The measurement component generates an output which is a digital representation of the angle between the sunline and the normal to the sensor face when the Sun is in the FOV of the command component, as shown in Fig. 6-13. The measurement component illustrated in Fig. 6-14 is a composite (similar to that flown on Nimbus-6, Adcole model 17032) that shows most of the features of interest. The Sun image is refracted by a material of index of refraction, n, which may be unity, and illuminates a pattern of slits. The slits are divided into a series of rows with a photocell beneath each row. Four classes of rows are illustrated: (1) an automatic threshold adjust (ATA), (2) a sign bit, (3) encoded bits (Gray code, described below, is shown), and (4) fine bits.
Because the photocell voltage is proportional to cos 0(0=Sun angle), a fixed threshold is inadequate for determining the voltage at which a bit is turned on. This is compensated for by use of the ATA slit, which is half the width of the other slits. Consequently, the ATA photocell output is half that from any other fully lit photocell independent of 0 as long as die Sun image is narrower than any reticle slit. A bit is turned "on" if its photocell voltage is greater than the ATA photocell voltage and, consequently, "on" denotes that a reticle slit is more than half illuminated (independent of the Sun angle).
The sign bit or most significant bit determines which side of the sensor the Sun is on. The encoded bits provide a discrete measure of the linear displacement of the Sun image relative to the sensor center line or mill. Several codes are used in Adcole sensors, including V-brush and Gray [Susskind, 1958]. Gray code, named after the inventor, is the most widely used and is compared with a binary code in Tabie 6-2 and Fig. 6-15. The advantage of a Gray code may be seen by comparing the binary and Gray codes for a Sun angle near -16 deg. As the Sun angle decreases across the transition, the binary cole changes from -001111 to -010000 and the Gray code from -101000 to -111000. Thus, five binary bits change but
Table 6-2. Gray-to-Binary Conversion. The most significant bit is the same in either binary or Gray code. Each succeeding binary bit is the complement of the corresponding Gray bit if the preceding binary bit is 1 or is the same if the preceding binary bit is 0. (See Section 8.4 for conversion algorithm.)
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