1011 1100 1101 1110 1111 10000 >0001 10010 10011 10100 10101

1110 1010 1011 1001 1000 11000 noot 11011 11010 lino 11111

only one Gray bit changes. By inspection, the Gray code is an equidistant code. That is, one and only one bit changes for each unit distance whereas one or more binary bits change for the same unit distance. Because some imperfection in the reticle pattern is inevitable and a transition may occur while the photocell is being interrogated for transmission, the possible decoded angles for a binary code near — 16»could range from —0 to — 16, whereas for a Gray code, only — 16 or - 15 is

Fig. 6-15. Gray and Binary Coded Reticle Patterns for a ±64-Deg FOV Digital Sun Sensor With a 1-Deg Least Significant Bit

binary code gray code

Fig. 6-15. Gray and Binary Coded Reticle Patterns for a ±64-Deg FOV Digital Sun Sensor With a 1-Deg Least Significant Bit possible. Algorithms for converting between Gray and binary codes are given in Section 8.4.

The calibration of the encoded bits is verified by plotting the output from each photocell versus Sun angle, as shown in Fig. 6-16 for the two least significant bits (LSBs). Note that the envelope of the sinusoidal output of both bits is roughly proportional to cos 0 and the ATA output follows the envelope with half the amplitude. A characteristic of the Gray code is that the peak output of one bit corresponds to alternate minima of the next lesser bit. The angular error at a bit on-off transition is typically half the LSB.

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