IEEE Geoscience and Remote Sensing Magazine - March 2015 - 17

◗ printed wiring board (PWB) trace and ground-plane

VI. FLIGHT RADAR TEST RESULTS
Our team has completed the flight hardware build and
integration-and-test phases for the SMAP radar subsystem. Some salient performance test results from the several month-long radar ambient and environmental testing
campaign are presented here which show compliance with
requirements for RFI suppression, radar system stability,
and radiated emissions limits.
A. RFI suppRessIon cApAbIlIty
The response of the radar to a frequency-swept RFI source
is measured to gauge how well the receiver tolerates strong,
out-of-band interference levels. Test results are shown
in Fig. 5 for the maximum echo case (-81 dBm input,
VV channel) and the minimum echo case (-106 dBm input, VV channel), where power levels are referenced to the
antenna port. In both cases, a strong RFI tone at -37 dBm
(prescribed at ~70 dB above minimum echo) is simultaneously injected into the flight radar antenna port and swept
from 1200-1300 MHz to characterize receiver susceptibility. The distortion seen in detected power from 1245-
1255 MHz represents the susceptible band of the receiver.
Outside this band, variability in detected signal power is
less than 0.01 dB for the maximum echo and 0.22 dB for
the minimum echo. The spike seen in the minimum echo
case at ~1204 MHz is due to the receiver 2 # 2 mixing
product between the RFI tone and local oscillator (tuned to
1160 MHz here) which falls within the IF passband centered
around 90 MHz. Even with mixing products taken into account, the flight receiver meets the instrument 0.4-dB RFI
error budget with approximately a factor-of-two margin.
march 2015

ieee Geoscience and remote sensing magazine

5.0

(VV_EC-VV_LB) vs. Temperature in
TVAC Ramp from -15C to 45C, 20130716
VV_EC-VV_LB

4.9
Power Diff (dB)

layout for low-impedance coupling of return currents
to their source, especially for high-speed digital signals,
◗ avoiding single-ended external interfaces, and using instead differential signal standards like RS-422 or LVDS
(low-voltage differential signal) or ground-isolated interfaces to minimize return current through the radar
chassis structure,
◗ use of high-speed digital signal terminations (especially source-series termination on output drivers) to avoid
multiple reflections and ringing on signal lines.
Techniques for maximizing shielding effectiveness include:
◗ closing off long, open slots in assembly-level shielding (e.g., using EMI gaskets) to prevent L-band leakage
through these apertures,
◗ cable harness shielding treatment - use of twisted
shielded pair wiring, EMI tape overwrap for doubleshielding on harnesses, and 360-degree shield termination to connector backshells,
◗ integrated mechanical layout of the high-power
amplifier sub-assemblies (low voltage power supply, RF deck) into a single enclosed assembly, so that
power line interfaces to the RF deck are internal to the
HPA chassis.

4.8

4.7

4.6
-30 -20 -10

0
10
20
Temperature (°C)

FIGURE 6. Thermal-vacuum testing of radar stability, using fiber

optic delay line (FODL) equipment to measure round-trip echo
pulsed power. The slope in the echo:loopback power-ratio curve
during a temperature ramp indicates -0.0043 dB/°C system error
sensitivity. From this slope and the predicted environment in orbit,
the radar measurement bias error is estimated at < 0.02 dB.

b. RAdAR systemAtIc stAbIlIty vs. tempeRAtuRe
During thermal-vacuum testing of the flight radar, we characterized radar measurement stability with respect to temperature using a custom fiber-optic delay line (FODL) test
configuration. The RF front-end temperature is ramped up
from -20 C to +45 C over several hours while the radar
continually collects loopback
and echo power data (Fig. 6).
End-to-end measurement of
THE LookUp TAbLE
echo pulse power is achieved by
AppRoAcH FoR
interfacing the radar transmitAUTomATIcALLy
out port to FODL equipment,
commAnDInG RF
which attenuates and delays
opERATInG FREqUEncy
the transmit pulse by ~100 us
bASED on oRbIT
before injecting it back into the
radar. The FODL, placed outside
LATITUDE-LonGITUDE
the thermal-vacuum chamber,
AnD on L-bAnD RFI
is temperature controlled to
SURVEyS ALLowS THE
regulate round-trip path loss
SmAp RADAR To
and delay time. The resulting
opERATE AnD ADApT In
power ratio between echo and
A cHAnGInG, GLobAL
loopback data over temperaRFI EnVIRonmEnT.
ture is an indicator of radar net
calibration stability. The slope
of the plotted echo:loopback
ratio curve (shown for the VV channel) over the cold-tohot temperature ramp shows a minute systematic bias
error sensitivity of -0.0043 dB/°C. For a worst-case
temperature variation of !2 °C over the SMAP orbit,
this sensitivity translates to 0.017 dB measurement bias
17

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