A Novel Dicke Microwave Radiometer without
Temperature Control for Reference Match Load
Yan Li, Liang Lang*, Qingxia Li, Siyuan Liu, Liangqi Gui
Huazhong University of Science & Technology, School of Electronic Information and Communications
1037 Luoyu Road, Wuhan 430074, China Email: L_Lang@mail.hust.edu.cn
Abstract- Dicke microwave radiometer usually adopts one
reference match load of constant desired temperature. In the
paper, a novel Dicke microwave radiometer without temperature
control for reference match load is presented. We choose to
monitor the physical temperature of reference match load instead
of to keep it constant, so the structure of the reference match load
is simplified greatly. At the same time, the accuracy and stability
of the radiometer are both improved because the accuracy of
measuring reference match load’s temperature is better than
keeping its temperature nowadays. At last, an XC band Dicke
microwave radiometer is designed, and some experiments of
brightness temperature measurement are performed, which
verified the principle of this kind of Dicke microwave radiometer.
I.
On contrary, technology of measuring temperature has been
improved greatly nowadays comparing the age when
R.H.Dicke developed Dicke type radiometer[7, 8]. Small drifts
of reference load temperature can be measured precisely by
temperature sensor embedded in the load. For example,
resolution and accuracy of temperature measurement by
platinum resistance thermometer sensors can reach to 0.0010C
and ±0.010C respectively, which are both better than the
resolution and accuracy of temperature preserving of reference
match load.
INTRODUCTION
Dicke microwave radiometer is basically a total-power
radiometer with a Dicke switch and a synchronous
demodulator. Because effects of low frequency gain variations
can be reduced greatly, Dicke radiometer can yield better
performance than total-power radiometer. Dicke microwave
radiometer has been studied for many applications in remote
sensing and passive imagery, such as soil moisture and sea
surface salinity retrieval[1], providing the “wet” troposphere
path delay for sea level determination[2], sea surface
temperature, wind speed, water vapor content derivation[3],
passive indoor or outdoor millimeter wave imaging[4, 5].
A block diagram of an unbalanced Dick-type microwave
radiometer is shown In Fig.1. The synchronous modulation
switches the receiver input between the antenna and a constant
noise source, and then output voltage of radiometer varies
directly with the difference between antenna temperature and
brightness temperature of constant noise source. The common
constant noise source is usually thermo-controlled reference
matched load at a known temperature whose brightness
temperature is equal to its physical temperature. In order to
achieve long-term stability for radiometer, reference matched
load must be kept a constant desired temperature with complex
construction. Usually the thermo-controlled matched load is
composed of the heating and cooling system, the microwave
termination, the temperature sensor and the heat preservation
material, which makes it bulky and cumbersome. Moreover, it
is still a challenge to keep match load temperature with good
stability for a long time. It is reported that temperature of an
internal calibrator based on two loads can be kept at
250 ± 0.2K and 370 ± 0.2K respectively[6]. And in our lab, the
temperature of match load for Dick-type radiometer can be
kept with a stability of 0.1K at room temperature environment.
CLK
Dicke
Switch
LNA
BP
Filter
LNA
+
Detector
Direct detection radiometer chain
OpAmp
Switch
LP
Filter
-
Constant
Match load
Fig. 1. Block diagram of a Dicke-type radiometer
We propose a novel Dicke microwave radiometer without
temperature control for reference match load, alternately the
temperature of reference matched load is monitored in real
time. The antenna temperature being measured can be
calculated correctly with temperature compensation algorithm.
The principle of the novel Dicke radiometer with temperature
compensation algorithm is introduced in Section II. In Section
III, in order to verify the temperature compensation algorithm,
an XC band Dicke radiometer with temperature sensor is
designed to perform a brightness temperature measurement
experiment, and some results and discussion are presented. At
last, some conclusions are given based above theory and
experiments in Section IV.
II. PRINCIPLE
A. Total Power Microwave Radiometer
Assuming a linear operation of total power microwave
radiometer, the relation between antenna temperature Ta being
measured and output voltage of radiometer Va can be written as
(1)
Ta = aVa + b
Where a and b are both constants. Using two reference
targets with known brightness temperature Ta1 and Ta2, the
constant a and b in (1) can be calculated, and the radiometer
978-1-4673-8983-9/16/$31.00 ©2016 IEEE
Authorized licensed use limited to: Universitaet Linz. Downloaded on May 28,2021 at 12:43:02 UTC from IEEE Xplore. Restrictions apply.
equation of the system is expressed as
T − Ta 2
(2)
(V a − V a1 ) + Ta 1
Ta = a 1
Va1 − Va 2
Where Va1 and Va2 are output voltages of radiometer
corresponding radiation source of two known brightness
temperature Ta1 and Ta2 correspondingly.
A. Dicke Microwave Radiometer
Supposing a linear operation of Dicke radiometer, the direct
measured input signal is not antenna temperature Ta but
difference between antenna temperature Ta and brightness
temperature of reference load Tref , which is written as
(3)
Δ Ta = Ta − Tref
III. EXPERIMENTS VALIDATION AND DISCUSSION
A. The XC band Dicke Radiometer and Calibration
In order to verify the principle in section Ⅱ, we design an XC
band Dicke microwave radiometer without temperature control
for reference math load, whose block diagram is shown in
Fig.2, and (9) is adopted. In Fig.2 reference match load and
temperature sensor are good contact and covered with insulated
foam, and the measured temperature signal is sampled by A/D
card and sent to data process unit for temperature
compensation.
Using the same calibration method as total power radiometer,
two known brightness temperature △Ta1= Ta1－Tref and △Ta2=
T a2－Tref are used. Va1 and Va2 are output voltages of Dicke
radiometer corresponding to △Ta1 and △Ta2 respectively, then
the radiometer equation of the system is expressed as
Δ Ta 1 − Δ Ta 2
(4)
(V a − V a1 ) + Δ Ta1
Δ Ta =
Va1 − Va 2
Replacing △Ta1= Ta1－Tref, △Ta2= T a2－Tref and (3) in (4),
we can get same equation as (2)
T − Ta 2
(5)
(V a − V a1 ) + Ta 1
Ta = a 1
Va1 − Va 2
If we choose to monitor reference load’s temperature instead
of to keep the temperature, the reference load’s temperature
will drift as time goes by. Reference load temperature is
assumed to be Tref at the moment of radiometer calibration, and
it will change to Tref + ΔTref at the moment of scene
measurement. So the (3) should be rewritten as
Δ Tam = Tam − (Tref + Δ Tref )
=
Δ Ta 1 − Δ T a 2
(V a − V a1 ) + Δ Ta1
Va1 − Va 2
(6)
Fig. 2.Block diagram of a Dicke-type radiometer with temperature sensor
A. The Validating Experiment and Results
After radiometer system calibration the XC band Dick
radiometer is used to measure brightness temperature of a layer
of basalt detritus, and the radiometer is the far left radiometer
in Fig.3.
Temperature of reference match load and output voltages of
the Dicke radiometer were both measured at a speed of one
sample every second. The experiment lasted 45 minutes and
the experimental data are shown in Fig.4 and Fig.5. It is
founded that temperature of reference match load increased
rapidly with time in Fig.4. Correspondingly, the output
voltages of Dicke radiometer also increase in general in Fig.5.
where Tam represents the antenna temperature when reference
load temperature has changed to Tref + △Tref, and (6) can be
written as a simplified form as
T − Ta 2
(7)
(V a − Va1 )+ Ta1 ] + Δ Tref
Tam = [ a1
Va1 − Va 2
Comparing (5) with (7), it will be found that
(8)
Tam = Ta + ΔTref
Equation (8) indicates clearly that antenna temperature being
measured will increase △ Tref when there is △ Tref drift of
reference load temperature comparing to moment of
radiometer calibration.
For some Dicke radiometer, the direct measured input signal
is the difference between brightness temperature of reference
load Tref and antenna temperature Ta, which is written as
(9)
Δ Ta = Tref − Ta
Using the above method, the same equation as (8) can be
deduced.
Fig.3. Scene of experiment measurement
Authorized licensed use limited to: Universitaet Linz. Downloaded on May 28,2021 at 12:43:02 UTC from IEEE Xplore. Restrictions apply.
permittivity and thickness. Because these parameters of basalt
detritus almost are almost invariant within 45 minutes, its
brightness temperature and antenna temperature should both be
constant. From results of Fig.6, the calculated antenna
temperature without temperature compensation algorithm
decreases about 8K in within 45 minutes. And when
temperature compensation algorithm is considered, the
calculated antenna temperature is very close to a constant,
which validate the correctness of the algorithm in some extent.
physical temperature / K
321
320
319
318
317
316
IV. CONCLUSION
315
In order to void designing complex constant reference match
load which is major advantages for the novel Dicke microwave
radiometer, a method of monitoring reference match load
physical temperature is adapted. By adding the temperature
drift of reference match load, antenna temperature can be
calculated correctly, which is verified by above experiments.
And because accuracy and precision of measuring temperature
are both better than keeping temperature, the accuracy and
stability of Dicke radiometer also can be improved.
314
0
10
20
30
40
time / minutes
Fig. 4. Physical temperature of reference match load
0.25
0.24
voltage / V
0.23
ACKNOWLEDGMENT
This work was supported by the Fundamental Research
Funds for the Central Universities of China (2014QN157) and
the Key Innovation Project of Science and Technology of
Hubei Province (2015AEA074)
0.22
0.21
0.2
0.19
REFERENCES
0.18
[1]
0
10
20
30
40
time / minutes
Fig. 5 Output voltages of Dick radiometer
Substituting temperature of reference match load in Fig.4
and output voltages in Fig.5 into radiometer system calibration
equation, the calculated brightness temperature of basalt
detritus with temperature compensation algorithm and without
temperature compensation algorithm are both shown in Fig.6.
antenna temperature / K
315
[2]
[3]
[4]
Ta with compensation
Ta without compensation
310
305
[5]
300
[6]
295
290
285
280
[7]
0
10
20
30
40
time / minutes
Fig. 6 Calculated antenna temperature with temperature compensation
algorithm and without temperature compensation algorithm
[8]
Acevo-Herrera, R., Aguasca, A., Bosch-Lluis, X., and Camps, A.: ‘On
the use of compact L-band Dicke radiometer (ariel) and UAV for soil
moisture and salinity map retrieval: 2008/2009 field experiments’, IEEE
International Geoscience and Remote Sensing Symposium, 2009, Vols 15, pp. 3109-3112
Eymard, L., Obligis, E., Tran, N., Karbou, F., and Dedieu, M.: ‘Longterm stability of ERS-2 and TOPEX microwave radiometer in-flight
calibration’, Ieee Transactions on Geoscience and Remote Sensing, 2005,
43, (5), pp. 1144-1158
Misra, T., Jha, A.M., Putrevu, D., Rao, J., Dave, D.B., and Rana, S.S.:
‘Ground calibration of multifrequency scanning microwave radiometer
(MSMR)’, Ieee Transactions on Geoscience and Remote Sensing, 2002,
40, (2), pp. 504-508
Weissbrodf, E., Kallfass, I., Hulsmann, A., Tessmann, A., Leuther, A.,
Massler, H., and Ambacher, O.: ‘W-band radiometer system with
switching front-end for multi-load calibration’, IEEE International
Geoscience and Remote Sensing Symposium (IGARSS), 2011, pp. 38433846
Gong, M., Jiao, L., Du, H., and Bo, L.: ‘Multiobjective immune
algorithm with nondominated neighbor-based selection’, Evolutionary
Computation, 2008, 16, (2), pp. 225-255
Macelloni, G., Brogioni, M., Pampaloni, P., Cagnati, A., and Drinkwater,
M.R.: ‘DOMEX 2004: An experimental campaign at Dome-C Antarctica
for the calibration of spaceborne low-frequency microwave radiometers’,
Ieee Transactions on Geoscience and Remote Sensing, 2006, 44, (10), pp.
2642-2653
R.H.Dick: ‘The measurement of thermal radiation at microwave
frequencies’, Rev. Sci.Instr., 1946, 17, pp. 268-275
Strom, L.: ‘The theoretical sensitivity of the Dicke radiometer’. Proc.
WESCON/57 Conference Record, Aug 1957 pp. 188-193Pages
A. Discussion
The brightness temperature of basalt detritus is affected
mainly by its parameters such as temperature profile, complex
Authorized licensed use limited to: Universitaet Linz. Downloaded on May 28,2021 at 12:43:02 UTC from IEEE Xplore. Restrictions apply.