The 7-track analog telemetry tapes from the ISIS-1 satellite were the second to be converted from an analog to a digital (A/D) format to produce digital topside ionograms. The ISIS-1 tapes were selected to address subjects and time periods of special interest. Differences between the ISIS-1 and ISIS-2 data formats are described below the format description.
Each ionogram file contains header information and sounder data including the averaged AGC (automatic gain control). The digitized sounder data (consisting of 8-bit receiver-amplitude values collected at a 40 kHz rate) contain the time and the frequency associated with each sounder pulse. (click the Background link for more information about the a/d process).
Two types of ionogram files are archived: (1) the full resolution ionogram and (2) a standard resolution version of the ionogram. The standard version is obtained by averaging over 4 full resolution receiver-amplitudes. The apparent-range resolution corresponds to 15 km (every 0.1 ms) for the standard files and to 3.75 km resolution (0.025 ms samples) for the full resolution ionograms.
The data are provided in OS2 binary. The standard resolution ionograms are also available in CDF format.
Items starting with "Satellite ID number" and ending with "Scan Line Number of Start of Swept Portion" are all contained in the first record. The record structure is indicated in the table below. Each record is preceded and followed by a 4 byte descriptor that contains the length of the record. For example the first record is 160 bytes in length. Therefore to get the entire record you would have to allow for 168 bytes (4+160+4).
The iongrams are given as x-y images where x is the "scan line time" (SLT) [or frequency] and y is the delay time (DT) [or apparent range].
The "Types" of data are I*1 (1 byte integer), I*4 (4 byte integer), R*4 (4 byte real) and R*8 (8 byte real).
Any value for the variables below found to be outside the valid value for that variable indicates the variable was undetermined. A description of the different modes can be found at the Mode Description Page
Word Description of Variable Type
------------------------------------------------------------------------
1 Satellite ID number (1:AL1,2:AL2,3:IS1,4:IS2) I*4
2 Station Code number (Two Digit Code) I*4
3 Transmitter Power Code (1:400W[prim.],2:400W[sec.]) I*4
4 Sounder/Receiver Code (0:OFF,1:ON) I*4
5 PRF code (0:30 PPS,1:60 PPS) I*4
6 DMODE Code (0:OFF,1:ON) I*4
7 GMODE Code (0:OFF,1:ON) I*4
8 Sounder Mixed Mode (0:OFF,1:ON) I*4
9 Fixed Freq. Code (0:OFF, 1:0.25MHz, 2:0.48, 3:1.00, 4:1.95, I*4
5:4.00, 6:9.303)
10 Year (Last 2 Digits) (Frame Sync UT) I*4
11 Day of Year (Frame Sync UT) I*4
12 Hour (Frame Sync UT) I*4
13 Minute (Frame Sync UT) I*4
14 Second (to the microsecond) (Frame Sync UT) R*8
15 LMT (HH) (At time of Frame Sync) I*4
16 LMT (MM) (At time of Frame Sync) I*4
17 GGLatitude (Deg) (At time of Frame Sync) R*4
18 GGLongitude (Deg) (At time of Frame Sync) R*4
19 Height (km) (At time of Frame Sync) R*4
20 GMLTM (HH) (At time of Frame Sync) I*4
21 GMLTM (MM) (At time of Frame Sync) I*4
22 GMLAT (Deg) (At time of Frame Sync) R*4
23 GMLONG (Deg) (At time of Frame Sync) R*4
24 Gyro Frequency (MHz) (At time of Frame Sync) R*4
25 Invariant Latitude (Deg) (At time of Frame Sync) R*4
26 Magnetic inclination, Dip (Deg) (At time of Frame Sync) I*4
27 Solar zenith angle (Deg) (At time of Frame Sync) I*4
28 Sunlight (1:SL,2:NSL) I*4
29 L Shell (At time of Frame Sync) R*4
30 CEP (0:OFF,1:ON) I*4
31 VLF Receiver (0:OFF,1:ON) I*4
32 SEA (0:OFF,1:ON) I*4
33 IMS1 (0:OFF,1:ON) I*4
34 IMS2 (0:OFF,1:ON) I*4
35 SPS (0:OFF,1:ON) I*4
36 EPD (0:OFF,1:ON) I*4
37 Scan Line Number of Start of Swept Portion I*4
------------ end of 1st record -------152 Bytes--------------------------
38 1st Freq. Marker: Frequency (MHz) R*8
39 1st Freq. Marker: Time (ms) (ms after Frame Sync) R*8
------------ end of 2nd record --------16 Bytes--------------------------
...... {repeated for each frequency marker} ......
------------ end of 22nd record -------16 Bytes--------------------------
82 22nd Freq. Marker: Frequency (MHz) R*8
83 22nd Freq. Marker: Time (ms) (ms after Frame Sync) R*8
------------ end of 23rd record -------16 Bytes--------------------------
84 Number of Scan Line Time (SLT) values [c] I*4
85 Number of Delay Time (DT) values [r] I*4
r=335 for avg and r=1340 for full ionogram files
------------ end of 24th record --------8 Bytes--------------------------
85+1 First Delay Time (in ms) R*8
...... {repeated for each delay time value} ......
85+r Last Delay Time (in ms) R*8
------------ end of 25th record -------8r Bytes--------------------------
85+r+1 First Apparant Range (in km) R*8
...... {repeated for each apparant range value} ......
85+2r Last Apparant Range (in km) R*8
------------ end of 26th record -------8r Bytes--------------------------
85+2r+1 1st SLT(in ms) R*8
85+2r+2 Frequency(in MHz) corresponding to 1st SLT R*8
85+2r+2+1 Amplitude for 1st SLT and 1st DT I*1
...... {repeated for all amplitudes for 1st SLT} ......
85+3r+2 Amplitude for 1st SLT and last DT I*1
------------ end of 27th record -----16+r Bytes--------------------------
85+3r+3 2nd SLT(in ms) R*8
85+3r+4 Frequency(in MHz) corresponding to 2nd SLT R*8
85+3r+4+1 Amplitude for 2nd SLT and 1st DT I*1
...... {repeated for all amplitudes for 2nd SLT} ......
85+4r+4 Amplitude for 2nd SLT and last DT I*1
------------ end of 28th record -----16+r Bytes-------------------------
....
...... {repeated for all SLT} ......
....
------------ end of (25+c)th record --16+r Bytes------------------------
85+(c+1)r+2(c-1)+1 Last SLT(in ms) R*8
85+(c+1)r+2(c-1)+2 Frequency(in MHz) corresponding to last SLT R*8
85+(c+1)r+2(c-1)+2+1 Amplitude for last SLT and 1st DT I*1
...... {repeated for all amplitudes for last SLT} ......
85+(c+2)r+2(c-1)+2 Amplitude for last SLT and last DT I*1
------------ end of (26+c)th record --16+r Bytes------------------------
Differences between the ISIS-1, ISIS-2, and Alouette-2 format:
The 1st record of ISIS-1 data files is 2 words (8 Bytes) shorter than the
1st record of ISIS-2 data files:
- "AIT Code" (ISIS-2, Word 9) is not included in ISIS-1.
- "Auroral Scanning Photometer" (ISIS-2 Word 38) is not included in ISIS-1.
The following words have different content:
- ISIS-1 Word 5, 32-36: "PRF Code", "SEA", "IMS1", "IMS2", "SPS", "EPD"
- ISIS-2 Word 5, 33-37: "Swept Frequency Range Code", "RPA", "IMS", "SPS", "EPD",
"Red Line Photometer"
Number of delay times (and apparent ranges) for the full ionogram file is 1340 for
ISIS-1 and Alouette 2, and 892 for ISIS-2
Number of delay times (and apparent ranges) for the avg. ionogram file is 335 for
ISIS-1 and Alouette 2, and 223 for ISIS-2
In the case of Alouette 2 there was no cpm data and information pertaining to instruments
other than the sounder are not provided here; also the sounder did not have different
operational modes. Rather than making major changes to the ISIS-1 format, however, the
identical format as used for ISIS 1 was used for Alouette 2 and the following default
values were used in the places where there were no values:
Words 6, 7, and 8 (for DMODE, GMODE, AND MIXED-MODE) are each set = 2 (valid values for ISIS 1
were 0, for off, and 1 for on)
Word 9 (for FIXED FREQ) is set = 0 (a valid value for ISIS 1 indicating the fixed frequency
operation was off)
Words 30-36 (for CEP, VLF, SEA, IMS1, IMS2, and EPD) are each set = 2 (valid values for
ISIS 1 were 0, for off, and 1 for on)
On the Alouette-2 headers, N/A was entered in the above places.