EarthCARE ATLID In Orbit
Characterisation, Calibration and Verification
2nd ESA EarthCARE Validation Workshop
Georgios Tzeremes
25-28 May 2021 (online)
ESA UNCLASSIFIED – For ESA Official Use Only1

Atmospheric Lidar - ATLID
Ø Atmospheric Lidar
Laser wavelength λ = 354.8 nm, linearly polarised
Ø High Spectral Resolution Lidar (HSRL) using
Fabry-Perot etalon centred on the laser centre
wavelength à separates molecular from particle
backscatter signals (lidar ratio measured)Ø 3 channels receiver :§ Rayleigh scatter§ co-polar Mie§ cross-polar
Ø Main products are profiles of§ molecular backscatter signal§ cloud and aerosol backscatter signal, co-polar§ cloud and aerosol backscatter signal, cross-polarØ Sampling: PRF 51 Hz (fixed), along-track 280m (2 shots),
vertical 103m up to 20km, 500m from 20 to 40 km
Ø Mass: 550 kg, Power: 465 W, Data rate 404 kb/sIndustry: ADST (F) + Leonardo (I)2

ATLID Switch on & IOCV activities• ~5 days• ATLID off• Heaters “protect” E-BEX interface• ~28 days
• ATLID in INI,SBY,LWU,RDY,DEC,RDY,LWU,MEA• Instrument Switch on, test of Op modes,
Dec mode (heat up E-BEX), certain
CAL/VAL activities• ~10 days (one laser, min reqs )• ATLID On, MEA mode, calibration modes• Cal / Val campaign, first ATLID products

3

Functional - Shooting Calibrations
Description

Instrument
interruption

Ground processing

Operationalconstrains

Periodicity &Duration

Dark Current
Calibration

YOn-line processing

S/C eclipse

Monthly/ 5 min

Emitted EnergyNOn ground datacalibration

No

ContinuousCoarse coalignment
calibrationY

Off-line processing

S/C eclipse

Beginning of life/20 minFine co-alignment
calibrationN

Off-line processing(After COC)

Monthly

Emission defocus
calibrationY

Off-line processing

No

Once per laser
initiation/ 70 hCoarse Spectral
calibration

YOn-line processing

No

Once per laser
initiation / 24 min

Fine spectral
calibration

YOn-line processing(After CSC)

Weekly/ 10 min* (x-talk verification)

*

*

*

*

Fibre spot position
on DFAY

Off-line processing

S/C eclipse

Beginning of life
/10 min4

ATLID Calibration/Validation Overview
ATLID In Orbit Validation has started with an extensive on-ground calibration approach, allowing an initial population of thelevel 1B algorithms. This sets a preliminary starting point for early in-orbit operation.
Key parameters such as spectral cross-talk, lidar constants are then routinely calibrated in flight in order to improve andcorrect for in-flight drifts.

measuredpre-flightmeasured
in-flight

applied inflight

Dark signal non-uniformity(DSNU)

üü (mon,cm)

ü from CCDB

Linearityüûü from CCDB

Backgroundüüü directlySpectral cross-talk (χ, ε)üüü directly

Polarisation cross-talk (ψ)üüü from CCDBLidar constant (K) – absolutecal.

üü (mon)ü from CCDB

mon = for monitoring, CCDB updates as needed cm = needs dedicated calibration mode5

In-Orbit Radiometric Calibration: Background Signal
Subtraction
Mode: Background signal subtraction
Verification objective: background noise estimation andSubtraction.
Periodicity: At each echo
Instrument mode: MES
Constraints: Background signal is acquired for a time windowof 67µs (10 kms long sample) above 100 kms and afterecho sample between -2 km and -12 km altitude.
Comment:• Linear interpolation between the two altitudes allows estimatingbackground level and offset for each echo sample.
• value subtracted from atmospheric echo samples.

6

In orbit: Spectral cross-talk Calibration
Mie (χ): A pure Rayleigh signal can be measured when observing the high layers of stratosphere (>30 km of altitude) where
only molecular backscattering occurs. This calibration of c can then be performed at each laser shot (500 km averaging).
Rayleigh (ε): A pure “Mie spectrum” can be measured for instance when observing a ground echo or the echo from a dense
cloud. In this case, the Rayleigh scattering contribution is negligible, or can be easily subtracted. Each time a sharp echo isrecorded, an estimation of e can be calculated, completing the calibration. A filter is applied to reject non-usable echoes.

7

In-Orbit Absolute Channels Calibration
Rayleigh channel absolute calibration•measuring the return from high atmospheric layers, between 30 and 40 km altitude.

•continuously monitored over the orbit with signal horizontal averaging over 500 km.

•night data are favored.

Mie co-polar channel absolute calibration (off-line processing)
Method 1 : absolute calibration for a pure Rayleigh signal :•the return from stratospheric layers (above 25 km) is used to estimate theMie co-polar channel response for a pure molecular backscattering.
•Only night data will be used.

•knowledge of the relative transmission of HSR etalon on Mie co-polar

and Rayleigh channels for a Rayleigh spectrum
Method 2 : absolute calibration for a pure Mie signal :•use of ground echoes from characterised sites ( ground albedo, opticalthickness of the atmosphere ).
•every 6 months and to monitor the trend in between using averaged groundechoes on poles and atmosphere optical thickness knowledge.
8In-Orbit Mie cross-polar Channel Calibration
The method proposed for Mie co-polar and Rayleigh channels is not applicable for the cross polar channel as the depolarisation of the scatteringtarget must be characterised. It is thus proposed to calibrate the cross polar channel in relation to Mie co-polar channel using the Earth backgroundlight in conjunction with the Mie co-polar channel, as already validated for CALIPSO.

* Proposed approach, synergetic to other EarthCARE instruments.
9

Thank you~123 seconds of light10