ciple was obtained in June 1973 in a meeting with Dr. Draper and one of his principal staff advisors and colleagues, John W. Hursh. Over the next year, the foundation was laid for his laboratory, which in the interim had become the independent Charles Stark Draper Laboratory, Inc., to undertake the necessary work through contract with the Survey. DEVELOPMENTAL HISTORY In the early 1970's, as national concerns grew over property developments in the flood plains of streams, the resolve to improve flood-plain management grew concurrently. This triggered in the minds of a few Survey hydrologists a desire to formalize a modest beginning research on improved techniques for flood-plain delineation. Thus in August 1971, Rolland W. Carter (ret.), then Chief, Surface Water Branch, and the late Roy E. Oltman, then Assistant Hydrologist for Research and Technical Coordination, arranged for the assignment of George W. Edelen, Jr., to the research problem of investigating and comparing various alternative procedures for flood-plain mapping. This investigation was to feature cost comparisons and was to include examination of the prospects for electromagnetic measurements from aircraft. June 1974, several all-day technical discussion sessions were held at Draper Laboratory. Participants included a number of their professionals as well as Survey professionals from the several Operating Divisions. The result was a "Request for Proposal" (RFP No. 5507) which the Survey delivered to Draper Laboratory on April 8, 1974. The RFP described a proposed 6-month engineering analysis, or technical feasibility study, and, in essence, circumscribed the research task with the following specifications: 1. Define the prospects for designing and building an airborne instrument system capable of a. Determining continuously in real time (virtually instantaneously) the three-coordinate position of the aircraft. b. Continuously profiling terrain along the vertical trace of flight path. c. Achieving absolute (referenced to local control) position and profile accuracies of ±0.5 ft (0.15 m) vertically and ±2 ft (0.61 m) horizontally. d. Deployment in relatively light fixed- or rotarywing aircraft. e. Operating at relatively low flight altitudes, arbitrarily defined as below 3,000 ft (914 m) above the terrain. f. Providing vertical pointing stabilization for a small camera. g. Being updated and adjusted in flight with respect to its precise orientation and position. h. Tying aircraft position data to ground-control points in local survey area. ject instrument system. Edelen's early work coincided with the Director's expressed interest in examining the technical feasibility of developing an airborne instrument system for surveying. Thus, Edelen served on the ad hoc interdivisional committee appointed to explore the feasibility issue. In the wake of that committee's report, Edelen arranged with the U.S. Navy Oceanographic Office to test fly laser distance-measuring equipment in a Lock- 2. Define flight and operating techniques for the subheed Constellation aircraft. The intent was to profile the terrain along U.S. Highway 60 west of Richmond, Va., and to monitor vertical motion of the aircraft with an accelerometer; however, severe air turbulence vitiated most results. This served mainly to underscore the merit in proceeding with an analytical appraisal of the principal features needed in an airborne instrument system geared to the Survey's unique field surveying and operating requirements. Preliminary work on such an appraisal had been started already, but, at this point, the effort was intensified with appropriate administrative sanction. Several early milestones in this developmental history are in the two preceding introductory sections and are not repeated here. It is pertinent to note, however, that Dr. Draper's June 1973 endorsement in principle of the proposed new airborne instrument system, coupled with an opportunity to view state-of-the-art inertial guidance hardware already tested and proven by Draper Laboratory, effectively spurred the earnest work to shape the initial research tasks. Thus, at widely spaced intervals in the period from June 1973 to An added stipulation, implicit in the foregoing specifications, limits field use of the proposed airborne instrument system to nonmountainous terrain. Furthermore, the specifications were drawn primarily from requirements of the flood-plain delineation problem, which entails fitting floods of given magnitudes, such as the 100-year flood or the 50-year flood, into the local stream-valley geometry. The fitting exercise uses precisely measured valley cross sections at a sequence of selected sites and processes the relevant data through what a hydrologist terms a "step-backwater computational procedure." In the field-investigation programs of the Survey alone, about 10,000 stream-valley areas require such flood-plain definition-a singular need that drives the stakes quite high for success in developing airborne instrumentation that can offer substantial work savings in manpower, time, and cost. Reflections of this particular Survey need are evident in each of several other Federal agency programs. With the research task defined, the heart of the proposed instrument system was seen to be the inertial navigator which establishes the three-coordinate reference-carrying capability. This suggested a versatility in using the system that would transcend the rather limited variety of field problems involving terrain profiling. Some expression of the range of field uses, identified as time progressed, is given in the section titled "Uses for the Instrument System" and is seen to embrace the study programs of all FieldOperating Divisions in the Survey. Furthermore, the time and costs projected by Draper Laboratory for the necessary contract work clustered principally around the inertial navigator. The laser profiler, therefore, was recognized, even at this early juncture, as a lowcost and relatively simple adjunct to the heart of the instrument system that would enable early flight testing on important problems that involved terrain profiling. Thus, when the need for a system acronym arose, project personnel coined "APT" to signify Aerial Profiling of Terrain. Effective June 24, 1974, the Survey contracted with Draper Laboratory (contract no. 14-08-0001-14548) for the referenced 6-month engineering analysis, and the author was designated Technical Officer for this contract. His counterpart at Draper Laboratory, John W. Hursh, was designated Principal Investigator. The Survey monitored the advancing work regularly and closely and stayed involved through technical discussions and field data inputs as questions arose. The Draper Laboratory study culminated in a 225-page open-file report (Desai and others, 1975). The significant findings in the report are summarized as follows: 1. An airborne instrument system can be built that will perform to the stipulated precision. 2. The airborne system will require update (position and velocity vector) information at 3-minute intervals during the flight mission. 3. The update information required by the system can be supplied by an onboard tracker component. 4. The inertial and tracker components should be built to function as one integral unit. 5. Inflight calibration of the system will require an inexpensive retroreflector mounted on each of three presurveyed ground-control points. These will be tracked, one at a time, by the tracker. 6. The gyroscopes and accelerometers in the system should be state-of-the-art quality (Draper Laboratory supplied) if the Survey's high-precision specifications are held; they can be available commercial quality if lower precision specifications can be tolerated. also were made for reviews by scientists at the Army Missile Laboratory, Huntsville, Ala. The reviews focused on two fundamental areas: The mathematical developments that undergird the concept of a tracker device for updating the inertial navigator in flight, and the performance capabilities of the Draper Laboratory gyroscopes (gyros) and accelerometers in terms of meeting the overall precision specified for the complete instrument system. The reviews completely supported the Draper Laboratory findings. To anticipate the ultimate need for a decision and commitment on when and how to continue contract work at Draper Laboratory, a briefing was arranged for the Survey Executive Committee, which comprised the Director, principal administrators in the Director's office, and the Division Chiefs. The briefing was given on December 18, 1974, and highlighted work progress and outlook. After a followup briefing on May 7, 1975, the committee recommended that the contract effort at Draper Laboratory be extended through completion of the general design for the full instrument system. That work phase, estimated to require 18 months, officially began June 28, 1975, with approval of the contract papers and allocation of an initial sum of money. In the hiatus that otherwise would have occurred between completion of the engineering analysis and start of the general design for the overall instrument system, the Survey contracted with Draper Laboratory for a series of laboratory experiments with laser devices. Primary purpose of the experiments was to accumulate pertinent operational data that would lead to the best design for the laser profiler. In this work, Draper Laboratory was aided by the cooperative consulting efforts of personnel in the Electronics Branch of Harry Diamond Laboratories. Results of this 8-month work phase are documented in the report by Mamon and others (1976). Concurrent with their recommendation for continuing the Draper Laboratory contract effort, the Executive Committee requested that cost-effectiveness data be assembled, based on the potential Survey uses of the proposed airborne instrument system. During summer 1975, those data were assembled and analyzed. Justification was shown for completing the design and venturing into the costly fabrication phase of the instrument-system development as budgetary constraints would allow. Highlights of the costeffectiveness analysis were given to the Executive Committee in a briefing on November 12, 1975; the Survey-envisioned uses for the instrument system came from that analysis. The Survey arranged for outside technical reviews The contract effort progressed through a series of by selected scientists in the Harry Diamond Laborato- distinct work phases, beginning with the described enries, part of the Army Materiel Command, in Washing-gineering analysis, as shown in figure 1. The figure ton, D.C. Through those Laboratories arrangements illustrates the sequence and time frames within which the phases were accomplished and also shows costs of the contract work by phase and fiscal year. When completed, each phase was documented in an open-file manuscript report which is included in the list of refer ences. To stay abreast of the evolving contract work and to recognize the diversity in potential field applications for the proposed airborne instrument system, the Survey chose to monitor progress directly through monthly or bimonthly review sessions at Draper Laboratory. These sessions commonly ran 1 or 2 days and involved the Survey Contract Technical Officer and at least one colleague each from the Geologic, National Mapping, and Water Resources Divisions, as well as a colleague from what was then the Survey's Office of Land Information Analysis. The individuals who have comprised that review team throughout most, if not all, of the contract work phases shown in figure 1 are Russell H. Brown, William H. Chapman, John M. De Noyer, William F. Hanna, and Charles E. Mongan. The indicated mix of Survey representation, commonly combined with one or two representatives, most consistently Zoltan G. Sztankay, from the Electronics Branch of Harry Diamond Laboratories, brought top-level review expertise to bear in the very relevant scientific domains of cartography, geodesy, geology, geophysics, hydrology, mathematics, physics, seismology, and sophisticated instrumentation design and development, the last involving especially the fields of acoustics, electromagnetics, electronics, and optics. Each review session featured an informal, but detailed, exchange of ideas, identification of problem areas, and discussion of the most promising avenues for resolution. On-the-spot WORK TASKS ENGINEERING decisions usually were made, thereby forestalling delays that inevitably would have accompanied attempts at resolution by correspondence or telephone. At about the two-thirds point in the 18-month design phase of the contract work, Draper Laboratory held a design-review session. On June 22 and 23, 1976, Draper Laboratory project personnel described their progress on the primary facets of the design effort. Handout paperbound text and illustration materials totaled more than 400 sheets. In addition to Survey attendance at the review session, representatives from six other Federal agencies and from the Geodetic Survey of Canada attended. The discussions surrounding the design review reemphasized and strengthened the favorable determinations of feasibility that had been developed earlier. The design phase terminated virtually on schedule with submittal of a 401-page open-file report (Draper Laboratory, 1977). The report gives the principal general design features for the overall proposed airborne instrument system, with significant backup detail to support important design choices. Part of this detail for the laser profiler evolved from two sets of limited flight tests jointly organized by the Survey and Draper Laboratory in September 1976 and April 1977, respectively. The flight-test results are in a key report by Youmans (1977). On March 31, 1977, the Survey contracted with Draper Laboratory for the design-verification phase of the contract effort. The phase was programmed originally for a 10-month period, with the Survey entitled to fund the work incrementally. Due to unavoidable budget delays, this phase was not completed until Septem INTEGRATE, CONFIRM, AND CHECK DESIGN FABRICATE AND ASSEMBLE ENTIRE SYSTEM TOTAL COSTS (x 1000) T TRANSITION QUARTER = FIGURE 1.—Fiscal year history of work on U.S. Geological Survey contract by Draper Laboratory, Cambridge, Mass., 1975–81. ber 30, 1978. Effective July 13, 1978, Lowell E. Starr was named the Survey Contracting Officer's Technical Representative, and Russell H. Brown and William H. Chapman were named his Technical Designees. The design-verification phase represented the last necessary step before undertaking the fabrication phase. Design verification embraced complete integration and checking of all general design features, as well as detailed laboratory testing of key components, circuits, and interface electronics. The end products were the drawings, assembly and test procedures, and process specifications ready for submittal to the fabrication facilities. The design-verification phase is documented in a narrative report published by Draper Laboratory (1978). Companion to the narrative report is a “Technical Data Package” that contains all approved drawings, source control drawings, process specifications, and software documentation. The package is part of the official body of open-file information maintained at Draper Laboratory, Cambridge, Mass., and at the Survey National Center, Reston, Va. With no break in time, the fabrication phase began and spanned the 3 fiscal years from October 1, 1978, to September 30, 1981 (see fig. 1). In the negotiations for this phase, it became evident very quickly that if Draper Laboratory fabricated, in the absence of any commercial suppliers, the principal inertial components (three gyros and three accelerometers), the contract costs quickly would overrun the Survey's limited budget resources. Conferences between the Survey and Draper Laboratory elicited an alternate and less costly fabrication plan for the prototype APT system, which was based on using gyros and accelerometers that were emerging as "surplus" to an Air Force contract. In adopting this plan, however, some instrument design constraints were relaxed, with no relaxation in overall precision, to accommodate the existing gyro and accelerometer hardware. The principal relaxed constraint concerned the operating temperature environment for the gyros and accelerometers, and this, thereby, invoked a need to build cooling coils into the housing of the inertial navigator unit. This, in turn, mandated a decision to air condition the aircraft cabin, with the end result that the complete prototype APT system would need to be fielded in an aircraft larger than that envisioned in the Draper Laboratory design report. Thus, this first system was designed to be flown, through its test, demonstration, and early operational phases, in a DeHavilland Twin Otter aircraft rather than the earlier envisioned Rockwell Commander twin-engine aircraft. The fabrication phase terminated on schedule and is documented in a five-volume report (Draper Laboratory, 1982). ACKNOWLEDGMENTS The authors acknowledge the insights and effective support of top managers and scientists in the Survey, Harry Diamond Laboratories of the U.S. Army Materiel Command, and Draper Laboratory. Many individuals and organizations contributed to the success of this instrument development project. Some are identified at pertinent places in this paper. However, in recognition of especially supportive and significant contributions, the following individuals, and their organizational affiliations, are gratefully identified: U.S. GEOLOGICAL SURVEY James R. Balsley, former Assistant Director, Land Resources (ret.)1 Rolland W. Carter, former Regional Hydrologist, Southeastern Region (ret.) Philip Cohen, Chief Hydrologist Joseph S. Cragwall, Jr., former Chief Hydrologist (ret.) Edward J. Cyran, Cartographer, National Mapping Division John M. De Noyer, former Chief, Earth Resources Observation System Program; currently Geologic Division O. Milton Hackett, former Associate Chief Hydrologist (ret.) Warren W. Hastings, former Assistant Chief Hydrol ogist, Research and Technical Coordination (ret.) Joseph T. Long, former Chief, Branch of Field Surveys, National Mapping Division (ret.) Hugh B. Loving, former Deputy Assistant Chief, Research and Technical Standards, National Mapping Division (dec.) Robert H. Lyddan, former Chief Topographer (ret.) Edward A. Moulder, former Assistant Chief Hydrologist, Research and Technical Coordination (dec.) Roy E. Oltman, former Assistant Chief Hydrologist, Research and Technical Coordination (dec.) William A. Radlinski, former Associate Director (ret.) Richard P. Sheldon, former Chief Geologist (ret.) Rupert B. Southard, former Chief, National Mapping Division (ret.) Lowell E. Starr, Assistant Chief for Research, National Mapping Division HARRY DIAMOND LABORATORIES Lyndon S. Cox, Plans and Operations Office Iret.-retired dec.-deceased res.-resigned Joseph M. Kirshner, former Chief, Fluid Systems blage of gyros and accelerometers. For it to function as Helmut Sommer, former Director, Research and and the Earth's rotation. The directions of these vectors are "remembered" by the gyros, which establish the needed references for pointing or orienting the James E. Spates, former Chief, Plans and Operations whole instrument. The manner in which the references Office Zoltan G. Sztankay, Chief, Near-Millimeter Wave U.S. ARMY MISSILE LABORATORY Charles A. Halijak, Professor, University of Ala bama Joseph S. Hunter, Aerospace Engineer, Inertial Sys tems Development J. V. Johnston, Research Aerospace Engineer, Inertial Systems Development (ret.) are created may be likened to an orthogonal set of x, y, z coordinate axes fixed on an inner or stable platform that actually supports three orthogonally mounted gyros. The stable platform is isolated from the disturbing influence of aircraft rotational motion (pitch, roll, and yaw) about the three axes by means of a System"). The rotational movement of individual gimgimbal-support structure (see section titled "Gimbal bals can be offset or compensated for through the actions of their individual torque motors, which, in turn, are controlled by signals from the gyros as relayed William W. Stripling, Supervising Aerospace Engi- through the appropriate servoamplifiers. H. V. White, Aerospace Engineer, Inertial Systems THE CHARLES STARK DRAPER LABORATORY William A. Drohan, former Associate Division Leo F. Hughes, former Section Chief (ret.) Benjamin S. Smith, former Section Chief (ret.) Douglas S. Youmans, former Staff Member (res.) INSTRUMENT SYSTEM OVERALL SYSTEM CONCEPT Some important clues have already been given regarding the nature of the overall airborne instrument system. From these, the realization should be emerging that, if the Survey earth science measurement criteria are to be met, the heart of the instrument system, the inertial navigator, must be able to determine instantaneously, very precisely, and continuously its position and its orientation. The former implies a capability for navigation, and the latter, for pointing. The uniqueness of an inertial navigator is that it combines both the navigation and the pointing capabilities in a computer and a mechanism known as an inertial measurement unit (IMU)2. The IMU is an assem An inertial navigator cannot operate perfectly, owing to inescapable built-in mechanical imperfections and imprecise knowledge of variations in the Earth's gravitational field along its path of motion (the flight path). Thus, the position and orientation data yielded by the instrument drift slowly away from the truth (the correct values). This can be overcome by providing high-quality position-data samples periodically from an independent source, and the frequency with which this is done is related to the desired overall measurement precision. To satisfy the Survey precision criteria, the requirement is to update the inertial navigator with position data at 3-minute intervals; the independent source for obtaining those data is a laser-ranging device called a tracker, mounted in the aircraft in the same structural housing as the IMU. During flight, the laser tracker locates and locks onto a retroreflector target mounted over a known ground-control point. While the target is in view (about 30 seconds), a stream of range and pointing-angle measurements is collected. The way in which the inertial navigator and tracker complement each other and combine to make an outstandingly high-quality instrument system can best be illustrated in the following manner. First, the inertial navigator is a device that can very easily recognize and measure high-frequency-type accelerations and angular rotations-in other words, the normal motions-of the carrying vehicle (the aircraft). Its performance is corrupted, however, by very low frequency angular rotations caused by very slow and random gyro drift and other small systematic errors. Second, the tracker is an 2The term "inertial measurement unit" tends to mislead, in the sense that inertia cannot be measured directly. The unit simply features mechanical components (gyros and accelerometers) which, in responding to the phenomenon of inertia, deliver the measurements needed to define a "specific force" vector (containing gravitational and nongravitational accelerations) acting upon the carrying vehicle. |