Thermal measuring and testing – Thermal calibration system
Reexamination Certificate
1999-08-23
2001-04-17
Gutierrez, Diego (Department: 2859)
Thermal measuring and testing
Thermal calibration system
C374S175000
Reexamination Certificate
active
06217210
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
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 an electronically adjustable noise source providing hot thermal radiation temperature from an output port and cold thermal radiation temperature from an input port.
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 that bound the brightness temperatures of the scene are used to calibrate a radiometer receiver. 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 mostly cold range of 10° K.-300° K. There are also specialized radiometer applications where an instrument is needed to measure hot brightness temperatures from forest fires and burning dumps. For these applications the radiometer must measure brightness temperatures in the range of 300° K. to greater than 1000° K. The ground-based systems may utilize closed cycle refrigeration such as a sterling cycle cooler with liquid nitrogen or is 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
FIGS. 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
150
. Currently, satellite-based systems use calibration techniques that are either externally-based (
FIGS. 1 and 2
) or internally-based (FIG.
3
).
Referring to
FIG. 1
, there is illustrated an externally-based calibration technique known as the sky horn approach. The sky horn approach utilizes a radiometer
150
which includes a first RF switch
10
connected to 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 warm thermal radiation temperature “Tw,” approximately 300° K. A precision thermistor
24
in thermal contact with the warm load
22
outputs an electrical hot thermal radiation temperature “Td” that is 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
FIG. 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 warm 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 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
150
.
Referring to
FIG. 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 FIG.
1
. However, the internal technique may utilize a thermoelectric 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 warm and cold thermal radiation temperatures “Tc” and “Tw” used in the internal is approach may only be 30° K. apart. The 30° K. difference between the cold and warm thermal radiation temperatures “Tc” and “Tw” does not cover the full range of earth brightness temperatures which are approximately 100° K. to 300° K., (exclusive of burning materials) therefore, measurement accuracy of the radiometer receiver
16
will likely degrade below the cold thermal radiation temperature “Tc.” Accordingly, there is a need for an adjustable calibration noise source to provide cold to hot thermal radiation temperatures from a waveguide or coaxial port. There is also a need to provide a noise source manufactured using microwave integrated circuit (MIC) and/or monolithic microwave integrated circuit (MMIC) technologies. These and other needs are satisfied by the adjustable calibration noise source of the present invention.
SUMMARY OF THE INVENTION
The present invention is a radiometer calibration system utilizing an electronically adjustable noise source and a method for calibrating a radiometer. The noise source includes a transistor configured as a noise equivalent circuit having a gate port, drain port and source port. A source inductance providing series feedback for the noise source has one end coupled to the source port of the noise equivalent circuit and another end connected to ground. A bias circuit controls the amount of DC bias applied to the noise equivalent model. In order to match the impedances in the noise source, an output impedance matching network is connected to the drain port and an input impedance matching network is connected to the gate port of the noise equivalent model. The output and input impedance networks have an output port and input port, respectively. The noise source terminates a matched load to the output port while an adjustable cold thermal radiation temperature is generated at the input port. Alternatively, a port switch may be used to terminate a matched load to the input port while an adjustable hot thermal radiation temperature is generated at the output port.
According to the present invention there is provided an adjustable noise source for calibrating ground-based, airborne, or satellite-based radiometers.
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 noise source implemented as an integrated circuit.
Further in accordance with the present invention there is provided a calibration system having a noise source for measuring the radiometer receiver transfer function or receiver linearity.
Further in accordance 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 wi
Dunleavy Lawrence P.
Lardizabal Steven M.
Roeder Robert S.
Smith Matthew C.
Baker & Botts L.L.P.
De Jesus Lydia M.
Gutierrez Diego
Raytheon Company
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