1 1 0 I 0 I I 0 1110 10 1 10 0 11110

Fig. 8-16. Conversion Between Gray and Binary Codes. The left-most (sign) bit may or may not require inversion, depending on the sensor convention.

pattern which results in a value of - 0; i.e., the sign bit indicates negative but the magnitude is zero. Because this value will be converted to +0 by most computers, a legitimate sensor reading (-0) will be converted to an erroneous value ( + 0). Consequently, such data are usually converted to a range of positive numbers so that each value remains unique. Thus, the range of legitimate values

— 63, -62,..., — 1, —0, +0, + 1,..., +62, +63 may be converted to

+ 0, + l,...,+62, +63, +64, +65,...,+ 126, + 127 by a judicious choice of the Gray-to-digital conversion scheme.

In addition, data is recorded in buckets, or integral steps, and may require that half a stepsize be added to or subtracted from the transmitted value, so that the converted value corresponds to the most probable value of the quantity measured.

Validating and Correcting Telemetry Data. Validation of telemetry data within the telemetry processor is done on a discrete point-by-point basis and is discussed in Section 9.1. In some instances, invalid data can be corrected based on other data, but corrections in the telemetry processor are usually minimal and highly spacecraft dependent. Invalid data can be deleted or flagged as incorrect, but it is generally left to attitude determination software to attempt corrections. If the data contain any parity bits, they can be checked by the telemetry processor or by attitude determination software. (See Section 8.2 for a discussion of parity bits.)

Time-Checking Telemetry Data. Time-checking of telemetry data usually consists of comparing the times associated with different data points for self-consistency. The values checked are usually the attached time, the telemetered count of the spacecraft clock, and/or the minor or major frame number. If one of the three values checked is in error, it can frequently be corrected by using one of the other two. Many algorithms have been developed to accomplish time checking; there is no general agreement as to the best type to use.* Upper and lower limit checking is also often performed (see Section 9.1).

Generating Segments of Valid Attitude Data. The telemetry processor, having performed all conversions and validation, generates segments of valid (or, in some cases, flagged) data which are passed to the attitude determination system for further processing. The segments generated may or may not contain data found to be invalid, depending on the option chosen by the operator. If he elects not to accept invalid data, the data segments must be generated with gaps at the beginning, middle, or end, depending on which data are invalid. Often gaps are left in segments of one type of data because another type of data was invalid during the gap and the first type is useless without the second.

Data segments are usually generated on the basis of an integral number of minor or major telemetry frames. For example, in the case of SAS-3, all major frames are initially flagged as containing invalid data, and as valid data are identified, they replace the flagged values. As a result, a major frame at the beginning of a pass may contain flag values at the beginning of the frame, and a major frame at the end may contain flags at the end, as shown in Fig. 8-17.

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