ATLID in-orbit crosstalkcharacterization.
Steps 1-2
L1 T,P
Norm_signals
Dem and
Land_flag

Read Data

Loop in along-track

Loop in sgment

Chi cor (T,P)
Loop inalong-track

Calculate cross-talkcorrection matrix
Loop in Segment

Loop in Segment

Find average
profiles and std of
excluding surface
points
Detect surface andForm surface mask

Find Chi_cor

Calculate C_lid*K
matrixFind eps STRAP
methodPerform x-talkcorrection andCalibration

Find Norm
calibration ratios
using average
Ray_clr ATB

Estimate Phi_perp
Find eps usingsurface return(if available)

Calculate
Ray_clr ATB

End loop in segment

End Loop inalong track

End loop in alongtrack

Find Chi andcalibration ratios
End loop in segment

End Loop insegment
Store results
Fit eps to alongtrack position

Steps 3-4

Fit Psi_perp toalong-track position

Fit Chi and Cal ratios
to along-trackposition

Steps 5-8

Steps 9-10

Step 11

Steps 12-14

D.P. Donovan, G-J van Zadelhoff (KNMI)
U. Wandinger, M. Haarig (Tropos)
F. Marnas (ESA-ESTEC)

What is cross-talk ?

0 N Ray   C1,1 NMie =  0 C2,2
N   00
 Cro  0  BRay 0  BMie, C3,3 
 BMie,⊥ This would be theidea case !

 N Ray   C1,1 C1,2 N Mie  =  C2,1 C2,2
 N  C
 Cro   3,1 C3,2

C1,3  BRay 
C2,3  BMie , C3,3 
 BMie ,⊥ 

But it is more likethis !

Modeling the Instrument transfer function
What is measured

What one wants to know (the input atmospheric signals)

cor N Ray Rayinp  cor −2 N Mie  = Clid R( z ) M det TRX  Mieinp cor  NCro Miein⊥p ⊥⊥  Ray (1 +  Ray )−1 (TRay +  RayTRay
TRay (1 − TpHSR ) TRay
(1 − TpHSR ) )⊥⊥
TRX = (1 −  Ray )(1 +  Ray ) −1 (TMie +  RayTMie
TMieTpHSR
TMieTpHSR )−1⊥⊥(1+)T+TTT()
RayCroRay CroCroCro

Some of these terms
are expected to be“constant”, others not !

And a good approximation to the correction Matrix is..
-1
M Crosstalk_Cor = M Crosstalk=Where it has been assumed that1
Det(M Crosstalk )⊥⊥ TCroTRay
TCroTMieCro
1− ⊥ò ⊥− 1
 ( ò − KCrosstalk ) 
TCroTMie TCroTRay⊥⊥  Ray(T+T)
TMie Cro Ray Cro
Cro−11K−1()   K CrosstalkCrosstalk⊥
TMieTCro ⊥⊥TCro Ray + TCro ) TCro Ray + TCro ) ((⊥Ray⊥Mie   K Crosstalk− 1
1 − ò  1 − K Crosstalk
TCroTCro



⊥TMie
 Ray  TMie⊥
TRay
 Ray  TRay⊥⊥(1 − TpHSR )TRay(TCro Ray + TCro )⊥ RayTCr⊥o (TRay Ray + TRay ) 1And whereò=

RRayTRay (1 − TpHSR )RMieTMieTpHSR

=RMieTMie (1 −  Ray )RRayTRay Ray =⊥TMieTpHSR RMie⊥
TCroRCro⊥ =TCro RCro
TMie RMieTpHSR

Useful to note that it is the ratios
and not so much the absolutevalues that are most important.

R-B scattering line shape is BOTH
pressure and Temperature dependent !
Simulation using Range of Temperatures and Pressures
Ray➔Mie cross-talk coefficient

Figure 2: R-B spectrum at 300mb and 220 K (Orange).

Figure 4: Chi correction factor with 300 mb and 220 K as the reference point.
Figure 3: R-B spectrum at 1000mb and 245 K (Orange).

Need for monitoring !
• ATLID will be periodically tuned by scanning the laser wavelength tomaximalize the Mie signal.
➔Cross-talk will be minimized• But what are the exact values and what happened between tuning
operations and when is re-tunning necessary ?
➔Need an effective means to monitor in orbit during normal operational mode!

How to go about characterization and correction ?
1.

Use Aerosol freeSignals (e.g.
above 30 km)

2.

?

3.

Use Aerosol freeSignals (e.g.
above 30 km andupper Trop)

Once we know ϵ and χ : Assume true value of ATBMie =0 and particle
Depol=0 and using expected value of Ray Depol then the polarization
cross-talk can be assessed.

4.

Use Aerosol freeSignals (e.g.
above 30 km)

Cross-talk corrected Relative signals can be absolutely calibrated !

Assume true value of ATBMie =0 Then ratio of Observed Mie/Rayyields χ (Ray ➔ Mie coefficient)

Once we know and χ we need to find ϵ. How ?

Mie➔ Rayleigh cross-talkcorrection/assessment…..
• Ray➔ Mie (χ) coefficient can be accurately determined using highaltitude (aerosol-free) signals.
• Mie➔Ray (ϵ) coefficient is more problematic.
• If true Ray return is << true Mie return, then ϵ can be determined but..
• Need to use suitably optically thick clouds !
• Identification of suitable cloudy regions can be non-trivial as• Zero true Ray return ➔ attenuation ➔ bad SNR |!

• Can not use ocean returns !
• Thus• A technique that can use a wide variety of clouds is necessary !

Assume BRay=0 method is problematic forclouds
If Ray input is assumed to be zero:➔ ϵ is given by the observed scattering ratio
But in realityò= + ( Rt − 1) − ( Ro − 1)
( Ro − 1)( Rt − 1)

Convergence is slow !
➔ Need optically thick clouds➔Attenuation will be a problem !

Aside: Maybe the maximum observed scattering ratio from clouds can beused…but this is likely very noise…

Some simple modelling….
X-talk corrected(ϵ/2 to ϵ x 1.5)
X-talk correctedWith X-talkWith X-talk

There are two main natural constrainsthat are relevant dictated by the physics ofthe lidar equation i.e.
1. The Pure Ray ATB profile must becontinuous at the layer boundaries
2. The Pure Ray ATB/atmos_den profile
can not have a positive slope w.r.t.
range from the lidar (eqiv. Negativeslope with altitude).
It is easy to see when one isover-correcting or…
under-correcting

The (very-very close to) correct value of epsilon is
the one which results in the``shortest’’ path

X-talk corrected(ϵ/2 to ϵ x 1.5)
X-talk correctedWith X-talkWith X-talk``shortest’’ ➔ min variance in extinction profile➔ smoothest path.

Why does STRAP work ? (a quantitative view)
Ratio of trial x-talkcorrected Ray channel
to expected clear-air
returnMinimize with respect to epsilon (equiv. Tau_ray)

Result is not exact…but very close to exact
In fact, it can be shown that, in general, finding the minimum leads to a value of epsilon a little different from thetrue value (less than 0.2 % error typically)
Numerator tends to be much smaller than thedenominator.

Steps 1-2
L1 T,P
Norm_signals
Dem andLand_flag

Read Data

Chi cor (T,P)
Loop inalong-track

Calculate cross-talkcorrection matrix
Loop in along-track

Loop in sgment

Loop in Segment

Find Chi_cor

Calculate C_lid*Kmatrix

Perform x-talkcorrection andCalibration

Find Norm
calibration ratios
using average
Ray_clr ATB

Estimate Phi_perp

End loop in alongtrack•Average the data horizontally using Ray above 30 km to determine
initial calibration coefficients and χ.

•Land-surface detection ➔ Masked average of Mie signal ➔Direct eps
determination of ϵ•Determination of ϵ using the STRAP technique•Appropriate along-track smoothing/interpolation of χ and ϵ values.
Taking account uncertainties and a-priori values.

•Examine Clear-air areas to assess polarization x-talk and calibration.

End loop in segment

End Loop insegment
Store results
Fit eps to alongtrack position

Steps 3-4

Simple average of the ATBs (before x-talk correction)

Find Chi andcalibration ratios
End loop in segment

End Loop inalong track•

Find eps STRAP
methodFind eps using
surface return(if available)
Calculate
Ray_clr ATB

Divide the frame into the desired number of segments

Loop in Segment

Find average
profiles and std of
excluding surfacepoints
Detect surface andForm surface mask•

Fit Psi_perp toalong-track position

Fit Chi and Cal ratios
to along-track
position

Steps 5-8

Steps 9-10

Step 11

Steps 12-14

Test using the Halifax scene
Example (Halifax Scene)

Step before cross-talk correction and atmospheric calibration
Simulated Mie channel normalized signal

Simulated Rayleigh normalized signal

Name | Short title | Aeolus Cal/Val & Science Workshop | 2-6 Nov. 2020 | Slide 14

Test using the Halifax scene
Example (Halifax Scene)

After cross-talk correction and atmospheric calibration
Simulated Mie ATB

Simulated Rayleigh ATBName | Short title | Aeolus Cal/Val & Science Workshop | 2-6 Nov. 2020 | Slide 15

Ray ➔ Mie cross-talk

Ratio of Mie andRay channels above
30 km can be used
as a pure Ray inputThis allows theRay➔Mie cross-talkto be assessed.

=RMieTMie (1 −  Ray )RRayTRay Ray

Best Path approach results

Curvature aroundminimum is being
used to assign anuncertainty

Low error
Higher errorò=Combined
STRAP
assessments(circles) andland-return
based
assessments(Triangles)

RRayTRay (1 − TpHSR )RMieTMieTpHSR

Land surface assessments

STRAP assessments

Now !

ECGP: Earlier !

Summary and Outlook• Problems with the L1 ATLID processor were noticed late…but not too late !
• Innovative robust methods were developed.
• ATBD and working prototype code have bee delivered to ESA for implementation• Further monitoring can be done (likely at L2 ) !
• Use of background solar signals above suitable targets could
help with the Depol characterization.
• Assessing the relationship between layer depol and layer
integrated attenuated backscatter in water clouds (borrowinga Calipso technique).

ATBD and prototype working Python application recently delivered to ESA !