IEEE - Aerospace and Electronic Systems - February 2023 - 21

Teng et al.
Figure 6.
Block diagram of the SSM/T-2 antenna system [4].
THZ ANTENNA SYSTEM FOR NOAA SERIES-AMSU-B
(ATMOSPHERIC REMOTE SENSING)
The fifth generation of the U.S. NOAA (National Oceanic
and Atmospheric Administration) polar operational environmental
satellites, NOAA 15, 16, and 17, were launched
in 1998, 2000, and 2002, respectively. The Advanced
Microwave Sounding Unit-B (AMSU-B) instruments [11],
[12] flying on the NOAA series were designed to obtain
humidity profiles in five channels spanning the height
range from the surface to about 42 km.
The AMSU-B antenna system consists of a scanning
dual-offset reflector antenna and a quasi-optical
network, as shown in Figure 7. The main antenna
inclined at 45 to direct the atmospheric radiation into
the QON; then the QON split the incoming broadband
signal into three separate beams; finally the corrugated
feedhorns pass the beams to the radiometers for signal
processing. The AMSU-B instrument covers the
86-92 GHz band (1 channel), the 148-152 GHz band
(1 channel), and the 175-191 GHz band (three channels),
respectively.
THEDUAL-REFLECTORANTENNA
As illustrated in Figure 7, the front-end dual-reflector
antenna consists of an off-axis paraboloidal primary
reflector and an off-axis hyperboloidal secondary reflector.
The beryllium main reflector with a 219-mm diameter
aperture is housed in a cylindrical shroud. The surface
roughness was specified to be <0.4 mm and the overall
departure from a paraboloid to be <10 mm. The halfpower
beamwidths of the antenna achieved 1.110% for
all channels, while the crosspolar relative to copolar peak
and sidelobe levels to be less than27 dB.
FEBRUARY 2023
THEQUASI-OPTICALNETWORK
The quasi-optical frequency demultiplexer uses two planar
dichroic filters to split the incoming broadband signal into
three bands, as shown in Figure 7. Two of the five channels
centered on 89, 150 GHz, and the 183 GHz receiver has filters
to split the band into three channels centered on
183.31.0, 183.33.0, and 183.37.0 GHz. As can be seen
from the optics of the 183 GHz channel (Figure 7), two
beam folding mirrors guide the rays from dichroic plate to
the corrugated feedhorn. The series ofbeam folding mirrors
enable all channels to have the same far-field beamwidth
despite oftheir different center frequencies.
The dichroic plates used in AMSU-B are waveguide
arrays, which are made of parallel-sided metal plates perforated
by arrays of equispaced circular holes. The plates
function as high-pass filters in transmission, and the transition
frequency is close to the cutoff frequency of the individual
waveguides. Transmission loss of the dichroic
frequency filter is achieved to be <0.5 dB.
The feedhorns for the 89 and 150 GHz bands are linearly
tapered conical corrugated horns, and for the 183 GHz band is
a shaped corrugated horn. The shaped horn can produce an
axially symmetric field with uniform phase over its aperture
plane within the frequency band, which could provide better
matching ofthe antenna patterns.
In this mission, the separation of multiple frequency
bands uses two planar dichroic filters. The inherently lowloss
QO front-ends was chosen and designed to meet the
broadband need.
THZ ANTENNA SYSTEM FOR ODIN-SMR
(ATMOSPHERIC REMOTE SENSING)
Odin satellite [7], [14], [15] was launched in February
2001, which was developed in cooperation between
IEEE A&E SYSTEMS MAGAZINE
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