1. microware remote sensing S. Cruz Pol
Radiometer Systems
microware remote sensing
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2. Microwave Sensors Radar (active sensor) Radiometer Tx Rx Rx
(passive sensor)
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3. 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 (system’s noise).
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4. Topics of Discussion Equivalent Noise Temperature Radiometer Operation
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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5. Topics of Discussion (cont.)
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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6. Radiometer’s 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.
Radiometer TA
TA’
Vout TB
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7. Noise voltage The noise voltage is the average=0 and the rms is
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8. Noisy resistor connected to a matched load is equivalent to… [ZL=(R+jX)*=R-jX]
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9. Ideal radiometer “Real” radiometer B, G radiometer TA Pn=k B G TA B, G
TA =200K
Pn=k B G (TA + TN)
TN =800K
Usually we want DT=1K,
so we need B=100MHz and t =10msec
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10. Equivalent Output Noise Temperature for any noise source
Ideal Bandpass Filter
B, G=1 ZL
Receiver
antenna
TE is defined for any noise source when connected to a matched load. The total noise at the output is
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11. Measurement Accuracy and Precision
Accuracy – how well are the values of calibration noise temperature known in the calibration curve of output corresponding to TA‘ .
Precision – smallest change in TA‘ that can be detected by the radiometer output.
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12. Noise Figure, F Measures degradation of noise through the device
is defined for To=290K (62.3oF!)
input signal
input thermal noise
Total output signal
Total output noise
Noise introduced by device
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13. Noise Figure, F Noise figure is usually expressed in dB
Solving for output noise power
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14. 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 To=290K (room temperature)
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15. Cascade System
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16. Noise for an Attenuator
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17. Superheterodyne receiver
G=23dB
F=7.5dB
RF amp
Grf ,Frf ,Trf
IF amp
Gif ,Fif ,Tif
Mixer
LM,FM,TM
Pni
Pno
G=30dB
F=2.3dB
G=30dB
F=3.2dB LO
Trf=290( )=638K
Tm=1,340K
Tif=203K
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18. Total Power Radiometer
Super-heterodyne receiver: uses a mixer, L.O. and IF to down-convert RF signal.
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19. Detection
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20. Noise voltage after IF amplifier
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21. Noise voltage after detectorIF x2
square-law
detector
represents the average value or dc, and sd represents the rms value of the ac component or the uncertainty of the measurement.
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22. Noise voltage after Integrator
Integrator (low pass filter) averages the signal over an interval of time t.
Integration of a signal with bandwidth B during that time, reduces the variance by a factor N=Bt, where B is the IF bandwidth. x2
integrator
Low-pass t, gLF
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23. Radiometric Resolution, DTx2
integrator
Low-pass t, gLF
The output voltage of the integrator is related to the average input power, Psys
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24. Receiver Gain variations
Noise-caused uncertainty
Gain-fluctuations uncertainty
Total rms uncertainty
Example p.368
T’Rec=600K
T’A=300K
B=100MHz
=0.01sec
Find the radiometric resolution, DT
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25. Pre-detection Section
Dicke Radiometer
Noise-Free
Pre-detection Section
Gain = G
Bandwidth = B
Dicke Switch
Synchronous Demodulator
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26. Dicke Radiometer
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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27. Dicke radiometer
The uncertainty in T due to noise when looking at the antenna or reference (half the integration time)
Unbalanced Dicke radiometer resolution
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28. Balanced Dicke
A balanced Dicke radiometer is designed so that TA’= Tref at all times. In this case,
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29. Balancing Techniques Reference Channel Control Antenna Noise Injection
Pulse Noise Injection
Gain Modulation
Automatic Gain Control
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30. Reference Channel Control
Force T’A= T ref
Switch driver and
Square-wave generator, fS
Pre-detection
G, B, TREC’
TA’
Vout
Integrator t
Synchronous
Demodulator
Tref
Feedback
and
Control circuit Vc
Variable
Attenuator
at ambient
temperature To L TN
Noise
Source
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31. Reference Channel Control
TN and To have to cover the range of values that are expected to be measured, TA’
If 50k<TA’< 300K
Use To= 300K and need cryogenic cooling to achieve TN =50K.
But L cannot be really unity, so need TN < 50K. To have this cold reference load, one can use
cryogenic cooled loads (liquid nitrogen submerged passive matched load)
active “cold” sources (COLDFET).
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32. Cryogenic-cooled Noise Source
When a passive (doesn’t require output 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 = 77.36°K
Used on ground based radiometers, but not convenient for satellites and airborne systems.
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33. Active “cold or hot” sources
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34. Active noise source-FET
The power delivered by a noise source is characterized using the ENR=excess 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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35. Antenna Noise Injection
Force T’A= T ref = T o
Vout
Synchronous
Demodulator
Tref
Coupler
Pre-detection
G, B, Trec’
Feedback
and
Control circuit
Switch driver and
Square-wave generator, fS
Integrator t L Vc TN
Noise
Source
TA’
TA”
T’N
Variable
Attenuator
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Fc = Coupling factor of the
directional coupler
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36. Antenna Noise Injection
Combining the equations and solving for L
from this equation, we see that To should be >TA’
If the control voltage is scaled so that Vc=1/L, then Vc will be proportional to the measured temperature,
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37. Antenna Noise Injection
For expected measured values between 50K and 300K, Tref is chosen to be To=310K, so
Since the noise temperature seen by the input switch is always To , the resolution is
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38. Example
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39. Pulse Noise Injection t Switch driver and Square-wave generator, fS
TA”
TA’
Vout
Coupler
Pre-detection
G, B, Trec’
Integrator t
Synchronous
Demodulator
TN’
Tref
f r
Feedback
and
Control circuit
Diode switch
Attenuation
Pulse-
Noise
Source TN
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40. Pulse Noise Injection tp tR
Pulse repetition frequency = fR = 1/tR
Pulse width = tp
Square-wave modulator frequency fS< fR/2
Switch ON – minimum attenuation
Switch Off – Maximum attenuation
Diode switch TN
TN’
Example:
For Lon = 2, Loff = 100 , To = 300K
and T’N = 1000K
We obtain Ton= 650K, Toff= 297K
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41. Pulse Noise Injection
Reference T is controlled by the frequency of a pulse
The repetition frequency is given by
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For Toff = To, is proportional to T’A
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42. 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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43. Dicke Switch Two types Switching rate, fS ,
Semiconductor diode switch, PIN
Ferrite circulator
Switching rate, fS ,
High enough so that GS remains constant over one cycle.
To satisfy sampling theorem, fS >2BLF (Same as saying that Integration time is t =1/2BLF)
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44. Dicke Input Switch Two important properties to consider Insertion loss
Isolation
Switching time
Temperature stability
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45. Radiometer Receiver Calibration
Most are linear systems
The radiometer is connected to two known loads, one cold (usually liquid N2), one hot.
Solve for a and b.
Cold load :satellites
use outer space ~2.7K
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46. 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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47. Uncertainty Principle for radiometers
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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48. Airborne scanning radiometer
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49. 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 takes to scan through one beamwidth in the transverse direction is the dwell time
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50. 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, large t
As a compromise, choose
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51. Radiometer Uncertainty Eq.
Radiometric resolution
Spatial resolution
Spectral
resolution
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52. WindSat first images @ Ka
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