Selected original analog telemetry tapes from three of the topside-sounder satellites of the International Satellites for Ionospheric Studies (ISIS) program, namely Alouette 2, ISIS I, and ISIS II, were used in an earlier project to produce more than million digital topside ionograms; the resulting digital topside ionograms from ISIS II were used to produce morethan 86,000 globally-distributed vertical topside ionospheric electron density profiles Ne(h)that cover a time span of more than a solar cycle. These Ne(h) were produced using the TOPIST auto-scaling software. Before attempting to automatically process Alouette-2 orISIS-I ionograms a data-enhancement project was initiated so as to increase the auto processing success rate. These enhancements were mainly to correct problems that often occurred during the analog-to-digital conversion of the original telemetry tapes. Here we present the status of, and results from, this ongoing enhancement effort

Benson, Robert F.

Bilitza, Dieter

Fung, Shing F.

Truhlik, Vladimir

Wang, Yongli

NASA Technical Reports Server

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 Status of and scientific results from the ISIS-I Topside Digital Ionogram 
Data Enhancement Project 
 
 
Robert F. Benson*(1), Dieter Bilitza(2), Shing F. Fung(1), Vladimir Truhlik(3), and Yongli 
Wang(4) 
(1) Geospace Physics Laboratory, Heliophysics Science Division, NASA/Goddard Space 
Flight Center, Greenbelt, Maryland 20771 USA, 
http://science.gsfc.nasa.gov/heliophysics/ 
(2) Heliospheric Physics Laboratory, Heliophysics Science Division, 
GMU/SWL/Goddard Space Flight Center, Greenbelt, Maryland 20771 USA 
(3) Institute of Atmospheric Physics, Czech Academy of Sciences, Prague, Czech 
Republic 
(4) Space Weather Laboratory, Heliophysics Science Division, UMBC/GPHI/Goddard 
Space Flight Center, Greenbelt, Maryland 20771 USA 
 
 
ABSTRACT 
 
Selected original analog telemetry tapes from three of the topside-sounder satellites of the 
International Satellites for Ionospheric Studies (ISIS) program, namely Alouette 2, ISIS I, 
and ISIS II, were used in an earlier project to produce more than ½ million digital topside 
ionograms; the resulting digital topside ionograms from ISIS II were used to produce more 
than 86,000 globally-distributed vertical topside ionospheric electron density profiles Ne(h) 
that cover a time span of more than a solar cycle. These Ne(h) were produced using the 
TOPIST auto-scaling software. Before attempting to automatically process Alouette-2 or 
ISIS-I ionograms a data-enhancement project was initiated so as to increase the auto-
processing success rate. These enhancements were mainly to correct problems that often 
occurred during the analog-to-digital conversion of the original telemetry tapes. Here we 
present the status of, and results from, this ongoing enhancement effort.  
 
 
1. INTRODUCTION 
 
The four polar-orbiting Canadian-built and U.S. launched topside-sounder satellites Alouette 
1, Alouette 2, ISIS I, and ISIS II were included in the six-satellite International Satellites for 
Ionospheric Studies (ISIS) program; the other two were the U.S. Explorer 20 and Explorer 31 
satellites [Jackson, 1986; Jackson and Warren, 1969]. The four Alouette/ISIS satellites were 
launched between 1962 and 1971 and each operated for 10 or more years to produce 60 
satellite-years of swept-frequency ionospheric topside sounder observations. These sounders 
were designed as analog systems and the data were recorded on analog 7-track telemetry 
tapes at a globally-distributed network of telemetry stations as illustrated in Figure 2 of 
Benson and Bilitza [2009]. Cost considerations prevented all of the sounder data from being 
converted to 35-mm film topside ionograms. An Analog-to-Digital (A/D) conversion project 
was conducted at the NASA/Goddard Space Flight Center (GSFC) to process some 16,000 of 
the original telemetry tapes from Alouette 2, ISIS I, and ISIS II; more than 588,000 digital 
topside ionograms were produced [Benson, 1996; Benson and Bilitza, 2009]. The ISIS-II 
https://ntrs.nasa.gov/search.jsp?R=20180004239 2019-08-31T15:18:01+00:00Z
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digital topside ionograms were automatically processed to produce more than 86,000 
globally-distributed vertical topside ionospheric electron density profiles Ne(h) using the 
Topside Ionogram Scaler with True height algorithm (TOPIST) [Bilitza et al., 2004; Huang 
et al., 2002]. Many of the ISIS-II digital ionograms could not be auto-processed due to 
problems encountered during the A/D operation. In addition, some files that were auto-
processed yielded erroneous topside Ne(h) due to incorrect starting conditions in the 
identification of ionospheric reflection traces; this situation led to improvements in the 
TOPIST software [Benson et al., 2012]. A data-enhancement project was then initiated to 
correct problems that often occurred during the A/D process so as to increase the TOPIST 
success rate [Benson et al., 2012; Wang et al., 2015]. First priority was given to ISIS I 
because of the large number of digital ionograms available, the data time span from 1969 to 
1982, and because of the similar ionogram format to that of ISIS II (a few seconds of fixed-
frequency operation followed by swept-frequency operation to 10 or 20 MHz). ISIS I is a 
valuable complement to ISIS II because it includes altitudes up to 3,500 km (ISIS II was in a 
circular 1,400-km orbit) where predictions based on the International Reference Ionosphere 
(IRI) [Bilitza, 2009] are in great need of improvement. 
 
 
2. TOPIST-PROCESSED ISIS-II TOPSIDE Ne(h) 
 
Most topside Ne(h) available after 1969 are from ISIS II. They are either from the hand 
scaling of 35-mm film ionograms or from the auto TOPIST-processing of digital ionograms 
but most are from the latter (see Figure 1a). Only slightly more than 1/3 of the available ISIS-
II topside digital ionograms (see Figure 1b) were capable of being auto-processed by 
TOPIST, and not all of the output files produced Ne(h) profiles (see caption). Many of the 
ISIS-II digital ionograms did not yield Ne(h) because some of the sounding modes were 
either passive or fixed-frequency operation. In the former, the sounder transmitter is off but 
the receiver is sweeping in frequency; in the latter, the sounder transmitter and receiver are 
held at a selected frequency. While ionograms resulting from these conditions are useful for 
investigating Ne gradients along the satellite orbit, auroral kilometric radiation and other 
natural emissions, sounder-stimulated plasma resonances, etc., they will not contain swept-
frequency ionospheric reflection traces to invert into Ne(h). In addition, some of the 
ionograms with reflection traces could not be auto-processed by TOPIST due to (1) 
incomplete reflection traces, (2) the presence of strong field-aligned reflection traces, spread 
F, or interference, (3) incorrect starting conditions due to the misidentification of wave 
cutoffs and sounder-stimulated plasma resonances, or (4) problems with the digital files due 
to difficulties encountered during the A/D processing of the analog telemetry tapes (and 
similar to those discussed in connection with the ISIS-I digital ionogram files in Section 3). 
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Figure 1. Data sets available from the NASA Space Physics Data Facility (SPDF) at 
http://spdf.gsfc.nasa.gov/isis/isis-status.html as of January 2009: (a) manually-scaled topside 
Ne(h) from Alouette-1, Alouette-2, ISIS-I, and ISIS-II 35-mm film topside ionograms and the 
number of digital files resulting from the processing of all ISIS-II digital ionograms using 
TOPIST, and (b) digital topside ionograms from Alouette 2, ISIS I, and ISIS II selected to 
obtain global coverage and to complement the earlier manually-scaled data (note the factor of 
2 change in scale relative to Figure 1a). Unfortunately, the label in Figure 1a is not correct in 
the case of ISIS II since all of the files processed by TOPIST did not yield Ne(h) results; see 
Figure 2, and the discussion in the caption and text, for the correct number of ISIS-II 
TOPIST Ne(h) broken down by quality codes. (Figure after [Benson and Bilitza, 2009].) 
 
TOPIST could not process many of the ISIS-II digital ionogram files and it was often not 
clear why the processing failed. There were 4 possible outcomes for those that were 
processed: (1) the file could be processed but the reflection trace data were not sufficient to 
produce a Ne(h), (2) a Ne(h) could be produced but it was of the lowest quality (q = 1) due to 
severe spread F or the reflection traces calculated from the derived Ne(h) did not agree very 
well with the auto-scaled traces, (3) the Ne(h) was considered to be of medium quality (q = 2) 
because, even though the conditions for q = 1 were not present, the deduced height of the F 
peak differed from the IRI model value by more than 50 km or the foF2 value was 
considered to be greater than the auto-scaled high-frequency cusp of the reflection trace, or 
(4) the Ne(h) was considered to be of the highest quality (q = 3) because neither the 
conditions for q = 1 or q = 2 were present. The distributions of these four possible outcomes 
over all the years covered by the ISIS-II digital ionogram data are presented in Figure 2. The 
totals indicate that when TOPIST could automatically process ISIS-II ionograms to produce 
Ne(h) that 60.5% of these profiles were either of medium or high quality, i.e., with q = 2 or 3. 
 
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Figure 2. Number of ISIS-II TOPIST Ne(h), broken down by quality codes: q=3 (highest 
quality; upper red-shaded areas), q = 2 (medium quality; next-lower green-shaded areas), q = 
1 (lowest quality; next-lower blue-shaded areas), or "no q" (or a q value but defective data so 
that no Ne(h) produced; the lowest black-shaded areas). There are 86,545 ISIS-II TOPIST 
Ne(h) (17,278 q = 3; 35,201 q = 2; 34,066 q = 1) and 28,725 ISIS-II TOPIST no-q files 
(including 225 with defective data), i.e., without Ne(h) profiles, for a total of 115,270 (the 
total given in Figure 1a included 6 TOPIST files with zero size). 
 
The large digital topside ionogram database, indicated in Figure 1b, has enabled efficient 
search procedures used to identify fixed-frequency ionograms recorded during specific 
geomagnetic conditions to support a theoretical interpretation of a sounder-stimulated plasma 
resonance that has remained a mystery for decades, i.e., the resonance at the electron 
gyrofrequency [Muldrew, 2006]. This large digital topside ionogram database, and the large 
topside Ne(h) database indicated in Figure 1a, has enabled changes in high-latitude Ne(h) 
stimulated by large magnetic storms to be compared with changes in solar-wind parameters 
[Benson et al., 2016]. The large TOPIST Ne(h) database has been used, together with higher-
altitude profiles obtained by the Radio Plasma Imager (RPI) on the IMAGE satellite, to 
improve the empirical IRI model in the topside ionosphere and to extend it into the 
plasmasphere [Reinisch et al., 2007]. 
 
3. VERSION-2 ISIS-I DIGITAL TOPSIDE IONOGRAMS 
 
We are currently producing version-2 ISIS-I topside digital ionograms to be accessible from 
the SPDF CDAWeb system and from the NASA Heliophysics Virtual Wave Observatory 
(VWO). A major goal is to produce as many topside ionospheric Ne(h) as possible so as to 
expand the Ne(h) profile coverage in latitude, longitude, and epoch available from the 
NASA/SPDF. First priority was given to producing more than 80,000 version-2 digital 
topside ionogram files for all of the available years of ISIS-I data from the Ottawa (OTT) 
telemetry station (from 1969-1983) [Wang et al., 2015]. Currently, we are processing ISIS-I 
digital topside ionogram files from the Quito (QUI), University of Alaska (ULA), and 
Santiago (SNT) telemetry stations. Figures 3 – 5, obtained from CDAWeb displays, illustrate 
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some of the differences between the original (version-1) files, designated as av for average 
files where the apparent-range information was averaged to yield 15-km resolution, and the 
enhanced files designated as av2. Both have good ionogram data but the lack of proper 
frame-sync detection in the av files often prevented the determination of frequency 
information. This lack of proper frame-sync detection is apparent by comparing the number 
of av and av2 altitude steps in Figures 3 and 4. Each altitude step corresponds to the satellite 
altitude at the time of ionogram frame sync-pulse detection during the A/D operation. After 
correcting the frame-sync detection it is possible to add the correct frequency information, as 
indicated in the right panels of Figures 3 – 5, allowing the ionograms to be processed 
automatically, by software such as TOPIST, to yield topside Ne(h). 
 
 
Figure 3. Comparing available frequency information (0.1 to 10 MHz in the top panels) and 
file start times (each at a different altitude in the range from 1,200 to 2,200 km in the bottom 
panels) for av (left panels) and av2 (right panels) for OTT ISIS-I data from 2334 to 2348 UT 
on 25 January 1976. The 1½-minute portions between the arrows are presented in Figure 4 
with the addition of sounder data. 
 
 
 
Figure 4. Same as Figure 3 except for only the OTT ISIS-I data from 2337:30 to 2339:00 UT 
on 25 January 1976, the altitude scales are different, and the sounder data appear in the top 
panels. The 10 s portions between the arrows are expanded in Figure 5. 
 
	 6	
 
Figure 5. Same as Figure 4 except for only the 10 s intervals of swept-frequency operation 
from 0.1 to 5 MHz during the OTT ISIS-I av (left) and av2 (right) ionograms from 2338:18 
to 2338:28 UT on 25 January 1976. The altitude is the value at the start of the file. 
 
4. RESULTS USING VERSION-2 ISIS-I IONOGRAMS 
 
Many of the version-2 ISIS-I digital topside ionograms have been processed by TOPIST and 
the resulting Ne(h) have been compared with those obtained from IRI2016. Some sample 
comparisons are given in Figure 6 and 7, for each q value, when the agreement in the topside 
ionosphere is good and not good, respectively. More such comparisons are planned, 
particularly for cases with q = 2 and 3, to determine the conditions that most often lead to 
good agreement and those that lead to poor agreement.  
 
 
Figure 6. TOPIST Ne(h) examples with q = 3 (left), 2 (middle), and 1 (right) with good 
topside agreement. 
 
 
Figure 7. Same as Figure 6 except for TOPIST Ne(h) examples where the agreement with 
IRI in the topside ionosphere is not good. 
	 7	
5. SUMMARY 
 
The 4 Alouette/ISIS topside sounder satellites combined to produce 60 satellite-years of 
swept-frequency topside ionospheric sounding spanning the time interval from 1962 to 1990. 
During the 1960s and 1970s skilled operators hand scaled approximately 177,000 of the 
millions of 35-mm film ionograms to produce vertical Ne(h). An analog-to-digital conversion 
project in the 1990s and 2000s led to more than 588,000 digital topside ionograms from the 
original Alouette-2, ISIS-I, and ISIS-II analog telemetry tapes; all of the ISIS-II digital 
ionograms were automatically processed by the TOPIST software and more than 86,000 
yielded topside Ne(h). An ISIS-I data enhancement project is underway to correct problems 
encountered during the A/D operation in order to maximize the number of files capable of 
processing by TOPIST. These problems were mainly caused by the lack of proper frame-
sync detection on many of the ionograms. During the first phase of this project all of the 
digital ionograms from the Ottawa telemetry station were converted to version-2 files (more 
than 80,000 files from 1969-1983). During the current phase, files from the Quito, University 
of Alaska, and Santiago telemetry stations are being converted to version-2 files. Many of the 
version-2 files have been processed by the TOPIST software to produce topside Ne(h). These 
vertical profiles have been compared with the IRI2016 model and examples were presented 
both when the agreement in the topside ionosphere was very good and when it was not good. 
In the later case the TOPIST Ne(h) values at high altitudes were considerably greater than the 
IRI2016 values; examples were shown where the differences were a factor of 2 or more at an 
altitude of 2,000 km. 
 
ACKNOWLEDGEMENTS This	 work	 was	 supported	 by	 the	 NASA	 Data	 Services	 Upgrade	 Program	 (NNH13ZDA001N-HIDEE	 and	 NNH15ZDA001N-HIDEE).	 V.T.	 was	 supported,	 in	 part,	 by	 grant	 1507281J	 of	 the	Grant	Agency	of	the	Czech	Republic.	
 
 
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Status of and Scientific Results from the ISIS-I Topside Digital Ionogram Data Enhancement Project

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