Vaisala RVP900, User Manual

Page: 180 / 484

7.2.5 Interference Filter
The interference filter is an optional processing step that can be applied to the raw (
I
,
Q
)
samples from the FIR filter chips.
The filter can remove strong but sporadic interfering signals that are occasionally received
from nearby man-made sources. The technique relies on the statistics of such interference
being noticeably different from that of weather.
For each range bin at which (
I
,
Q
) data are available, the interference filter algorithm uses
the received power (in dB) from the 3 most recent pulses. This 3-pulse algorithm is only
intended to remove interference that arrives on isolated pulses, and for which there are at
least 2 clear pulses in between. Interference that tends to arrive in bursts are not rejected.
�
� − 2
, �
� − 1
, ��� �
�
where:
�
�
= 10log
10
�
�
2
+ �
�
2
If the 3 pulse powers have the property that:
�
� − 1
− �
� − 2
< �
1
��� �
� − 1
− �
� − 2
> �
2
( Alg. 1 )
1)
then (
I
n
,
Q
n
) is replaced by (
I
n-1
,
Q
n-1
). Here
C
1
and
C
2
are constants that can be tuned by
the user to match the type of interference that is anticipated, and the error rates that can be
tolerated. For some environments, it is possible that good results can be obtained with
C
1
=
C
2
; but RVP900 does not force that restriction.
Two variations on the fundamental algorithm are also defined. The CFGINTF command
allows you to choose algorithms to use, and to tune the 2 threshold constants. You may also
do this from the
Mp setup menu. See 8.24 Configure Interference Filter (CFGINTF)
(page 310) and 5.2.4 Mp — Processing Options (page 107) .
�
� − 1
− �
� − 2
< �
1
��� �
�
− �
� − 1
> �
2
( Alg. 2 )
�
� − 1
− �
� − 2
< �
1
��� �
�
− ������ �
� − 1
, �
� − 2
> �
2
( Alg. 3 )
Where
LinAvg()
denotes the decibel value of the linear average of the two decibel
powers. The
Alg.2
and
Alg.3
algorithms also include the receiver noise level(s) as part of
their decision criteria. When power levels are compared in the algorithms, any power that is
less than the noise level is first set equal to that noise level. This makes the filters more
robust and properly tunable, so that interference is more successfully rejected on top of
blank receiver noise.
1) The JMA internal specification for Interference Filter algorithm for use on Chitose airport Doppler weather radar is the basis for
Alg.1
RVP900 User Guide M211322EN-J
178