These Ionograms were digitized from the original ISIS-2 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/s. This Sample Rate is higher than the Nyquist Frequency of 30 kHz. The Sample Frequency of 40 kHz provides a Measurement every 25 µs corresponding to an Apparent Range Interval equal to 3.75 km from c*t/2, where c is the Speed of Light. Ionograms with this Sample Rate are designated as 'Full' Ionograms because they have the full 3.75 km Apparent-Range Resolution. The Ionograms used for most Analyses, and those available from CDAWeb, were produced by averaging every four Samples of the Sounder-Receiver Video Amplitude Output to yield an average Value every 100 µs corresponding to an Apparent-Range Resolution of 15 km. These Ionogram Files are referred to as 'Average' Files with Standard Resolution. Each Ionogram consists of a Fixed-Frequency Portion and a Swept-Frequency Portion. The Time Resolution between Ionograms is typically 14 or 22 s depending on the Frequency Sweep Range.

Satellite ID: 1=Alouette 1, 2=Alouette 2, 3=ISIS 1, 4=ISIS 2

Two-Digit Telemetry Station Code

Transmitter Power Code: 1=Primary, 400 W, 2=Secondary, 400 W

Sounder-Receiver Code: 0=Off, 1=On

Swept Frequency Range Code: 0=Normal, 1=Extended

DMODE, Transmission On/Off for Alternate Frame Pairs: 0=Off, 1=On

GMODE, Transmission of Alternate Frames of Fixed Frequency Ionograms and Normal Combined Fixed Frequency and Swept Frequency Ionograms: 0=Off, 1=On

Sounder Mixed Mode, Transmission at a Fixed Frequency while the Sounder-Receiver sweeps over the Swept Frequency Range: 0=Off, 1=On

Automatic Ionogram Transmission, AIT, Mode, Automatic Sounder Operation every 3 min: 0=Off, 1=On

Fixed Frequency Code: 0=Off, 1=0.12 MHz, 2=0.48 MHz, 3=1.00 MHz, 4=1.95 MHz, 5=4.00 MHz, 6=9.303 MHz

Year of Ionogram Frame Sync

Day of Year of Ionogram Frame Sync

Hour of Day of Ionogram Frame Sync, UT

Minute of Hour of Ionogram Frame Sync, UT

Second of Minute of Ionogram Frame Sync, UT

Local Mean Time, LMT, at the Time of Ionogram Frame Sync, Format: HH,MM

Geographic Coordinates, GEO, Latitude, Longitude, Height, at the Time of Ionogram Frame Sync

Geomagnetic Local Time, GMLMT, Format: HH,MM, at the Time of Ionogram Frame Sync

Geomagnetic Latitude, MAG, at the Time of Ionogram Frame Sync

Geomagnetic Longitude, MAG, at the Time of Ionogram Frame Sync

Invariant Latitude, at the Time of Ionogram Frame Sync

Magnetic Inclination, Dip Angle of Magnetic Field Direction, at the Time of Ionogram Frame Sync

Solar Zenith Angle, SZA, at the Time of Ionogram Frame Sync

L Shell, McIlwain Parameter, at the Time of Ionogram Frame Sync

Spacecraft Sunlight: 0=Out, 1=In, at the Time of Ionogram Frame Sync

Cylindrical Electrostatic Probe, CEP, Instrument Status: 0=Off, 1=On

Very Low Frequency, VLF, 0.05 to 30 kHz Receiver, Instrument Status: 0=Off, 1=On

Retarding Potential Analyzer, RPA, Instrument Status: 0=Off, 1=On

Ion Mass Spectrometer, IMS, Instrument Status: 0=Off, 1=On

Soft Particle Spectrometer, SPS, Instrument Status: 0=Off, 1=On

Energetic Particle Detector, EPD, Instrument Status: 0=Off, 1=On

Red Line Photometer, RLP, Instrument Status: 0=Off, 1=On

Auroral Scanning Photometer, ASP, Instrument Status: 0=Off, 1=On

Scan Line Number of the Start of Swept Frequency Portion of the Ionogram, this seperates the Fixed Frequency and Swept Frequency Portions of the Ionogram

Times of Ionogram Frequency Markers, Milliseconds after Frame Sync

Ionogram Frequency Marker Valuess

Number of Delay Times per Ionogram Scan Line. 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 the other designated as Average. The Full File has 1340 Delay Times, one every 0.025 ms. The Average Files are derived from the Full Files by averaging the Amplitude Values in four 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.

Number of Ionogram Scan Line Frequencie Steps. Ionogram Files can be of different Lengths. In some the Swept Frequency Portion covers the Range from 0.1 Mhz to 10 MHz, in others from 0.1 Mhz to 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 versus Delay Time Scan Lines per Ionogram can vary.

Delay Time Values used in each Scan Line

Virtual Height Values corresponding to Delay Times used in each Scan Line

Epoch Time

Sounder Amplitude at 223 Virtual Heights for Fixed Frequency and Swept Frequency, Ionogram. The Sounder-Receiver Linear Video Amplitude, from 0 to 255 Telemetry Units, in each Apparent Range Bin of each Scan Line covering the Fixed Frequency and Swept Frequency Regions. This Range corresponds to a Linear Sounder-Receiver Video Amplitude Output Range from 0 V 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 Pulse Code Modulation, PCM, Data and thus is only available if PCM Data were available. The AGC Voltage, which ranges from 0 V to 5.12 V, is obtained from a Linear Interpolation between the AGC Trace displayed between the 4005 km Apparent Range, at 26.7 ms Delay Time, as 5.12 V and the 4395 km, at 29.3 ms Delay Time, as 0 V. 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 an thus 26 Intervals covering the 0 V to 5.12 V Range; thus each Row in Virtual Range above, i.e. 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.

Interpolated Fixed and Swept Frequencies for each Scan Line, in the Swept Frequency Portion of the Ionogram they correspond to interpolated Values between the Frequency Markers

Satellite Altitude

Electron Local Gyrofrequency, at the Time of Ionogram Frame Sync