Measuring Mach 8: sub-millimeter survey system
The Benchmark Survey System replaces legacy NGA conventional survey techniques with a GPS-only system, maintaining sub-millimeter positioning accuracy while increasing throughput by 400 percent, with an 85 percent decrease in manpower. Results agree with recent NGA precise survey data to better than 0.6 mm horizontal and better than 1.0 mm vertical.
**********
Since 1976, the National Geospatial-Intelligence Agency (NGA) has supported precise geodetic survey applications at the Holloman High Speed Test Track (HHSTT) at Holloman Air Force Base, New Mexico. Each survey element had individual stringent standard error criteria developed to meet or exceed the Air Force’s accuracy requirements; current NGA survey techniques at the HHSTT yield an accuracy of approximately 1 millimeter (2-sigma, each axis).
The HHSTT supports ground-level hypersonic testing of various propulsion systems, aerodynamic platforms, ejection seat systems, weapons systems, and targets. Its 9.6-mile length (15.5 kilometers) supports testing at speeds in excess of Mach 8; the HHSTT holds the current land speed world record at 9,465 feet/second (2,885 meters/second, or 6,453 miles per hour).
The HHSTT is a pair of continuously welded heavy duty crane rail sections, spaced approximately 2 meters apart and supported by a U-shaped concrete foundation. Track alignment fixtures are located every 52 inches (1.3 meters) along the entire length of each rail to secure it to the foundation. These fixtures permit incremental lateral and vertical adjustment of the rails. FIGURE 1 provides a cross-section view of the HHSTT.
The very high velocities obtained on the HHSTT mandate exacting control of the rails’ straightness and alignment. The Air Force routinely manipulates the rail, meter-by-meter via the track alignment fixtures, to maintain straightness and rail-to-rail spacing. All measurements made in support of these adjustment activities are referenced to 1,018 survey benchmarks installed along the entire length of the track. The Air Force requires survey positions for all the benchmarks that are accurate at the single-millimeter level, relative to a local reference frame.
Legacy Methods
The NGA has coupled precise geodetic survey instrumentation and standardized acquisition methods to meet customer challenges at locations throughout the world for several decades. In 1974, NGA (then the Defense Mapping Agency) conducted a study of HHSTT and the geodetic survey requirements imposed by the Air Force’s 6585th Test Group (now the 46th Test Group) at Holloman. This study determined that an eight-man crew, with a large inventory of geodetic survey equipment, would be needed on a continuing basis to meet the production schedules of the HHSTT, based on a shorter 35,000-foot (10.7 km) track length at the time.
FIGURE 2 depicts the evolution of this process from 1976 to the present. NGAs overall goal at the HHSTT has been to establish a database capable of serving as a tool to continuously monitor the HHSTT and its environment; the NGA geodetic survey results allow the 46th Test Group to keep the two HHSTT rails precisely aligned in support of ever-increasing test velocities.
Benchmark Survey System
Although the measurement quality achieved through NGA’s current survey process is satisfactory, the process is manpower-intensive and slow. This has motivated NGA to seek a more efficient method. In July 2004, under the sponsorship of NGA, Applied Research Laboratories, The University of Texas at Austin (ARL:UT) began the development of an alternative GPS-only survey system for the HHSTT, called the Benchmark Survey System (BSS). Two fundamental requirements were imposed on the BSS at the outset of the development: high accuracy and high throughput. The BSS has been designed to measure benchmark-to-benchmark vectors to an accuracy of 1.0 millimeter or better (1-sigma, horizontal and vertical components), and to support a full survey of the HHSTT in six weeks with a crew of four.
ARL:UT has developed and deployed the entire system; it includes commercial hardware components such as the GPS receiver and antenna, custom-designed mounting hardware specific to the HHSTT, and custom data processing software consisting of both a graphical interface and a GPS network position estimation application. In this article, we compare BSS results with a recent NGA survey, showing that the BSS has achieved the accuracy and throughput goals in the requirements.
Operating Principles. BSS hardware consists of 25 self-contained GPS data collection and storage units, or survey nodes, deployed together so that GPS observations are recorded concurrendy at 25 benchmarks along the HHSTT. Each survey node consists of a ruggedized electronics enclosure containing a GPS receiver, a single-board computer, a rechargeable battery pack and supporting electronics, plus a GPS antenna with a mechanical mount that supports the antenna and provides precise control of its location relative to the survey benchmark.