In 1990, US forces arrived in the Persian Gulf with a cornucopia of navigation technologies: not just GPS but also LORAN, TACAN, TERCOM (for cruise missiles), and inertial navigation systems which used laser, electrostatic, or mechanical gyroscopes, as well as old-fashioned manual tools like maps and compasses. So why were US surveyors heading off into the Saudi desert?
The surveyors were from the 30th Engineers Battalion (Topographic), which was deployed to provide map production and distribution, surveying, and terrain analysis services to the theatre. The survey platoon’s work was being done on behalf of the Corps and divisional artillery, which had their own particular navigational needs. Unlike fighter or helicopter pilots, field artillery gunners didn’t have the opportunity to see their targets and make last-minute adjustments to their own aim. Unlike bomber crews or cruise missiles, their fire missions were not planned well in advance using specialized materials. To provide precise positioning information to the guns, each artillery battalion in the Gulf was equipped with two Position and Azimuth Determining Systems (PADS), truck-mounted inertial navigation systems that keep an ongoing track of the unit’s positions. At the heart of the PADS was the standard US Navy inertial navigation system, the AN/ASN-92 Carrier Inertial Navigation System (CAINS).
Like all inertial navigation systems, PADS had a tendency to drift over time. That meant that it required regular refreshes using a pre-surveyed location, or control point. The initial specifications for PADS were to achieve a horizontal position accuracy of 20 meters over 6 hours and 220 kilometers. Actual horizontal accuracy seems to have been far better, more like 5 meters. One reason for the high accuracy was that, unlike an airplane, the vehicle carrying the PADS could come to a complete stop, during which the system detect and compensate for some of the errors by the accelerometers in the horizontal plane.
Unfortunately, the US had exactly one control point in Saudi Arabia, at Dharan airbase (Army Reserve historian John Brinkerhoff says this and several other point surveyed were done with “Doppler based methods.” I assume that means using the TRANSIT satellite system, which determined location on the basis of Doppler shift). Starting from that control point, the 30th’s surveyors extended a network of new control points northwards and westwards towards the Iraqi border. Conventional line-of-sight survey methods would have been too slow, but the surveyors had received four GPS receivers in 1989 and soon got more from the Engineer Topographic Laboratories to equip a follow-up team of surveyors. Eventually, their survey covered 10,000 square kilometers and included 95 control points. Relative GPS positioning took about two hours (according to Brinkerhoff) and offered accuracy to about 10 centimerers (compared to 17 meters for regular GPS use). Absolute positioning – done more rarely – required four hours of data collection and provided accuracy of 1–5 meters.
When the ground war began on 24 February 1991, the two survey teams tried to stay ahead of the artillery, which meant driving unescorted into the desert and marking new control points with steel pickets with reflectors (for daytime) and blinking lights (for night-time). Providing location data through headquarters was too slow, so the surveyors took to handing it directly to the artillery’s own surveyors or just tacking it to the pickets. By the ceasefire on March 1 they had surveyed all the way to 30 km west of Basra. Where the artillery outran the control points they used their own GPS receivers to make a “good enough” control point and reinitialized the battalion PADS there, so all the artillery batteries would at least share a common datum. One thing PADS could do and GPS couldn’t was provide directional information (azimuth), so units that outran their PADS capabilities had to use celestial observations or magnetic compasses to determine direction.
What the 30th Battalion and the artillery’s surveyors did in the Gulf was different enough from traditional survey methods that the some in the army already used a different phrase, “point positioning,” to describe it. In the 1968–1978 history for the Engineer Topographic Laboratories, which designed army surveying equipment, PADS was one of three surveying and land navigation instruments singled out as part of this new paradigm (the others were a a light gyroscope theodolite with the acronym SIAGL and the Analytical Photogrammetric Positioning System).
Brinkerhoff tells the story of the 30th’s surveyors as the meeting of the high and low tech, but the work really relied on a whole range of technology. Most of the GPS surveying was relative positioning that was anchored to previous Doppler surveying. Position and azimuth information was carried forward by inertial navigation, and the position of the firing battery was paired with target information from a forward observer equipped with GPS, an inertial navigation system, or a paper map or from aerial photography which could be interpreted using the aeroplane’s own navigation system or a photointerpreter’s tool like APPS. GPS surveying and navigation did not stay wrapped up with all these other navigational tools for long. The technology was flexible enough to be used in place of many of them. But in the early 1990s, GPS’s success was contingent on these other systems too.
Sources Notes: The story of the 30th and its surveyors appears in John Brinkerhoff’s monograph United States Army Reserve in Operation Desert Storm. Engineer Support at Echelons Above Corps: The 416th Engineer Command (printed in 1992). Further details appear in the Army Corps of Engineers history Supporting the Troops: The U.S. Army Corps of Engineers in the Persian Gulf War (1996) by Janet A. McDonnell and “The Topographic Challenge of DESERT SHIELD and DESERT STORM” by Edward J. Wright in the March 1992 issue of Military Review. Reflections on how the artillery used PADS and GPS in the Gulf come from the October 1991 issue of Field Artillery, a special issue on “Redlegs in the Gulf.” Technical details for PADS are from the ETL History Update, 1968–1978 by Edward C. Ezell (1979).