Home' RTCA Documents for Review : DO-220A Change 1 Contents Appendix A
DO-220A Change 1
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Number of Database Points
The NASA microburst data is provided with a fine grid structure. The equipment
manufacturer need not use every data base point for each iteration of the weather signal
computation, but use enough to show sufficient (i.e. convergent) database representation.
If some data points are not used, describe how the NASA database is used in the
calculation, including the number of database points accessed in computation, and
inaccuracies that may be apparent from any approximations.
Standard Deviation of Rain Velocities Calculated by ADWRS
To be consistent with nature, a lower limit on the calculated rain standard deviation is
required. A reasonable value of 3 m/s is given in Appendix C.
Radar System Manufacturer Arguments in Support of Superposition
In general, it is good design practice that pulse Doppler radars exhibit linear, stable, and
spurious-free operation in performance of their function as a measurement system. Such a
radar system would be a very nearly perfect linear radar sensor and would include all
effects of platform motion and/or near departure from linearity. The effects of any non-
linearity will be included in the recorded clutter data.
The use of the NASA weather wind database allows a calculation of intercepted power, as
calculated from first principles of the radar range equation. Superposition of the windfield
data with recorded clutter raises some issues that need to be satisfactorily addressed by the
applicant using the prescribed test method. These issues include the potential generation of
spurious signals when a large windfield signal is superimposed with clutter and an
accounting of the appearance of the windfield signal in the digital processes.
In general, mainbeam clutter would be expected as the largest signal in the system and the
recording of it and any products excited by it would be sufficient. However, it remains to
account for the target signal if the weather signal is also very large. The radar system
manufacturer is expected to furnish an analytical and experimental account of linearity,
etc. sufficient to show that weather signals may be calculated from first principles (i.e.
radar range equation).
Superposition of a calculated weather signal return with a recorded clutter (moving clutter
such as cars and stationary clutter such as buildings) or interference signal requires an
account of how the weather signal parameters (Doppler and reflectivity spatial distribution)
will be used to represent the amplitude and phase of that computed signal with reference
to the equipment under test. It is necessary to state what the Doppler offset and the
amplitude conversion scales are for such signals in the presence of the recorded signals
(i.e. an accounting of the effects of aircraft motion).
The use of the NASA weather-wind database allows a calculation of intercepted power, as
calculated from the first principles of the radar range equation. Describe the (scaling)
conversion of that intercepted power into machine quantized (sampled data) input word
values, for example, at the digital word converter or the spectrum analyzer filter bank, in
the presence of large or saturating signal levels.
The calculated weather/hazard return signal is summed with the recorded clutter data prior
to envelope detection or other non-linearity in the digital processor. If the clutter data is
not merged at the same point in the digital process that it was collected, validate any
interleaving processes on it, e.g. FFT.
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