A feasibility study conducted in 1994 at the NASA Goddard Space Flight Center (GSFC), using the facilities of the Data Evaluation Laboratory (DEL) within what was then the Mission Operations and Data Systems Directorate (Code 500), concluded that about 3500 ISIS analog telemetry tapes could be processed into digital ionograms per year. The digitization was performed using an A/D converter board and software device driver compatible with the OS/2 operating system used by a 486-based Programmable Telemetry Processor (PTP) in the DEL. A dedicated PTP was assembled in the DEL for the ISIS A/D operation. Considerable software development has led to a reliable system of producing digital ionograms even under noisy conditions.
During the A/D operation, ionogram frame sync pulses are identified, flagged and the corresponding PB4 time code is determined. This time is used to identify the ionograms and related files. (The PB4 time code is the output of a time-decoder card which receives the NASA 36-bit time code (added at the telemetry station) as the input.) The ionogram line sync pulses are also identified and flagged. These line-sync flags are used to identify the start of columns for data arrays used to generate digital ionograms. The flags are stripped off during the process of setting up these arrays.
The output from each satellite pass over a telemetry station as a result of the A/D operation is 2n + 3 files where n represents the number of ionograms on the pass. These files are the following:
(1) A log file used by the National Space Science Data Center (NSSDC) to
ingest the data into the NDADS nearline data system.
(2) An archive file containing header information pertinent to each satellite orbit (or pass) recorded by a given telemetry station. This information is obtained from a combination of operator input during the A/D process and data extracted from (and calculated from) the PCM track on the telemetry tape. It is incorporated into the final data output of the A/D process. In general, this information applies to the entire satellite pass (some could change during a pass as the result of up-link commands changing instrument operating modes).
(3) A file with the PCM data (annotated with the NASA 36-bit time code) for each satellite pass containing header, time, AGC, frequency-marker information and data from all other instruments.
(4) n files with full-resolution digitized sounder-data (for each of the n ionograms on the satellite pass) each containing sounder data, header information and averaged AGC (automatic gain control) information in the form of a trace on the digital ionogram modeled after the traces displayed at the bottom of 35 mm ISIS 1 & 2 film ionograms. The digitized sounder data (consisting of 8-bit receiver-amplitude values collected at a 40 kHz rate) contains the time and the frequency associated with each sounder pulse. The frequency of each sounder pulse is determined from a 3rd degree interpolation between the times of the frequency markers as identified in the sounder video during the A/D operation.
(5) n averaged files (with averaged 8-bit sounder-data for each of the n ionograms on the satellite pass) each containing the same information as in (4) above but each sounder receiver-amplitude value will correspond to an average of 4 of the 40 kHz 8-bit values. Thus the apparent-range resolution corresponds to 15 km for these samples every 0.1 ms rather than the 3.75 km resolution (for the 0.025 ms samples) in (4) above. These files are designed so as to be suitable for input to standard display software capable of adjusting ionogram intensity levels, expanding segments of interest, making amplitude scans, etc. They will also be designed so that image files can be produced at the NSSDC in response to an on-line request and forwarded to the user.
The NSSDC/SPDF prepared the scripts, software and the setup for the data flow from the DEL to the NSSDC for automated ingest onto its NDADS near-line archive and for the conversion of the average ionogram files into Common Data Format (CDF) files. The NSSDC/SPDF has also prepared the WWW pages presented here.
For more detailed information, see:
+ DESCRIPTION OF ISIS-2 A/D PROGRAMS
+ DESCRIPTION OF AVERAGE AND NON-AVERAGE IONOGRAM FILES: ISIS-1,ISIS-2
+ DESCRIPTION OF ISIS-2 PASS HEADER
+ ISIS-2 SOUNDER AMPLITUDE INFORMATION
+ FREQUENCY DETERMINATION ON ALOUETTE & ISIS DIGITAL IONOGRAMS