6/6/2014upr, mayagüez campus radiometer systems inel 6669 microware remote sensing s. x-pol
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04/10/23 UPR, Mayagüez Campus
Radiometer Systems
INEL 6669 microware remote sensing
S. X-Pol
04/10/23
RxTx Rx
Radar
(active sensor)Radiometer
(passive sensor)
Microwave Sensors
04/10/23 UPR, Mayagüez Campus
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).
04/10/23 UPR, Mayagüez Campus
Topics of Discussion
Equivalent Noise TemperatureNoise Figure & Noise Temperature
Cascaded SystemNoise for AttenuatorSuper-heterodyne Receiver
System Noise Power at AntennaRadiometer Operation
Measurement Accuracy and PrecisionEffects of Rx Gain Variations
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Topics of Discussion…
Dicke RadiometerBalancing Techniques
Reference -Channel ControlAntenna-Channel Noise-InjectionPulse Noise-InjectionGain-Modulation
Automatic-Gain Control (AGC) Noise-Adding radiometerPractical Considerations &Calibration
Techniques
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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 T.
Rad
iom
eter
TA
TA’
Vout
TB
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Noise voltage
The noise voltage is
the average=0 and the rms is
kTBRkThf
hfBR
e
hfBRV
JeansRayleighkThfn 4
/
4
1
4/
kTBRVV nrms 422
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Noisy resistor connected to a matched loadis equivalent to… [ZL=(R+jX)*=R-jX]
kTBR
kTBR
R
VVIVP rmsrms
nn
4
4
22
Independent of f and R!,
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Equivalent Output Noise Temperature for any noise source
BkTP Eno
TE is defined for any noise source when connected to a matched load. The total noise at the output is
ATIdeal Bandpass Filter
B, G=1
ZL
BkTP AA'
Receiverantenna
'AT
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Noise Figure, F Measures degradation of noise through the device
is defined for To=290K (62.3oF!, this = winter in Puerto Rico.)
noso
nisi
oo
ii
PP
PP
NS
NSF
/
/
/
/
oE TFT )1(
Total output signal
Total output noise
Noise introduced by device
input signal
input thermal noise
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Noise Figure, F
Noise figure is usually expressed in dB
Solving for output noise power
nonino
siso
PGPP
GPP
BGkT
P
BkT
PBGkT
GPP
PP
PP
PPF
o
no
o
noo
niso
nosi
noso
nisi
1
1
/
/
FFdB log10
niono FGPBFGkTP BGkTFP ono )1(
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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 To=290K (room temperature)
BGkTBGkTFP Eiono )1(
oEoE TTFTFT /1or )1(
Examples: 1dB NF is
and 3dB NF is What is TE for F=2dB?
170K
75K
288K
04/10/23 UPR, Mayagüez Campus
Cascade System
BTTkGG
PPGG
BG
TTTkGG
PGPGGPGGP
E
Ein
EE
EEnino
121
21
1
21121
2212121
1
21 G
TTT E
EE
1
21
11
G
FF
T
TF
o
E
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Noise of a cascade system
12121
3
1
21 ...
1...
11
N
N
GGG
F
GG
F
G
FFF
12121
3
1
21 ...
...
N
ENEEEE GGG
T
GG
T
G
TTT
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Noise for an Attenuator
BkTBkTLPLP
PBkTL
P
BkTP
P
PGL
EpnoE
nopno
pno
o
i
)1(
1
1/1
LTTLF
TLT
where
op
pE
/)1(1
)1(
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Antenna, TL and Rx
1
21'
G
TTT E
EREC
dBLKTKT
Example
prec 5.,290,50
:
KT
yields
REC 5.91'
...
ReceiverReceiver
TTE2E2
Transmission
Line, TE1
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
IF<RF), 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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)t] cos[-)t]-cos[(t)t)sin(2sin( RFLORFLORFLO
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RF amp G rf ,F rf ,T rf
Superheterodyne receiver
...MRF
IF
RF
MRFREC GG
T
G
TTT
MixerLM,FM,TM
IF amp G if ,F if ,T if
LO
Pni Pno
G=30dBF=2.3dB
G=23dBF=7.5dB
G=30dBF=3.2dB
Example:Trf=290(10.32-1)=638KTm=1,340KTif=203KTREC=?
KTREC 34.639...20010
203
10
1340638
33
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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:
ReceiverReceiver Transmission
Line
ARECsys PPP ''
BLTTLkBkTP
BTTkBkTP
TTT
RECpRECREC
plAlAA
plAlA
)1(''
and
)1(''
then,
)1(' Given
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Equivalent System noise power at antenna terminals
Then the total noise for the system is:
ReceiverReceiver Transmission
Line
RECpplAlsys
RECpplAlsys
sysRECASYS
LTTLTTT
or
BLTTLTTkBkT
BkTPPP
)1()1(
)1()1(
''
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 aboveBkTP
P
P
P
P
sysSYS
REC
REC
A
A
'
'
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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) T
04/10/23 UPR, Mayagüez Campus
Total Power Radiometer
Super-heterodyne receiver: uses a mixer, L.O. and IF to down-convert RF signal. Usually BRF>BIF
04/10/23 UPR, Mayagüez Campus
Detection- power spectra @:
deviation standard theis where
voltagespositivefor )(
envelope
withmean, zero with noiseGaussian
as drepresente is voltageIF the
noise thermalis spectraPower
'' where
''
2
2
22
eV
ee
SYSIF
RECASYS
SYS
RECASYS
eV
Vp
BGkTP
TTT
BkT
PPP
From Ulaby, Moore & Fung, 1986
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Noise voltage after IF amplifier
deviation standard theis where
2
1R Assuming
2
is voltagesquare noise for the mean value theshown that becan It
22
22
eIF
e
VP
V
IFddedd
dd
edd
d
PCCVCV
μV/μW CC
VCV
V
22
2
2
is oltagedetector v of valueaverage The
7 e.g. constant,detector theis where
is detector, law-square theofoutput The
SYSdGkBTC
IF
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Noise voltage after detector, Vd
V
σ
V
V
V
eV
Vp
dVVpdVVp
d
d
dd
dd
dd
V
V
d
d
eedd
d
d
1
or 1/ So,
is of variancethe
1
)(Then
)()(
Since
22
22
represents the average value or dc, and d represents the rms value of the ac component or the uncertainty of the measurement.
dV
IFx2
square-lawdetector
Ve Vd
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Noise voltage after Integrator
Integrator (low pass filter) averages the signal over an interval of time .
Integration of a signal with bandwidth B during that time, reduces the variance by a factor N=Bwhere B is the IF bandwidth.
BV
σ
BV
σ
V
σ
out
out
IFd
d
out
out
1
or
1
filter pass-low theofoutput at the voltageThe
2
2
2
2
x2
integrator
Low-pass , gLF
VoutVdVe
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Radiometric Resolution, T
The output voltage of the integrator is related to the average input power, Psys
VgV dLFout
x2
integrator
Low-pass , gLF
SYSS
SYSdLF
TG
GkBTCg
BT
T
V
σ
sys
sys
out
out
1
B
TT
B
TT RECAsys
sys
''
VoutVdVe
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.htmlhttp://www.millitech.com/pdfs/Radiometer.pdf
04/10/23
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Receiver Gain variations
Noise-caused uncertainty
Gain-fluctuations uncertainty
Total rms uncertainty
B
TT SYS
N
S
SSYSG G
GTT
22GN TTT
Example p.368T’Rec=600KT’A=300KB=100MHz=0.01sec
Find the radiometric resolution, T
01.
S
S
G
G
Gain Variations and 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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2
222
222
'2/
'
2/
'' refA
S
SRECrefRECA
GrefNantN
TTG
G
B
TT
B
TT
TTTT
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Dickie Radiometer
Noise-caused uncertainty
Gain-fluctuations uncertainty
Total rms uncertainty
B
TT SYS
N
S
SSYSG G
GTT
22GN TTT
QuizT’Rec=500KT’A=150KB=100MHz=1msec
Find the radiometric resolution, T
unknownG
G
S
S
2
222
222
''2''2
refAS
SRECrefRECA
GrefNantN
TTG
G
B
TTTT
TTTT
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Dicke Radiometer
•Dicke Switch
•Synchronous Demodulator
Noise-Free
Pre-detection Section
Gain = G
Bandwidth = B
Switching rate, fs= 1/s
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Dicke Radiometer
'''2
1RECREFRECASout TTTTGV
REFAS
SG TT
G
GT
'
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
The uncertainty in T due to noise when looking at the antenna or reference (half the integration time)
Unbalanced Dicke radiometer resolution
2
222
222
''2''2
refAS
SRECrefRECA
refNantNG
TTG
G
B
TTTT
TTTT
B
TTT RECref
refN
'2
B
TT
B
TTT RECARECA
antN
''2
2/
''
Give example: B=100MHz, =1s, T’rec= 700K, G/G=.01, Tref=300K for T’A=0K and 300K, for Total P radiometer and Dicke radiometer
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Balanced Dicke
ideal
RECASYS
refAS
SRECrefRECA
refNantNG
TB
TT
B
TT
TTG
G
B
TTTT
TTTT
2''22
''2''2
222
222
A balanced Dicke radiometer is designed so that TA’= Tref at all times. In this case,
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Balancing Techniques
Reference Channel ControlAntenna Noise InjectionPulse Noise InjectionGain ModulationAutomatic Gain Control
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Reference Channel Control
oN
ref TLL
TT
11
VoutSynchronousDemodulator
Tref
Pre-detection
G, B, TREC’
Feedback
and
Control circuit
Switch driver andSquare-wave generator, fS
Integrator
LVariableAttenuatorat ambient
temperature
To
Vc
TN
Noise Source
TA’
oref
Nref
refA
TTL
TTL
TT
if
1 if
'
Force T’A= T ref
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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 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); backward terminated LNA can
provide active cold source.
oAN TTT '
04/10/23
Cryogenic-cooled Noise Source
When a passive (doesn’t 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 = 77.36°K
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/5-049text.html
04/10/23
Ideal radiometer
“Real” radiometer
Usually we wantT=1K,so we need B=100MHz and =10msec
B
TTT NA
B, G
radiometerTA
Pn=k B G TA
B, G
radiometer
TA =200K
Pn=k B G (TA + TN)
TN =800K
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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.
ENRENR
T
T
kBT
TTkB
P
PPENR
dB
o
N
o
oN
o
on
log10
1)(
)(
Example for 9,460K: ENR= 15 dB
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Antenna Noise Injection
cA
c
NA
orefA
FT
F
TT
TTT
11'
'"
"
VariableAttenuator
VoutSynchronousDemodulator
Tref
Coupler Pre-detection
G, B, Trec’
Feedback
and
Control circuit
Switch driver andSquare-wave generator, fS
Integrator
L Vc
TN
Noise Source
TA’TA”
L
T
LTT N
oN
11'
T’N
Force T”A= T ref = T o
Fc = Coupling factor of the directional coupler
*Measures vc
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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,
'1 AoC
oN
TTF
TTL
'1Ao
oN
CC TT
TT
FV
'
AT
04/10/23
Example: Antenna Noise Injectio
K
B
TTT
L
KT
F
KTK
RECo
N
c
A
02.2'2
50-1.93between vary tohas
ENR) (22dB 000,50
100)(Coupler ldirectiona dB20
30050 '
'1 from
AoC
oN
TTF
TTL
04/10/23
Example: Antenna Noise Injection
If 50K< TA’< 300K, need to choose To>300K, say To=310K
If Fc=100(20dB) and Tn=50,000K
Find L variation needed:
'1 AoC
oN
TTF
TTL
KTforL
KTforL
A
A
3002.50
5093.1'
'
04/10/23
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
B
TTT RECo '2
L
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Pulse Noise Injection
poON
AoCR TT
TTFf
'
'1
cA
c
NA
orefA
FT
F
TT
TTT
11'
'"
"
LT
L
TT o
NN
11'
VoutSynchronousDemodulator
Tref
Coupler Pre-detection
G, B, Trec’
Feedback
and
Control circuit
Switch driver andSquare-wave generator, fS
Integrator
Pu
lse-
Atte
nu
atio
n
Dio
de
sw
itch
f r
TNNoise Source
TA’TA”
TN’
offo
off
Noff L
TL
TT
11'
*Measures fr
ono
on
Non L
TL
TT
11'
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Pulse Noise Injection
for
0for
'
'
'
RpOFF
pON
N
tT
tT
T
R
p Pulse repetition frequency = fR = 1/R
Pulse width is constant = p
Square-wave modulator frequency fS< fR/2
Switch ON – minimum attenuation
Switch Off – Maximum attenuation
off
N
offoOFF L
T
LTT
11'
Example:For Lon = 2, Loff = 100 , To = 300K
and TN = 1000K,
We obtain Ton= 650K, Toff= 307K
Diode switch
TN
TN’
T’on
T’off
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Pulse Noise Injection
Reference T is controlled by the frequency of a pulse
The repetition frequency is given by
''' )1( OFFRpRpONN TffTT
c
NA
coA F
TT
FTT
''" 1
1
poON
Aoc
pOFFON
ACOFFoCR TT
TTF
TT
TFTTFf
'
'
''
'' ))(1(1
For Toff = To, is proportional to T’A
04/10/23
Example; Pulse Noise-Injection
With:
needed rangefrequency Find
300'60
50
5.1
20
315
10
sec20p
KTK
dBL
dBL
dBENR
KT
dBF
A
off
on
o
c
HzfKT
kHzfKT
KT
KT
KT
F
Answers
rA
rA
on
off
N
c
302,300'
5,60'
22615
315
815,31
10
:
04/10/23
Gain-Modulation
Vout
SynchronousDemodulator
Pre-detection
G, B, Trec’
Control circuit
Switch driver andSquare-wave generator, fS
Integrator
v c
Tref
TA’
*Measures vc
Fixed attenuator
Lo
Variable attenuator
Lv
ocref
cA
vc
crefv
cAo
vout
LTT
TT
Lv
TTL
TTL
Lv
11
: thatso voltagecontrol theScale
11
:condition hemaintain t to vary 0, for
'Re
'
'Re
'
'Re
''Re
'
Drawback: slow variations of receiver noise temperature, yields error in reading.
04/10/23
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.
04/10/23
Automatic Gain-Control (AGC)
Vagc
SynchronousDemodulator
2fs
Pre-detection
G, B, Trec’
Feedback
amplifier
Switch driver andSquare-wave generator, fS
Integrator
Gv
Reference
Switch
2fs
T2T1
gv
SynchronousDemodulator
fs
fs
Hach radiometer: insensitive to variations from G, and Trec’.
04/10/23
Dicke Switch
Two types Semiconductor diode switch, PINFerrite 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 =1/2BLF)
http://envisat.esa.int/instruments/mwr/descr/charact.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/MyEracSite.htm
04/10/23
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 :satellites
use outer space ~2.7K
)(
)(
bTai
bTaicold
calcoldout
hotcal
hotout
rcAout fvbTai or or )( '
hotoutv
coldoutv
hotcalTcold
calT
04/10/23
Imaging Considerations
Scanning configurationsElectronic (beam steering)
Phase-array (uses PIN diode or ferrite phase-shifters, are expensive, lossy)
Frequency controlled
Mechanical (antenna rotation or feed rotation)Cross-track scanningConical scanning (push-broom) has less
variation in the angle of incidence than cross-track
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Uncertainty Principle for radiometers
For a given integration time, , there is a trade-off between spectral resolution, B andradiometric resolution, T
For a stationary radiometer, make larger.
For a moving radiometer, is limited since it will also affect the spatial resolution. (next)
B
MT
M= figure of merit
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Airborne scanning radiometer
04/10/23
Airborne scanning
Consider a platform at height h, moving at speed u, antenna scanning from angles s and –s , with beamwidth , along-track resolution, x
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
1
2
ts
Sd
t
21
u
xt
1
04/10/23
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
For better radiometric resolution, need large
As a compromise, choose
hu
xt
ssd
22
21
hx
suhx 2
B
MT
d
04/10/23
Radiometer Uncertainty Eq.
Equating, we obtain;
suhMBxT 2
Radiometric resolution
Spatial resolution
Spectralresolution
This equation applies for this specific scanning configuration.
04/10/23
Problem 6.6 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 T’rec=1000K and Tref=300K. Take antenna efficiency k=1.5 [k /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 T be?
= 0.1678 K
04/10/23
WindSat first images @ Ka