IEEE Geoscience and Remote Sensing Magazine - March 2015 - 14

dar signal passband. The
radar electronics design includes supplemental telemetry functions for flagging
pulsed RFI:
Channel Center Freq.
1) The RF/digital receiver
Spacing Dfch = 1.5 MHz
design includes "receive
power monitor" telemetry
circuits for detecting RFI
f (MHz)
RF Center
outside of the IF passband
Frequency f0
f0(max)
which is not obviously
f0(min)
1297.75
identifiable in the detect1217.25
MHz
ed output, but nonetheMHz
Noise Bandwidth
less causes compression
Bn = 1.2 MHz nom
errors in the echo power
measurement. An alarm
bit is set in the radar sciVideo Signal Bandwidth
B0 = 4.2 MHz
ence data packet on a PRIby-PRI basis whenever
strong out-of-band interf (MHz)
ference causes gain compression in the front-end
RF amplifiers, upstream
of the IF filter.
2) The digital design also
includes an A/D convertFIguRE 3. SMAP radar frequency plan and channel spacing. Transmit V- and H-polarized echo
er saturation-flag alarm.
measurement channels are tuned together at 3.0-MHz fixed frequency spacing, over an absolute
This telemetry bit is set
tuning range of 1217.25-1297.75 MHz. The tuning step resolution is 1.25 MHz, yielding 63 unique V/H
whenever strong in-band
frequency pairs. A "noise-only" channel in-between echo channel frequencies is also filtered and
RFI causes one or more
power-detected in the digital electronics for noise subtraction processing.
voltage samples to be
clipped
in
the
receive
window
of a given range line.
revolution - or one frequency update for every 22.5° azi3) Noise-only power measurements in the digital processor
muth segment in the conical scan.
are likewise accumulated on PRI time scales (~350-us
The receiver chain functional implementation is shown
intervals) to facilitate threshold detection of short pulsed
in Fig. 4. Each V- and H-pol receive leg uses a single-stage
RFI in ground processing.
superheterodyne downconverter and field-programmable
These telemetry flags help to identify and remove corrupted
gate array (FPGA) back-end digital processor. The analog
echo range line and noise-only measurements in the downand digital filter stages are configured as a series of funlinked data. Alarm reporting by the hardware at PRI timenels; filter bandwidth becomes progressively narrower
resolution allows bad range lines to be discarded before
down the amplification chain to help maintain receiver
blocks of raw data are averaged or processed further. Science
linearity in the presence of RFI. A broad (1200-1300 MHz)
data is therefore recoverable in a coherent processing interpre-select filter precedes the first low-noise amplifier
val, even if hit by occasional short bursts of RFI.
(LNA) to suppress interference outside the radar allocated
spectrum. Next, a higher-order IF bandpass filter (90-MHz
IV. BACKSCATTER CALIBRATION STABILITY
center frequency, 6-MHz bandwidth) provides sharp rollAs the radar frequency hops, transmitter and receiver gain
off (80+ dB attenuation at 7-MHz center frequency offset)
varies as a function of operating frequency. The gain/loss
for rejecting RFI which is outside the signal band but still
of active and passive RF components in the chain also drift
within the radar spectrum. Finally, after A/D conversion,
over temperature and over their operating life. Gain comFPGA digital narrowband filtering (1.2-MHz bandwidth
pensation must therefore be applied to accurately recover
per channel) provides very sharp rolloff (60+ dB attenusurface backscatter coefficients. The SMAP radar design uses
ation at 1-MHz offset) to extract the individual co-pol,
a loopback calibration approach, building off of technology
cross-pol and noise-only measurement channels.
inherited from the Aquarius/SAC-D scatterometer instruWhile the operations approach is to avoid known noisy
ment [9], to reduce these sources of measurement bias and
regions of the spectrum, there will always be times when
drift in the radar subsystem to less than the 0.1-dB calibraout-of-band terrestrial RFI is strong enough to cause gain
tion stability requirement.
compression effects, or when RFI falls directly in the ra14

Tx-V

n-0

Tx-H

f0 - 1.5 MHz

f0 + 1.5 MHz

f0 Tuning Range:
1218.75-1296.25 MHz at 1.25-MHz Steps
(63 Frequency Pairs, 77.5-MHz Tuning Range)

ieee Geoscience and remote sensing magazine

march 2015

Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - March 2015