Geoscience Reference
In-Depth Information
Fig. 3. Block diagram of the receiver (without signal processing part) and the transmitter.
3.1 Digital receiver
The analogue signal
is sampled at a rate of 71.9 MHz producing a
stream (time
t
i
)
of 14 bit numbers. These are multiplied (Fig. 4) with sin(
ω
if
t
i
) and cos(
ω
if
t
i
)
and digitally filtered to obtain the base band I and Q components (at times
t
k
). Although
the nominal short pulse duration is 1.57 μs same as sample spacing in range, 155 samples
spaced at ~ 13.8 ns over 2.15 μs interval are used for multiplication and filtering (in the
long pulse mode the number of samples is 470 over a 6.53 μs interval). The digital low
pass filter is adjusted to match the shape of the transmitted long or short pulse. Matching
is achieved by passing the attenuated transmitted pulse (”burst”) through the receiver
and taking the discrete Fourier transform of the output. The inverse of this transform
gives the coefficients of the matched impulse response filter. Amplitude and phase of the
“burst” is sampled upon each transmission to monitor power, compensate for phase
instabilities, and use in phase codes for mitigating range ambiguities. The timing diagram
(Fig. 5) illustrates the relations between transmitted sequence, digital oscillator samples,
the sampled sequence from a point scatterer and its I and Q values (after the matched
filter).
At
()cos(
t
t
)
if
d
3.2 Transmitted sequences and volume scans
Several volume coverage patterns are available. With the exception of one all utilize the
short pulse. The exception has a uniform sequence of long pulses at the longest PRT for
observations in clear air or snow where weak reflections are from insects, birds, ice and/or
refractive index fluctuations. For storm observations the volume coverage patterns have
three distinct modes depending on the elevation.
