Measurements and Processing
Data on target missiles or satellites are recorded in each radar channel by photographing a five-inch (127 mm) intensity-modulated oscilloscope with the camera shutter open on a 35 mm film moving approximately five inches per minute. The range of an individual target is represented by its location across the width of the film, the time by a dotdash code along the length. In addition to this positional information, the target's approximate radial velocity (velocity in the direction of observation) was determined by measuring the Doppler frequency shift in the radar signal when it is returned. The doppler shift is found to within 500 cycles by determining which of eighteen frequency filters covering successive bands 500 cycles per second wide will pass the return signal. This measurement of radial velocity runs from -4 to -f-4 nautical miles (7 km) per second in increments of 0.219-nautical-mile (0.406 km). All these data, together with the elevation and azimuth of the observing beam, are automatically converted to serial form, encoded in standard teleprinter code, and punched on paper tape for transmission.
Data was thus received at Wright-Patterson Foreign Technology Division (FTD) first by teleprinter and then on film, the latter accompanied by logs giving data on the target as read by site personnel and data on equipment performance such as peak transmitted power, frequency, and receiver sensitivity. Upon arrival, the film when was edited and marked to facilitate reading on the "Oscar" (preliminary processing) equipment. Targets are sorted by differences in range and rate of range change, and the returns on each were numbered in time sequence.
The FTD Oscar equipment consisted of a film reader which gave time and range data in analog form, a converter unit which changed them to digital form, and an IBM printing card punch which received the digital data. The Oscar equipment and human operator thus generated a deck of IBM cards for computer processing which contains the history of each target's position through time.
The first step in the computer processing is to translate Oscar units into actual radar range, "Z" (Greenwich mean) time, and beam number, the latter fixing the azimuth and elevation of the return. During this first step, three separate quality-control checks are made on each IBM card to eliminate erroneous data.
Those observations that succeed in passing all these tests are taken to the second step of computer processing, with fitting of a second-degree polynomial curve to the raw range/time data in accordance with least squares criteria. In this method, a mathematical function is fit to best approximate a series of observations where the sum of squares of its residuals (deviations from the raw data) is least. If there is systematic irregularity in the reliability of the data, the residuals are weighted accordingly.
A standard deviation from this curve is established, and any raw datum point showing a deviation as large as three times the standard is discarded. Then second-degree curves are similarly fitted to the azimuth/time and elevation/time data. The three second-degree polynomials - for range/time, azimuth/time, and elevation/time - are used to generate a value for position and velocity at mean time of observation, and on the basis of these values an initial estimate of the elliptical trajectory is made.
In computing the elliptical path, the earth is physically considered a rotating homogeneous sphere and geometrically considered an ellipsoid -that is, its equatorial bulge is ignored in the gravitational computation but not with respect to intersections of its surface. An ellipse not intersecting the Earth's surface represents a satellite orbit; one intersecting the Earth's surface describes a trajectory above the point of intersection.
The parameters of the ellipse are iterated with the computer, establishing a best-fit ellipse constrained by a weighted least-squares criterion. Along this ellipse the target's track is computed -the history through time of latitude, longitude, altitude, and such velocity and angular parameters as may be of interest. A missile's actual range is probably shorter than that of its computed trajectory because of its non-elliptical thrusting path and atmospheric drag after its reentry. The difference is on the order of 10-nautical-mile (19 km) to 25 nautical miles (46 km) for short and medium range missiles, 50-nautical-mile (93 km) for ICBM's.
Read more about this topic: AN/FPS-17