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Definitions
A Index
classification categories
A
≤ 7 category is QUIET
A
≥ 8 and A ≤ 15 then category is UNSETTLED
A
≥ 16 and A ≤ 30 then category is ACTIVE
A
≥ 31 and A ≤ 50 then category is MINOR STORM
A ≥ 51
then category is MAJOR STORM
DIFS
A BAE Systems Advanced Technology Centre HF Forecasting
system, which provides HF and SATCOM operators with daily
forecasts of predicted ionospheric conditions. The system uses
in-house algorithms and an externally developed model (GISM) to
generate HF and SATCOM forecasts.
GISM
A
program developed by IEEA under contract with ESA, which models
scintillation that occurs in the ionosphere due to solar
activity. It is composed of two models, one providing the mean
errors, based on a solution of ray tracing equations. The second,
which is used in DIFS is based on a multiple phase screen (MPS)
technique and provides estimates of the S4 index.
GISM
Scintillation Chart Categories
S4
≤ 0.25 then category is QUIET
S4
> 0.25 and S4 ≤
0.5 then category is MODERATE
S4
> 0.5 and S4
≤ 1 then category is DISTURBED
S4
>1 then category is SEVERE
S4Index
The
signal scintillation, which is the root-mean-square of the power
P divided by the average power , see equation below[1].
WinHF
A
BAE Systems Advanced Technology Centre HF propagation prediction
tool, based on cyclic space weather trends, diurnal, seasonal and
geographic information and the ITU-R REC533 algorithms, which
helps to establish and maintain good communications. It may be
fed with daily inputs of ICF, obtained from the DIFS, which
thereby modifies the model on a daily basis to take into account
current ionospheric conditions.
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HF
Forecast Interpretation
The
message is divided into a summary of past conditions followed by
the forecast of future developments. These main sections
have the following components:
Past
Summary
Past summary information applies to the 24
hour period starting and ending at the times and dates indicated.
MUF
variations are deduced from up to six ionospheric sounding
stations in the area of interest and expressed as a percentage
deviation of the last 24 hours of foF2 data from the median foF2
data. This is spilt into 6 hourly time blocks and for each one
an overall MUF condition is determined based on a set of
thresholds, where UP, NORMAL and DOWN indicate whether the
frequencies are going to be greater than, the same as or less
than the median conditions, Normal day-to-day fluctuations in
the ionospheric f2-region are in the range (80-120) percent.
MUF depressions give rise to operational difficulties.
ICF
is a scaling factor produced by DIFS, derived from the
percentage deviations over the last 24 hours of foF2 from the
median values. It indicates the daily ionospheric conditions and
can be used in WinHF, a propagation prediction tool, to scale
assigned frequencies. Under normal conditions the value of the
ICF is defined to be 10, in geomagnetic storms this can drop to
as low as 6 or in very good conditions can be as much as 14
LUF
depends on the users’ radio equipment and link, and
ionospheric conditions. It is based on the sum of the K Indices
for the day before the forecast, the current month number and
any strong absorption events that may have occurred. A
qualitative indication (DOWN, NORMAL, UP) of LUF variations is
given. Only daytime LUF is described as night-time LUF is much
less variable.
SWF
are caused by intense short-lived (typically 20 minutes) periods
of abnormally high signal attenuation. They are caused by
solar flares. Only daylit circuits are affected. Short wave
fadeouts are most common in sunspot maximum years. The past
summary lists these events by date/time groups. Even those
occurring in the European night are reported.
Geomagnetic
Activity is related to ionospheric behaviour and HF skywave link
performance. Magnetic disturbances are associated with periods
of circuit 'outage'. The past summary reports geomagnetic
activity in terms of a daily index called 'A', the Sunspot
Number Index and 10cm Flux Value.
A
Index is taken from external data sources and varies in the
range 0 to 400. The A index is displayed with a qualitative
descriptor, where as follows:
A
≤ 7 category is QUIET
A
≥ 8 and A ≤ 15 then category is UNSETTLED
A
≥ 16 and A ≤ 30 then category is ACTIVE
A
≥ 31 and A ≤ 50 then category is MINOR STORM
A
≥ 51 then category is MAJOR STORM
Sunspot
Number Index is taken from the values
reported by external data providers.
A plain
language summary of conditions completes the past summary.
Forecast
Forecast
information applies to the 24 hour period after the past summary
period, starting and ending at the times and dates indicated.
MUF
variations for four 6 hour time blocks are forecast
on a three-point scale:
The
categories are based on the observed MUF conditions, which are
then translated to a score and modified to take into account the
occurrence of cyclic solar effects, SSCs, very high or low SSN
and 10cm flux deviations, X-ray flares and geomagnetic Ap and K
indices.
ICF
is derived from the percentage
deviations over the last 24 hours of foF2 from the median
values. Then as number of correction factor algorithms are
applied to take into account of other factors such as:
Daytime
LUF is forecast on a qualitative three-point scale (DOWN,
NORMAL, UP)
It
is helpful to know the likelihood of experiencing a SWF.
This is given as a percentage SWFs probability based on a
consideration of solar activity. For example a forecast of ten
percent indicates that during the next 24 hours there is a one
in ten chance of experiencing one or more SWFs in Western
Europe.
A
indices are displayed for the forecast date and the
following two GMT days, and are taken directly from an external
data source. A descriptive comment accompanies the values
(determined by the thresholds as shown in the Past Summary) to
explain the conditions which are predicted to occur.
A plain
language description completes the forecast information.
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SATCOM
Forecast Interpretation
The
SATCOM forecast comprises of one section which shows the next 24
hours predictions of ionospheric conditions. It is split into
four 6-hour periods and displays a category for Polar, Mid and
Equatorial regions.
Below the
predictions is an explanation of the categories as follows:
Q
- Quiet. Low probability of fading expected on SATCOM Circuits
M
- Moderate. Moderate fading possible on circuits
D
- Disturbed. Reasonably high fading probable on circuits
S - Severe.
High probability of outages
Eight
3 hourly K Indices are displayed covering the past 24
hours and vary in the range 0 – 9 where values grater than
5 indicate high geomagnetic activity. These are taken from
external data sources.
A
Index is taken from external data sources and varies in the
range 0 to 400. The A index is displayed with a qualitative
descriptor, where as follows:
A
≤ 7 category is QUIET
A
≥ 8 and A ≤ 15 then category is UNSETTLED
A
≥ 16 and A ≤ 30 then category is ACTIVE
A
≥ 31 and A ≤ 50 then category is MINOR STORM
A ≥ 51
then category is MAJOR STORM
Predicted
A Indices are displayed for the forecast date and the
following two GMT days, and are taken directly from an external
data source.
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[1]
Briggs, B.H.
and Parkin, J.A., On the Variation of Radio Star and Satellite
Scintillation with Zenith Angle, J. Atmosphere. Terr. Phys., 40,
p713, 1963.
For more details
on receiving the Daily Ionospheric Forecast, the SATCOM forecast
or the HF propagation prediction program (WINHF) please contact:
Nigel Wheadon (Tel: +44 (0)1245 242143)
Site Owner:
nigel.wheadon@baesystems.com
Date
Last Modified: 15th October 2008.
Copyright BAE SYSTEMS 2008,
all rights reserved.
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