MIRAS: The SMOS Instrument

ICMARS 2010Jodhpur – India December 16th, 2010 1/46

SMOS: Principles of Operation & First Results

SMOS: SMOS: Principles of Operation & First ResultsPrinciples of Operation & First Results

Prof. A. CampsDept. de Teoria del Senyal i Comunicacions

Universitat Politècnica de Catalunya and IEEC/CRAE-UPC

E-mail: [email protected]



Outline of the presentation:

1. Basic principles2. Imaging in Synthetic Aperture Radiometers:

2.1. Synthetic Aperture Radiometers2.2. Image Reconstruction Algorithms: Ideal Case

3. The SMOS Mission4. MIRAS instrument description

4.1. Array topology4.2. Receivers’ architecture4.3. NIR architecture4.4. DIgital COrrelator System (DICOS)4.5. CAlibration System (CAS)

5. Instrument Performance5.1. Angular Resolution5.2. Radiometric Performance: definition of terms5.3. Image Formation Through a Fourier Synthesis Process

5.4. Imaging Modes: Dual-polarization and full-polarimetric 6. Geolocalization: from director cosines grid to Earth reference frame grid and Retrieval of Geophysical Parameters



21

2 21 2

,( , )

1

*B ph recnn

T TF , ,FT

Channel 2

Channel 1

0( , ) ( , )u v x y

*1 2

1 2 1 2

1 12

B

V u,v = t b t bk BB GG

H1(f)

H2(f)

b1(t)

b2(t)

*1 2

12

bb

Complex Correlator

= antenna spacing normalized to the wavelength

sinsin,cossin,

Baseline

• Spatial resolution is achieved by cross-correlating the signals collected by a number of antennas• Antennas can have a wide beam or a narrow one in one or two directions

( , ) ( , )V u v F T Ideal case: - Identical antenna patterns

- Negligible spatial decorrelation - No antenna positioning errors

2D Fourier Transform

1. Basic Principles



2.1. Synthetic Aperture Radiometers using Fourier Synthesis:

VLA, New Mexico, Socorro

ESTAR(1 D Aperture Synthesis)

NASA

Radioastronomy Earth Observation (concept proposed in 1983 by LeVine & Good)

MIRAS(2 D Aperture Synthesis)

ESA

( , ) ( , )V u v F T

2. Imaging in Synthetic Aperture Radiometers



• Differences between radio-astronomy and Earth observation:

- Large antenna spacing

- Very narrow field of view (FOV)

- Obliquity factor (1/cos ) can be approximated by 1

- Antenna patterns are approximatedly constant (amplitude and phase)

over the FOV

- Typically quasi-point sources imaged over cold background

super-resolution image reconstruction algorithms can be used



. After the successful results of ESTAR radiometer (1988),

the European Space Agency starts in 1993 the first feasibility studies to apply synthetic aperture microwave radiometry in two dimensions:

. MIRAS concept is born: Microwave Imaging Radiometer by Aperture Synthesis

. First studies (1993-95): led by Matra Marconi Space as the prime contractor

. 1995 Soil Moisture and Ocean Salinity Workshop (ESTEC, the Netherlands)Aperture Synthesis Microwave Radiometry is the only technique capable of measuring soil moisture and ocean salinity with enough accuracy and spatial resolution.

SSS image derived from the ’“Electronically Steered Thinned Array Radiometer (ESTAR)”. Error = 0.3 psu (D. M. LeVine et al., NASA Goddard).



Antenna Positions Spatial frequencies (u,v)

u

v

Periodic extension

2

2 2

,( , )

1

n B ph recF , T TT

( , ) ( , )V u v F T

21 elements + 2 redundant elements/arm Antenna spacing d = 0.875

Hexagonal grid in (u,v) planeNyquist criterion: d< / 3

Overlapping of 1 alias

Alias-free Alias-free Field Of View Field Of View

(AF-FOV)(AF-FOV)

Overlapping of 2 aliases

2.2. Image Reconstruction Algorithms: Ideal Case



In SMOS the “alias-free FOV” can be enlarged since part of the alias images are the “cold” sky (including the galaxy!) TB image limited by Earth replicas

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.2

0.4

0.6

0.8

1

Boresight Distance: 912.65 Km

Boresight Incidence Angle: 36.35º

-3 -2 -1 0 1 2 3

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

hsat=755.6 Km, =32.00º, d= 0.89

Extension ofAlias-Free FOV

-800 -600 -400 -200 0 200 400 600 800-200

0

200

400

600

800

1000

1200

Cross track coordinate (Km)

Alo

ng t

rack

coo

rdin

ate(

Km

)

hsat=755.9 Km, tilt=32.50 , d= 0.88

10

20

30

40

50

60

1.2032.85

2.4788.01

2.2178.62

1.7661.13

1.4348.48

1.2039.69

1.0534.97

1.2335.52

1.4939.34

2.3181.55

2.1374.95

1.7258.77

1.4046.29

1.1937.53

1.1032.62

1.3935.77

2.80101.26

2.2681.39

1.8465.47

1.5053.04

1.2544.31

5.1290.57

1.1340.12

1.2740.33

1.3951.36

1.3448.11 1.3948.00

- Pixel axial ratio a/b- Spatial resolution defined as geometric mean of axes

Iso-incidence angle contours



SMOS is a challenge:Particularities of 2D aperture synthesis radiometers:1) New type of instrument: - Review of the fundamental equation- Detail error model & error correction (calibration) algorithms- Image reconstruction algorithms

2) New type of observations:- Multi-look and multi-angle observations: . different pixel size and orientation . different noise and precision for each pixel- Polarization mixing:

. Earth reference frame antenna reference frame

3) New L-band and multiangular ocean and soil emission models : - Wide range of incidence angles (0º-60º)

4) New geophysical parameter retrieval algorithmstaking into account issues 1, 2 and 3 above

3. The SMOS Mission



Scientific measurements require a - Sun-synchronous, - dawn/dusk, and- quasi circular orbit.

Orbital parameters:• Mean altitude = 755.5 km• Eccentricity = 0.001165• Mean inclination = 98.416º• Local Time Asc. Node =6 AM• Argument of Perigee = 90º• Mean Anomaly = 306.3º

Note: The SUN is nearly always visible (97 % of the time) !!!

SMOS Mission: SMOS

Proba-2

Transformed SS-19 missile



HUB

ARM A

ARM C ARM B

CMN

PD

NS (DISTRIBUTED)

NS (CENTRALISED)

LICEF

LICEF/NIR

Optical Splitter

Optical Splitter

Optical Splitter

CMN

1ST SEGMENT OF ARM C

CMN

PD

Optical Splitter

PD

CMN

1ST SEGMENT OF ARM A

LICEF

LICEF/NIR

1ST SEGMENT OF ARM B

LICEF/NIR

LCF_A_21

LCF_A_16 LCF_A_15

LCF_A_10 LCF_A_09

LCF_A_04 LCF_A_03 LCF_A_02 LCF_A_01

NIR_AB_02

LCF_AB_01

HUB UnitIn the arms In the Hub Total

LICEF 3 x 3 x 6 3 x 4 66

LICEF/NIR - 3 x 1 3

CMN 3 x 3 x 1 3 x 1 12

CCU - 1 1

NS (distributed) 3 x 3 x 1 - 9

NS (centralised) - 1 1

PD (2 to 8) 3 x 3 x 1 3 x 1 12

Optical Splitter (1 to 8) 3 x 3 x 1 3 x 1 12

Optical Splitter (2 to 12) - 1 1

TRANSMITTERS - 2 2

FILTERS - 2 2

X-ANTENNA - 1 1

UnitIn the arms In the Hub Total

LICEF 3 x 3 x 6 3 x 4 66

LICEF/NIR - 3 x 1 3

CMN 3 x 3 x 1 3 x 1 12

CCU - 1 1

NS (distributed) 3 x 3 x 1 - 9

NS (centralised) - 1 1

PD (2 to 8) 3 x 3 x 1 3 x 1 12

Optical Splitter (1 to 8) 3 x 3 x 1 3 x 1 12

Optical Splitter (2 to 12) - 1 1

TRANSMITTERS - 2 2

FILTERS - 2 2

X-ANTENNA - 1 1

UnitUnitUnitIn the armsIn the armsIn the arms In the HubIn the HubIn the Hub TotalTotalTotal

LICEFLICEF 3 x 3 x 63 x 3 x 6 3 x 43 x 4 6666

LICEF/NIRLICEF/NIR -- 3 x 13 x 1 33

CMNCMN 3 x 3 x 13 x 3 x 1 3 x 13 x 1 1212

CCUCCU -- 11 11

NS (distributed)NS (distributed) 3 x 3 x 13 x 3 x 1 -- 99

NS (centralised)NS (centralised) -- 11 11

PD (2 to 8)PD (2 to 8) 3 x 3 x 13 x 3 x 1 3 x 13 x 1 1212

Optical Splitter (1 to 8)Optical Splitter (1 to 8) 3 x 3 x 13 x 3 x 1 3 x 13 x 1 1212

Optical Splitter (2 to 12)Optical Splitter (2 to 12) -- 11 11

TRANSMITTERSTRANSMITTERS -- 22 22

FILTERSFILTERS -- 22 22

X-ANTENNAX-ANTENNA -- 11 11

4.1. Array topology• 69 antenna elements (LICEF)

• Equally distributed over the 3 arms and hub

• The acquired signal is transmitted to a central correlator unit, which computes the complex cross-correlations of all signal pairs.

4. MIRAS instrument description



MIRAS consists of a central structure (hub) with 15 elements, and 3 deployable arms, each one having 3 segments with 6 antennas each.

[credits EADS-CASA]



4.2. Receivers’ architecture:

HVCU

SWITCH ISOL LNA BPF RFAMP MIXER

IFFILTER

ATTENSLOPECORR.

IF AMPs1BITADC

IFFILTER

ATTENSLOPECORR.

IF AMPs1BITADC

SYNTH

1396 MHz

PMS

1404-1423 MHz

8-27 MHz

DI

TI

TQ

DQ

REF55.84 MHz

VCOMAIN PATH GAIN = 100 dBPMS PATH GAIN = 65 dB

TRF

ANTENNA

I

Q

DICOS

DICOS

OffsetUNCALjCAL eA

A outT aV b • PMS acts as a total Power Radiometer in each LICEF

• Needed to denormalize the “normalized” correlations (1 bit/2 level)



LICEF: the LIght and Cost Effective Front-end

[credits MIER Comunicaciones]



4.3. NIR architectureThe Noise Injection Radiometer (NIR) is fully polarimetric and operates at 1.4 GHz

3 NIRs in the hub for redundancy.

Functions:• precise measurement of Vpq(0,0) = TApq for mean value of TBpq(,) image. • measurement of noise temperature level of the reference noise source of Calibration Subsystem (CAS) absolute amplitude reference

1st LICEF unit (V-pol)

2nd LICEF unit (H-pol)

Controller unit (switches, noise injection...)

Correlated noise inputs (from Noise Distribution Network)

allow phase/amplitude calibration of receivers as LICEFs & for 3rd and 4th Stokes parameters measurements

[credits HUT]



SMOS NIR:

T NA + TA = TUT NA + TA = TREF + TNR

Normal mode of operation: Calibrating internal noise source mode:

known (cold sky) ?

[Colliander et al., 2005]

[credits HUT]



4.4. DIgital COrrelator System (DICOS)

Digital signals from each LICEF are transmitted to DICOS to compute the complex cross-correlations of all signal pairs.

1 bit ADC (comparator) in each LICEF

Correlator = = NOT-XOR + up-counter



• Lower half: II-correlations: Nr,Nc Zr r Vr

• Upper half: IQ-correlations: Ni,Nc Zi i Vi

• Diagonal: IQ-correlations of same

element (q: quadrature errors)

• Correlations of I and Q signals with 0’s and 1’s

to compensate comparators’ threshold errors

• Correlations of 0’s and 0’s and 1’s and 1’s = Ncmax

• NCmax = 65437 for dual-pol mode (= fCLK · int)

NCmax = 43625 for full-pol mode

•Total number of products:•2556 correlations Ik-Ij •2556 correlations Ik-Qj•72 correlations Ik-Qk•72 correlations I-0•72 correlations Q-0 •72 correlations I-1 •48 correlations Q-1 •36 control correlations between 1 and 0 channels (4 for each ASIC)



CCU: the Correlator and Control Unit

[credits EADS-CASA]



4.5. CAlibration System (CAS)

Noise sources needed to calibrate the instrument.

HUB ARMS



* source 0* source 2* source 5* source 8

124

25

48

49

72

A

B

C

Correlated noise is injected to the receivers in two steps:first the “even” sources and then using the “odd” ones

Centralized and distributed calibration

These receivers belong tothe NIR (□: H-channel) and

do not form additional baselines

Overlapping between elements(phase & amplitude tracking

along the arms)

Overlapping between elements(phase & amplitude tracking

among arms)

Centralized Calibration(separable & non-separable

errors can be corrected) Distributed Calibration(only separable errors

can be corrected)



OVERALL SEGMENT ARCHITECTURE

[credits EADS-CASA]



6 LICEF / segment

[credits EADS-CASA]



MOHA

[credits EADS-CASA]



CAS

[credits EADS-CASA]



CMN

[credits EADS-CASA]



• The retrieved image is the 2D convolution of the original T(,) image

with the instrument’s impulse response or equivalent array factor:

' 2 ' 2

20 ' ' 0

1

2

2 ' '0

1ˆ , , , , ', ' ' '

3

2

', ' ,

mn mn

mn mn

j u vmn mn mn mn

m n

j u v

mn mnm n

T s W u v V u v e T AF d dK

s d

AF s W u v e

5.1. Angular Resolution

2 2

*B 1 2

122 2

011 2

T , ( ) ( )1( , ) d d

1

phrec n n -j 2 u +vn

T F , F , u vV u v = er

f

• The “ideal” brightness temperature image is formed by an inverse (discrete) Fourier transform of the measured visibility samples (B = 0):

Equivalent Array Factor:same response as for an array of elements at (u,v) positions

(except for the |(.)|2)

1 0 0Τ̂ , * ΤH, = F W u v V u, v = AF , ,

Τ̂ ,

5. Instrument Performance



Response with rectangular window

Response with Blackmann window(rotational symmetry)

3 maxmax

max

2; 10% 15

2 3

43%

rectdB

EL

e for uu

u N d

MBE

3 31.48

90%

Blackmann rectdB dB

MBE

'sec1A main lobe ondary lobesT MBE T MBE T

W(umn,vmn): window to weight the visibility samples:• reduces side lobes• widens main lobe• increases main beam efficiency (MBE)



5.2. Radiometric Performance: definition of terms

Error maps: TB(,,t)

Random errors

(noise due to finite integration time)

Temporal average Zero 0

Temporal standard deviation

Radiometric sensitivity

Systematic errors

(instrumental errors)

Spatial average Radiometric bias

(scene bias)

Spatial standard deviation

Radiometric accuracy

(pixel bias)

1M

t,,T̂t,,T̂T

M

1i

2

tBiB

ysensitivit

N

1iiiB

tiiBbias ,Tt,,T̂

N1

T

2N

i i i iB Bti 1

accuracy

T̂ , ,t T ,T

N 1



Cut for =0N

tB

TTdT w

a

eff

RAB

222 1

),(2

3),(

Dashed lines. Theoretical formula:

Radiometric Sensitivity over ocean

[credits I. Corbella]



Scene Bias < 0.1 K

Accuracy < 0.5 K

Moon

Galaxy (yellowish)

Galaxy Alias

Galactic radio-source (TBC)

Cosmic Background Radiation at 3.3 K

Sun Alias

[credits DEIMOS]



• 45 deg singularity discarded

• All points with the same incidence angle averaged

Fresnel

V

H

Y

X

T

T

T

T

22

22

cossin

sincos

Incidence angle dependenceSingularity in the transformation antenna to Earth reference frame (dual-pol mode)




5.3. Image Formation Through a Fourier Synthesis Process

Even in the ideal case:- Antenna spacing > /3 aliasing- Gibbs phenomenon near the sharp transitions (mainly alias borders)

In the real case:- Antenna patterns are different - Receivers’ frequency responses are different ( FWF different)- Antenna positioning errors (u,v,w)real different from (u,v,0)ideal

IHFFT cannot be used as image reconstruction methodMore sophisticated algorithms must be devisedBut it will be good that the second ones tend to IHFFT in ideal conditions

… and obviously instrumental errors must be calibrated first!



1) Receivers relative calibration (image “contrast”)- Error model (distorsions, artifacts, blurring…)- Internal references (Tcorr, Tuncorr,…)

TB imaging in a single snap-shot (1 integration time = 1.2 s / polarization in dual-pol):

Aperture Synthesis Radiometer: 2 step calibration

TB imaging pixel by pixel through antenna scan:

Real Aperture Radiometer:1 step calibration

1) Absolute calibrationExternal references:

Thot, Tcold

2) Absolute Calibration (image accuracy)- External references (FTT, OTT…)- Thot/Tcold, ground truth, external calibration…

*** Image Reconstruction Algorithm *** *** Imaging by (e.g.) conical scan ***



Calibration Concept: Brief sketch

• Items that need calibration:

- NIR Gain and Offset

- PMS gain and offset (receiver and baseline amplitude errors)

- Fringe-washing function FWF (amplitude and phase errors)

- Noise that is injected to receivers during calibration

- Correlator Offsets

• Types of Calibration:

– Internal: injection of correlated or uncorrelated noise to the receivers

– External: observation of known target:

• NIR absolute calibration

• Flat-Target Transformation: to calibrate antenna pattern errors

– CAS Calibration: performed by NIR during internal calibration

– Correlator Calibration: injecting known signals



a. MIRAS internal calibration

Instrumental errors correction: set of measurements and mathematical relations to remove instrumental errors

INTERNAL INSTRUMENT CALIBRATION

• Characterizes the instrument behavior independently of the input signal. • It can be characterized by suitable internal known signals injected at its input: correlated/uncorrelated and hot/cold noise injection.

Error model



MIRAS Internal calibration

LI CEF

NI R

switchi

q

To correlator

ma

trix

Antenna

H

V

ADCIF-i

C

U

IF-q ADC

LO distribution

Clock distribution

Mixer

RF

Noise

distributionnetw

ork

PMS

Reference receiver(Noise Injection

Radiometer)

Lv

TN

TS1

C

H

V

TS2

Noise source

0

N

Matched load

Hot

Warm

kk offCC

syskk vTGv

k

offkACsys G

vvT k

k

Calibrated visibility:

njkSjkkj TSSTSST *00

*00

ˆ

)1(2

0

2

0 knSkN STTSTk

CR

CN

CCsys kkk

TTT

(*)

(*)

PMS gain PMS offset

Correlation amplitude



2 2

2 2*

12 1 2 1212 12 12

12 12 1

2 201 2 1

22 2

, 11, ,

1

exp 2 1

pq rec pqpqnp np

fore

T TV F

u v w

u v w

F rf

j d d

Formulation of the Problem:Instrument Equation After Internal Calibration

-30 -20 -10 0 10 20 300.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

time (ns)

Amplitude of fringe-washing functions

-30 -20 -10 0 10 20 30-6

-4

-2

0

2

4

6

8

time (ns)

De

g

Phase of fringe-washing function


To be corrected using the Flat Target Response

( , ) ( , ) ( , )pp pp ppij ij ijV u v V u v V u v

, , ,decW u v V u v G T



The Flat Target Response:

-The Flat Target Response is defined by:

2 2

, , , * 2 ( ), , 2 2

1

1 1( , ;1) ( , ) ( , ) d d

1

pq p q j u vij n i n j ij

oi j

u vV u v F F r e

f

,( , ; ) ( , ; 1)pq pqij o r o r ijV u v T T T T V u v

( , ) ( , ) ( , )pp pp ppij ij ijV u v V u v V u v

defining:

( , ) ( , ; )pp

pp ppB rij ij P r

P r

T TV u v V u v T T

T T

Then the differential visibilities to be processed are:



• Once in a month (every week during commissioning) the platform rotates to point to the cold sky

• External calibration is used to correct for elements not included in internal calibration: switch and antenna losses

• Also the Noise Injection Radiometer (NIR) is calibrated and the Flat Target Response (FTR) measured

HERE IT GOES THE ANIMATION. T_X_skylook2.gif

HERE IT GOES THE ANIMATION. T_Y_skylook2.gif

External calibration




5.4. Imaging Modes: Dual-polarization and full-polarimetric

Dual-polarization radiometer:

MIRAS has dual-pol antennas, but only one receiver polarizations have to be measured sequentially, with an integration time of 1.2 s each

[credits M. Martin-Neira]



Full-polarimetric mode: (selected as operational mode for SMOS)

[credits M. Martin-Neira]



6. Geolocalization: from director cosines grid to Earth reference frame grid

• ISEA family of grids seem to be the best option for the SMOS Products, but EASE-Grid has

come to be popular amongst many of the Earth Observation missions of the USA, namely AQUA

(NASA/NASDA) and AQUARIUS (NASA), which are particularly interesting for comparison with the SMOS

products.

• Spatial partitioning of EASE-Grid is square-based and ISEA can be triangular,

hexagonal or diamond-based:

- In its hexagonal form, ISEA has a higher degree of compactness, quantize the plane with the smallest

average error and provides the greatest angular resolution. -ISEA hexagonal possesses uniform adjacency with its neighbors, unlike the square EASE-Grid.

• Both grids have uniform alignment and are based on a spherical Earth assumption.

• ISEA hexagonal at aperture 4 and resolution 9 (15km) is made up of 2,621,442 points

and the EASE-Grid at 12km has 3,244,518 points.

• EASE-Grid is congruent, whereas ISEA is not congruent, being impossible to decompose

a hexagon into smaller hexagons or aggregate hexagons into larger ones. This would be a

negative feature for real-time re-gridding, but in SMOS the grids will be pre-generated.

6. Geolocalization and Retrieval of Geophysical Parameters



L1 processor L2 processor L3 processor

Auxiliary data

Multi-angular emission models

OS map (1 overpass)

SM map (1 overpass)

Spatio-temporal averaging

Snap-shot 1

Snap-shot 2

Snap-shot 3

Snap-shot 4

• Atmospheric and foreign sources corrections• Use of multiangular information:

1. Th & Tv or Tx and Ty + Faraday and geometric rotations corrections:

Earth Antenna: retrieval in antenna ref frame,Antenna Earth: retrieval in Earth ref frame,

2. First Stokes parameter: I = Tx+Ty=Th+Tv. (invariant to rotations)



• Sample SMOS data: Pixel in different positions of SMOS swath

OS retrieval:(pin 5)

(pin 3)



Sample results of the application of the downscaling algorithm to a SMOS image covering the Murrumbidgee catchment, South-Eastern Australia, on January 19, 2010 (6 am). First row: 40 km SMOS soil moisture [m3/m3] over Murrumbidgee (left), and zoom into Yanco site (right). Second row: 1 km downscaled soil moisture [m3/m3] over Murrumbidgee (left), and zoom into Yanco site (right). Dots indicate the location of the soil moisture permanent stations within the Murrumbidgee catchment used for validation purposes with colors representing their measurement at the exact SMOS acquisition time (only within Yanco site). Empty areas in the images correspond to non-retrieved soil moisture or clouds masking MODIS Ts measurements.

60 km

(b) MODIS NDVI [m3/m3]

(c) MODIS LST [m3/m3]

(a) Murrumbidgee catchment

(a) 60 x 60 km Yanco site in the Murrumbidgee catchment, South-Eastern Australia, (b) 1 km MODIS NDVI, and (c) and LST [K] on January 19, 2010.

1 km downscaled SMOS soil moisture [m3/m3] using MODIS VIS/IR data

40 km SMOS soil moisture [m3/m3]

• Sample SMOS data over Australia: Murrumbidge catchement



Thanks for your attention!

MIRAS: The SMOS Instrument

Technology

Transcript of MIRAS: The SMOS Instrument