The development of a small, lightweight, low-power, seven channel passive microwave radiometer system for use on the Defense Meteorological Satellite Program (DMSP) was described. The 50-60 GHz sensor system operates in the region of an intense atmospheric oxygen absorption band to provide atmospheric temperature profiles to 30 kilometer altitudes on a global basis

Louappe, M. E.

Paradis, K. A.

English

NASA Technical Reports Server

N 7 8 - 1 4 5 6 8
A MULTICHANNEL PASSIVE MICROWAVE ATMOSPHERIC
TEMPERATURE SOUNDING SYSTEM
M. E. Louapre and K. A. Paradis
Aerojet ElectroSystems Company
Azusa, California
ABSTRACT
The development of a small, lightweight, low-power,
seven-channel passive microwave radiometer system for use
on the Defense Meteorological Satellite Program (DMSP) is
described. This 50-60 GHz sensor system operates in the
region of an intense atmospheric oxygen absorption band to
provide atmospheric temperature profiles to 30 kilometer
altitudes on a global basis.
1. INTRODUCTION
Knowledge of the temperature distribution of the atmosphere on a global scale is a key
factor in the prediction of various weather parameters by meteorologists. Although some
such profile information is currently available from radiosonde, rocketsonde and infrared
atmospheric sounding systems, each of these systems has its obvious limitations. This
paper describes a multichannel passive microwave radiometric sensor system that has been
developed (Ref . 1) for use on the Defense Meteorological Satellite Program (DMSP) to gather
such temperature distribution information on a global basis. Atmospheric temperatures for
pressure levels of 1000, 850, 700, 500, 400, 300, 250, 200, 150, 100, 70, 50, 30, 20 and
10 mb; 14 thicknesses between these levels; and the temperature and pressure of the tropo-
pause will ultimately be provided.
The radiometric characteristics of the atmosphere are determined primarily by the
absorption and emission characteristics of the oxygen molecule in the 50-60 GHz band of the
frequency spectrum. Since the mixing ratio of oxygen is constant in the atmosphere, the
contribution of a particular layer of the atmosphere to the total radiation detected by a
spaceborne radiometer operating in this frequency range is primarily controlled by the air
temperature and the product of two competing factors — namely, the emission per unit
volume, which decreases monotonically with height due to the decreasing air density, and
the transmission factor of the atmosphere above the layer under consideration, which
increases with height. The product results in a function (the weighting function) which typi-
cally peaks at some height and decreases with distance away from the peaking altitude. By
choosing frequencies with different absorption coefficients, weighting functions emphasizing
radiances from preselected atmospheric layers may be obtained. The problem of profiling
the temperature of the atmosphere then reduces to finding the most probable profile which,
when weighted by the appropriate weighting functions, produces the measured brightness
temperature values of the selected frequencies.
Since the underlying earth surface also contributes to the detected radiation at those fre-
quencies responding to the lowest portion of the atmosphere, the software used in the extrac-
tion of atmospheric information from the microwave data must be designed to eliminate the
effects of the surface. A unique, highly efficient algorithm that accounts for varying surface
emiss ivities, lower tropospheric effects, and terrain height variability has been developed to
do this (Ref. 2).
Specification of the altitudes at which the weighting functions are to peak and the full width
half maximum of the weighting functions establishes the center frequency and bandwidth of
the individual sensor channels. At the higher peaking altitudes where the pressure broad-
ening is reduced and the line widths are extremely narrow, both frequency and bandwidth
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selections must reflect a consideration of both software and hardware roles. Consequently,
narrow bandwidths and a high degree of frequency stability are required at these altitudes.
Profiling of the atmosphere from sea level to an altitude of 30 kilometers has resulted in
the selection of seven channels ranging in frequency from 50. 5 GHz to 59. 4 GHz as shown in
Table I A requirement for a maximum calibration uncertainty of 1 K and maximum NETD
for the arious channels of 0. 4 to 0. 6 K has established the electrical performance require-
ments of the sensor system. Stringent weight, power and size limitations of 11. 5 kg, 18 watts
and 0. 20 x 0. 36 x 0. 41 meters respectively, have also restricted the physical characteristics
of the sensor.
This paper describes the design and performance of a lightweight, low-power, seven-
channel radiometer sensor system that satisfies these requirements.
2. SYSTEM CONFIGURATION
The Multichannel Passive Microwave Temperature Sounder System, shown in Figure 1 and
in simplified block diagram form in Figure 2, consists of a mechanically scanned reflector
antenna, a stepper motor/gear antenna drive, a seven-channel superheterodyne receiver, and
signal processing/timing circuitry. Precise total system inflight calibration is achieved by
viewing known radiometric temperatures through the antenna system. An accurately known
ambient load (300 K nominal) and the cosmic background (2. 7 K) are used for this purpose.
These absolute calibration points make it possible to determine the sensed antenna tempera-
tures in an absolute manner.
The mechanically scanned reflector antenna system shown in Figure 3 consists of a lens-
corrected, corrugated feed horn, a shrouded reflector and an orthogonal mode transducer
(OMT). It provides a planar scan by a rotation of the reflector surface; this allows the
antenna feed system to remain fixed. A metallic hood shrouds the reflector to eliminate back
radiation and provides a guided interface to the calibration system. A thermal shroud is
also incorporated into the design to shade the inflight calibration source apertures during
scan and reduce thermal gradients at the calibration apertures.
As seen in Figure 4, the antenna beam is step-scanned through seven discrete earth-
looking positions at a fixed, programmed rate, then is rapidly rotated, first to the warm
calibration reference, then to the cold calibration reference, before the scanning sequence is
repeated. Key scan parameters are detailed in Table II. Because the calibration paths are
characterized by low reflective and dissipative losses, the equivalent antenna input tempera-
ture during calibration can be accurately determined.
The antenna input brightness temperature (Ti) is determined from the relationship
TH - T_
T = T - (V - V 1i XH V,, - Vu < i H'w rl
where TH = warm calibration input temperature, TC cold calibration input temperature,
VH = output voltage during warm calibration, VQ = output voltage during cold calibration,
Vj = output voltage corresponding to Tj . (The modulator temperature is not required in the
above expression for Ti because of the thermal characteristics of the instrument and the
periodicity of calibration. )
Signals in the 50 to 60 GHz frequency range are received by the antenna system and sub-
sequently split into two orthogonal polarizations by an orthogonal mode transducer (OMT).
The use of an OMT allows both orthogonal polarizations to be utilized, thereby establishing
the initial level of frequency channelization. One polarization is used for the lower altitudes,
Channels 1-4; the other polarization is used for the upper altitudes, Channels 5-7.
From the OMT the received signals are applied to a unique broadband, low-loss dual
resistive vane modulator. This device serves to chop the input signal and does not, as men-
tioned previously, influence the determination of Tj . With an attenuation that exceeds
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50 dB and an insertion loss of less than 0. 3 dB over the entire frequency band this modulator
approximates an ideal switch and thus avoids the switching errors associated with fe r r i te
type modulators.
After modulation, one of the signals is applied to an RF diplexer, the other to an RF band-
pass filter. The RF diplexer splits the signal into two bands containing Channels 1, and 2, 3,
and 4 frequencies, respectively. Specifics of the channel peaking altitude, frequency, band-
width and NETD were shown previously in Table I. The Channel 1 band is down-converted,
amplified, fi l tered, and subsequently detected. Down-conversion is accomplished in a broad-
band balanced mixer in which GaAs Schottky-barrier diodes are used. Similarly the Channel
2, 3, and 4 band is down-converted and amplified in a common mixer-IF amplifier and then
applied to an IF triplexer which splits the signal into three discrete channels of information,
which are then detected by square-law detectors.
The signal from the other half of the dual-vane modulator, i. e. , the signal applied to the
RF bandpass filter, contains Channels 5, 6, and 7. The filter serves to remove the corre-
sponding image frequencies. After filtering, these channels are treated in a manner identi-
cal to that used for Channels 2, 3 and 4. The overall receiver bandwidth and channelization
characteristics are shown in Figure 5.
A single frequency stabilized Gunn oscillator is used as the local oscillator to conserve
power and weight. The frequency of the oscillator was chosen such that potentially detri-
mental EMI signals would fall outside the filtered IF passbands. Stability specifications for
the oscillator are based on the profiling accuracy required over a two-year life of the instru-
ment. The fundamental Gunn oscillator is matched and locked to a high-Q Invar cavity to
achieve the required long-term frequency stability. Since the stabilization network is totally
passive, long-term reliability is assured.
After detection, the seven channels of information are processed by individually program-
mable stepped automatic gain control (SAGC) attenuators. The SAGC maintains the system
gain such that the analog-to-digital converter resolution capability is optimized for any pos-
sible variation in the predetection gain. After gain conditioning, the signals are processed
by individual synchronous demodulator integrate-and-dump circuits. These integrated sig-
nals are then multiplexed into a common 12-bit analog-to-digital (A/D) converter and subse-
quently demultiplexed into data storage and readout circuits. Critical points throughout the
system are thermally monitored by temperature sensor readout channels which are multi-
plexed into the A/D converter for subsequent readout.
The system operates in both a synchronized mode and an independent or automatic mode.
In the synchronized mode, an external synchronization signal causes the sensor to initiate
scanning in a step-synchronized fashion with an onboard IR sensor. When the system is
operated in the automatic mode, antenna scanning is completely independent of the external
synchronization signal.
3. CALIBRATION
As described previously, the inflight calibration system, shown schematically in Figure 6,
consists of two noise sources. A cold path views the cosmic background(2. 7 K) for one cali-
bration point and a built-in "warm" load (^300 K) is observed for the other calibration point.
The radiometer (including the antenna) is calibrated once during each 32-second scan cycle.
The inflight calibration system is a well-matched, closed-path configuration with very low
dissipative wall losses. A shroud on the reflector allows direct coupling to both the cold
path and warm load. The cold path is an oversize circular transmission line that is used to
restrict the radiometer field of view so that extraneous input signals due to both the sur-
rounding spacecraft and the earth's atmosphere are minimized. Due to the location of the
sensor on the spacecraft it is not possible for the sensor antenna to view the sky directly.
Therefore, it is necessary to utilize a reflecting miter bend in the cold path to direct the
antenna pattern in the proper direction. The warm load is an extended microwave radiator
made up of a large number of tapered absorbing sections and is designed to provide a stable
blackbody temperature source at approximately 300 K. An accurate measurement of the sur-
face temperature of the load is provided as a result of the warm load thermal design. A
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shroud allows direct coupling to the antenna and a sun shield located on the antenna reflector
prevents the warm load from viewing the sun, thereby enhancing the thermal stability of this
load.
Precision primary temperature standards are used for laboratory calibration of the instru-
ment. In these standards the surface temperature of the microwave load is measured very
precisely and the reflection characteristics are extremely low. Two primary standards
stabilized at approximately 78 K and 300 K are used to calibrate the radiometer in the labor-
atory under stable thermal-vacuum conditions. A remotely controlled, semiautomatic pri-
mary standard transport mechanism. Figure 7, is used in conjunction with the radiometer to
accomplish the calibration process. Once the radiometer has been calibrated with at least
two primary standards, the calibration and linearity of the instrument is verified by viewing
additional standards at temperatures between the calibration extremes.
4. PACKAGING DESCRIPTION
To achieve the high packaging density required by system weight and size limitations, a
compartmentized design has been used. As shown in Figure 8, the RF components of the
receiver and the antenna feed horn are mounted to a honeycomb shelf. Use of a shelf allows
the antenna reflector to be mounted high in the instrument, thereby avoiding radiation pattern
interference from the instrument walls. Use of the shelf also allows the feed horn, orthog-
onal mode transducer and the modulator to be mounted close together on the same surface,
reducing RF losses. A higher than usual packaging density for RF components is achieved by
packaging the three mixers and IF amplifiers in three parallel routes on each side of the
modulator. The IF filter and triplexers, as well as the seven detector/preamplifiers are
located in the forward part of the lower compartment. All post detection circuitry is mounted
on printed circuit cards located in the rear portion of this lower compartment.
5. SYSTEM PERFORMANCE
The overall measured performance of the sensor system is summarized in Table III. The
final size, weight and power consumption of the sensor system are 0. 2 x 0. 28 x 0. 41 meters,
11. 2 kilograms, and 14. 2 watts respectively. An antenna beam efficiency in excess of 95. 8
percent and a beamwidth of 14. 4 degrees or less were achieved at all seven frequencies and
all seven beam positions.
6. REFERENCES
1. United States Air Force, Space and Missile Systems Organization, Contract F04701-75-
C-0090.
2. Rigone, J. L. and A. P. Stogryn, "Data Processing for the DMSP Microwave Radiometer
System," Eleventh International Symposium on Remote Sensing of Environment, April 1977,
Poster Session P-123.
TABLE I. CHANNEL PARAMETER REQUIREMENTS
Channel Peaking Height (km) Frequency (GHz) Bandwidth (MHz) NETD
1 0 (window) 50. 5 400 0. 6
2 2 53.2 400 0.4
3 6 54.35 400 0.4
4 10 54.9 400 0.4
5 30 58.4 115 ' 0.5
6 16 58.825 400 0.4
7 22 59.4 250 0.4
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TABLE II. KEY SCAN PARAMETERS
Scan Type
Cross-Track Positions
Calibration Positions
Total Cross-Track Scan
Total Scan Period
Dwell Time (Cross-Track and
Calibration Positions)
Cross-Track Nadir
7
2-Cosmic Background and ^300 K
-H360
32 Seconds
2. 7 Seconds
TABLE III. MEASURED PERFORMANCE
Channel
No.
1
2
3
4
5
6
7
Center Freq
GHz
50. 5
53. 2
54. 35
54. 9
58. 4
58. 825
59. 4
Bandwidth
MHz
400
400
400
400
115
400
250
NETD
K
0. 32
o. 24
0. 36
0. 22
0. 39
0. 29
0. 31
Calib Uncertainty
K
0. 25
0. 21
0. 24
0. 13
0. 06
0. 24
0. 13
Figure 1. Multichannel Passive Microwave Temperature
Sounder System
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A multichannel passive microwave atmospheric temperature sounding system

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