FIGURE 33.-Recurrence intervals and callup sequence for the five synchronous tasks in the aerial profiling of terrain system. BRATE AND ALINE mode (fig. 35) show the same basic structure and organization as for the STARTUP AND TEST mode, with the important addition of the special actions needed to calibrate and aline the APT system. The STANDBY mode continues the use of the same computer-action structure to hold the system in its alined configuration ready to perform a flight survey and to allow time for entering flight-mission data that will be used by the computer in the flight modes

that will follow.

Computer processing actions for all four flight modes-EN ROUTE, SEARCH, AERIAL CALIBRATE, and AERIAL SURVEY-have enough similarities in basic structure and organization so that they are shown conveniently in a single diagram (fig. 36). Figures 34 through 36 faithfully portray all computer processing actions given in table 2, although some actions have been lumped to improve clarity. It should be emphasized, however, that the computeraction linkages and flow of data shown in these figures are, to some degree, schematic, or analogous, to "snapshots" of many actions that are commingling in apparent kaleidoscopic fashion. An infinite number of such snapshots are possible, but the several chosen serve to illustrate important features of the many necessary computer actions. To delve any deeper into the real complexities of the software development is well beyond the intended scope of this paper.

Laser Profiler

As already described in tracing the history of this instrument development project, the beginning centered on the innocent question, "Is it possible to measure terrain profiles to a specified high degree of precision from an aircraft?" The precision requirements in fixing aircraft position in the vertical and horizontal coordinate directions were what gave pause in answering the question. An immediate "yes" seemed possible, insofar as the instantaneous electromagnetic measurement of distance from aircraft to terrain was concerned. Subsequent events showed, however, that even this partial answer was brash to a degree because the design of the laser profiler encountered its own set of unique challenges.

Measurement of distance or length is so fundamental a problem that much attention is given to it in most studies of basic physics. Earth scientists in general are familiar with the platinum-iridium bar, exactly 1 m in length, which, for many years, has been maintained under precisely stipulated and controlled conditions at the International Bureau of Weights and Measures to represent a standard unit of length used throughout most of the scientific world. In 1960, the eleventh gathering of the General Conference of Weights and Measures on the International System of Units (SI) redefined the meter in the more rigorous terms of a specified number of wavelengths of electromagnetic radiation corresponding to the decay of the krypton-86 Thus, in this part of the overall instrument developatom between two particular activity or product levels. Thus, in this part of the overall instrument development project, it was fundamentally sound to set out to measure a length with a laser beam of light.

The basic processes involved in such a measurement technique require a capability for timing the known velocity of light propagation [about 186,281 mi/s (299,790 km/s)]. If the length measurement is to be precise to ±0.5 ft (0.15 m), this means the timemeasuring capability must reach a precision of 1 ns, or the time required for the light beam to advance about 1 ft. Tremendous developments in timekeeping by electronic means now make it possible to obtain "off-theshelf" equipment equal to the nanosecond timing task; special purpose equipment can do even better.

In essence, the measurement technique entails generation of a light beam by a laser source. The light thus produced has some very unique and desirable properties, the ones of principal interest being the spectral and temporal coherence of the beam. Thus, when the light beam is generated, successive wave forms have the same frequency, phase, amplitude, and direction. Although the particular laser chosen for the terrainprofiling task does not have as high a degree of coherence as most other lasers, it can be packaged so compactly that its small emitting area literally can be

FIGURE 34. Data flow for the STARTUP AND TEST mode of the software for the aerial profiling of terrain system.