Windshear and microburst conditions exist when the data generated by the system exceeds
the airport-specific threshold values for possible windshear conditions. The data is based on
horizontal wind speed and wind direction measurements. Windshear and microburst alerts
are generated when the calculated windshear conditions exist.
The following paragraphs briefly describe the operation of the formulas used to calculate
and present windshear conditions:
Edge divergence is calculated by making the dot product of the edge direction vector and
the velocity differential between the endpoints of the edge. Triangle divergence is
calculated by computing the derivative, in the x and y directions, of the linear interpolation
functions for the wind components, based on the wind at the vertices of the triangle.
These values are compared to airport-specific thresholds to determine if persistent
windshear conditions exist. If this is the case, the headwind loss estimate and the location of
the first encounter is calculated and a microburst alert is generated. Typically, the limit for a
microburst alert is a 30
‑
knot loss. If headwind loss or gain is less than the microburst limit
(typically 30 knots), but more than windshear limit (typically 15 knots), a windshear alert is
issued.
In cases where there are multiple windshear condition situations, the cases are arbitrated
and only the most hazardous alert for that runway is issued. The arbitration order is:
1. Microburst
2. Windshear with large loss
3. Windshear with large gain
4. Windshear with small loss
5. Windshear with small gain
In these cases the location reported is the location of the first encounter with any of these
phenomena.
2.9 Regulatory Compliances
The AviMet systems are designed according to the recommendations of international air
traffic authorities as presented in the following sources.
The algorithm used for calculations in the Vaisala AviMet
â
Low-Level Windshear Alert
System (LLWAS) is the latest version of the phase III windshear algorithm developed for the
U.S. Federal Aviation Administration (FAA) by UCAR:
• United States Patent number 5,221,924:
Wind shear alert system
, W. Wilson, UCAR, CO
• United States Patent number 5,208,587:
Low level wind-shear alert system
, L. Cornman,
UCAR, CO
• FAA LLWAS-III Specification
The wind sensors of the LLWAS system are positioned and implemented in accordance with
the M.I.T., Lincoln Laboratories and the National Center for Atmospheric Research system
design guidelines specified in:
•
Siting Guidelines for Low Level Windshear Alert System (LLWAS) Remote Facilities
. FAA
Document Nr 6560.21A.
Low-Level Windshear Alert System User Guide
M210885EN-K
14
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