microware remote sensing S. Cruz Pol Radiometer Systems microware remote sensing S. Cruz Pol INEL 6069

Microwave Sensors Radar (active sensor) Radiometer Tx Rx Rx (passive sensor) UPR, Mayagüez Campus INEL 6069

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). UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus INEL 6069

Noise voltage The noise voltage is the average=0 and the rms is UPR, Mayagüez Campus INEL 6069

Noisy resistor connected to a matched load is equivalent to… [ZL=(R+jX)*=R-jX] UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus INEL 6069

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. UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus INEL 6069

Noise Figure, F Noise figure is usually expressed in dB Solving for output noise power UPR, Mayagüez Campus INEL 6069

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) UPR, Mayagüez Campus INEL 6069

Cascade System UPR, Mayagüez Campus INEL 6069

Noise for an Attenuator UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus INEL 6069

Total Power Radiometer Super-heterodyne receiver: uses a mixer, L.O. and IF to down-convert RF signal. UPR, Mayagüez Campus INEL 6069

Detection UPR, Mayagüez Campus INEL 6069

Noise voltage after IF amplifier UPR, Mayagüez Campus INEL 6069

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. UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus INEL 6069

Radiometric Resolution, DT x2 integrator Low-pass t, gLF The output voltage of the integrator is related to the average input power, Psys UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus INEL 6069

Pre-detection Section Dicke Radiometer Noise-Free Pre-detection Section Gain = G Bandwidth = B Dicke Switch Synchronous Demodulator UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus INEL 6069

Dicke radiometer The uncertainty in T due to noise when looking at the antenna or reference (half the integration time) Unbalanced Dicke radiometer resolution UPR, Mayagüez Campus INEL 6069

Balanced Dicke A balanced Dicke radiometer is designed so that TA’= Tref at all times. In this case, UPR, Mayagüez Campus INEL 6069

Balancing Techniques Reference Channel Control Antenna Noise Injection Pulse Noise Injection Gain Modulation Automatic Gain Control UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus INEL 6069

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). UPR, Mayagüez Campus INEL 6069

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. UPR, Mayagüez Campus INEL 6069

Active “cold or hot” sources http://www.maurymw.com/ http://sbir.gsfc.nasa.gov/SBIR/successes/ss/5-049text.html UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus Fc = Coupling factor of the directional coupler INEL 6069

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, UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus INEL 6069

Example UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus INEL 6069

Pulse Noise Injection Reference T is controlled by the frequency of a pulse The repetition frequency is given by UPR, Mayagüez Campus For Toff = To, is proportional to T’A INEL 6069

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. UPR, Mayagüez Campus INEL 6069

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) http://envisat.esa.int/instruments/mwr/descr/charact.html UPR, Mayagüez Campus INEL 6069

Dicke Input Switch Two important properties to consider Insertion loss Isolation Switching time Temperature stability http://www.erac.wegalink.com/members/DaleHughes/MyEracSite.htm UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus INEL 6069

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) UPR, Mayagüez Campus INEL 6069

Airborne scanning radiometer UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus INEL 6069

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 UPR, Mayagüez Campus INEL 6069

Radiometer Uncertainty Eq. Radiometric resolution Spatial resolution Spectral resolution UPR, Mayagüez Campus INEL 6069

WindSat first images @ Ka UPR, Mayagüez Campus INEL 6069