Group Repetition Period

iffltmi_uuai]_UBIB_uuuil_imimi i

U slave y emission -J ' delay 1

Fig. 8-13. Typical Loran-C Pulse Trains time within the slave station. When data are processed in near real time, these errors are ignored. When portable STDN stations are out of range of direct master or slave signals, delayed signals reflected off the Earth's ionosphere are used. These reflections, called skywaves, are less reliable because their arrival time depends on local atmospheric conditions between the transmitter and the receiver. The pulsed transmission described above facilitates distinguishing between direct signals and skywaves, which typically lag direct signals by 30 to 40 /tsec. Loop antennas are used at STDN receivers to directionalize reception; therefore, signals from only one Loran-C station are received at a time.

Time tagging errors other than those in the timing scheme itself include (I) propagation time from the spacecraft to the tracking station, (2) electronic hardware and software delays during the time-tagging process, (3) uncertainties in the position of the spacecraft, and (4) uncertainties in the position of the tracking station. These errors are typically small compared with the timing accuracies required by attitude determination and control software and are usually neglected. Their magnitudes are on the order of milliseconds. (The propagation delay from the Moon is ~1.3 sec.)

When required, all NASA STDN tracking stations can also employ a cesium beam frequency standard as the primary source for time and time interval measurements, with rubidium atomic frequency standards as a first backup. At some tracking stations, a highly stable oven-controlled quartz crystal frequency standard provides a secondary backup. All stations provide automatic switchover from primary to secondary timing source in the event of low signal amplitude from the primary source. Secondary timing sources are phase-locked to the primary, eliminating frequency and time jumps during switchover [Scott, 1974],

For precise attitude determination and experimental data processing, the actual time of measurement of each data type is determined from the time tag. In this determination, considerations are made for time delays caused by pulse shaping within the electronics, the location of the data within the minor frame, the time delay between the time the sensor measurement was made and the time the sensor was sampled, ard so forth (see Section 8.1).

Time Tagging of Near-Real-Time Data. Data received from the spacecraft in near real time and transmitted from the tracking station to a receiving station via NASCOM (Section 8.1) are usually time tagged by the tracking station, but can be tagged by the receiving station. In either case, the process is as described above. Attitude determination software must handle random erroneous times in this case, since the tracking or receiving station software does not have time to detect and correct incorrect times. Limit checks on the times are typically sufficient.

Time Tagging of Playback Data. When a spacecraft does not have continuous tracking station coverage, which is generally the case for low-Earth orbits, and continuous attitude information is required, the data are stored on tape recorders onboard the spacecraft and played back while over a tracking station. Time tagging of data in this case is done either by the tracking station or the receiving station at Goddard Space Flight Center and is generally accomplished by correlating playback data with near-real-time data. During the period of the orbit when a tracking station is not available, all data are recorded for later playback. When a tracking station is available, the tape recorder continues recording, and telemetry data are simultaneously transmitted to the ground in real time. This continues for a fraction of the station pass, after which real-time data are neither transmitted nor recorded, unless there is a second tape recorder available. The recorded data are dumped, or played back at high speed (usually a factor of at least five faster than they were recorded), and are transmitted to the ground. After the recorder has been dumped, it resumes recording and the process is subsequently repeated.

The tracking or receiving station time tags the tape recorded data by searching for the data segment which was recorded simultaneously with the transmitted real-time data. After a match or correlation is found, the data segment can be tagged with ground time, since the real-time data were tagged with ground time as they were received. Once this segment of recorded data has been tagged with ground time, the rest of the recorded data can be tagged by working backward from the known segment. Periods of missing data, or data dropout, can be detected by examining the spacecraft clock count in the recorded data. This process will be illustrated by the time tagging scheme used for the RAE-2 spacecraft.

The Radio Astronomy Explorer-2 (RAE-2) spacecraft is in a lunar orbit. It is unable to transmit data while on the far side of the Moon, so it contains two tape recorders to record and then transmit data when the spacecraft is in view of tracking stations. The spacecraft clock count register is updated by one count every 20 minutes, and is read into the telemetry stream during 1 of 10 calibration frames occurring at 20-minute intervals along with other spacecraft housekeeping information, such as battery temperatures. The sequence of events during transmission to tracking stations is depicted in Fig. 8-14. When acquisition of signal at a tracking station occurs, the tape recorder in use continues to record data while the station receives real-time data. When a calibration sequence is received, a command is transmitted to the spacecraft which causes (I) the tape recorder to begin dumping its recorded data, (2) the other tape recorder to begin recording data, and (3) real-time data transmission to cease. The tape recorder dump requires 45 minutes, after which tape recording and real-time transmission resume. A ground command then turns the tape recorder off again, while real-time transmission continues until loss of signal [Grant and Comberiate 1973; Ferris, 1973J.



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