The ISIS-2 DIGITAL TOPSIDE SOUNDER DATA SEARCH PAGE allows ISIS-2 digital ionograms to be located based on date, time, geographic or magnetic parameters corresponding to the satellite position at the ionogram frame sync time, status of the ionospheric topside sounder or other ISIS-2 instruments, telemetry station, satellite orbit number, or sunlight/shadow condition. The default condition is to list all ionograms that satisfy the search criteria. By selecting "PASS" after the choice :TYPE OF SEARCH:", only the satellite passes containing such ionograms will be listed (a "pass" being defined as the portion of an orbit within telemetry range of a given telemetry station).

The definitions, formats, and acceptable limits for the input parameters are the following:


Date: year and day-of-year, 71001 to 90365 (Note: the time period covers all days in the year of launch and the year of satellite turn off to allow for "renegade" ionograms caused by erroneous time codes - see discussion at the end of this help page.)

Universal Time (UT): HHMM, 0000 to 2400 (Note: min > max produces search through midnight.)

Local Mean Time (LMT): HHMM, 0000 to 2400 (Note: min > max produces search through midnight.)

Magnetic Local Time (MLT): HHMM, 0000 to 2400 (Note: min > max produces search throught midnight.)

Magnetic dip (DIP): degrees, -90 to 90

Solar zenith angle at the satellite (CHI): degrees, 0 to 180

Geographic latitude (GGLAT): degrees measured positive to the north, -90 to 90

Geographic longitude (GGLON): degrees measured positive to the east, -180 to 180

Altitude (or height HGT): km, 1300 to 1500

Geomagnetic latitude (GMLAT): degrees measured positive to the north, -90 to 90

Geomagnetic longitude (GMLON): degrees measured positive to the east, -180 to 180

Electron gyrofrequency (FH): MHz, 0.3 to 1.1

Invariant latitude (INVLAT): degrees, -90 to 90

L value (L): Earth radii, 0.9 to 99,999.99 (Note: since L tends toward infinity as INVLAT tends toward either -90 or 90, large values exist in the data base. L values in the search output listing up to 999.98 will correspond to the values in the database; L = 999.99 in the search output listing corresponds to values > or = 999.99).


DMODE: topside sounder alternates between sounder transmitter and receiver both on for two ionograms, and sounder transmitter off but receiver on for two ionograms (fixed- plus swept- frequency operation in each case).

GMODE: topside sounder alternates between one ionogram at a fixed frequency and one "normal" ionogram (fixed- plus swept- frequency operation).

MIXMODE: sounder transmitter operates at one of the available six fixed frequencies even when the sounder receiver is sweeping.

AITMODE: sounder operation once every three minutes.

CEP: Cylindrical Electrostatic Probe

VLF: Very Low Frequency receiver

RPA: Retarding Potential Analyzer

IMS: Ion Mass Spectrometer

SPS: Soft Particle Spectrometer

EPD: Energetic Particle Detectors

RLP: atomic oxygen Red Line Photometer

ASP: Auroral Scanning Photometer (3914 and 5577 angstroms)

SWEPT FREQ: indicates the range of the quasi-logarithmic sweep during the swept-frequency portion of the ionogram, i.e., from 0.1 MHz to either 10 (normal) or 20 MHz (extended)

FIXED FREQ: indicates the value of the fixed-frequency during the fixed-frequency portion of the ionogram. There are ten possible selections for a search. The first one "any of the following" is a default value that accepts any possibility. The next eight are the result of information obtained from the PCM data. These choices are "fixed frequency off", fixed frequency = 0.12, 0.48, 1.0, 1.95, 4.0 or 9.303 MHz, or "fixed frequency unknown", i.e., there was fixed-frequency operation but the value was not known at the time the PCM information was produced. The last selection is "fixed frequency undetermined" and indicates that information could not be extracted from the pcm data during the A/D operation. Note: 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.


STATION: three-letter telemetry station code (see table and figure describing the telemetry stations)
 Location on Worldmap

ORBIT: satellite orbit number (also referred to as a pass number over a telemetry station) as recorded on the telemetry-station log sheet.

SUNLIGHT: indicates whether the satellite is in sunlight or shadow


A typical search, involving several parameter selections, of the ISIS-2 ionogram-header data base of 29,896 ionograms used in the search program as of September, 1999 takes approximately 1/2 minute.

Ionogram search output: file name,  satellite orbit number (ORB),local mean time (LMT), geographic latitude (GGLAT), geographic longitude (GGLON), geographic altitude (ALT), magnetic local time (MLT), invariant latitude (INVLAT), L value(L), magnetic dip (DIP), electron gyrofrequency (FH), solar zenith angle (CHI), geomagnetic latitude (GMLAT), and geomagnetic longitude (GMLON) for each ionogram satisfying the search criteria.

The orbit number will be zero if the information was not available from the telemetry-station log sheet. (Note: some ISIS-2 ionograms processed before 11 March 1997 had zero values for all of the world-map parameters listed after the orbit number if certain conditions were not satisfied during the  A/D operation  A search involving a non-zero request on one of these world-map parameters will ensure the retrieval of only ionograms with world-map information.)

Pass search output: each output line corresponds to an ionogram from one satellite orbit (or pass) over a telemetry station. (Note that a pass consists of a sequence of consecutive ionograms, typically 20 to 40, over a given telemetry station.) The output corresponds to the first ionogram of the pass that satisfied the search criteria. There may be additional ionograms in the pass that also satisfied the search criteria.

File name description: the satellite identifier (I2 for ISIS 2), the type of file (AV for average - see description below), the three-letter station code, and the year, day number, hour, minute and second corresponding to the ionogram frame-sync time. An example of a Common Data Format(CDF) file corresponding to the ISIS-2 average ionogram from the Falkland Island station, referred to as SOL (for South Atlantic), recorded in 1972, day 303 at 0247:05 UT is given below:


where a wild card is used after the UT time so as to cover the designation for the latest file version number.

Meaning of average (AV) and full (FU) files: During the A/D operation, the sounder-receiver video output amplitude is digitized at the rate of 40 kHz yielding amplitude samples every 0.025 ms corresponding to an apparent-range resolution of 3.75 km. In order to produce smaller ionogram files (than these full (FU) files), that will be adequate for most purposes such as producing electron-density profiles, averaged files (AV) were produced where each sounder receiver-amplitude value corresponds to an average of 4 of the 40 kHz samples. These averaged samples, every 0.1 ms, correspond to an apparent-range resolution of 15 km.

If binary ionogram files are desired rather than CDF files, the information in the CDF file name returned from a data search can be used to order these files. Examples of these file names, corresponding to the above CDF file name, are given below:


for the average binary file, and


for the full binary file.

Corresponding header (HDR) and pulse-code modulation (PCM) files:  each contain information pertaining to the entire satellite pass over the telemetry station and can be retrieved using:




where the time includes a wild card rather than 024705 because it corresponds to the beginning of the satellite pass over a station (of duration typically 15 minutes) as recorded by the A/D operator (based on the telemetry station log sheet) and is unlikely to correspond to an exact ionogram frame-sync time.

 Description of average and full files
 Description of header files

Anomalous values retrieved from data search:

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.

For assistance, contact:

Dr. Robert F. Benson
Code 692
Laboratory for Extraterrestrial Physics
Goddard Space Flight Center
Greenbelt, MD 20771