(1) Time code errors are occasionally encountered in the data on the telemetry tape. These errors are often in the day number. (Note: information on the year is not included in the time-code information on the telemetry tape, it comes from the information on the station log sheet included with the tape.) The result of such an error is that the ionogram ends up separated from its neighbors, gets tagged with incorrect world map information, and ends up in a location corresponding to the erroneous time. These anomalous values are usually easy to identify in the output of a data search because they will have a latitude, longitude or other world-map value not appropriate for the telemetry station. If one of these ionograms contains important information to a user, the problem can be corrected with a little tender loving care.
(2) During the first eleven months of ISIS-2 digital data sent to the NSSDC (from 22 October 1996 to 27 September 1997) the digital ionogram sent to the NSSDC erroneously indicated 0.25 MHz (the value used in ISIS 1) for the first fixed-frequency value. These data contain a variety of stations and dates. A search, based on a fixed-frequency value of 0.12, will retrieve these data along with data correctly labeled as 0.12 MHz. Any ISIS-2 data retrieved with a fixed-frequency value equal to 0.25 should be changed to 0.12 MHz.
(3) At times, an ionogram frame-sync pulse is not detected by the software during the A/D operation within a normal time interval of an ionogram. Under such conditions, a sync pulse is inserted and a file of typical ionogram length is produced. This process is repeated until a true frame-sync pulse is detected in the data. During such operation, ionograms are produced but their start times will not correspond to the normal beginning of an ionogram, i.e., they will appear truncated. Since these inserted times did not correspond to true ionogram start times, the initial procedure during the A/D operation was to not look up the satellite world-map information in these cases. This approach, however, led to zero values for all of the world-map parameters if these ionograms were included in the output of a data search. This condition was true up until 11 March 1997. After that date, the software-inserted time for ionograms with frame-sync problems was used to obtain world-map information. (Note: there is a completely independent situation that can lead to a zero value for the orbit number, namely, if the information was not available from the telemetry-station log sheet.)