Acronyms, Definitions and Forecast Descriptions

Acronyms

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Definitions

• 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:
• DOWN (less than 80 percent of normal),
• NORMAL
• UP (more than 120 percent of normal)



• 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:

  • X flares
  • SSCs
  • High or Mid planetary A indices
  • High K-Index




• 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.

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: 27th March 2006.
Copyright BAE SYSTEMS 2006, all rights reserved.