The Ionospheric Informatics Working Group (IIWG) of URSI Commission G had recommended the CHARS format as a standard for ionogram data dessimination and archival in 1989. This format was then accepted as the URSI standard at the URSI general Assmebly in Prague, 1990. This report present the general description of the CHARS format with the latest updates as of November 1997.
Historically, the maximum length of individual text lines in a CHARS file was set to 120 characters so that it still could be printed without wrapping. The number of lines in a file is determined by the number of days in the month, the number of measurements made each day, and the number of characteristics being archived.
The structure of the CHARS file is shown in Table 1. It consists of two headers, Station Header and Data Header, followed by the main Data Group.
Then, a repeating format of (12A10) is used to list the Names of the particular characteristics being archived. There are K of them, hence more that one line might be required to fit all names. For example if one were archiving only the critical frequencies foF2, foF1, and foE, K would be three, and the characteristics list would be ' foF2' ' foF1' ' foE'. A list of the names of the characteristics, the units, and URSI codes taken from UAG23 [Piggott and Rawer, 1978] are given in Table 2. The URSI list has been enhanced with characteristics that are scaled by the Digisonde ARTIST [Reinisch and Huang, 1983; Tang et al., 1989]. The Chebyshev coefficients [Huang and Reinisch. 1996] used to represent the electron profile of ionosphere are also given, also the best B0 and B1 parameters [Reinisch and Huang, 1997] for the IRI F2 profile. and the calculated ionospheric electron content.
Next, the Units corresponding to the characteristics list (see Table 2) are given in the file, these are in (12A10) repeating format. The last lines of the Data Header are for the URSI codes specified for each of the characteristics (see Table 2) and are written in (60A2) repeating format.
From the information in this Data Header one knows immediately how many data for the time or for each characteristic are to be read. From the number of measurements for each day the time data can be separated into the times for the individual days of the month and the measured characteristics can uniquely be associated with a given time on a given day.
Finally, the Data Header contains
the measurement times for the month. With uneven time spacing the measurement
times must be recorded to associate with the reported characteristics. This
requires that hours, minutes, and seconds of each measurement be entered into
the database. To conserve space, the times are written once per month and
the reported characteristics are written to correspond to these times. The
measurement times are written in a (30(3I2)) repeating format corresponding
to the hours, minutes, and seconds, HHMMSS, of the measurements.
The number of lines needed for this is determined by the data sampling rate
for the month.

FORTRAN Format 

Station Header  A30  Station Name 
A5  Station code  
I4  Meridian time used by station on records  
F5.1  Latitude N  
F5.1  Longitude E  
A10  Scaling type: Manual/Automatic  
A10  Data editing: Edited/Nonedited/Mixed  
A30  Ionosonde system name  
Data Header  30I4... *  Year 
Month  
Number of days in the month, M  
Number of Characteristics, K  
Numbers of measurements total  
Numbers of measurements for each of the M days (N_{i}, i=1..M)  
12A10...*  Names  
12A10...*  Units  
12A10...*  List of corresponding URSI codes  
20(3I2)...*  Measurement times HH:MM:SS for each of M days, (N_{i} values)  
Data Group  24(I3,2A1)...*  N_{1} values of characteristic 1 (Day 1) 
N_{2} values of characteristic 1 (Day 2)  
...  
N_{M} values of characteristic 1 (Day M)  
24(I3,2A1)  24 hourly medians for characteristic 1  
24(I2,I3)  24 x 2 hourly counts and ranges  
24(I3,2A1)  24 hourly upper quartiles  
24(I3,2A1)  24 hourly lower quartiles  
24(I3,2A1)  24 hourly upper deciles  
24(I3,2A1)  24 hourly lower deciles  
Repeated for characteristic 2  
...  
Repeated for characteristic K 

ARTIST Name 
# 
URSI Name 
# 




F2  foF2  1  foF2  00  .1  MHz  1.11  The ordinary wave critical frequency of the highest stratification in the F region 
fxF2  01  .1  MHz  1.11  The extraordinary wave critical frequency  
fzF2  02  .1  MHz  1.11  The zmode wave critical frequency  
M(D)  3  M3000F2  03  .01  1.50  The maximum usable frequency at a defined distance divided by the critical frequency of that layer  
h'F2  12  h'F2  04  km  1.33  The minimum virtual height of the ordinary wave trace for the highest stable stratification in the F region  
hpF2  05  km  1.41  The virtual height of the ordinary wave mode at the frequency given by 0.834 of foF2 (or other 7.34)  
h'Ox  06  km  1.39  The virtual height of the x trace at foF2  
MUF(D)  4  MUF3000F2  07  .1  MHz  1.5C  The standard transmission curve for 3000 km  
hc  08  km  1.42  The height of the maximum obtained by fitting a theoretical h'F curve for the parabola of best fit to the observed ordinary wave trace near foF2 and correcting for underlying ionization  
ScaleF2  40  qc  09  km  7.34  Scale height  
F1  foF1  2  foF1  10  .01  MHz  1.13  The ordinary wave F1 critical frequency 
fxF1  11  .01  MHz  1.13  The extraordinary wave F1 critical frequency  
12  
M3000F1  13  .01  MHz  1.50  See Code 03  
h'F1  14  km  1.30  The minimum virtual height of reflection at a point where the trace is horizontal  
15  
h'F  11  h'F  16  km  1.32  The minimum virtual height of the ordinary wave trace taken as a whole  
MUF3000F1  17  .1  MHz  1.5C  See Code 07  
18  
19  
E  foE  9  foE  20  .01  MHz  1.14  The ordinary wave critical frequency of the lowest thick layer which causes a discontinuity 
fxE  fxE  21  .01  MHz  1.14  The extraordinary wave critical frequency of the lowest thick layer which causes a discontinuity  
foE2  22  .01  MHz  1.16  The critical frequency of an occulting thick layer which sometimes appears between the normal E and F1 layers  
foEa  44  foEa  23  .01  MHz  The critical frequency of night time auroral E layer  
h'E  13  h'E  24  km  1.34  The minimum virtual height of the normal E layer trace  
25  
h'E2  26  km  1.36  The minimum virtual height of the E2 layer trace  
h'Ea  45  h'Ea  27  km  The minimum virtual height of the night time auroral E layer trace  
28  
29  
Es  foEs  6  foEs  30  .1  MHz  1.17  The highest ordinary wave frequency at which a mainly continuous Es trace is observed 
fxEs  31  .1  MHz  1.17  The highest extraordinary wave frequency at which a mainly continuous Es trace is observed  
fbEs  48  fbEs  32  .1  MHz  1.18  The blanketing frequency of the Es layer  
ftEs  33  .1  MHz  Top frequency Es any mode.  
h'Es  14  h'Es  34  km  1.35  The minimum height of the trace used to give foEs  
fminEs  34  fminEs  35  .1  MHz  The minimum frequency of the trace used to give foEs  
Type Es  49  Type Es  36  7.26  A characterization of the shape of the Es trace  
37  
38  
39  
Other 1  foF1.5  40  .01  MHz  1.12  The ordinary wave critical frequency of the intermediate stratification between F1 and F2  
41  
fmin  5  fmin  42  .1  MHz  1.19  The lowest frequency at which echo traces are observed on the ionogram  
M3000F1.5  43  .01  MHz  1.50  See Code O3  
h'F1.5  44  km  1.38  The minimum virtual height of the ordinary wave trace between foF1 and foF1.5 (equals h'F2 7.34)  
45  
46  
fm2  47  .1  MHz  1.14  The minimum frequency of the second order trace  
hm  48  km  7.34  The height of the maximum density of the F2 layer calculated by the Titheridge method  
fm2  47  .1  MHz  1.25  The minimum frequency of the third order trace  
Spread F, Oblique  foI  50  .1  MHz  1.26  The top ordinary wave frequency of spread F traces  
fxI  10  fxI  51  .1  MHz  1.21  The top frequency of spread F traces  
fmI  52  .1  MHz  1.23  The lowest frequency of spread F traces  
M3000I  53  .01  MHz  1.50  See Code 03  
h'I  54  km  1.37  The minimum slant range of the spread F traces  
foP  46  foP  55  .1  MHz  Highest ordinary wave critical frequency of F region patch trace  
h'P  47  h'P  56  km  Minimum virtual height of the trace used to determine foP  
dfs  57  .1  MHz  1.22  The frequency spread of the scatter pattern  
58  7.34  Frequency range of spread fxIfoF2  
59  
N(h) Titheridge  fhpF2  30  fh'F2  60  .1  MHz  7.34  The frequency at which h'F2 is measured 
fhpF  29  fh'F  61  .1  MHz  7.34  The frequency at which h'F is measured  
62  
h'mF1  63  km  7.34  The maximum virtual height in the omode F1 cusp  
h1  64  km  7.34  True height at f1 Titheridge method  
h2  65  km  7.34  True height at f2 Titheridge method  
h3  66  km  7.34  True height at f3 Titheridge method  
h4  67  km  7.34  True height at f4 Titheridge method  
h5  68  km  7.34  True height at f5 Titheridge method  
H  69  km  7.34  Effective scale height at hmF2 Titheridge method  
T.E.C.  I2000  70  10^{16}  e/m2  7.34  Ionospheric electron content Faraday technique  
I  71  10^{16}  e/m2  7.34  Total electron content to geostationary satellite  
ITEC  39  I1000  72  10^{16}  e/m2  7.34  Ionospheric electron content to height 1000 km using Digisonde technique  
73  
74  
75  
76  
77  
78  
T  79  10^{16}  e/m2  7.34  Total subpeak content Titheridge method  
Other 2  fminF  7  FMINF  80  .1  MHz  Minimum frequency of F trace (50 kHz increments) Equals fbEs when E present  
fminE  8  FMINE  81  .1  MHz  Minimum frequency of E trace (50 kHz increments).  
zE  15  HOM  82  km  Parabolic E layer peak height  
yE  16  yE  83  km  Parabolic E layer semithickness  
QF  17  QF  84  km  Average range spread of F trace  
QE  18  QE  85  km  Average range spread of E trace  
FF  22  FF  86  .01  MHz  Frequency spread between fxF2 and fxI  
FE  23  FE  87  .01  MHz  As FF but considered beyond foE  
fMUF(D)  25  fMUF3000  88  .01  MHz  MUF(D)/obliquity factor  
hpMUF(D)  26  h'MUF3000  89  km  Virtual height at fMUF  
N(h)  zmE  15  zmE  90  km  Peak height E layer  
zmF1  33  zmF1  91  km  Peak height F1 layer  
zmF2  32  zmF2  92  km  Peak height F2 layer  
zhalfNm  34  zhalfNm  93  km  True height at half peak electron density  
yF2  37  yF2  94  km  Parabolic F2 layer semithickness  
yF1  38  yF1  95  km  Parabolic F1 layer semithickness  
96  
97  
98  
99  
Digisonde profile, F2 layer  [A0F2]  A0  km  Coefficient A0, truncated to integer km  
<A0F2>  A1  m  A0  [A0], truncation remainder  
[A1F2]  A2  km  Coefficient A1, truncated  
<A1F2>  A3  m  A1  [A1]  
[A2F2]  A4  km  Coefficient A2, truncated  
<A2F2>  A5  m  A2  [A2]  
[A3F2]  A6  km  Coefficient A3, truncated  
<A3F2>  A7  m  A3  [A3]  
[A4F2]  A8  km  Coefficient A4, truncated  
<A4F2>  A9  m  A4  [A4]  
[fsF2]  AA  MHz  starting frequency, truncated  
<fsF2>  AB  kHz  fs  [fs]  
[fmF2]  AC  MHz  ending frequency, truncated  
<fmF2>  AD  kHz  fm  [fm]  
[hmF2]  AE  km  peak height, truncated  
<hmF2>  AF  m  hm  [hm]  
EppF2  AG  0.1  km  error per point, an average mismatch of original h'(f) trace and the trace reconstructed from the calculated profile  
Digisonde profile, F1 layer  [A0F1]  B0  km  Coefficient A0, truncated to integer km  
<A0F1>  B1  m  A0  [A0], truncation remainder  
[A1F1]  B2  km  Coefficient A1  
<A1F1>  B3  m  A1  [A1]  
[A2F1]  B4  km  Coefficient A2  
<A2F1>  B5  m  A2  [A2]  
[A3F1]  B6  km  Coefficient A3  
<A3F1>  B7  m  A3  [A3]  
[A4F1]  B8  km  Coefficient A4  
<A4F1>  B9  m  A4  [A4]  
[fsF1]  BA  MHz  starting frequency of the layer, truncate  
<fsF1>  BB  kHz  fs  [fs]  
[fmF1]  BC  MHz  ending frequency fm  
<fmF1>  BD  kHz  fm  [fm]  
[hmF1]  BE  km  peak height  
<hmF1>  BF  m  hm  [hm]  
EppF1  BG  0.1  km  error per point, an average mismatch of original h'(f) trace and the trace reconstructed from the calculated profile  
Digisonde profile, E layer  [A0E]  C0  km  Coefficient A0, truncated to integer km  
<A0E>  C1  m  A0  [A0], truncation remainder  
[A1E]  C2  km  Coefficient A1  
<A1E>  C3  m  A1  [A1]  
[A2E]  C4  km  Coefficient A2  
<A2E>  C5  m  A2  [A2]  
[W]  C6  km  Valley width [W], truncated  
<W>  C7  m  W  [W]  
[D]  C8  km  Valley depth [D], truncated  
<D>  C9  m  D  [D]  
[fsE]  CA  MHz  starting frequency  
<fsE>  CB  kHz  fs  [fs]  
[fmE]  CC  MHz  ending frequency  
<fmE>  CD  kHz  fm  [fm]  
[hmE]  CE  km  peak height  
<hmE>  CF  m  hm  [hm]  
EppE  CG  0.1  km  error per point, an average mismatch of original h'(f) trace and the trace reconstructed from the calculated profile  
ValleyID  CH  Valley Model ID  
IRI  B0  41  B0  D0  km  IRI Thickness parameter  
B1  42  B1  D1  0.1  IRI Profile Shape parameter  
D1  43  D1  D2  0.1  IRI Profile Shape parameter, F1 layer  
D3  
D4  
D5  
D6  
D7  
D8  
D9 
The IIWG Workshop suggested the use of two slashes, //, in place of the qualifying and descriptive letters for monthly characteristics data that were autoscaled but not validated or "edited", i.e. where no quality control procedure has been applied. This code has been extended to consider data that have been edited but no descriptive or qualifying letters introduced. With two positions to fill and the use of a single or double slash there are four codes which can be defined. The first is no slashes implying the use of the descriptive or qualifying letters. The next is the use of two slashes which signifies no editing. The third choice is to put a slash in the first position followed by a blank. This is used to signify autoscaled data that have been edited but no descriptive or qualifying letters are used. The last possibility is a blank in the first position followed by the slash. This is not currently used thus it leaves the possibility for future extension of the code The codes are summarized in Table 3.
Symbolic code  Description 
Q D  Qualifying and descriptive letters used according to UAG #23A. 
/  Data, edited but no qualifying and descriptive letters used. 
/  No current meaning, for future extension. 
/ /  Autoscaled data, no editing, no qualifying and descriptive letters used. 
The above sections are repeated for each characteristic given in the "characteristics list." This completes the CHARS file, i.e. a month of characteristics data.
Gamache R. R. and B. W. Reinisch, "Ionogram Characteristics at Uneven Data Rates," Presented at URSI Working Group G.4 Ionospheric Informatics International Workshop, July, 1989., University of Lowell Center for Atmospheric Research, 1989b.
Gamache R. R. and B. W. Reinisch, "Ionospheric Characteristics Data Format for Archiving at the World Data Centers", University of Lowell Center for Atmospheric Research, Sci.Report 467, 1994.
Reinisch B.W., and X. Huang, Automatic calculation of electron density profiles from digital ionograms, 3, Processing of bottomside ionograms, Radio Sci., 18, 472492, 1983.
Tang, J., R.R. Gamache, and B.W. Reinisch, Progress on ARTIST improvements, Sci. Rep. No. 14, GLTR890185, Air Force Geophys. Lab., Hanscom AFB, Mass., 1989.