These ionograms were digitized from the original ISIS-1 7-track analog telemetry tapes using the facilities of the former Data Evaluation Laboratory at the NASA/GSFC. This data restoration project is headed by Dr. R.F. Benson (NASA/GSFC). Ionograms were digitized at the rate of 40,000 16-bit samples/sec. This sample rate is higher than the Nyquist frequency of 30 kHz. The sample frequency of 40 kHz provides a measurement every 25 microseconds corresponding to an apparent range (c*t/2) interval of 3.75 km. Each ionogram consists of a fixed-frequency and and a swept-frequency portion. The time resolution between ionograms is typically 29 seconds.

sat.-ID: 1=AL1, 2=AL2, 3=ISIS1, 4=ISIS2

two-digit telemetry station code

trans. power code: 1=Prim(400W) 2=Sec(400W)

sounder/receiver code: 0=off, 1=on

pulse per second code: 0= 30 pps, 1= 60 pps

DMODE (transm. on/off for alternate frame pairs): 0=off, 1=on

GMODE (transmission of alternate frames of fixed-frequency ionograms and normal combined fixed- and swept-frequency ionograms): 0=off, 1=on

sounder mixed mode (transmission at a fixed-frequency while the sounder receiver sweeps over swept-frequency range): 0=off, 1=on

fixed frequency code: 0=off, 1=0.25, 2=0.48, 3=1.00, 4=1.95, 5=4.00, 6=9.303 MHz

Year of ionogram frame sync in UT

Day of Year of ionogram frame sync in UT

Hour of ionogram frame sync in UT

Minute of hour of ionogram frame sync in UT

Second of minute of ionogram frame sync in UT

Local Mean Time (hh,mm) at time of ionogram frame sync

geographic coordinates (Lat,Long,Hgt) at time of ionogram frame sync

Geomagnetic Local Time (HHMM) at time of ionogram frame sync

geomagnetic latitude at time of ionogram frame sync

geomagnetic longitude at time of ionogram frame sync

invariant latitude at time of ionogram frame sync

Dip angle of mag field direction at time of ionogram frame sync

Solar Zenith Angle at time of ionogram frame sync

L shell (McIlwain parameter) at time of ionogram frame sync

spacecraft in(=1)/out(=2) sunlight at time of ionogram frame sync

CEP instrument on(1)/off(0) status

VLF receiver on(1)/off(0) status

SEA instrument on(1)/off(0) status

IMS1 instrument on(1)/off(0) status

IMS2 instrument on(1)/off(0) status

SPS instrument on(1)/off(0) status

EPD instrument on(1)/off(0) status

scan line number of start of swept-frequency portion of the ionogram - seperates the fixed- and swept-frequency portions

msec after frame sync - time of frequency markers

ionogram frequency markers

each sounder-pulse transmission is followed by a scan of the sounder-receiver video amplitude output values for each delay time monitored after the transmitted pulse; these scans are called scan lines. There are two types of ISIS-1 binary ionogram files, one designated as "full" and one as "average" The full file has 1340 delay times (one every 0.025ms). The average files are derived from the full files by averaging the amplitude values in 4 delay-time bins. Thus the average binary files contain 335 time-delay bins within each scan line. The cdf files were made from these average binary files.

Ionogram files can be of different length. In some the swept-frequency portion covers the range 0.1 - 10 MHz, in others 0.1 - 20 MHz. Also, the proper start of each ionogram file, marked by a frame-sync pulse, was not always detected during the analog-to-digital (A/D) conversion. In such cases a frame sync was inserted to limit the file size. Thus the number of sounder-receiver video amplitude vs. delay time scan lines per ionogram can vary.

delay-time values used in each scan line

apparent-range values used in each scan line

Time

the sounder receiver linear video amplitude, from 0 to 255 telemetry units, in each apparent-range bin of each scan line covering the fixed- and swept-frequency regions. This range corresponds to a linear sounder-receiver video amplitude output range from zero to 4.5 V. This calibration is based on the video calibration pulse (of 3.0 V or 170 telemetry units) at the end of the line scan following each sounder pulse (the combined calibration pulses from all of the line scans produces a colored stripe at the bottom of the digital ionograms). In order to derive an input power level corresponding to the receiver video output it is necessary to know the sounder-receiver automatic-gain-control (AGC) voltage. This voltage was obtained from the pcm data and thus is only available if pcm data were available. The AGC voltage, which ranges from 0 to 5.12 volts, is obtained from a linear interpolation between the AGC trace displayed between 4005 km apparent range (26.7 ms delay time) as 5.12 V and 4395 km (29.3 ms delay time) as 0 volts. (On the full ionograms these numbers are 4001.250 km or 26.675 ms, and 4398.750 km or 29.325 ms.) On the average ionograms, which are available as either CDF or binary files from the SPDF or the NSSDC, the range resolution is 0.1 ms (corresponding to a virtual-range resolution of 15 km) there are 27 rows (26 intervals) covering the 0 to 5.12 V range; thus each row in virtual range above (decreasing delay time) the zero level (at 29.3 ms delay time) corresponds to an increase of 5.12/26 V of AGC with a quantization uncertainty of approximately plus or minus 0.1 V. Two rows have zero video-output amplitude values (on the average ionograms) in order that a clear AGC trace is visible on the ionograms. The row with the greatest virtual range corresponds to the actual AGC value. (On the full ionograms, which are only available as binary files from the NSSDC, the range resolution is 0.025 ms, corresponding to a virtual-range resolution of 3.75 km, and three rows have zero video-output amplitude values; the middle row corresponds to the actual AGC value.)

frequency of each scan line; in the swept-frequency portion of the ionogram they correspond to interpolated values between the frequency markers

Electron Gyrofrequency at time of ionogram frame sync