Note: Descriptions are shown in the official language in which they were submitted.
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RADIOMETER SYSTEM AND METHOD OF
CALTBRATING RADTOMETER RECEIVER
This is a division of co-pending Canadian Patent
Application No. 2,274,473, filed June 3, 1999.
TECHNICAL FIELD
The present invention relates to calibration of a
radiometer with reference temperatures from a noise
source and, more particularly, to calibration of a
radiometer with reference temperatures from a microwave
active solid state noise source providing hot and cold
reference noise temperatures.
BACKGROUND OF THE INVENTION
Radiometers are used to measure thermal radiation
or brightness temperatures emitted from a segment of a
remote object. The segment is commonly referred to as
a scene and may be a portion of the earth's surface.
Like most sophisticated instrumentation, radiometers
require periodic calibration to insure accurate
measurements. In practice, at least two known
calibration temperatures, which bound the brightness
temperatures of the scene, are used to calibrate a
radiometer rece3.ver. The lowest and highest
calibration temperatures are referred to as cold and
hot thermal radiation temperatures, respectively.
Radiometers are generally ground-based, airborne
or satellite-based systems that measure brightness
temperatures in the range of 10°K-300°K. The ground-
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based system may utilize closed cycle refrigeration such
as a sterling cycle cooler with liquid nitrogen or liquid
helium to generate cold thermal radiation temperatures
"Tc". The closed cycle refrigeration systems are not
~ considered practical for the satellite-based systems.
Referring to FIGURES 1-3, there are illustrated
three traditional satellite-based systems for measuring
the brightness temperature "Ta" emitted from a portion of
the earth's surface and received by an antenna 36. The
brightness temperature "Ta" is then transmitted through
an antenna feed 32 on an antenna-earth scene line 12 to
a radiometer receiver 16 of the radiometer system 150.
Currently, satellite-based systems use calibration
techniques that are either externally-based (FIGURES 1
and 2) or internally-based (FIGURE 3).
Referring to FIGURE 1, there is illustrated an
externally-based calibration technique known as the sky
horn approach. The sky horn approach utilizes a
radiometer system 150 which includes a first RF switch 10
that connects either the antenna-earth scene line 12 or
a calibration line 14 to the radiometer receiver 16. In
the calibration line 14 a second RF switch 18 alternately
switches between a sky horn 20 and an internal warm load
22. The sky horn 20 outputs the cold space thermal
radiation temperature "Tc," approximately 2.7°K, and the
internal warm load 22 generates a hot thermal radiation
temperature "Tw," approximately 300°K. A precision
thermistor 24 in thermal contact with the warm load 22
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outputs an electrical hot thermal radiation temperature
"Td" that i~ equivalent to the hot thermal radiation
temperature "Tw". The electrical hot thermal radiation
temperature "Td" is utilized in the calibration of the
' radiometer receiver 16.
The sky horn approach is a complex and expensive way
to calibrate the radiometer receiver 16. The main
problem is that the antenna-earth scene line 12 and
calibration line 14 are separate lines, thereby requiring
precise knowledge of the RF losses, mismatch losses and
physical temperatures of each line to accurately
calibrate the radiometer receiver 16. Also, the use of
the sky horn 20 adds to the complexity of the
calibration, because of possible interference of the sky
horn pattern by a spacecraft or contamination caused by
the earth or sun.
Referring to Figure 2, there is illustrated another
externally-based calibration technique for satellite-
based systems using an antenna scanner 26. The antenna
scanner 26 is a mechanical mechanism employed during a
calibration mode to alternately couple a reflector plate
28 or an absorption target 30 to respectively feed a cold
thermal radiation temperature "Tc" or a hot thermal
radiation temperature "Tw" to the antenna feed 32. The
antenna feed 32 is connected. to the radiometer receiver
16. During an antenna mode when the brightness
temperature "Ta" is measured, the antenna scanner 26
connects the antenna-earth scene line 12 to the
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radiometer receiver 16. The antenna scanner 26 does have
an advantage over the sky horn approach in that only one
RF path is utilized. However, the antenna scanner 26 is
complex, bulky and adds significant size and weight to
the radiometer system 150.
Referring to FIGURE 3, there is illustrated an
internally-based calibration technique that may be used
in a satellite-based system. The internal approach is
very similar to the sky horn approach discussed
previously and illustrated in FIGURE 1. However, the
internal technique may utilize a thermo-electric cooler
34 to generate a cold thermal radiation temperature "Tc"
of approximately 270°K, instead of the sky horn 20 used
in the sky horn approach. However, the hot and cold
thermal radiation temperatures "Tc" and "Tw" used in the
internal approach may only be 30°K apart. The 30°K
difference between the cold and hot thermal radiation
temperatures "Tc" and "Tw" does not cover the full range
of earth brightness temperatures which are approximately
100°K to 300°K, therefore, measurement accuracy of the
radiometer receiver 26 will likely degrade below the cold
thermal radiation temperature "Tc."
Accordingly, there is a need for a calibration noise
source to provide cold and hot thermal radiation
temperatures for calibrating a radiometer. There is also
a need to provide a noise source that may be manufactured
using microwave integrated circuit LMIC) or monolithic
microwave integrated circuit (MMIC) technologies. These
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and other needs are satisfied by the calibration noise
source of the present invention.
SOI~MARY OF T8E INVENTION
The present invention is a radiometer calibration
system utilizing a microwave active noise source and a
method for calibrating a radiometer. The noise source
includes a transistor configured as a noise equivalent
model having a gate port, drain port and source port.
During calibration of the radiometer the drain port is
terminated and a bias circuit applies DC bias to the
noise equivalent model. The~bias circuit controls the DC
bias such that a hot thermal radiation temperature and a
cold thermal radiation temperature are alternately output
to the gate port of the noise equivalent model. A source
inductance coupled to the source port of the noise
equivalent model provides series feedback for the noise
source. To match impedances to the noise equivalent
model, an output matching impedance network is connected
to the drain port and an input matching impedance network
is connected to the gate port. The input matching
impedance network includes an input port for outputting
the hot thermal radiation temperature and the cold
thermal radiation temperature utilized to calibrate the
radiometer.
According to the present invention there is provided
a calibration system having a noise source for
calibration of ground-based, airborne or satellite-based
radiometers.
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Also in accordance with the present invention
there is provided a noise source that functions in the
millimeter and microwave spectrum.
Further in accordance with the present invention
there is provided a calibration system having a noise
source implemented as an integrated circuit.
Further in a~~cordance with the present invention
there is provided a calibration system having a noise
source with a built-in-test capability providing noise
figure measurements.
In accordance with the present invention there is
also provided a radiometer having adjustable
calibration time intervals to maximize the measurements
of earth scenes.
In accordance with one aspect of the present
invention there is provided a radiometer system
comprising: a radiometer receiver; a switch for
selecting between an antenna mode and a calibration
mode, said antenna mode enabling a brightness
temperature received from an antenna to be applied to
the radiometer receiver; a microwave active noise
source connected to the radiometer receiver through
said switch when in the calibration mode, said noise
source alternatively outputting a cold thermal
radiation temperature or a hot thermal radiation
temperature; a driver for controlling the operation of
said switch; and a microprocessor coupled to the
radiometer receiver, the noise source and said driver
for correcting the output of said radiometer receiver,
said microprocessor storing a series of initial
reference calibration data to be accessed by the
microprocessor during the calibration mode to adjust
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the uncorrected output voltage from the radiometer
receiver.
BRIEF DESCRIPTION OF THE DRANINGS
The present invention, taken in conjunction with
the invention disclosed in co-pending Canadian Patent
Application No. 2,2?4,473, filed June 3, 1999, will be
discussed in detail hereinbelow with the aid of the
accompanying drawings, wherein:
FIGURE 1 is a schematic representation of PRIOR
ART illustrating a sky horn approach for calibrating a
satellite-based radiometer;
FIGURE 2 is a schematic representation of PRIOR
ART illustrating a calibration technique using an
antenna scanner;
FIGURE 3 is a schematic representation of PRIOR
ART illustrating an internally-based calibration
technique using a thermo-electric cooler;
CA 02411995 2002-12-30
FIGURE 4 is a schematic representation of the
present invention illustrating a satellite-based
radiometer calibration system incorporating a noise
source;
FIGURES 5A-5D illustrate calibration curves for use
with the present radiometer calibration system;
FIGURES 6A and 6B are illustrations of calibration
and bias commands transmitted by a microprocessor to the
driver and the noise source, respectively, illustrated in
FIGURE 4;
FIGURE 7 is a sirnplif ied schematic of a microwave
active solid state cold/hot noise source implemented as
a microwave integrated circuit;
FIGURE 8 is a graph of a thermal radiation curve for
a noise source operating at l9GHz and 22GHz with DC bias
applied; and
FIGURE 9 is a graph illustrating a noise figure
measurement of the radiometer receiver illustrated in
FIGURE 7.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring to Figure 4 there is disclosed a noise
source 100 for calibration of a radiometer in accordance
with the present invention.
Although the noise source 100 will be described
incorporated with a radiometer calibration system 150,
those skilled in the art will appreciate that such
application is only one of many for utilizing the noise
source of the present invention. Accordingly, the
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described noise source 100 should not be construed in a
limiting manner.
Major contributing errors associated with
calibrating satellite-based radiometers arise from the
following factors: (1) cold calibration brightness
temperature; (2) warm calibration brightness temperature;
(3) radiometer receiver transfer function; (4) ground
retrieval algorithm; and (5) antenna brightness
temperature. Each of the major contributing errors must
be separately addressed and combined in establishing an
overall accuracy scheme for the radiometer calibration
system 150. The errors associated with the cold and warm
calibration brightness temperatures are addressed by the
noise source 100. A detailed description of the noise
source 100 will be discussed after describing the
interaction of the noise source with the radiometer
calibration system 150.
Referring to FIGURE 4, there is illustrated a
schematic representation of the satellite-based
radiometer calibration system 150 incorporating the noise
source 100. The brightness temperature "Ta" emitted from
a segment of the earth's surface is received by the
antenna reflector 36 and transmitted to the antenna feed
32. The antenna feed 32 outputs the brightness
temperature "Ta" on the antenna- earth scene line 12.
The antenna-earth scene line 12 is connected to a
selector switch 62 for switching either the antenna earth
scene line 12 or a.calibration line 64 to an input
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terminal 66 of the radiometer receiver 16. The
calibration line 64 connects the noise source 100 to the
radiometer receiver 16. The selector switch 62 is
preferably a low loss RF ferrite switch.
A driver 68 actuates and controls the selector
switch 62 according to commands received from a
microprocessor 70. Initially, the microprocessor 70
receives a "test command" signal from an external source
(not shown) on line 80; the test command triggers the
calibration sequence.
Referring to FIGURE6 4, 6A and 6B, the
microprocessor 70 transmits a command on line 72 to the
driver 68 to actuate either an antenna mode 82 or
calibration mode 84 (FIGURE 6A). In the antenna mode 82
the selector switch 62 is actuated to connect the
antenna-earth scene line 12 to the input terminal 66 of
the radiometer receiver 16. In the calibration mode 84
the selector switch 62 is actuated to connect the
calibration line 64 to the input terminal 66 of the
radiometer receiver 16. Selection of the calibration
mode 84 at selected time intervals for short durations
maximizes measurements of the brightness temperatures
"Ta".
The microprocessor 70 also transmits a bias command
signal 86 (FIGURE 6B) on line 74 to the noise source 100.
The noise source 100, responsive to the bias command
signal, alternately outputs the cold thermal radiation
temperature "Tc" and the hot thermal radiation
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temperature "Tw". Alternating between the hot and cold
thermal radiation temperatures "Tc", "Tw" occurs during
the calibration mode 84. The noise source 100 does not
output the cold thermal radiation temperature "Tc" or the
hot thermal radiation temperature "Tw" during the antenna
mode 82.
Referring again to FIGURE 4, the noise source 100
includes a correction precision thermistor 76 in thermal
contact with the noise source and connected to the
microprocessor 70 by a line 78. The correction precision
thermistor 76 provides compensation for changes in the
physical temperature "Td" of the noise source 100. A
thermal insulation blanket 71 may be provided to
encompass the noise source 100. The compensation, DC
bias, and the correction precision thermistor 76 are
elements of the noise source 100 and will be discussed in
greater detail later.
Prior to using the radiometer calibration systems
150, the noise source 100 is initially calibrated with a
laboratory radiometer (not shown). During the initial
calibration of the radiometer calibration system 150
there is generated a series of reference calibration
curves which are stored in the microprocessor 70. The
calibration curves are accessed by the microprocessor 70
during the calibration mode 84 to adjust the uncorrected
output voltage from the radiometer receiver 16 on line 98
to the corrected output voltage on line 99. Referring to
Figures 5A, 5B, SC, and 5D, the calibration curves
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include a precision thermistor calibration curve 88, a
noise source radiation temperature drift curve 90, a
radiometer calibration curve 92 and a corrected
radiometer calibration curve 94.
The precision thermistor curve 88 (FIG. 5A)
illustrates the change in the voltage "Vd" versus the
temperature "Td" of the noise source 100 sensed along
signal line 78. "Vd" is a calibrated thermistor output
voltage corresponding to the known physical temperature
"Td".
The noise source radiation temperature drift curve
90 (FIG. 5B) sensed along line 64 and radiometer
calibration curve 92 (FIG. 5C) sensed along line 98 are
combined into the corrected radiometer calibration curve
94 (FIG. 5D). The corrected radiometer calibration curve
94 represents the amount of correction required of the
uncorrected output voltage generated by the radiometer
receiver 16 on line 98 and input to the microprocessor
70. The radiometer calibration curve 92 (FIG. 5C)
illustrates the radiometer calibration performed during
the calibration mode 84. The uncertainty is due to the
variation in the physical temperature "Td" of the noise
source 100. The microprocessor 70 utilizing data
represented by the precision thermistor curve 88 adjusts
the uncorrected output voltage to generate a corrected
output voltage on line 99. The corrected output voltage
represents the correct output by taking into
consideration the physical temperature "Td" of the noise
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source 100. The shift in the calibration curves 88, 90,
92 and 94 have been exaggerated to illustrate the
correction procedures of the radiometer calibration
system 150. The data represented by the calibration
curves 88, 90, 92, and 94 have been utilized to calibrate
the output signal of the radiometer receiver 16 when
operating in the antenna mode 82.
Referring to FIGURE 7, there is illustrated a
simplified schematic of the active solid state cold/hot
noise source 100 implemented as a microwave integrated
circuit. The microwave integrated circuit utilizes
either microwave integrated circuit (MIC) or monolithic
microwave integrated circuit (MMIC) technologies. The
noise source 100 is designed to operate in the microwave
and millimeter wave spectrum having a frequency range of
18-40 GHz.
The noise source 100 includes an input port 110
where the hat thermal radiation temperature "Tw" and the
cold thermal radiation temperature "Tc" are reflected to
the calibration line 64 (FIGURE 4) and applied to the
radiometer receiver 16 (FIGURE 4) during the calibration
mode 84. The input port 110 is an element of an input
matching impedance network 112 which includes a plurality
of input transmission lines configured and sized to match
the impedances of the input port and a noise equivalent
model 114. The plurality of input~transmission lines are
preferably manufactured with A1203 and are approximately
0.015" thick. The input matching impedance network 112
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is connected to a gate port 116 of the noise equivalent
model 114.
The noise equivalent model 114 is a microwave active
circuit designed to simulate noise temperatures such as
the hot and cold thermal radiation temperatures "Tw" and
"Tc" When DC bias is applied. A report regarding the
simulation of noise temgeratures using a model was
presented at the 1995 IEEE International Microwave
Symposium by P.B. winson, S.M. Landizabal, and L.P.
Dunleavy entitled "A Table Based Bias and Temperature
Dependent Small Signal and Noise Equivalent Circuit
Model."
The term "noise temperature" is an expression for
the noise power spectral density at a specified frequency
and is derived from Planck's blackbody formula. The
average energy of an oscillator at a temperature T is:
<Ea = ~~ (1)
exp (hf JkT) - 1
where f is the frequency; h is Planck's constant; and k
is the thermal conductivity. At high temperatures and
low frequencies <E> approaches kT so the power in a
bandwidth B will be P=kTB (Nyquist's formula). A
quantity ~ =P/kB is taken as a convenient unit of thermal
noise power spectral density and is referred to as "noise
temperature."
The noise equivalent model 114 utilizes a field
effect transistor (FET) having a drain port 118, a source
port 120 and the gate port 116. The drain port 118 is
terminated during the operation of the noise source 10o.
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The source port 120 is connected to a source inductance
122 to provide series feedback for the noise source 100,
where the source inductance 122 is preferably in the
range of 20-700 pH.
A bias circuit 128 generates a DC bias that is
applied to the noise source 100, during the calibration
mode 84. The microprocessor 70 adjusts the magnitude of
the DC bias to change the values of the cold and hot
thermal radiation temperatures "Tc" and "Tw". More
l0 particularly, the DC bias corresponds to the bias command
signal 74 transmitted from the microprocessor 70 (FTGURE
4) such that a hot thermal radiation temperature "Tw" and
a cold thermal radiation temperature "Tc" are alternately
reflected to the gate port 116 of the noise equivalent
model 114.
A stabilizing compensation circuit 130 in contact
with the noise equivalent model 114 and connected to the
microprocessor 70 (FIGURE 4) provides further control of
the DC bias. The stabilizing circuit 130 includes the
precision thermistor 76 and measures the physical
temperature "Td" of the noise source 100. If the
stabilizing compensation circuit 130 is not used then
fluctuations in the physical temperature "Td" of the
noise source 100 may adversely effect the performance of
the noise source.
An output matching impedance network 124 includes a
load 126 and a plurality of output transmission lines
configured and sized to match the impedances of the load
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and the drain port 118 of the noise equivalent model 114.
The output matching network 124 is connected to the drain
port 118. The plurality of output transmission lines are
preferably manufactured from A1203 and are approximately
o.ols" thick.
Referring to FIGURE 8, there is illustrated a graph
of a thermal radiation curve for the noise source 100
operating at l9GHz and 22GHz with DC bias. The typical
noise temperatures generated by the noise source 100 are
in the range of 100°K to 1400°K.
Referring to FIGURE 9 there is a graph illustrating
a noise figure measurement of the radiometer receiver 16.
Noise figure measurement is the process of quantitatively
determining the ratio of the total noise power per unit
bandwidth at the output of the noise source 100 to the
portion of the noise power due to the input termination.
at the standard noise temperature of 290°K. The noise
figure (F) equation may be represented by the following
equation:
F = Tr/To + 1 (2)
where "Tr" is the receiver naise temperature and "To"
represents the temperature of the radiometer receiver 16.
"To" is measured using a receiver precision thermistor
(not shown) mounted on RF components in the radiometer
receiver 16.
The following equations are derived by referring to
FIGURE 9 and are relevant in calculating the noise figure
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measurement utilizing a linear radiometer receiver 16
discussed above:
For a linear radiometer receiver --
yo - yc yw - Vc
Tin - Tc = Tw - Tc ' (3)
Tin = Tc or Tw applied to the
radiometer receiver (4)
For Tin = 0
Vr = Vc - Tc [ T~, ~ and ( 5 )
Tr = Vc ~ V-~, - Tc ( 6 )
The noise figure is expressed by:
F = Tr/To + 1 (where To ~ 290°K (ambient)) (7)
where' "Vc°, "Vr" and "Vw" axe the radiometer output
voltages corresponding to "Tc", "Tr" and "Tw",
respectively.
While the present invention has been described with
reference to the illustrated embodiment, it is not
intended to limit the invention but, on the contrary, it
is intended to cover such alternatives, modifications and
equivalents as may be included in the spirit and scope of
the invention as defined in the following claims.
