Geodesy for GPS

On Tuesday, I wrote about how commercial users helped make GPS ubiquitous in the US military.  I want to add a quick coda to that, or maybe a prelude.  Having ultra-precise navigational fixes is only useful if you have ultra-precise maps to match.  If you read Steve Call’s Danger Close. Tactical Air Controllers in Afghanistan and Iraq (and you should), you’ll be amazed by how often forward air controllers were unable to call for close air support because of poor maps.  Getting your location off by a few meters isn’t a big deal when your getting directions to drive from Ohio to Connecticut, but it matters a lot more when you’re trying to make sure bombs hit the target and not you.

Part of the problem is that while maps are flat, the earth is a bumpy ellipse.  That means that map coordinates are a conversion from three-dimensional reality to a two-dimensional abstraction.  In the case of GPS, the abstraction is World Geodetic System 84, and how we got there is an interesting story.  Its a story that almost no one tells – mostly because it requires you to talk about geodesy, and talking about geodesy is the kind of thing that makes people run away from you at parties.  The sweet science that deals with “the measurement and representation of the Earth” (thank you Wikipedia), it uses terms like geoid and reference ellipsoid.  During the Cold War, geodesy became vital because you couldn’t aim inter-continental weapons like ballistic missiles without having a precise distance to your target – and that required precision mapping that no one had ever accomplished before.

The first blow in the geodesic Cold War was struck in late 1944, when Floyd Hough of the Army Map Service seized 90 tons of German geodesic reference data.  The German data was important because it included survey data for large chunks of the Soviet Union, territory where American surveyors had never been and wouldn’t be allowed.  Meshed with precise measurements the Americans already had for Western Europe and the Americas (which could be combined using HIRAN radio data for the distance across the Atlantic), that gave the Americans precise data connecting launch sites in the United States with targets in the Soviet Union.  When the CIA began launching spy satellites at the end of the 1950s they carried mapping cameras to help plot the positions of the secret facilities the main cameras were photographing (a version of the second-generation camera ended up on the Apollo spacecraft to help map the moon).

The work of converting all this information into a single, comprehensive database was done by two organizations: the Army Map Service (AMS) and the Air Force’s Aeronautical Chart and Information Center (ACIC).  Each used slightly different methods, and their datums were about 50 meters apart.  Unable to choose between them, but convinced that the military needed a single set of map data to avoid confusion, the Department of Defense split the difference in 1960.  The first World Geodetic System, WGS 60, was “the arithmetic mean of the 1959 solutions by AMS and ACIC, rounded off to the nearest meter for datum shifts.”  Using the combined datum, the ACIC was able to continue its work of nailing down the position of Soviet targets.  With the help of Corona’s mapping cameras (and a special mapping-only spy satellite, the KH-5 Argon), errors in target information went between 2-3 miles (at best) and 30 miles (at worst) to 300-450 feet by 1965.  By the time the whole geodetic system was revised in 1974, both AMS and ACIC had been absorbed – along with the Navy’s Oceanographic Office – into the newly-formed Defense Mapping Agency (DMA).  Merged with the government’s imagery analysis centers in 1996 to create the National Imagery and Mapping Agency (NIMA), today the DMA is part of the National Geospatial-Intelligence Agency (NGA).  NGA does far more than just maps – you can read about the agency’s contribution to the killing of Osama bin Laden here – but its geodetic responsibilities are the hidden foundations for everything that GPS can do.

Postscript: Yes, the story of geodesy is important, but the real reason I wrote this was to give a shout out to my alma mater, Ohio State University.  Faculty at OSU worked closely with the Air Force’s mappers from the very beginning, leading to a department of Geodetic Science being created in 1961.  The first Gravity Survey of the State of Ohio was published in 1956, and many of the Air Force’s staff earned PhDs at OSU.  Go Bucks!


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