Radiometer Systems

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Radiometer Systems

• INEL 6669
• microware remote sensing
• S. X-Pol

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Microwave Sensors
Radar (active sensor)
Radiometer (passive sensor)
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Radiometers

• Radiometers are very sensitive receivers that
  measure thermal electromagnetic emission (noise)
  from material media.
• The design of the radiometer allows measurement
  of signals smaller than the noise introduced by
  the radiometer (systems noise).

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Topics of Discussion

• Equivalent Noise Temperature
• Noise Figure Noise Temperature
• Cascaded System
• Noise for Attenuator
• Super-heterodyne Receiver
• System Noise Power at Antenna
• Radiometer Operation
• Measurement Accuracy and Precision
• Effects of Rx Gain Variations

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Topics of Discussion

• Dicke Radiometer
• Balancing Techniques
• Reference -Channel Control
• Antenna-Channel Noise-Injection
• Pulse Noise-Injection
• Gain-Modulation
• Automatic-Gain Control (AGC)
• Noise-Adding radiometer
• Practical Considerations Calibration Techniques

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Radiometers Task Measure antenna temperature,
TA which is proportional to TB, with sufficient
radiometric resolution and accuracy

• TA varies with time.
• An estimate of TA is found from
• Vout and
• the radiometer resolution DT.

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Noise voltage

• The noise voltage is
• the average0 and the rms is

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Noisy resistor connected to a matched loadis
equivalent to ZL(RjX)R-jX
Independent of f and R!,
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Equivalent Output Noise Temperature for any noise
source
TE is defined for any noise source when connected
to a matched load. The total noise at the output
is
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Noise Figure, F

• Measures degradation of noise through the device
• is defined for To290K (62.3oF!, this winter in
  Puerto Rico.)

Total output signal Total output noise
Noise introduced by device
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Noise Figure, F

• Noise figure is usually expressed in dB
• Solving for output noise power

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Equivalent input noise TE

• Noise due to device is referred to the input of
  the device by definition
• So the effective input noise temp of the device
  is
• Where, to avoid confusion, the definition of
  noise has been standardized by choosing To290K
  (room temperature)

75K
Examples 1dB NF is and 3dB NF is
What is TE for F2dB?
288K
170K
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Cascade System
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Noise of a cascade system
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Noise for an Attenuator
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Antenna, TL and Rx
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Superheterodyne Receivers

• Rx in which the RF amplifier is followed by a
  mixer that multiplies the RF signal by a sine
  wave of frequency LO generated by a local
  oscillator (LO).  The product of two sine waves
  contains the sum and difference frequency
  components
• The difference frequency is called the
  intermediate frequency (IF).  The advantages of
  superheterodyne receivers include
• doing most of the amplification at lower
  frequencies (since IFltRF), which is usually
  easier, and
• precise control of the RF range covered via
  tuning only the local oscillator so that back-end
  devices following the un-tuned IF amplifier,
  multichannel filter banks or digital
  spectrometers for example, can operate over fixed
  frequency ranges.

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Superheterodyne receiver
G23dB F7.5dB
RF amp Grf ,Frf ,Trf
IF amp Gif ,Fif ,Tif
Mixer GM,FM,TM
Pni
Pno
G30dB F2.3dB
G30dB F3.2dB
LO
Example Trf290(10.32-1)638K Tm1,340K Tif203K
TREC?
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Equivalent System noise power at antenna terminals

• Taking into consideration the losses at the
  antenna and T.L. with a physical temperature of
  Tp

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Equivalent System noise power at antenna terminals

• Then the total noise for the system is

For radiometer , Psys Prec For Radar, S/N
Pr/Psys
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Summary

• Antenna
• Antenna losses
• Receiver
• Receiver T.L.
• All of the above

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Measurement Accuracy and Precision

• Accuracy (certeza) how well are the values of
  calibration noise temperature known in the
  calibration curve of output corresponding to TA
  . (absolute cal.)
• Precision (precisión) smallest change in TA
  that can be detected by the radiometer
  output.(sensitivity) DT

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Total Power Radiometer
Super-heterodyne receiver uses a mixer, L.O. and
IF to down-convert RF signal. Usually BRFgtBIF
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Detection- power spectra _at_
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Noise voltage after IF amplifier
The instantaneous IF voltage has a time-varying
envelop ve(t) and phase angle f(t)
with zero average
The average IF power is equal to the average of
the square of vIF(t)
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Noise voltage after detector, Vd
The detector voltage is proportional to the
square of the envelop voltage
Ve
Vd
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Noise voltage after Integrator
Ve

• For averaging the radiometer uses an Integrator
  (low pass filter). It averages the signal over an
  interval of time t with voltage gain gI.
• Integration of a signal with bandwidth B during
  that time, reduces the variance by a factor NBt,
  where B is the IF bandwidth.

Vd
Vout
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Radiometric Resolution, DT
Ve
Vd

• The output voltage of the integrator is related
  to the average input power, Psys

Vout
GS is the overall system gain factor.
Which can be solved forTA
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Noise averaging

• By averaging a large number N of independent
  noise samples, an ideal radiometer can determine
  the average noise power and detect a faint source
  that increases the antenna temperature by a tiny
  fraction of the total noise power.
• http//www.cv.nrao.edu/course/astr534/Radiometers.
  html
• http//www.millitech.com/pdfs/Radiometer.pdf

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The IF voltage

• Is a sum of noise signals with same frequency
• In phase-domain
• Since summing Ns random noise sources, Ve has
  probability density function pdf given by (see
  section 5.7 Ulaby Long 2013)
• With an associated standard-deviation
• to mean ratio

Before integration
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The detection voltage Vd has a DC component and
an AC component.

• The DC component is proportional to the Tsys
• The AC component are related to the fluctuations
  related to the statistical uncertainties of
  measurement.

Before integration the uncertainty is so large
that its equal to the signal we want to detect.
So we need to filter the AC AC component which
is equivalent to integrating (averaging) over
time.
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Integration

• Averaging over a B bandwidth and during t time,
  reduces the variance by a factor NBt
• Total rms uncertainty

Still have fluctuations after LPF but are smaller
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Radiometric Sensitivity

• Since and
  then
• The Noise-caused uncertainty
• Its the minimum (statistically) detectable
  change in radiometric antenna temperature of the
  observed scene.

Radiometric Sensitivity (or resolution)
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Total-power radiometer

• This doesnt take into account variations in Gain
• Its also known as
• Where the bandwidth is called the predetection
  bandwidth and given a nonuniform transfer
  funcition is given by

Ideal total-power radiometer
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Receiver Gain variations

• DT is due to various causes
• Noise-caused uncertainty
• Gain-fluctuations uncertainty
• Total rms uncertainty

Total-power radiometer resolution including gain
variations
Also, Try with 10-5 gain variation and no RF amp
(TREC3000K)
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Gain Variations and the Dicke radiometer

• As you can see gain variations in practical
  radiometers, fluctuations in atmospheric
  emission, and confusion by unresolved radio
  sources may significantly degrade the actual
  sensitivity compared with the sensitivity
  predicted by the ideal radiometer equation.
• One way to minimize the effects of fluctuations
  in both receiver gain and atmospheric emission is
  to make a differential measurement by comparing
  signals from two adjacent feeds. The method of
  switching rapidly between beams or loads is
  called Dicke switching after Robert Dicke, its
  inventor. Using a double throw switch.

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Dicke radiometer
Unity-gain amplifiers (-) ()
The radiometer voltage is
The switching rate is fs switching period ts is
much shorter than integration time.
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Dicke Radiometer
Noise-Free Pre-detection Section Gain
G Bandwidth B
Switching rate, fs 1/ts

• Dicke Switch
• Synchronous Demodulator

Nyquist sampling theorem
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Dicke radiometer
The radiometer switches rapidly between reference
and antenna using the Dicke switching
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Dicke Radiometer resolution
The output voltage of the low pass filter in a
Dicke radiometer looks at reference and antenna
at equal periods of time with the minus sign for
half the period it looks at the reference load
(synchronous detector), so The receiver noise
temperature cancels out and the total uncertainty
in T due to gain variations is
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Dicke radiometer resolution

• The uncertainty in T due to noise when looking at
  the antenna or reference (half the integration
  time)
• Unbalanced Dicke radiometer resolution

Example B100MHz, t1s, Trec 700K, DG/G.01,
Tref300K for TA0K and 300K, for Total P
radiometer and Dicke radiometer
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Balanced Dicke
A balanced Dicke radiometer is designed so that
TA Tref at all times. In this case,
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Balancing Techniques

• Reference Channel Control
• Antenna Noise Injection
• Pulse Noise Injection
• Gain Modulation
• Automatic Gain Control

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Reference Channel Control
Force TA T ref
Switch driver and Square-wave generator, fS
Vout ?
Pre-detection G, B, TREC
Vout
TA
Integrator t
Synchronous Demodulator
Tref
Measures vc
Feedback and Control circuit
Vc
Variable Attenuator at ambient temperature To
L
TN
Noise Source
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Reference Channel Control

• TN and To have to cover the range of values that
  are expected to be measured, TA
• If 50kltTAlt 300K
• Use To 300K and need cryogenic cooling to
  achieve TN 50K.
• But L cannot be really unity, so need TN lt 50K.
  To have this cold reference load, one can use
• cryogenic cooled loads (liquid nitrogen submerged
  passive matched load)
• active cold sources (COLDFET) backward
  terminated LNA can provide active cold source.

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Cryogenic-cooled Noise Source

• When a passive (doesnt require power to work)
  noise source such as a matched load, is kept at a
  physical temperature Tp , it delivers an average
  noise power equal to kTpB
• Liquid N2 boiling point 77.36K
• Used on ground based radiometers, but not
  convenient for satellites and airborne systems.

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Active cold or hot sources

• http//www.maurymw.com/
• http//sbir.gsfc.nasa.gov/SBIR/successes/ss/5-049t
  ext.html

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Active noise source FET

• The power delivered by a noise source is
  characterized using the ENRexcess noise ratio
• where TN is the noise temperature of the source
  and To is its physical temperature.

Example for 9,460K ENR 15 dB
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Antenna Noise Injection
Measures vc
Force TA T ref T o
TN
Variable Attenuator
Fc Coupling factor of the
directional coupler
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Antenna Noise Injection

• Combining the equations and solving for L
• from this equation, we see that To should be gtTA
• If the control voltage is scaled so that Vc1/L,
  then Vc will be proportional to the measured
  temperature,

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Example Antenna Noise Injection
Find the necessary values of the Attenuator L, to
measure this range of Temperatures and the
resolution for this balanced Dicke radiometer
given
Choose To310K
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Example Antenna Noise Injection

• If 50Klt TAlt 300K, need to choose Togt300K, say
  To310K
• If Fc100(20dB) and Tn50,000K
• Find L variation needed

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Antenna Noise InjectionResolution

• For expected measured values between 50K and
  300K, Tref is chosen to be To310K, so
• Since the noise temperature seen by the input
  switch is always To , the resolution is

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Pulse Noise Injection
Measures fr
Switch driver and Square-wave generator, fS
TA
Vout
TA
Coupler
Pre-detection G, B, Trec
Integrator t
Synchronous Demodulator
TN
Tref
Feedback and Control circuit
f r
Pulse- Attenuation Diode switch
Noise Source
TN
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Pulse Noise Injection

• Reference T is controlled by the frequency of a
  pulse
• The repetition frequency is given by

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Pulse Noise Injection
Ton
Pulse repetition frequency fR 1/tR Pulse
width is constant tp Square-wave modulator
frequency fSlt fR/2
Toff
Switch ON minimum attenuation Switch Off
Maximum attenuation
ExampleFor Lon 2, Loff 100, tp 40 ms, To
300K and TN 1000K, F20dB
We obtain Ton 650K, Toff 307K
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Example Pulse Noise-Injection

• With

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Summary
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Summary
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Cont Source Microwave Radiometer Resolution
Optimization Using Variable Observation Times,
by Adriano Camps and Jose Miguel Tarongí
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Gain-Modulation
Measures vc
Switch driver and Square-wave generator, fS
TA
Fixed attenuator Lo
Synchronous Demodulator
Pre-detection G, B, Trec
Tref
Variable attenuator Lv
v c
Integrator t
Control circuit
Vout
Drawback slow variations of receiver noise
temperature, yields error in reading.
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Automatic-Gain-Control AGC

• Feedback is used to stabilize Receiver Gain
• Use sample-AGC not continuous-AGC since this
  would eliminate all variations including those
  from signal, TA.
• Sample-AGC Vout is monitored only during
  half-cycles of the Dicke switch period when it
  looks at the reference load.
• Hach in 1968 extended this to a
  two-reference-temperature AGC radiometer, which
  provides continuous calibration. This was used
  in RadScat on board of Skylab satellite in 1973.

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Automatic Gain-Control (AGC)
Switch driver and Square-wave generator, fS
2fs
Synchronous Demodulator 2fs
Pre-detection G, B, Trec
gv
Reference Switch
Integrator t
Synchronous Demodulator fs
fs
T2
Gv
T1
Feedback amplifier
Vagc
Hach radiometer insensitive to variations from
G, and Trec.
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Dicke Switch

• Two types
• Semiconductor diode switch, PIN
• Ferrite circulator
• Switching rate, fS ,
• High enough so that GS remains constant over one
  cycle.
• To satisfy sampling theorem, fS gt2BLF
• http//envisat.esa.int/instruments/mwr/descr/chara
  ct.html

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Dicke Input Switch

• Important properties to consider
• Insertion loss
• Isolation
• Switching time
• Temperature stability

http//www.erac.wegalink.com/members/DaleHughes/My
EracSite.htm
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Radiometer Receiver Calibration

• Most are linear systems
• Hach-radiometer is connected to two known loads,
  one cold (usually liquid N2), one hot.
• Solve for a and b.
• Cold load on satellites
• use outer space 2.7K
• http//ipnpr.jpl.nasa.gov/progress_report/42-154/1
  54G.pdf

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Imaging Considerations

• Scanning configurations
• Electronic (beam steering)
• Phase-array (uses PIN diode or ferrite
  phase-shifters, are expensive, lossy)
• Frequency controlled
• Mechanical (antenna rotation or feed rotation)
• Cross-track scanning
• Conical scanning (push-broom) has less variation
  in the angle of incidence than cross-track

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Uncertainty Principle for radiometers
M figure of merit

• For a given integration time, t, there is a
  trade-off between
• spectral resolution, B and
• radiometric resolution, DT
• For a stationary radiometer, make t larger.
• For a moving radiometer, t is limited since it
  will also affect the spatial resolution. (next)

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Airborne scanning radiometer
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Airborne scanning

• Consider a platform at height h, moving at speed
  u, antenna scanning from angles qs and qs , with
  beamwidth b, along-track resolution, Dx
• The time it takes to travel one beamwidth in
  forward direction is
• The angular scanning rate is
• The time it stays while scanning through one (1)
  beamwidth in the transverse direction is the
  dwell time

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Dwell time

• Is defined as the time that a point on the ground
  is observed by the antenna beamwidth. Using
• For better spatial resolution, small t
• For better radiometric resolution, need large t
• As a compromise, choose

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Radiometer Uncertainty Eq.

• Equating, we obtain

Radiometric resolution
This equation applies for this specific scanning
configuration.
Spatial resolution
Spectral resolution
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Problem

• A 1GHz balanced Dicke radiometer with a 100 MHz
  bandwidth is to be flown on a satellite at an
  altitude of 600 km with average speed of 7.5
  km/s.
• The radiometer uses a 10-m diameter antenna, and
  the receiver is characterized by Trec1000K and
  Tref300K. Take antenna efficiency k1.5 b?k
  l/l
• The radiometer integration time is chosen to be
  equal to 0.1 of the dwell time of the antenna
  beam for a point on the ground. If the antenna
  is fixed so that its main beam is always pointed
  in the nadir direction,
• What will DT be?

0.1678 K
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WindSat first images _at_ Ka