upper Air Data via OPNOTES from the Meteorological Detachment on board aircraft carriers and/or command ships.

B. BALLISTIC DENSITY

1. The effective density value reflects the average densities through which the projectile passes. At a target range of 25,000 yards, a 1 percent change in density can cause a 135 yard error.

2. Sources of ballistic density:

a. Meteorological message

b. Surface observation

(1) Nomogram (Found in reference (b))

(2) This is called surface density (Normally used for very short ranges)

3. Zero setting on the fire control computer. Standard density is 1013.25mb. Calculations can provide the deviation from this standard.

a. In some systems a zero entered into the computer means that we default to standard density for calculations.

C. REQUESTING BALLISTIC METEOROLOGICAL FORECAST MESSAGES

1. Figure 1.9-1 reflects the areas of cognizance and origin for Meteorological messages.

2. Request will be in message format and should be sent by requesting unit three to four working days prior to actual forecast date needed on board.

3. Sample request message

a. FM: (requesting unit)

b. TO: FLENUMMETOCCEN MONTEREY CA

c. Classification //N03140//

(1) This message could be classified because ship position information, as well as intended ship movement in specific latitude, longitude, month, date, and time is given.

(2) //N03140// - is the SSIC. Used on all naval messages and official correspondence dealing with ballistic wind and density information.

d. Subj: UPPER AIR DATA

e. Start time - stop time - period

(1) Start: in DTG format that the forecast will be required for use.

(2) Stop: in DTG format that you want to receive the last message on board (multiple messages).

(3) Period: the number of hours and periodicity that you wish to receive messages.

f. Required NLT (No Later Than)

(1) Required to be received by requesting unit on board.

(2) Notifies FLENUMMETOCCEN of time the forecast should be received by requesting unit.

g. Position

(1) This is the latitude and longitude of the area in which the firing will occur.

(2) Up to five separate areas can be listed.

(3) Do not specify OPAREAs on this line. They mean nothing to the civilians who write the forecasts.

h. Note: The information that you receive from either the cognizant OCCEN or FLENUMMETOCCEN could be dated. It is important to remember that the Upper Air Data being provided to you is the most recent data from any ship that was/is in the area requested.

3. Upper air data can also be requested via a plain langauge GENADMIN message to FLENUMMETOCCEN. The orginator simply requests upper air data for a specific area on specific dates.

D. USING UPPER AIR DATA

1. Upon receipt of an Upper Air Data message, the information provided will be entered into a computer that has the Geophysical Fleet Mission Program Library (GFMPL) program. This is a multi-purpose program that can provide the following information:

a. Integrated Refractive Effect Prediction System (IREPS) data

b. Acoustic Propogation information

c. Ballistic wind data

2. GFMPL will provide values for ballistic wind and density that are usable in both the MK 86 GFCS and the MK 34 GFCS.

E. OTHER TYPES OF BALLISTIC METEROLOGICAL MESSAGES

1. NATO forecast

a. A formatted message that contains several groups of numbers. Each group is a six digit set which, after interpretation, will provide ballistic wind and density information for the area requested.

b. The NATO forecast is rarely used by the U.S. Navy, however, is still used by some NATO navies.

2. U.S. Navy forecast

a. A formatted message that provides ballistic wind and density information for a particular area.

b. Although the U.S. Navy forecast is much simpler to interpret than the NATO forecast, it is rarely used by U.S. Navy ships. Most Navy ships have GFMPL which greatly simplifies ballistic wind and density computations and precludes the use of NATO and U.S. Navy forecast messages.

F. INITIAL BALLISTIC CORRECTIONS

1. The object of initial ballistic correction is to estimate an Initial Velocity (IV) based on measurements of gun wear and the noted variations from range table standards.

2. These estimates are entered into the fire control computer to predict aim point for the weapon system.

3. The initial ballistic correction is only an estimate; however, the closer the estimate the more accurate the solution from the computer.

G. REFERENCES FOR INITIAL BALLISTIC CORRECTIONS (IBC)

1. Range tables for the applicable projectile and system configuration is required

2. SW225-AB-MMO-010 - Ballistic Wind and Density for Naval Guns

3. Current Ballistic Wind Density (BWD) from forecast message.

H. METHODS OF COMPUTING IBC

1. Figure 1.9-2 is the Initial Ballistic Corrections computation sheet and associated format.

2. Exterior ballistic section (known values)

a. Line A1

(1) Mount number

(2) Exercise

(3) Date fired

(4) Projectile

(5) Fuze type

(6) DODIC / NALC

b. Line A2 - Barometric pressure

c. Line A3 - Surface air temperature

d. Line A7 - Projectile weight

e. Line A8 - Target range

(1) Expected radar or optical range to the target

3. Exterior ballistic (unknown variables)

a. Line A4 - Ballistic density

(1) Refer to source priority chart for best ballistic input density.

(2) Surface density is determined using air temperature vs. barometric pressure.

b. Line A5 - Equivalent density correction

c. Line A6 - Ballistic/surface wind speed, wind direction

(1) Information is obtained from ballistic Meteorological (MET) message or by surface means as applicable.

d. Line A9 - Total Density

(1) Total density is the summation of line A4 and A5.

e. Line A10 - Range Error

(1) The range error of the Mean Point of Impact (MPI) is determined from the Pre-Action Calibrations (PAC) firings.

4. Interior ballistic (known)

a. Line B1 - Powder temperature

(1) Powder temperature is the average of the last three magazine temperature readings, measured in Deg F.

b. Line B2 - Powder index

(1) Powder index describes the type of powder being fired, which is found on the container. Majority of the designations are Smokeless Powder (SP).

c. Line B3 - Measured Projectile Seating Distance (PSD) reading.

5. Interior ballistic (unknown)

a. Line B4 - Velocity loss vs. PSD data

b. Line B5 - Velocity loss adjusted for powder type

c. Line B6 - Velocity loss (total) as a result of gun barrel wear

6. Initial velocity corrections (refer to section C on IBC chart)

a. Line C - Expected velocity based on seating distance

7. Range corrections (refer to section D on IBC chart)

a. Line D - Computes range corrections for other non-standard conditions resulting in range spot

8. PSD measurements must be conducted prior to each gunnery exercise, or incorrect velocity loss calculations may result.

9. The MK 34 GFCS utilizes a velocimeter which consists of an antenna unit mounted on the gun barrel slide, and is used to measure IV of 5"/54 caliber projectiles fired from the gun mount. This velocity data can then be used to confirm or update the previous IV that was used for ballistic computations.