The bottom line for accurate massed fires: Common Grid

FA Journal, Jan-Feb, 2003

Chief Warrant Officer Three Xavier Herrera

Since the introduction of the global positioning system (GPS) and its integration with inertial navigation systems (INS), many of today's Field Artillery weapons and target acquisition (TA) platforms can quickly and accurately perform self-location and orientation without relying on external survey support. Fielding of systems, such as the gun laying and positioning system (GLPS), modular azimuth positioning system/hybrid (MAPS/H) and the position navigation unit (PNU), have dramatically reduced the number of survey personnel and equipment assets in the Army's FA. US Marine Corps artillery units seemingly will follow suit.

This transformation has redefined the role survey personnel play on the modem battlefield. The primary function of the survey section for many years had been to provide common grid. Under normal operating conditions, platforms with self-location systems no longer require a surveyed firing position to emplace, but they still must maintain a common grid with each other to mass fires and achieve the desired effects on target.

The fire support community must be careful not to dismiss the need to maintain common grid based on platforms' self-location capabilities. With the introduction of digital maps and other digital products into our automated command, control and communication systems, it is imperative that warfighters understand common grid to employ FA and TA systems globally. Without common grid, units can't achieve the desired effects without wasting ammunition and manpower or inflicting damage to the wrong target.

This article discusses common grid and common survey and their targeting issues and provides references and recommendations to ensure accurate, massed fires--time-on-target.

Why is common grid required? Common Grid is required to permit the massing of fires, delivery of surprise observed fires, delivery of effective unobserved fires, and transmission of target data from one unit to another in order to aggressively neutralize or destroy enemy targets. (Quoted from the "Field Artillery Position and Navigation Plan" written by the Field Artillery School, Fort Sill, Oklahoma, Page 1. It is online at http://sill-www.army.mil/famag in the "Go-to-War Primer.")

Common grid is the foundation upon which the success of the artillery is built. However, until Operation Desert Storm, most fire supporters never concerned themselves with common grid. During ground combat operations in Desert Storm, rounds missed some targets by up to 750 meters. The culprit was the lack of common grid, specifically due to multiple datums, ellipsoids and grid zones referenced by the maps our joint forces were using.

In once instance in Desert Storm, an aerial observer located an enemy unit and transmitted a request for fire to the supported artillery headquarters for processing. The target coordinate was transmitted to a Navy ship positioned off the coast for prosecution. The ship fired two salvos (rounds) only to have the aerial observer report that the rounds missed the target by 527 meters. Why? They missed because the aerial observer and the artillery headquarters were operating on one datum (Nahrwan Datum) while the ship was operating on another datum (WGS84 Datum). This is known as a "datum shift."

As the result of many similar incidents, the Target Acquisition Department of the Field Artillery School investigated the datum-to-datum capabilities in Field Artillery systems in 1991. Those findings determined that our datum-to-datum capabilities were inadequate, that Field Artillerymen did not understand the subject, that the FA had no standard position navigation (POS/NAV) system requirements and that datums needed to be included as part of any position coordinate description.

Today, many of these same common grid issues continue. To gain an appreciation for common grid, Field Artillerymen must first review the five requirements for accurate predicted fire. (See Figure 1.) These five requirements are equally important; however, three of the five must be referenced to a common grid to be of value: accurate target location, accurate weapon location and accurate meteorological data.

Common grid allows synchronization of geographic information between all sensors, scouts and forward observers (FOs); weapons locating radars; meteorological measuring systems; delivery systems; and automated command, control and communication systems to mass fires. As our weapons achieve greater ranges, inaccuracies caused by not having a common grid become greater.

What is common grid? Common grid is the sum of several components: the geodetic system, coordinate/grid system, map projection and common or relative survey.

Geodetic System. Within the geodetic system are the datum and ellipsoid. A datum is a mathematical model for the surface of the earth used in mapping a region. There are horizontal datums and vertical datums. All maps do not reference the same datum or ellipsoid (to which the datum is referenced); in fact, there are still more than 1,000 different datums in use around the world today. No single operating system is programmed to operate in all datums; however, some allow the user to define datum entries.