The ionospheric topside sounders of the International Satellites for Ionospheric Studies (ISIS) program were designed as analog systems. More than 16,000 of the original telemetry tapes from three satellites were used to produce topside digital ionograms, via an analog-to-digital (A/D) conversion process, suitable for modern analysis techniques. Unfortunately, many of the resulting digital topside ionogram files could not be auto-processed to produce topside Ne(h) profiles because of problems encountered during the A/D process. Software has been written to resolve these problems and here we report on (1) the first application of this software to a significant portion of the ISIS-1 digital topside-ionogram database, (2) software improvements motivated by this activity, (3) N(sub e)(h) profiles automatically produced from these corrected ISIS-1 digital ionogram files, and (4) the availability via the Virtual Wave Observatory (VWO) of the corrected ISIS-1 digital topside ionogram files for research. We will also demonstrate the use of these N(sub e)(h) profiles for making refinements in the International Reference Ionosphere (IRI) and in the determination of transition heights from Oxygen ion to Hydrogen ion

Benson, Robert F.

Bilitza, Dieter

Chu, Philip

Fung, Shing. F.

Huang, Xueqin

Truhlik, Vladimir

Wang, Yongli

NASA Technical Reports Server

 1 
Data Services Upgrade: Perfecting the ISIS-I topside digital ionogram database 
 
Yongli Wang1, Robert F. Benson2, Dieter Bilitza3, Shing. F. Fung2, Philip Chu1, Xueqin 
Huang4, and Vladimir Truhlik5 
 
1UMBC/GPHI/Space Weather Laboratory, Code 674, Heliophysics Science Division, Greenbelt, 
MD 20771 
2NASA/Goddard Space Flight Center, Geospace Physics Laboratory, Code 673, Heliophysics 
Science Division, Greenbelt, MD 20771 
3GMU/SWL/Heliospheric Physics Laboratory, Code 672, Heliophysics Science Division, 
Greenbelt, MD 20771 
4University of Massachusetts Lowell, Center for Atmospheric Research, Lowell, MA 01854 
5Institute of Atmospheric Physics, Academy of Sciences of the Czech Republic, Praha, Czech 
Republic. 
 
ABSTRACT 
 
The ionospheric topside sounders of the International Satellites for Ionospheric 
Studies (ISIS) program were designed as analog systems. More than 16,000 of the 
original telemetry tapes from three satellites were used to produce topside digital 
ionograms, via an analog-to-digital (A/D) conversion process, suitable for modern 
analysis techniques. Unfortunately, many of the resulting digital topside ionogram files 
could not be auto-processed to produce topside Ne(h) profiles because of problems 
encountered during the A/D process. Software has been written to resolve these problems 
and here we report on (1) the first application of this software to a significant portion of 
the ISIS-1 digital topside-ionogram database, (2) software improvements motivated by 
this activity, (3) Ne(h) profiles automatically produced from these corrected ISIS-1 digital 
ionogram files, and (4) the availability via the Virtual Wave Observatory (VWO) of the 
corrected ISIS-1 digital topside ionogram files for research. We will also demonstrate the 
use of these Ne(h) profiles for making refinements in the International Reference 
Ionosphere (IRI) and in the determination of transition heights from O+ to H+. 
 
 
1. INTRODUCTION 
 
The ionospheric topside sounder on the ISIS-1 satellite was designed as an analog 
system, as were the other ionospheric radio sounders included in the International 
Satellites for Ionospheric Studies (ISIS) program. The plan was to display the data on 35-
mm film for analysis by visual inspection. The long operational lifetime of ISIS 1 (21 
years) and cost considerations, however, prevented all of the sounder data from being 
converted to such film records. After the termination of the ISIS program in 1990, more 
than 16,000 of the original telemetry tapes from three of the topside-sounder satellites 
(Alouette 2, ISIS 1, and ISIS 2) were converted directly into digital records. The goal was 
to select a large number of telemetry tapes that were never processed to produce 35-mm 
film topside ionograms and to use them to produce topside digital ionograms suitable for 
modern analysis techniques, thus essentially creating new satellite missions with old data 
[Benson and Bilitza, 2009]. See also the web page for the ISIS/Alouette Topside Sounder 
https://ntrs.nasa.gov/search.jsp?R=20160005846 2019-08-31T03:28:41+00:00Z
 2 
Data Restoration Project home page at spdf.gsfc.nasa.gov/isis/isis-status.html. Based on 
experience gained with ISIS 2, the analog-to-digital (A/D) conversion process was 
modified for ISIS 1 in an attempt to increase the number of digital topside ionograms that 
could be successfully auto-processed to produce topside vertical electron-density profiles 
Ne(h). The TOPIST algorithm (TOPside Ionogram Scaler with True-height) [Bilitza et 
al., 2004; Huang et al., 2002], developed for the ISIS-2 digital topside ionograms, was 
modified so as to be able to automatically process the more-recently produced ISIS-1 
digital topside ionograms. As with ISIS 2, however, many of the digital ISIS-1 topside 
ionogram files could not be auto-processed by TOPIST to produce topside Ne(h) profiles 
because of problems with the digital files. Software has been written to resolve these 
problems, which originated during the earlier A/D process, as described in Benson et al. 
[2012]. In the following sections we first illustrate the problems being addressed. Next 
we describe the software used to correct these problems and illustrate the results of these 
corrections. Finally we will present topside Ne(h) profiles, automatically-produced using 
the TOPIST software on the corrected ISIS-1 ionogram files, and illustrate the 
application of these profiles in refining the International Reference Ionosphere (IRI) and 
in determining O+/H+ transition heights. 
 
 
2. TOPSIDE-IONOGRAM A/D CONVERSION-PROCESS PROBLEMS 
 
Several problems were encountered during the A/D conversion of the ionogram data 
contained on the original 7-track analog telemetry tapes. These include timing errors in 
the data files and, at times, the lack of proper detection of ionogram frame-sync pulses 
and frequency markers. Timing errors were caused by bit-code errors in reading the time 
code during the A/D process resulting in incorrect spacecraft position and magnetic field 
information. Some of these erroneous values could easily be detected as described at 
spdf.gsfc.nasa.gov/isis/isis-status.html. Other timing problems were caused by variations 
in the speeds of the tape drives used in the recording and play back of the telemetry data. 
When the ionogram frame sync pulse was not properly detected then, in most cases, the 
frequency marks were also not properly detected and programs such as TOPIST could 
not automatically process the ionospheric reflection traces into topside Ne(h) profiles. 
Even when frame syncs were properly identified, proper frequency-marker identification 
was not always guaranteed because of interference signals and strong plasma resonances 
that could be falsely identified as frequency markers during the A/D process. An 
experienced observer can often easily correct these problems but it is desirable to have 
software capable of increasing the efficiency of the process. The next section will 
illustrate the correction procedure. 
 
 
3. SOFTWARE-AIDED MANUAL CORRECTION OF PROBLEMS 
 
Software was developed to aid in detecting timing-overlap problems, fixing frame-
sync detection problems, identifying frequency marks, and for the interpolation between 
frequency markers to assign the proper frequency value to each sounder amplitude scan 
line. 
 3 
In order to detect timing-overlap problems, i.e., when an ionogram starts prior to the 
end of the previous ionogram, ionogram time spans are plotted on UT-altitude plots. 
Neighboring ionograms with large overlaps can then be visually identified easily with the 
help of efficient survey software developed for this purpose. Careful examination of these 
overlaps is conducted and those ionograms causing the overlaps may be removed to make 
sure the final ionograms are as complete as possible and are free from any large overlaps. 
Many ionograms have frame-sync detection problems. As a result, one ionogram can 
be split into multiple ionograms and there can be multiple ionograms in one ionogram 
file. To fix this problem, we developed visual ionogram segmentation software to 
split/join ionograms appropriately so that each data file only has one unique ionogram. 
An example illustrating the problems to be corrected, and a display from the software 
developed to resolve the problem, is given in Figure 1. 
 
 
Figure 1. An example of a version-1 ISIS-1 digital ionogram in need of correction 
because of incorrect segmentation, i.e., the proper ionogram frame sync pulse was not 
detected during the A/D process. Three consecutive version-1 ionogram files are 
displayed in this illustration of the software developed to address this problem. 
 
 
 4 
An illustration of an ionogram after correcting the segmentation problem is given in 
Figure 2. Note that there are still problems with the frequency markers. 
 
 
Figure 2. An example of a version-1 ISIS-1 digital ionogram with proper 
segmentation (middle panel) but in need of frequency-marker correction.  
 
Even with version-1 ionograms with proper segmentation, there are often cases where 
ionogram frequency markers are missing or wrongly placed as illustrated in Figure 3. 
These markers have to be fixed to produce high-quality ionograms for scientific research. 
We developed efficient software for easy addition/deletion of markers. Standard markers 
are plotted as a background  (red dots in Figure 3) that can be shifted relative to the 
current markers to provide a useful reference during marker correction. An automatic 
pattern-fitting program is used to automatically assign standard frequency values to the 
final markers. 
There are five distinct frequency ranges: 0.1-2 MHz, 2-3 MHz, 3-5 MHz, 5-6 MHz, 
and 6-20 MHz. Frequencies falling outside of the time marker time ranges are set to 
negative values. Within each frequency range, we request at least two markers to allow 
effective frequency interpolation and frequencies outside of these markers will be set to 
 5 
negative values. In the first, third, and fifty ranges, 3rd order polynomial interpolation is 
conducted when there are more than three markers, 2nd order polynomial interpolation is 
conducted when there are three markers, and linear interpolation is conducted when there 
are two markers. In the second and fourth ranges, linear interpolation is conducted when 
there are two markers at least. In addition, in the second range, an extra linear mapping: 
(2.0,2.0), (2.165,2.1), (2.39,2.3), (2.75,2.7), (3.0,3.0) is conducted, and in the fourth 
range, an extra linear mapping: (5.0,5.0), (5.52,5.5), (6.0,6.0) is conducted to account for 
special instrument adjustment as described in connection with Table A2 of Benson and 
Bilitza [2009]. Such an adjustment is not incorporated in the Version 1 ISIS 1 files, so 
each user application has to provide its own mapping. A corrected final version-2 
ionogram is illustrated in Figure 4. 
 
 
Figure 3. An example of a version-1 ISIS-1 digital ionogram in need of frequency-
mark correction because of false frequency-marker detection due to interference. The red 
dots correspond to the expected location of the frequency markers based on a reference 
ionogram; the white dots correspond to the frequency markers detected during the A/D 
process. 
 
 
 6 
 
Figure 4. A software display of a final version-2 ISIS-1 digital topside ionogram with 
the yellow bar at the bottom indicating the range of correct frequency values. 
 
The resulting version-2 ISIS-1 files will be suitable for the analysis of plasma-wave 
phenomena, as briefly reviewed in Section 3 of Benson [2010], and for the automatic 
processing of the ionogram reflection traces into Ne(h) profiles as illustrated in the next 
section. 
 
 
4. AUTOMATICALLY-PRODUCED Ne(h) PROFILES 
 
The number of digital ionograms available, as illustrated in Figure 2 of Benson and 
Bilitza [2009], calls for an automated analysis for the production of Ne(h) profiles. These 
profiles are of fundamental importance to improving the International Reference 
Ionosphere (IRI) and for the automatic determination of transition heights from O+ to H+. 
The resulting version-2 ISIS-1 topside ionogram files produced during this project will be 
available from spdf.gsfc.nasa.gov/isis/isis-status.html and also from the Virtual Wave 
Observatory (VWO) [Fung, 2010]. As an illustration of the use of the version-2 ISIS-1 
digital topside-ionogram files, all of the available version-1 files from the Quito telemetry 
station recorded during 1976 were processed through the TOPIST software. The resulting 
profiles, edited to remove files that appeared to have non-physical results, e.g., weird 
shape or the height of the F layer too low, are presented in Figure 5 corresponding to 
TOPIST-determined quality flags 1 (worst), 2 (medium), and 3 (best). 
 7 
 
Figure 5. Left: 24 selected TOPIST-processed Ne(h) profiles with quality flag = 1, 
Middle: 22 selected TOPIST-processed Ne(h) profiles with quality flag = 2, Right: 2 
TOPIST-processed Ne(h) profiles with quality flag = 3, from QUI during 1976. 
 
One important application of such topside profiles is the determination of the upper 
transition height (Ht); it corresponds to the altitude where the distribution of dominant 
ions changes from the topside ionosphere (O+) to the plasmasphere (H+ and He+). It is a 
key parameter since magnetospheric-ionospheric interaction processes are important in 
this transition region. We used a method developed by Webb et al. [2006] to determine 
Ht. This procedure assumes diffusive-equilibrium, uses an electron temperature model 
(IRI) as an input, and employs a least-square fit to Ne profiles to determine ion (O
+ and 
H+) profiles and from them Ht. 
A preliminary test using TOPIST ISIS-1 topside Ne(h) profiles from the equatorial 
telemetry station QUI was performed. We have selected only profiles from ±15° diplat. 
Only two local times were populated (5.4 h and 15.9 h) by the data represented in Figure 
5. The processed data show an average Ht of about 530 km for 5.4 h LT and about 920 
km for 15.9 h LT. Solar activity was very low; the PF10.7 index was about 71. The 
AEIKion-13 model ([Truhlik et al., 2015]) produced values of about 600 km and 800 km 
at these local times, respectively. Thus, there is a reasonable agreement between the data 
and the model as illustrated in Figure 6. The availability of more high-quality topside 
Ne(h) profiles will enable improvements to current models of Ht. 
 
  
Figure 6. Local time dependence of the processed upper transition height (red * - data, 
green Δ - AEIKion-13 model) 
 
 8 
Acknowledgements 
We are grateful to Howard Leckner for assistance in the data processing. This work was 
supported by the NASA Data Services Upgrade Program via NASA Grant 13-
HIDEE13_2-0019. V.T. was supported, in part, by grant 1507281J of the Grant Agency 
of the Czech Republic. 
 
REFERENCES 
 
Benson, R. F. (2010), Four decade of space-borne radio sounding, Radio Sci. Bull., 
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Benson, R. F., and D. Bilitza (2009), New satellite mission with old data: Rescuing a 
unique data set, Radio Sci., 44, RS0A04, doi:10.1029/2008RS004036. 
Benson, R. F., V. Truhlik, X. Huang, Y. Wang, and D. Bilitza (2012), Improving the 
automatic inversion of digital Alouette/ISIS ionogram-reflection traces into topside 
electron-density profiles, Radio Sci., 47, RS0L04, doi:10.1029/2011RS004963. 
Bilitza, D., X. Huang, B. W. Reinisch, R. F. Benson, H. K. Hills, and W. B. Schar 
(2004), Topside ionogram scaler with true height algorithm (TOPIST): Automated 
processing of ISIS topside ionograms, Radio Sci., 39, RS1S27, 
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