Between Sensor and Shooter: The Point Positioning Data Base, Part One

In 2002, after the kick-off of the US invasion of Afghanistan but before the invasion of Iraq, US Air Force Chief of Staff General John Jumper gave a speech in which he said that “The sum of all wisdom is a cursor over the target.” Two years later, the Air Force was demonstrating how, using an XML schema, troops could pass target coordinates to a bomb-armed F-15E flying overhead in an entirely machine-to-machine process in which each device involved communicated seamlessly with each other. The system was called, in honor of General Jumper’s speech, “Cursor on Target.”

Describing the basic process in a “Senior Leader Perspective” for the Air Force’s Air & Space Power Journal, project supporter Brigadier General Raymond A. Shulstad explained how target coordinates input into a laptop in the hands of a forward air controller in the field pass directly to a map workstation in the air operations center (the command center that coordinates air operations in a theater) and then, after the operator clicks on those coordinates on the map, appear automatically on the F-15’s head-up display.

Even Shulstad’s article only touches lightly on the element of “Cursor on Target” that’s absent in its name: the map (or the geodetic information) over which both cursor and target are positioned. The workstations in the air operations center that are in the middle of the observer-to-airplane communication are not just there to monitor what’s happening and to ensure that the coordinates really are a target (as opposed to friendly forces or a hospital). They are also there to provide information that’s missing from the initial observer’s message – the three-dimensional positioning needed to ensure that the bomb truly strikes its target.

Michael W. Kometer, who was an Air Force lieutenant colonel when he wrote an excellent book on some of these issues, explains that process, usually called “mensuration,” as follows:

Mensuration is a process that translates coordinates from a flat chart to take into consideration the elevation of the target. If a weapon could fly to a target directly perpendicular to the earth, mensuration would not be necessary. Since it approaches at an angle, however, a weapon could be long or short if the target is at a low or high elevation.

Today, mensuration software is a corporate product. The industry standard for military operations is a Northrop Grumman product called RainStorm that has its own openly accessible support page on the internet. But that wasn’t always the case. The map hiding within Cursor on Target only came into existence after a long process that began in a very different place: among the US Army’s field artillery crews.

Before APPS
The best place to begin the story is in the years immediately after the Second World War, when the US Army Corps of Engineers began trying to create a comprehensive coordinate system (or datum) that would cover not just Europe but the entire world. Until the Second World War, most nations did their own mapping using their own national datums – which meant that trying to plot directions and distances using coordinates from two different national surveys could lead to significant errors unless one did the math to convert coordinates from one datum into the other (sacrificing time and accuracy in the process).

The discrepancies only got worse the further the distances involved, which was a particular problem if you were trying to plot transatlantic flight paths or the trajectories of intercontinental ballistic missiles. The solution was to establish a new global coordinate system with maps to match – a process that triggered a massive turf war between the US Army and Air Force’s surveying and mapping centers, but eventually led to first World Geodetic System (WGS 60).

In Europe and North America, the US or its allies could do new surveys and create new maps whose coordinates were in WGS 60. For the Soviet Union’s territory, whose maps were closely controlled, the Americans had to turn to satellite photography. Measuring positions and distances using these photographs relied on a process called stereophotogrammetry. By examining a pair of images of the same target through a binocular fitting that showed each image to one eye, the photo interpreter saw a three-dimensional image on which he or she could make accurate measurements of distance. Connecting those measurements to known points, or using simultaneous stellar photography to determine the location of the camera-carrying satellite, let US cartographers create maps of the Soviet Union without ever setting foot on the ground.

Stereophotogrammetry was slow and calculation-intensive, which is why the CIA’s first computer was purchased by the photo interpretation center to speed up the process. But in the late 1960s the US Army’s Engineering Topographic Laboratory (ETL) began to experiment with ways of bringing that same capability to the army in the field. Using off-the-shelf commercial components they were able to build a mobile system that let any photo interpreter establish the precise position – within a matter of meters – of any object that had been captured on reconnaissance photography.

APPS and PPDB
The process began with a pair of film chips and a matching data cassette that carried the information to calculate the geographic coordinates of any point on the film. Aligning the pair of film chips under a stereo comparator (in which the user sees each image with one eye), the photo interpreter saw a three-dimensional image equivalent to the reconnaissance imagery of the target. Picking out the precise point of interest seen on the original photo, the operator zapped that information to an attached computer – a Hewlett Packard 9810A desk calculator – that consulted the magnetic tape and prints out the geographic coordinates. (When I say “zap,” I’m not kidding. An electric current ran from the cursor the operator saw onto a wire grid underneath the photographs. The voltage of the current told the computer where on the photograph the pointer is.) The entire system weighed 109 kg and could be set up on a standard-sized desk. It was called the Analytical Photogrammetric Positioning System (APPS); the tapes and film chips were known together as the Point Positioning Data Base (PPDB).

The Analytical Photogrammetric Positioning System. From Army Research & Development, May-June 1976, p.24

The Analytical Photogrammetric Positioning System. From Army Research & Development, May-June 1976, p.24

The first prototype APPS consoles were built from off-the-shelf parts and delivered to the Army in 1972–3. They offered a substantial improvement in measurement accuracy over previous techniques. In tests by Raytheon and Human Factors Research, Inc. the average error in position measurement using APPS was only 5–6 meters. ETL seems to have originally expected APPS to be used by artillery and surveying units, but it soon also became a tool for the brand-new MGM-52 Lance missile. That made sense because the Lance missile was both long-ranged and lacked any terminal guidance – without any way of homing on its target it needed to be launched with precise information on the target’s location.

Before the decade was over the original APPS was being replaced by a more advanced device, the APPS-IV, built by Autometrics, Inc., but the general principles involved remained the same. Look at two film chips through a stereo comparator, locate the point of interest, then use the attached computer to establish its coordinates. A handy tool, for sure, but not the basis for a revolution in military operations.

So what changed? The answer was GPS.

Forward to Part Two

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