Standard Archiving Output (SAO) Format

Introduction

Automatic scaling of ionogram data has come a long way and the quality of the autoscaled data has reached a remarkable level. Consequently the time has arrived to directly transfer ionosonde data to the World Data Centers using the Internet. We have begun to equip the Digisondes with Internet connections. The first Internet links were established between the Okinawa Digisonde (CRL, Japan) and the WDC-C2 in Tokyo, the Millstone Hill Digisonde (UML, USA) and the WDC-A in Boulder, Colorado, and Chilton Digisonde (RAL, GB) and the WDC-C1 in Chilton. All data generated in the Digisonde are made available for electronic transfer: ionogram data, scaled data, and drift data.

Starting in 1987, the Ionospheric Informatics Working Group (IIWG) of Commission G of URSI has developed recommendations for the data formats to be used for dissemination and archiving of scaled ionogram data and for the monthly ionospheric characteristics. The IIWG abstained (wisely) from trying to develop a common data format for the system-dependent ionogram and drift data.

The attached report gives a detailed description of the Standard Archiving Output (SAO) format. Each SAO (text) file contains the scaled data for one ionogram including the echo traces h'(f), echo amplitudes, frequency and range spread, etc. and the electron density profile.

The upgraded or new Digisondes produce the SAO files in real time for local recording and/or electronic transfer. The older Digisondes generate only binary files, but offline editing results are usually stored in the SAO format. Since these Digisonde ionograms SAO files are now becoming available to any user either through the WDC sites or via the web pages of the connected Digisonde stations it seems important to publish a description of the SAO format.

The SAO format was originally designed for storing Digisonde ionograms scaled by autoscaling software ARTIST and edited using ADEP utility. However, in subsequent releases a special effort was made to generalize SAO design so that it can hold scaled data produced by other sounder systems. With release of version 4.1, the degree of format universality became high enough to promote SAO as a standard format for exchange of scaled ionogram data.

SAO Format version 4.3

The SAO file structure has remained the same since it was developed by the IIWG in 1989, but the content has been expanded in subsequent releases. The following is a description of the SAO format version 4.3 [Gamache et al., 1996].

A SAO file is an ASCII text file with a maximum line length of 120 characters. In order to concisely describe the database some definitions are necessary. The nomenclature is as follows:

File a collection of many Records
Record all data for a single observation (ionogram) 
Group  all Lines of a datum type 
Line  a sequence of Elements of a datum type, CR/LF terminated 
Element  a single datum in the specified format 

The Record structure is composed of two basic components: a Data Index and Data. The format and size of the Data Index is fixed. It describes the contents of the Data in the Record. The Data component of each Record contains a varying number of Groups as indicated by the Data Index. The format and length of data varies from one Group to the next; however, all data Elements within a single Group are of the same type and length. The number of characters in a given Group can easily exceed the 120 characters per line limit. In this case, the output overflows to succeeding lines, thus a data Group may extend over several Lines.

This format design allows storing variable amount of information per ionogram, depending not only on ionospheric conditions, but also on sounder system specifics. There is only a subset of Groups that have to be present in a Record. As explained below, all others may be omitted and their corresponding index in the Data Index section set to zero. Data systems engineers have to decide which Groups to use to report data available from their sounders, if different from Digisonde. There are three situations, described in detail below, where system-specific data can be readily ingested using existing SAO-4 format:

Groups 63 to 79 are currently vacant for specification formats of other data items currently missing from SAO-4. Each addition of a Group has to be accompanied with a new release of SAO 4 format (versions 4.2, 4.3,... ) which contains format specification for the new Group. If necessary. the number of vacant Groups may be expanded by addition of new line(s) in the Data Index.

Data Index

The Data Index contains 80 three digit integers. The position in the list corresponds to the data for the data Group number. These are shown in Table 1. The first integer is the number of Elements in the data Group 1, Geophysical Constants, in the current Record. The second integer represents the number of Elements in the second data Group, System Description, etc. A value of zero indicates that there is no data for the Group in the Record. Position 80 of the Data Index array is not used to specify the format of the data to follow. It is reserved for the SAO version indicator:

    0        SAO-3
    1        SAO-3.1
    2        SAO-4.0
    3        SAO-4.1
    4        SAO-4.2
    5        SAO-4.3

If the demand for vacant Groups grows beyond the existing limit, the Data Index will have to expand and include more lines. The 80th element of the Data Index will still be used as the Version Indicator so that the reading logic will be aware of extra index lines.

Column Req. of Table 1 indicates which Groups are required to specify in a minimum content SAO-4 file. Red "x" marks indicate mandatory groups. If trace points are availble for output in the file, each trace has to be specified with at least two groups (virtual heights and frequencies) as indicated by a "xx" cyan marks.
 

Table 1. SAO Record Format
Group Req. FORTRAN Format
Description
 Reference





 
x
 2(40I3)  DATA FILE INDEX 
1
x
 16F7.3  GEOPHYSICAL CONSTANTS  Table 2
2
A120  SYSTEM DESCRIPTION AND OPERATOR'S MESSAGE
3
x
 120A1  TIME STAMP AND SOUNDER SETTINGS  Table 3,4,5
4
x
 15F8.3 SCALED IONOSPHERIC CHARACTERISTICS  Table 6,7
5
60I2  ANALYSIS FLAGS  Table 8
6
16F7.3  DOPPLER TRANSLATION TABLE 








O-TRACE POINTS - F2 LAYER
7
xx
 15F8.3  VIRTUAL HEIGHTS 
8
15F8.3  TRUE HEIGHTS 
9
40I3  AMPLITUDES 
10
120I1  DOPPLER NUMBERS 
11
xx
 15F8.3  FREQUENCIES 








O-TRACE POINTS - F1 LAYER
12
xx
 15F8.3  VIRTUAL HEIGHTS 
13
15F8.3  TRUE HEIGHTS 
14
40I3  AMPLITUDES 
15
120I1  DOPPLER NUMBERS 
16
xx
 15F8.3  FREQUENCIES








O-TRACE POINTS - E LAYER
17
xx
 15F8.3  VIRTUAL HEIGHTS 
18
15F8.3  TRUE HEIGHTS 
19
40I3  AMPLITUDES 
20
120I1  DOPPLER NUMBERS 
21
xx
 15F8.3  FREQUENCIES 








X-TRACE POINTS - F2 LAYER
22
15F8.3  VIRTUAL HEIGHTS 
23
40I3  AMPLITUDES 
24
120I1  DOPPLER NUMBERS 
25
15F8.3  FREQUENCIES 








X-TRACE POINTS - F1 LAYER
26
15F8.3  VIRTUAL HEIGHTS 
27
40I3  AMPLITUDES 
28
120I1  DOPPLER NUMBERS 
29
15F8.3  FREQUENCIES 








X-TRACE POINTS - E LAYER
30
15F8.3  VIRTUAL HEIGHTS 
31
40I3  AMPLITUDES 
32
120I1  DOPPLER NUMBERS 
33
15F8.3  FREQUENCIES





34
40I3  MEDIAN AMPLITUDES OF F ECHOES 
35
40I3  MEDIAN AMPLITUDES OF E ECHOES 
36
40I3  MEDIAN AMPLITUDES OF ES ECHOES 
37
10E11.6E1  TRUE HEIGHTS COEFFICIENTS F2 LAYER UMLCAR METHOD  Table 9
38
10E11.6E1  TRUE HEIGHTS COEFFICIENTS F1 LAYER UMLCAR METHOD  Table 9
39
10E11.6E1  TRUE HEIGHTS COEFFICIENTS E LAYER UMLCAR METHOD  Table 9
40
6E20.12E2 QUAZI-PARABOLIC SEGMENTS FITTED TO THE PROFILE Table 10
41
120I1  EDIT FLAGS - CHARACTERISTICS  Table 12
42
10E11.6E1  VALLEY DESCRIPTION - W,D UMLCAR MODEL 








O-TRACE POINTS - Es LAYER
43
15F8.3  VIRTUAL HEIGHTS 
44
40I3  AMPLITUDES 
45
120I1  DOPPLER NUMBERS 
46
15F8.3  FREQUENCIES 








O-TRACE POINTS - E AURORAL LAYER
47
15F8.3  VIRTUAL HEIGHTS 
48
40I3  AMPLITUDES 
49
120I1  DOPPLER NUMBERS 
50
15F8.3  FREQUENCIES 








TRUE HEIGHT PROFILE
51
15F8.3 TRUE HEIGHTS
52
15F8.3 PLASMA FREQUENCIES
53
15E8.3E1 ELECTRON DENSITIES [e/cm3]








URSI QUALIFYING AND DESCRIPTIVE LETTERS
54
120A1 QUALIFYING LETTERS
55
120A1 DESCRIPTIVE LETTERS
56
120I1 EDIT FLAGS - TRACES AND PROFILE Table 13








AURORAL E_LAYER PROFILE DATA
57
10E11.6E1  TRUE HEIGHTS COEFFICIENTS Ea LAYER UMLCAR METHOD  Table 9
58
15F8.3 TRUE HEIGHTS
59
15F8.3 PLASMA FREQUENCIES
60
15E8.3E1 ELECTRON DENSITIES [e/cm3]





80
-- (Reserved)

Group 1: Geophysical Constants

The values of the Geophysical Constants shown in Table 2 are specified for the station producing the data in the file. Frequencies are in MHz, angles are in degrees.

Table 2. Geophysical Constants
Position  Req
Description
1 
x
 Gyrofrequency (MHz) 
2 
x
 Dip angle (-90.0 to 90.0 degrees) 
3 
x
 Geographic Latitude (-90.0 to +90.0 degrees) 
4 
x
 Geographic Longitude East(0.0 to 359.9 degrees) 
5 
Sunspot Number for the current year 

Group 2: System Description and Operator's Message

This Group allows the user to give a description of the system which recorded the data and to store a free format text message. The Group 2 is given in A120 format, so the Data Index entry for the Group 2 counts total number of 120-character Lines of text. One text line is used to store system description; if an operator's message is given, it takes another text line. Thus, the Data Index can be 0 (no information), 1 (system description) or 2 (system description and operator's message).

The minimum contents of the System Description line should include sounder model and station IDs. To accomodate all possible station-specific information in an organized and flexible fashion, the concept of a token is introduced. System Description line is arranged in comma-separated tokens, where each token consists of a registered keyword and a data field. The first token is always the sounder model, local station ID and URSI station code number. One space character separates sounder model and IDs. Station IDs are separated by a forward slash. Local station ID is determined by host institution or sounder manufacturer. URSI station code number is assigned through World Data Center A for Solar-Terrestrial Physics, contact person Raymond O. Conkright.

For example, the System Description Line for a UMLCAR Digisonde Portable Sounder may look like this:

DPS-4 042/MHJ45, ARTIST 1297, NH 1.3, ADEP 2.19

It contains four tokens:

Thus, each item that the data support engineer needs in include into the SAO-4 System Description line has to form a token where the item is preceeded by a keyword. Another example can be given for a DISS sounder:

DISS 038/, NAME Wallops Island, WMOID HIGL BTGS 04231, ARTIST 0790, NH 1.3, ADEP 2.19

The SAO reading routine works as a simple string parser. It has to get the first word in the System Description line to identify the sounder system. Then, depending on the sounder model, it can scan the rest of the line for keywords and fill appropriate structures with corresponding data field contents. If the sounder model could not be identified, then the system Description line is used only as single text line, without analysis of individual tokens.

Group 3: Timestamp and Sounder Settings

Group 3 contains three fileds: Version Indicator, Timestamp of the measurement and a Sounder Settings. Only the first two fields are required in the minimum contents of the Group. In the minimum case, the Version Indicator should be set to AA as shown in the Table 3.

  Table 3. Minimum Contents of Group 3
Number Req. Description Possible Values
1-2.
x
 Version Indicator  AA 
3-6.
x
 4 digit Year.  (1976-...) 
7-9.
x
 Day of Year  (1-366) 
10-11.
x
 Month  (1-12) 
12-13.
x
 Day of Month  (1-31) 
14-15.
x
 Hour [All times and dates correspond to UT.]  (0-23) 
16-17.
x
 Minutes  (0-59) 
18-19.
x
 Seconds  (0-59) 

The Sounder Settings field is intended to allow users to assign codes that identify how the measurement is made with reference to particular sounders. For each particular sounder system, the format of System Preface Parameters Group must be personalized and a unique two-letter Version Indicator should be chosen to distinguish it from other sounder systems. The Version Indicator is then stored in the first two positions of the Group 3.

DPS data is represented by "FF" Version Indicator, and "FE" is allocated for Digisonde 256 data. Example formats of this Group for Digisonde Portable Sounder (DPS) and Digisonde 256 are shown in Table 4 and Table 5, respectively.
 
 

Table 4. DPS System Preface Parameters
Number Description Possible Values
1-2. Version Indicator  FF 
3-6. 4 digit Year.  (1976-...) 
7-9. Day of Year  (1-366) 
10-11. Month  (1-12) 
12-13. Day of Month  (1-31) 
14-15. Hour [All times and dates correspond to UT.]  (0-23) 
16-17. Minutes  (0-59) 
18-19. Seconds  (0-59) 
20-22. Receiver Station ID (three digits)  (000-999) 
23-25. Transmitter Station ID.  (000-999) 
26. DPS Schedule  (1-6) 
27. DPS Program  (1-7) 
28-32. Start Frequency, 1 kHz resolution  (01000 - 45000) 
33-36. Coarse Frequency Step, 1 kHz resolution  (1-2000) 
37-41. Stop Frequency, 1 kHz resolution  (01000 - 45000) 
42-45. DPS Fine Frequency Step, 1 kHz resolution  (0000 - 9999) 
46.  Multiplexing disabled [0 - multiplexing enabled, 1 - disabled].  (0,1) 
47. Number of DPS Small Steps in a scan  (1 to F) 
48. DPS Phase Code  (1-4, 9-C) 
49. Alternative antenna setup [0 - standard, 1 - alternative].  (0,1) 
50. DPS Antenna Options  (0 to F) 
51. Total FFT samples [power of 2]  (3-7) 
52. DPS Radio Silent Mode [ 1 - no transmission ]  (0,1) 
53-55. Pulse Repetition Rate (pps)  (0-999) 
56-59. Range Start, 1 km resolution  (0-9999) 
60. DPS Range Increment [2 - 2.5 km, 5 - 5 km, A - 10 km]  (2,5,A) 
61-64. Number of ranges  (1-9999) 
65-68. Scan Delay, 15 km units  (0-1500) 
69. DPS Base Gain  (0-F, encoded) 
70. DPS Frequency Search Enabled  (0,1) 
71. DPS Operating Mode [ 0 - Vertical beam, 5 - multi-beam ionogram ]  (0-7) 
72. ARTIST Enabled  (0,1) 
73. DPS Data Format [ 1 - MMM, 4 - RSF, 5 - SBF ]  (0-6) 
74. On-line printer selection [ 0 - no printer,1 - b/w, 2 - color ]  (0,1,2) 
75-76. Ionogram thresholded for FTP transfer [0-no thresholding]  (0-20, encoded) 
77. High interference condition [ 1 - extra 12 dB attenuation ]  (0,1)

 
Table 5. Digisonde 256 System Preface Parameters
Number Code Description Possible Values
1-2.
-
 Version Indicator  FE 
3-6.
-
 4 digit Year.  (1976-...) 
7-9.
-
 Day of Year  (1-366) 
10-11.
-
 Month  (1-12) 
12-13.
-
 Day of Month  (1-31) 
14-15.
-
 Hour [All times and dates correspond to UT.]  (0-23) 
16-17.
-
 Minutes  (0-59) 
18-19.
-
 Seconds  (0-59) 
20-30.
-
 Digisonde Preface Timestamp  YYDDDHHMMSS 
31.
S
 Program Set  (1-3) 
32.
P
 Program Type  (A,B,C,F,G) 
33-38.
J
 Journal  encoded 
39-44.
F
 Nominal Frequency, 100 Hz resolution  (001000 - 045000) 
45-51.
P#
 Output Controls  encoded 
52-53.
SS
 Start Frequency, 1 MHz resolution  (00-10) 
54.
Q
 Frequency Increment  (0-9,A-C,encoded) 
55-56. 
UU
 Stop frequency, 1 MHz resolution  (01-30) 
57-59.
CAB
 Test Output  encoded 
60-62.
V
 Station ID  (000-999) 
63.
X
 Phase Code  (0-F, encoded) 
64.
L
 Antenna Azimuth  (0-F, encoded) 
65.
Z
 Antenna Scan  (0-7, encoded) 
66.
T
 Antenna Option and Doppler Spacing  (0-F, encoded) 
67.
N
 Number of Samples  (1-8) 
68.
R
 Repetition Rate  (0,2-8,A,B, encoded) 
69.
W
 Pulse width and code  (0-7, encoded) 
70.
K
 Time control  encoded 
71.
I*
 Frequency correction  (0-4, encoded) 
72.
G*
 Gain correction  (0-7, encoded) 
73.
H
 Range increment  (0-3,8-C, encoded) 
74.
E
 Range start  (0-7, encoded) 
75.
I
 Frequency Search  (0-7, encoded) 
76.
G
 Nominal Gain  (0-F, encoded) 
77.
-
 Spare 0

Group 4: Scaled Ionospheric Characteristics

The Scaled Ionospheric Characteristics may be obtained by ARTIST, ADEP, some other autoscaling or editing/validating software, or typed in manually. All numbers represent either frequency in Megahertz or altitude in kilometers except as indicated in Table 6. The format F8.3 (DDDD.DDD) is used to report the characteristics which is equivalent to 1 kHz precision in frequencies and 1 m precision in heights. The accuracy of the stored values is usually 1 ionogram pixel (frequency step or height increment) except as indicated in Table 6.

There are currently 49 Scaled Ionospheric Characteristics defined. It is possible to report less than 48 characteristics and indicate that in the Data Index section of the record. Otherwise, all characteristics which are not scaled for a particular ionogram must be set to a default "No reading" value. which is 999.900 MHz for frequencies and 9999.000 km for heights.

Table 6. Scaled Ionospheric Characteristics
# Description Units Accuracy No reading
1  foF2 : F2 layer critical frequency, including the adjustment by the true height profile algorithm  MHz  at least quarter of frequency increment  9999.000
2  foF1 : F1 layer critical frequency  MHz  1 frequency increment  9999.000 
3  M(D) = MUF(D)/foF2  -  -  9999.000
4  MUF(D) : Maximum usable frequency for ground distance D  MHz  1 frequency increment  9999.000 
5  fmin: minimum frequency of ionogram echoes  MHz  1 frequency increment  9999.000 
6  foEs : Es layer critical frequency  MHz  1 frequency increment  9999.000 
7  fminF : Minimum frequency of F-layer echoes  MHz  1 frequency increment  9999.000 
8  fminE : Minimum frequency of E-layer echoes  MHz  1 frequency increment  9999.000 
9  foE : E layer critical frequency  MHz  1 frequency increment  9999.000 
10  fxI : Maximum frequency of F-trace  MHz  1 frequency increment  9999.000 
11  h'F : Minimum virtual height of F trace  km  1 height increment  9999.000 
12  h'F2 : Minimum virtual height of F2 trace  km  1 height increment  9999.000 
13  h'E : Minimum virtual height of E trace  km  1 height increment  9999.000 
14  h'Es : Minimum virtual height of Es trace  km  1 height increment  9999.000 
15  zmE : Peak height of E-layer  km  1 height increment  9999.000 
16  yE : Half thickness of E layer  km  1 height increment  9999.000
17  QF : Average range spread of F layer  km  1 height increment  9999.000
18  QE : Average range spread of E layer  km  1 height increment  9999.000
19  DownF : Lowering of F trace to the leading edge  km  1 height increment  9999.000 
20  DownE : Lowering of E trace to the leading edge  km  1 height increment  9999.000
21  DownEs : Lowering of Es trace to the leading edge  km  1 height increment  9999.000
22  FF : Frequency spread between fxF2 and fxI  MHz  1 frequency increment  9999.000 
23  FE : Frequency spread beyond foE  MHz  1 frequency increment  9999.000 
24  D : Distance for MUF calculation  km  1 km  9999.000
25  fMUF : MUF/OblFactor  MHz  1 frequency increment  9999.000
26  h'(fMUF) : Virtual height at MUF/OblFactor frequency  MHz  1 height increment  9999.000
27  delta_foF2 : Adjustment to the scaled foF2 during profile inversion  MHz  1 kHz  9999.000
28  foEp : predicted value of foE  MHz  ±0.3 MHz  9999.000
29  f(h'F) : frequency at which h'F occurs  MHz  1 frequency increment  9999.000
30  f(h'F2) : frequency at which h'F2 occurs  MHz  1 frequency increment  9999.000
31  foF1p : predicted value of foF1  MHz  ± 0.5 MHz  9999.000
32  peak height of F2 layer  km    9999.000
33  peak height of F1 layer  km    9999.000
34  zhalfNm : the true height at half the maximum density in the F2 layer  km  1 km  9999.000
35  foF2p : predicted value of foF2  MHz  ± 2.0 MHz  9999.000
36  fminEs : minimum frequency of Es layer  MHz  1 frequency increment  9999.000
37  yF2 : half thickness of the F2 layer, parabolic model  km  100 m  9999.000
38  yF1 : half thickness of the F1 layer, parabolic model  km  100 m  9999.000
39  TEC : total electron content  1016 m-2 -  9999.000
40  Scale height at the F2 peak  km  1km  9999.000
41  B0, IRI thickness parameter  km  -  9999.000
42  B1, IRI profile shape parameter  -  -  9999.000
43  D1, IRI profile shape parameter, F1 layer  - - 9999.000
44  foEa, critical frequency of auroral E layer  MHz  1 frequency increment  9999.000
45  h'Ea, minimum virtual height of auroral E layer trace  km  1 height increment  9999.000
46  foP, highest ordinary wave critical frequency of F region patch trace  MHz  1 frequency increment  9999.000
47  h'P, minimum virtual height of the trace used to determine foP  km  1 height increment  9999.000
48 fbEs, blanketing frequency of Es layer MHz 1 frequency increment 9999.000
49 Type Es - See Table 7 9999.000
Type Es is a letter characteristic which has to be reported in the Table 6 as a number using Lookup Table 7.
 

Table 7. Lookup Table for Type Es Characteristic
Type Es Value reported in Group 4
Description
A  1.0  Auroral
C  2.0  Cusp
D  3.0  below 95 km 
F 4.0 Flat 
H  5.0  Height discontinuity with normal E 
K  6.0  in the presence of night E 
L  7.0  Flat Es below E 
N 8.0 Non-standard
Q 9.0 Diffuse and non-blanketing
R 10.0 Retardation

Group 5: ARTIST Analysis Flags

The ARTIST Analysis Flags are a sequence of two digit integers (60I2 format) which indicate and qualify some of the ARTIST scaled results. Table 8 is a description of the flags and the meaning of their possible values.
 
 
 

Table 8. ARTIST Flags
Position Content
Description



1 1  foE scaled using E-region trace data 

2  No E-region trace obtained, only predicted foE available

3 No E-region trace obtained, but foE scaled using F trace



2 0  No F trace scaled 

1 E layer profile only

2 Separate soliutions for E and F layers

4 Frequency range error in E trace

5 Frequency range error in F2 trace

6 Frequency range error in F1 trace

7 Physically unreasonable E trace

8 Physically unreasonable F2 trace

9 Physically unreasonable F1 trace

10 F1 layer solution too thick

11 Oscillating solution in F1 layer

12 F2 trace too short

13 F1 trace too short

18 Oscillating solution in F1 layer

25 Root in F1 layer too severe to correct

26 Root in F2 layer too severe to correct



3    Not used



4 0  foF1 not scaled 

1  foF1 scaled



5 0  No AWS Qualifier applies 

1  Blanketing Sporadic E 

2  Non-Deviative Absorption 

3  Equipment Outage 

4  foF2 greater than equipment limits 

5  fmin lower than equipment limits 

6  Spread F 

7  foF2 less than foF1 

8  Interference 

9  Deviative absorption 



6-9   Not used



10 11-55  Confidence level: two digits, each ranging from 1 (highest confidence) to 5 (lowest confidence) 



11-19   Not used 



20   Internal ARTIST use 

Group 6: Doppler Translation Table

The Doppler Translation Table is a sequence of floating point numbers in the 16F7.3 format which convert the trace Doppler Number into a Doppler frequency in Hertz. These numbers should be read into a floating point array. Using the Doppler Number as an index to that array will result in the Doppler shift for the scaled trace point in question. The first element of the Doppler translation table corresponds to the Doppler number 0.

Trace Points

The following Groups include ionogram trace information obtained in some automated or interactive manner. The data format and content is identical for any of the F2, F1, E, or Es traces with either ordinary (O) or extraordinary (X) polarization although not all traces may be present in any one ionogram. For example, the ARTIST program currently does not scale the complete X-traces, however space has been provided for implementation of this feature at a later date.

The data for each trace are contained in five Groups. For the F2 O-trace they are in Groups 7, 8, 9, 10, and 11; for the F1 O-trace they are in Groups 12, 13, 14, 15, and 16; etc. (see Table 1). The groups for sporadic E, auroral night E leayrs and all extraordinary data groups do not contain the true height group. Also, Groups 51, 52, and 53 are reserved for an accurate representation of the electron density profile, including the valley. There is a one-to-one positional correspondence between elements in these five Groups, in that the first Virtual Height, True Height, Amplitude, Doppler Number and Frequency all correspond to the first Trace point on the ionogram. The same is true of the second point, and so on throughout the entire trace.

Autoscaling or editing software may interpolate or extrapolate missing trace points to maintain consistent frequency stepping within the trace or provide better accuracy of the scaled characteristics. Because of explicit specification of all trace point frequencies in the SAO format, the interpolated or extrapolated points may be omitted. However, in this case the value of true hieght obtained for that frequency will be missing as well. If included, the interpolated/extrapolated points shall be reported with amplitude set to 0 and Doppler number set to 9.

Groups 7, 12, 17, 22, 26, 30, 43, 47: Trace Virtual Heights

This group consists of a number of Virtual Heights in 15F8.3 format for the layer indicated. The number of these heights depends upon the length of the trace on the corresponding ionogram. Virtual Heights are reported in kilometers of altitude.

Groups 8, 13, 18: True Heights

This group consists of a number of True Heights in 15F8.3 format for the layer indicated. The number of these heights depends upon the length of the trace on the corresponding ionogram (compare to complete profiles specification in Groups 51-53). True Heights are reported in kilometers of altitude. Virtual heights of 0 km can be present in this group as "no-value" filler of missing trace points added to preserve continuous frequency stepping.

Groups 9, 14, 19, 23, 27, 31, 44, 48: Trace Amplitudes

The amplitude in dB of each trace point is recorded in 40I3 format.

Groups 10, 15, 20, 24, 28, 32, 45, 49: Trace Doppler Numbers

The Doppler Number, as measured by the Digisonde, for each trace point is recorded here in 120I1 format. To convert this number to an actual Doppler shift in Hertz, use this integer as the index to the Doppler Translation Table provided in Group 6. Index for 8 element Doppler Translation Table runs from 0 to 7. Value 9 is reported for interpolated or extrapolated points where information about Doppler frequency shift is unavailable.

Groups 11, 16, 21, 25, 29, 33, 46, 50: Trace Frequencies

The frequency (in MHz) of the trace point is given in this Group in the 15F8.3 format. Originally, this Group was provided for the possibility of uneven frequency stepping and would normally be left empty for Digisonde ionograms with a constant frequency step. This is no longer acceptable. The sounder settings which are required to restore linear step frequencies can be obtained only from a valid Sounder Settings Group 3 and Scaled Characteristics Group 4 and may appear to be missing for some sounder systems.

Group 34: Median Amplitude of F Echo

These values are an amplitude in dB for the F trace. It is calculated every integer MHz between fminF and foF2. See Code 4 for fmin and foF2. The Median Amplitude is calculated by taking the median of the trace amplitudes over a 0.5 MHz in frequency by five height range rectangle and then scaling this median value to appear as if it were at 100 km altitude.

Group 35: Median Amplitude of E Echo

Same as per Code 34, but for the E echo between fminE and foE.

Group 36: Median Amplitude of Es Echo

Same as per Code 34, but for the Es echo between fminE and foEs.

Group 37: True Height Coefficients for the F2 Layer

The True Height Data for F2 layer from the UMLCAR method are stored in the E11.6E1 format. There are up to 10 elements. The meaning of each element is given in Table 9.

Table 9. True Height Coefficients
Position Parameter
Description
1  fstart  Start frequency (MHz) of the F2 layer 
2  fend  The end frequency of the F2 layer 
3  zpeak  The height of the peak of the F2 layer 
4  dev  The fitting error in km/point. 
5-9  A0-A4  Shifted Chebyshev polynomial coefficients 
10  zhalfNm  Height at half peak electron density 

Group 38: True Height Coefficients for the F1 Layer

The True Height Data for the F1 layer from the UMLCAR method have the same format as those for the F2 layer (Group 37) above with the exception of zhalfNm (see Table 9).

Group 39: True Height Coefficients for the E Layer

The True Height Data for the E layer from the UMLCAR method have a format very similar to that for the F2 and F1 layers (Codes 37 and 38) above. The difference lies in that there are only seven elements stored in this Group. The first four parameters are fstart, fend, zpeak and dev as defined for the F2 layer. There are, however, only three coefficients for the shifted Chebyshev polynomials (A0 - A2) for the E layer true height.

Group 40: Quazi-Parabolic Segments Fitted to the Profile

An arbitrary number of parabolic segments may be fitted to the profile to approximate its shape. Each segment can be expressed as: where If n segments are fitted to the profile, the Group 40 will contain n+1 entries.  The first n entries store 6 values per segment (R1, R2, A, B, C, and fitting error E) in the E20.12E2 format, and the last lines contains the Earth radius, as is shown in Table 10.
Table 10. QP Segments reported in Group 40
# Value 1 Value 2 Value 3 Value 4 Value 5 Value 6
1 R11 R12 A1 B1 C1 E1
2 R21 R22 A2 B2 C2 E2
...





n Rn1 Rn2 An Bn Cn En
n+1 Re
-
-
-
-
-

The Earth radius, Re,  is the actual value used in the fitting process and is given in SAO file to ensure proper restoring of the profile shape.
 

Group 41: Edit Flags: Characteristics

The edit flags are written in 120I1 format and are used to indicate whether the reported ionospheric characteristics are result of autoscaling, manual input, or long-term prediction. One edit flag is a sum of three indicators, EDITED(1), PREDICTED(2) and VALIDATED(4). Table 11 shows possible combinations of the indicators.
 
Table 11. Edit Flag (characteristics) and its possible meanings
EDITED PREDICTED VALIDATED EDIT FLAG VALUE
Description 
0
0
0
0+0+0 = 0
 autoscaled value
0
0
4
0+0+4 = 4
 autoscaled value, validated by an operator
1
0
4
1+0+4 = 5
 manually specified value; the autoscaled value was incorrect or unavailable 
0
2
0
0+2+0 = 2
 long-term prediction

The position in the edit flag list corresponds to the order of the characteristics listed in Table 6. A complete list is given in Table 12. The edit flags may be used to set the slash (/) indicators in the URSI-IIWG characteristics database, if the indicators are not given in the Groups 54-55.

Table 12. Edit Flags: Characteristics
# Scaled Characteristic
Description 
1  foF2  F2 layer critical frequency 
2  foF1  F1 layer critical frequency 
3  M(D)  M-factor, MUF(D)/foF2, for distance D 
4  MUF(D)  Maximum usable frequency for distance D 
5  fmin  Minimum frequency for E or F echoes 
6  foEs  Es layer critical frequency 
7  fminF  Minimum frequency of F-trace 
8  fminE  Minimum frequency of E-trace 
9 foE  E layer critical frequency 
10  fxI  Maximum frequency of F-trace 
11  h'F  Minimum virtual height of F trace 
12  h'F2  Minimum virtual height of F2 trace 
13  h'E  Minimum virtual height of E trace 
14  h'Es  Minimum virtual height of Es layer 
15  HOM  Peak of E layer using parabolic model 
16  Ym  Corresponding half thickness of E layer 
17  QF  Average range spread of F-trace 
18  QE  Average range spread of E-trace 
19  Down F2  Lowering of F-trace maximum to leading edge 
20  Down E  Lowering of E-trace maximum to leading edge 
21  Down Es  Lowering of Es-trace maximum to leading edge 
22  FF  Frequency spread between fxF2 and fxI 
23  FE  As FF but considered beyond foE 
24  D  Distance used for MUF calculation 
25  fMUF(D)  MUF(D)/obliquity factor( 
26  h'MUF(D)  Virtual height at fMUF 
27  foF2c  correction to add to foF2 to get actual foF2 
28  foEp  Predicted foE 
29  f(h'F)  Frequency at which hminF occurs 
30  f(h'F2)  Frequency at which hminF2 occurs 
31  foF1p  Predicted foF1 
32  Zpeak  Peak height F2 layer 
33  ZpeakF1  Peak height F2 layer 
34  zhalfnm  Height at half peak electron density 
35  foF2p  Predicted foF2 
36  fminEs  Minimum frequency of Es layer 
37  YF2  Half-thickness of F2 layer in parabolic model 
38  YF1  Half-thickness of F1 layer in parabolic model 
39  TEC  Total electron content 
40  HscaleF2  Scale height at F2 peak 
41  B0  IRI thickness parameter 
42  B1  IRI profile shape parameter 
43  D1  IRI F1 profile shape parameter 
44  foEa  Critical frequency of auroral E layer 
45  h'Ea  Minimum virtual height of auroral E layer trace 
46  foP  Highest ordinary wave critical frequency of F region patch trace 
47  h'P  Minimum virtual height of the trace used to determine foP 
48 fbEs Blanketing frequency of Es layer
49 Type Es Type of Es layer

Group 42: Valley Characteristics UMLCAR model

The current content for this Group is two parameters describing the width and depth of the valley region in the UMLCAR model.

Group 51-53: Regular True Height Profile

The complete true height profile of electron density up to 1000 km is given here, including all layers and the valley. The profile is reported with the true height as the argument of the N(h) function, i.e. all heights within the valid range are scanned with a fixed increment, say, 1 km, and put in Group 51. Corresponding frequencies and electron densities are given in Group 52 and Group 53. Also, a few additional height points are reported in the groups: all peak heights of the layers and the starting height of the profile. The additional points might not be multiples of the height increment. One-to-one positional correspondence of individual elements in Groups 51-53 is preserved, so that, for example, the first element of Groups 51-53 refers to the starting height of the profile.

The height increment and coverage for the profile specification is determined by the program which created the SAO file.

Group 54-55: Qualifying and Descriptive Letters

These two groups store URSI Qualifying (Group 54) and Descriptive (Group 55) letters [URSI Handbook of Ionogram Interpretation and Reduction, 1972] using 120A1 format. The letters are used by manual scaling operators to reflect reliability of measurement and indicate the presence of certain ionospheric phenomena. The layout of the Groups 54-55 corresponds to Table 6 (Scaled Ionospheric Characteristics). The number of items stored in the Groups 54 and 55 must be the same as in Group 6.

When no qualifying or descriptive letter is applied to a characteristic but its value has been verified or edited, the correspoding entry in the Group 54 should read "/" (forward slash) and Group 55 should read " " (space) [see IIWG regulations, Table 3, here]. For autoscaled data, the IIWG regulations suggest storing "/" in both groups, but SAO-4 file created by the autoscaling software may simply omit Groups 54 and 55 and report only Group 41 (Edit Flags).

Group 56: Edit Flags: Traces and Profile

The edit flags are written in 120I1 format and correspond to whether ionogram traces and profile were modified as a result of manual scaling of the data. Autoscaling software must not report this group to distinguish it from the manual editing/validating. If no trace points were adjusted and profile was not recalculated in the process of manual editing/validation, the Group 56 must still be reported with all zero settings to distinguish it from autoscaled data.

Table 13. Edit Flags: Traces and Profile
# Name
Description 
1  F2 trace  F2 trace points were edited 
2  F1 trace  F1 trace points were edited 
3  E trace  E trace points were edited 
4  z(h)  true height was recalculated with edited traces 
5  Es trace  Es trace points were edited 

Group 57: True Height Coefficients for the Ea Layer

The True Height Data for the E auroral layer from the UMLCAR method have a format identical to Group 39 for E layer above.

Group 58-60: Auroral True Height Profile

The complete true height profile of electron density up to 1000 km is given here, including all layers and the valley. The profile is reported with the true height as the argument of the N(h) function, i.e. all heights within the valid range are scanned with a fixed increment, say, 1 km, and put in Group 58. Corresponding frequencies and electron densities are given in Group 59 and Group 60. Also, a few additional height points are reported in the groups: all peak heights of the layers and the starting height of the profile. The additional points might not be multiples of the height increment. One-to-one positional correspondence of individual elements in Groups 58-60 is preserved, so that, for example, the first element of Groups 58-60 refers to the starting height of the profile.

References

Gamache R. R., I.A. Galkin,  and B. W. Reinisch, "A Database Record Structure for Ionogram Data", University of Lowell Center for Atmospheric Research, UMLCAR 96-01, 1996.

URSI Handbook of Ionogram Interpretation and Reduction. Ed. W.R.Pigott and K.Rawer. WDC-A for STP, 1972.


Last Modified: September 22, 2006