Radiometer systems
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Radiometer Systems. microware remote sensing S. Cruz Pol. Tx. Rx. Rx. Microwave Sensors. Radar (active sensor). Radiometer (passive sensor). Radiometers. Radiometers are very sensitive receivers that measure thermal electromagnetic emission (noise) from material media.

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Radiometer Systems

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Radiometer systems

Radiometer Systems

microware remote sensing

S. Cruz Pol


Radiometer systems

Tx

Rx

Rx

Microwave Sensors

Radar

(active sensor)

Radiometer

(passive sensor)

UPR, Mayagüez Campus


Radiometers

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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Topics of discussion

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 cont

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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Radiometer systems

TA’

TA

Radiometer

TB

Vout

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.

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Noise voltage

Noise voltage

  • The noise voltage is

  • the average=0 and the rms is

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Noisy resistor connected to a matched load is equivalent to z l r jx r jx

Noisy resistor connected to a matched loadis equivalent to… [ZL=(R+jX)*=R-jX]

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Radiometer systems

B, G

radiometer

TA

Pn=k B G TA

TA =200K

B, G

radiometer

Pn=k B G (TA + TN)

TN =800K

  • Ideal radiometer

  • “Real” radiometer

Usually we want DT=1K,

so we need B=100MHz and t =10msec

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Equivalent output noise temperature for any noise source

Receiver

antenna

Ideal Bandpass Filter

B, G=1

ZL

Equivalent Output Noise Temperature for any noise source

TEis defined for any noise source when connected to a matched load. The total noise at the outputis

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Measurement accuracy and precision

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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Noise figure f

input signal

input thermal noise

Noise Figure, F

Measures degradation of noise through the device

  • is defined for To=290K (62.3oF!)

Total output signal

Total output noise

Noise introduced by device

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Noise figure f1

Noise Figure, F

  • Noise figure is usually expressed in dB

  • Solving for output noise power

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Equivalent input noise t e

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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Cascade system

Cascade System

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Noise for an attenuator

Noise for an Attenuator

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Superheterodyne receiver

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(10.32-1)=638K

Tm=1,340K

Tif=203K

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Total power radiometer

Total Power Radiometer

Super-heterodyne receiver: uses a mixer, L.O. and IF to down-convert RF signal.

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Detection

Detection

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Noise voltage after if amplifier

IF

Noise voltage after IF amplifier

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Noise voltage after detector

IF

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.

Noise voltage after detector

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Noise voltage after integrator

Low-pass t, gLF

x2

integrator

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.

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Radiometric resolution d t

Low-pass t, gLF

x2

integrator

Radiometric Resolution, DT

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

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Receiver gain variations

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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Dicke radiometer

Dicke Radiometer

Noise-Free

Pre-detection Section

Gain = G

Bandwidth = B

  • Dicke Switch

  • Synchronous Demodulator

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Dicke radiometer1

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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Dicke radiometer2

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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Balanced dicke

Balanced Dicke

A balanced Dicke radiometer is designed so that TA’= Tref at all times. In this case,

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Balancing techniques

Balancing Techniques

  • Reference Channel Control

  • Antenna Noise Injection

  • Pulse Noise Injection

  • Gain Modulation

  • Automatic Gain Control

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Reference channel control

Reference Channel Control

Force T’A= T ref

Switch driver and

Square-wave generator, fS

Pre-detection

G, B, TREC’

Vout

TA’

Integrator

t

Synchronous

Demodulator

Tref

Feedback

and

Control circuit

Vc

Variable

Attenuator

at ambient

temperature

To

L

TN

Noise

Source

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Reference channel control1

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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Cryogenic cooled noise source

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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Active cold or hot sources

Active “cold or hot” sources

  • http://www.maurymw.com/

  • http://sbir.gsfc.nasa.gov/SBIR/successes/ss/5-049text.html

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Active noise source fet

Active noise source-FET

  • The power delivered by a noise source is characterized using the ENR=excess noise ratio

    where TNis 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

Switch driver and

Square-wave generator, fS

TA”

TA’

Vout

Pre-detection

G, B, Trec’

Coupler

Integrator

t

Synchronous

Demodulator

Tref

L

Feedback

and

Control circuit

Vc

TN

Noise

Source

Antenna Noise Injection

Force T’A= T ref = T o

T’N

Variable

Attenuator

Fc = Coupling factor of the

directional coupler

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Antenna noise injection1

Antenna Noise Injection

  • Combining the equations and solving for L

    from this equation, we see that Toshould be>TA’

  • If the control voltage is scaled so that Vc=1/L, then Vcwill be proportional to the measured temperature,

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Antenna noise injection2

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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Example

Example

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Pulse noise injection

Pulse Noise Injection

Switch driver and

Square-wave generator, fS

TA”

TA’

Vout

Pre-detection

G, B, Trec’

Coupler

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 injection1

tp

tR

TN’

Diode switch

TN

Pulse Noise Injection

Pulse repetition frequency = fR = 1/tR

Pulse width = tp

Square-wave modulator frequency fS< fR/2

Switch ON – minimum attenuation

Switch Off – Maximum attenuation

Example:

For Lon = 2, Loff = 100 , To = 300K

and T’N = 1000K

We obtain Ton= 650K, Toff= 297K

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Pulse noise injection2

Pulse Noise Injection

  • Reference T is controlled by the frequency of a pulse

  • The repetition frequency is given by

For Toff = To, is proportional to T’A

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Automatic gain control agc

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: Voutis 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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Dicke switch

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 >2BLF (Same as saying that Integration time is t =1/2BLF)

      http://envisat.esa.int/instruments/mwr/descr/charact.html

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Dicke input switch

Dicke Input Switch

Two important properties to consider

  • Insertion loss

  • Isolation

  • Switching time

  • Temperature stability

http://www.erac.wegalink.com/members/DaleHughes/MyEracSite.htm

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Radiometer receiver calibration

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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Imaging considerations

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

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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Airborne scanning radiometer

Airborne scanning radiometer

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Airborne scanning

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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Dwell time

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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Radiometer uncertainty eq

Radiometer Uncertainty Eq.

Radiometric resolution

Spatial resolution

Spectral

resolution

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Windsat first images @ ka

WindSat first images @ Ka

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