Vaisala RVP900, User Manual

Page: 234 / 484

�
� + 1
− �
�
=
∫
� + 1 ��
� + 1 �� + ����
�
0
+ �� ⅆ� −
∫
���
��� + ����
�
0
+ �� ⅆ� = ��
�
�
���
which is independent of both
T
o
and
n
. Thus, a linear FM input signal produces a fixed (
I
,
Q
)
phase difference from pulse-to-pulse at any given range. The magnitude of the phase
difference is proportional to the range, and the slope is(
M
Ʈ
s
) cycles for each second of
delay in range. For example, if the test signal generator is sweeping 100 KHz every two
seconds, then the velocity observed at a range of 300 km at 250 Hz PRF is:
�
� + 1
− �
�
= 100���
2sec
× 1
250sec × 300�� ×
6.6�sec
1��
= 0.40 ������
We would thus observe a velocity of (0.8 ×
Vu
) at 300 km, where
Vu
is the unambiguous
Doppler velocity in meters/sec. Note that these phase difference calculations have made no
assumptions about the RVP900 processing mode, and thus are valid in all major modes
(PPP, FFT, DPRT, RPH), as well as in all Dual-PRF unfolding modes.
This simple FM signal generator also produces valid second trip velocities that can be seen
during Random Phase processing. This follows from the above analysis because we've never
assumed that
Ʈ
bin was smaller than
Ʈ
s, that is, it is fine for the range bin to be located in
any higher-order trip.
7.10.2 Verifying PHIDP and KDP
The PHIDP and KDP processing algorithms can be tested using CW signal sources at IF.
In the alternating-transmitter single-receiver case, a single FM signal generator is modulated
with an RVP900 polarization select line so that slightly different frequencies are generated
for the H and V pulses. A maximum FM depth of several kilohertz is all that is required.
In the dual-receiver case, two (unmodulated) signal generators are used for each of the H
and V intermediate frequencies, and one or the other is detuned slightly from its correct
center frequency.
In either case the frequency difference that produces a KDP value of 1.0 degree/km is:
1.0deg ���
��
× 1
360
������
deg ��� × 299792
��
sec ��� = 833
������
sec ���
7.10.3 Verifying RHOH, RHOV, and RHOHV
These terms measure the normalized cross-channel covariance in a polarization radar. They
all are computed having the form:
����� =
< �
�
�
�
�
�
* >
< �
�
2
�
�
2
>
Where the �
�
�
and �
�
�
are complex (
I
,
Q
) vectors from two receiver channels A and B, and
"<>" denotes expected value. This suggests that some form of amplitude modulation (AM)
of the input signal might be helpful.
RVP900 User Guide M211322EN-J
232