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URSI Qualifying and Descriptive Letters for Ionospheric Parameters

Format

A five symbol format is used throughout. In all tables except Es types the first three symbols give the numerical value of the parameter. The fourth symbol is a qualifying letter and the fifth is a descriptive letter. Qualifying letters are used to indicate the nature or degree of uncertainty in a value. They are always accompanied by a descriptive letter with the possible exception of the median value. Descriptive letters are used to indicate the presence of a phenomenon or the reason for the use of a qualifying letter. Where only a descriptive letter appears in the five symbol group, a numerical value has not been possible within the limits imposed by the qualifying letters.

Qualifying Letters

A   Less than. Used only with fbEs.
D   Actual value greater than the given numerical value by between 5 and 20%.
E   Actual value less than the given numerical value by between 5 and 20%.
I   Missing value replaced by an interpolated value.
J   Ordinary component deduced from the extraordinary component.
M   Interpretation of measurements questionable because ordinary and 
    extraordinary components are not distinguishable.
O   Extraordinary component deduced from ordinary component. Used for x 
    characteristics only.
T   Actual observation is inconsistent or doubtful. The value has been 
    determined from a sequence of observations.
U   Doubtful value with an uncertainty of between 2 and 5%.
Z   Measurement deduced from the third magneto-ionic component (Z).

Descriptive Letters

A   Measurement influenced by, or impossible because of, the presence of a 
    lower, thin layer. For example, Es.
B   Measurement influenced by, or impossible because of, absorption near fmin.
C   Measurement influenced by, or impossible because of, any non-ionospheric 
    reason. For example equipment failure, local transmitters, rain and snow 
    static.
D   Measurement influenced by, or impossible because of, the upper limit of 
    frequency range recorded.
E   Measurement influenced by, or impossible because of, the lower limit of 
    frequency range recorded.
F   Measurement influenced by, or impossible because of, the presence of spread
    echoes.
G   Measurement influenced by, or impossible because of, the electron density 
    being too small for accurate observation.
H   Measurement influenced by, or impossible because of, the presence of 
    stratification.
K   Indicates particle E layer present.
L   Measurement influenced by or impossible because the trace has no 
    sufficiently definite cusp between layers.
M   Measurement influenced or impossible because the ordinary and extraordinary
    components are not distinguishable.
N   Measurement influenced or impossible because of conditions which cannot be 
    interpreted.
O   Measurement refers to the ordinary component.
P   Man-made perturbations of the observed parameter; or spur-type spread-F 
    present. 
Q   Range spread present.
R   Measurement influenced by, or impossible because of, attenuation near a 
    critical frequency.
S   Measurement influenced by, or impossible because of, atmospheric or 
    broadcast interference.
T   Actual observation is inconsistent. The value has been determined from a 
    sequence of observations.
V   Measurement influenced by the presence of a forked trace.
W   Measurement influenced or impossible because the echo lies outside the 
    height range recorded.
X   Measurement refers to the extraordinary component.
Y   Measurement influenced by, or impossible because of, the presence of 
    lacuna or a severe F layer tilt.
Z   Indicates the presence of the third magneto-ionic component (Z).
The following descriptive letters are used to show spread-F types and take precedence over all other letters:
F   Frequency spread present. Is used in foF2 and fxI tables only.
L   Mixed spread present. foF2 and fxI tables only.
P   Polar spur present. Is used in fxI table only.
Q   Range spread present.  Is used mainly in h'F and h'F2 tables, but appears 
    occasionally in foF2 and fxI tables.

Es Type Tables

In the Es type table, the first letter denotes the type of layer from which foes has been evaluated. This letter is followed by the number of multiple reflections from the layer. Other Es layers are recorded in order of number of multiples.

Es Types

F   A clean Es trace which shows no appreciable increase of height with 
    frequency. Applied only to night-time Es.
L   A flat Es trace below the normal E or particle E minimum virtual height. 
    Mainly applied to Es in the daytime.
C   An Es trace showing a relatively symmetrical cusp at or below the critical 
    frequency, foE.  Applied only in the daytime.
H   An Es trace showing a discontinuity in height with the normal E layer 
    trace. The cusp is not symmetrical, the height of the low frequency part 
    of the Es trace is higher than that of the high frequency normal E layer 
    trace.  Applied only in the daytime.
Q   An Es trace which is diffuse and non-blanketing over a wide frequency range.
R   An Es trace showing an increase in virtual height at the high frequency 
    end but which becomes partially transparent below foEs.
K   Denotes the presence of a particle E layer, similar in appearance to normal
    E, which obscures higher layers up to its critical frequency.
A   Denotes all types of very spread Es traces. The typical pattern shows a 
    well-defined flat or gradually rising lower edge with stratified or diffuse
    traces present above it.
S   A diffuse Es trace whose virtual height rises steadily with frequency.
D   A weak diffuse trace at or below 95 km associated with high absorption and 
    consequently high fmin.
N   An Es trace which does not fall into any of the standard categories above.