1.1

EarthCARE mission and satellite

EarthCARE is the third ESA Earth Explorer Core Mission. It is being implemented in collaboration with the Japan Aerospace Exploration Agency (JAXA), which is providing the Cloud Profiling Radar [9]. The European instruments are the Atmospheric Lidar, the Multi-Spectral Imager (whose status is discussed in a sister paper of the same conference session [10]) and the Broad Band Radiometer. The common platform allows the instruments to collect co-registered observations from all four instruments and allows the instrument data to be processed individually or synergistically [8].

The 2 tons satellite will be placed in a sun-synchronous orbit of around 400 km, with a descending node crossing time of 14:00 and a repeat cycle of 25 days. The mission lifetime is 3 years, with sufficient consumables for a year extension. EarthCARE builds on the success of the Cloudsat/Calipso mission, with improved performance and collocated radiation observations. The crux of the EarthCARE mission is its ability to collect simultaneous, co-registered observations from the four instruments mounted on a common platform. These will measure the vertical structure and horizontal distribution of cloud and aerosol fields, together with the outgoing radiation, over all climate zones. The instruments will operate individually and in synergy, with all fields of view directed towards the satellite ground track. MSI field of view (FOV) covers nadir but is pointed slighted to one side of the ground track, and BBR incorporates additionally a forward and a backward pointing FOV. The scientific objective of the mission is to improve understanding of cloud-aerosol radiation interactions and Earth emitted thermal and reflected solar radiation, so that they can be modelled with better reliability in climate and in numerical weather prediction models [8].

Figure 1:

EarthCARE and payload observation geometry

The following Figure 2 (left) shows EarthCARE satellite with all instruments integrated in Airbus Friedrichshafen Cleanroom. It gives an impression of ATLID Instrument protected by closed covers and accessibility inside the platform, at height of approx. 3,5m above floor ground. ATLID PFM instrument has been delivered by Airbus Toulouse to Airbus Friedrichshafen in March 2020 and integrated on the platform in May 2020. The challenging integration by sliding the 550kg instrument in a platform compartment with less than 2cm clearance can be seen in Figure 2 (right). An additional unexpected challenge has been the first Corona lockdown constraints in this period.

Figure 2.

EarthCARE satellite with all instruments integrated, CPR reflector antenna deployed, in Airbus Friedrichshafen cleanroom (left). ATLID integration into the EarthCARE platform (right).

Following unexpected failure of ATLID laser electronics in June 2021, the ATLID was dismounted from the platform, for laser electronic repair, and then remounted back on the platform in March 2022. After each ATLID integration on the platform, a set of ATLID IPCs has been executed to confirm unchanged good health and performance of the instrument.

Today (Aug 2022), the EarthCARE satellite is complete and has been transported to the environmental test facility in Noordwijk (ETS). The satellite environmental test campaign will start in October 2022 by satellite vibration and acoustic testing, after completion of some remaining CPR retrofit over the summer period.

1.2

ATLID instrument

The ATLID instrument is the first UV LIDAR with High Spectral Resolution capability [1]. The Lidar emission in ultraviolet (UV) at 355nm has been chosen because of the higher molecular scattering compared to that from longer wavelength light; enhancing the Rayleigh signal in this manner helps to distinguish between the backscattering from aerosol particles, with monochromatic backscattering (Mie), and the molecular backscattering (Rayleigh) that is spectrally broadened by a few GHz. This distinction allows quantifying the extinction-to-backscatter ratio. The spectral separation is performed via the High Spectral Resolution Etalon (HSRE) included in the telescope focal plane of the instrument. A polarizer optic allows to also quantify the cross polarized backscattered signal, such that it is possible also to distinguish certain aerosol type with oriented shapes.

The double capability of ATLID with its 3 channels (Co-polarized Mie, Co-polarized Rayleigh, Cross-polarized) leads to unique unprecedented LIDAR products. Previous LIDAR in orbit have not yet combined the HSRE capability with the polarization backscattering measurement. The CALIPSO is a LIDAR mission with double wavelength (1064nm and 532nm) and with two polarization sensing but with no UV wavelength and no HSRL capability, and the AEOLUS mission provides indirect backscattering products with Mie and Rayleigh separation but only with one single polarization [1], [11].

The ATLID LIDAR data provides backscattered signals measured with vertical resolution from 100m (0 to 20km) to 500m (20 to 40km); Figure 3. The UV laser power of 35mJ is emitted in 35ns pulses with a pulse repetition frequency (PRF) of 51Hz within a typical 36μrad total emission cone angle. In baseline settings, the backscattered signals are accumulated on MCCD after two laser shots, leading to a ground sampling of 285m from two typical 14m diameter laser footprints. The main performance requirements are assessed with on ground averaging of 10km of these accumulations [1].

Figure 3.

ATLID sampling scheme for vertical range and ground sampling

1.3

ATLID design description

The ATLID instrument is bistatic LIDAR architecture: two independent paths for emission assembly (TxA) and for receiver assembly (RxA); the Figure 4 presents the overview of the design described just below.

Figure 4.

ATLID block diagram description and ATLID Optical Flight Model placed on stable structure assembly

The TxA is based on a tripled Nd:YAG diode pumped MOPA (Master Oscillator Power Amplifier) laser developed by Leonardo (Pomezia), and an external beam expander (EBEX) developed by Sodern (Limeil Brévannes). The laser emission timing is synchronized with the detection chain [1].

The RxA is a 620mm aperture SiC telescope with attached SiC focal plane assembly. The atmospheric echo is filtered from solar background via narrow band filter and via blocking filter limiting the instrument field of view to equivalent 66.5μrad total cone angle. The echo is then entering the HSRE section (developed by RUAG) which extracts cross-polarization signal and spectrally splits the co-polarized signal into two channels: the HSRE Fabry Perot etalon transmitted signal is collected by the particulate backscatter (so called “Mie”) channel, while the reflected signal is collected on the molecular backscatter (“Rayleigh”) channel. Fabry Perot function combined with the backscattered spectrum induces signal spectral cross talk between the channels that is corrected in data processing. The signal is fiber coupled and acquired on a Memory CCD sensor that ensures very low read out noise, while keeping high quantum efficiency [1].

Both RxA and TxA are mounted on a common stable structure assembly that maintains good passive co-alignment performance of their two independent optical paths, while an active co-alignment closed control loop corrects the remaining thermo-elastic line of sight perturbation with a 10min low reaction time. The co-alignment loop commands a beam steering mechanism (BSM) inside the laser in order to co-align the TxA LoS on the RxA LoS. The control loop inputs are based on the centroiding results from a sampled beam on a co-alignment sensor (CAS) placed inside the focal plane [1].

1.4

ATLID Instrument Performance Checks (IPCs) – General aspects and objectives

ATLID PFM has been delivered after completion of all environmental and performance qualification testing on ATLID instrument level (except ATLID radiated EMC qualification, to be achieved by satellite level EMC test campaign). Main ATLID performances have been characterized and the instrument level qualification testing has shown excellent adherence to the ATLID performance specification.

The ATLID LIDAR performance of backscatter absolute retrieval accuracy is based on the relative retrieval accuracy on each channel and the absolute calibration of the LIDAR constant. The Lidar retrieval accuracy shown in Figure 5 is given at 10 km altitude sample, on a 10 km horizontal integration length, for three types of scenes: a sub visible cirrus, a thin cirrus and a depolarization cirrus. Further details on ATLID instrument level performances and test results can be found in [1].

Figure 5.

ATLID LIDAR performance of backscatter absolute retrieval accuracy, based on the relative retrieval accuracy on each channel and the absolute calibration of the LIDAR constant and characterization of the receive channel crosstalks

The following “key contributors” to ATLID Lidar performance have been identified, in order to become subject of five dedicated ATLID IPCs.

The ATLID IPCs are developed by the instrument supplier to be an instrument performance check with “reasonable complexity” for execution also in satellite level AIT (check with possibly reduced accuracy compared with full instrument performance test). ATLID IPC repetition at satellite level within the success criteria shall confirm:

“Reasonable complexity” of ATLID IPC definition with satellite AIT environment has considered:

The following table shows the repetitions of the 5 ATLID IPCs along instrument AIT and satellite AIT.

Figure 6.

ATLID IPC repetitions along instrument and satellite AIT

The next chapter will present in more detail for these 5 ATLID IPCs the IPC objective, required OGSE, measurement principle and measurement steps, evaluation steps, measurement accuracy, IPC success criteria (linked to resulting sensitivity to ATLID Lidar performance), as well as the possible ATLID degradation mechanism that shall be sensed by the IPC.

2.1

IPC-1 for ATLID Laser Beam Line of Sight (LoS) stability

Objective of this test is the health check of the coalignment and stability of both nominal and redundant ATLID laser beam lines of sight (TxA Los) wrt receiver line of sight (Rx-Tx coalignment).

The ATLID instrument line of sight is defined by the receive chain (RxA) line of site (LoS), accessible via optical reference alignment cube on the ATLID FPA SiC baseplate, see Figure 7.

Figure 7

ATLID receive chain RxA line of sight and ATLID laser beam line of sight (TxA – LoS).

The TxA-LoS will be measured wrt to the RxA LoS. For this measurement, a specific TxA IPC OGSE has been designed, that can be mounted on ATLID emission baffles, see Figure 8.

Figure 8.

TxA IPC OGSE Design and photo of OGSE mounted on ATLID emission baffle.

The IPC measurement of the stability of the Laser Beam line of sight (TxA-LoS) consists of 3 measurement steps:

The measurement accuracy of the TxA LoS to RxA LoS stability has been determined to be +/-87μrad (quadratic summation of +/-50μrad +for the 2-theodolite measurement of alignment cubes and +/-71μrad for absolute TxA LoS measurement wrt E-Baffle). The success criterion for satellite level IPC for laser beam line of sight stability was defined to be approx. 3 times the measurement accuracy, i.e. +/-230μrad. One notes the ATLID beam steering mechanism capability to re-adjust laser beam line of sight by +/-450μrad in outer space. In addition to the individual measurement accuracy of each ATLID IPC, the actual IPC results are carefully checked for any trend evolution even within above formal test success criterion.

Degradation of the stability of TxA-LoS wrt to the RxA LoS would reveal any mechanical degradation of ATLID TxA – RxA alignment or any physical degradation inside ATLID power laser head, of which the good health is closely linked to the actual TxA beam orientation.

Figure 8 shows more detailed configuration drawing of TxA IPC OGSE and a photo of the actual TxA IPC OGSE.

Figure 9 gives an impression of the satellite level ATLID Laser Beam Line of Sight (LoS) stability measurement step, by theodolite measurements. Increased complexity due to the height of ATLID position inside the platform is directly visible. With this setup, it is 2 days to complete the IPC for both redundancies, this is the longest ATLID IPC.

Figure 9.

TxA IPC setup for alignment cube measurement with theodolites, installed on tripods at 3,5m height.

Figure 10 shows the available test results, from instrument and satellite level IPC repetitions. One can see the final TxA LoS error calculated from the contributors of:

Figure 10.

ATLID Los error measurement result and trend for measurements in instrument AIT (grey color) and satellite AIT (orange color)

Instrument level tests colored in grey take first instrument level measurement as reference. Satellite level tests colored in orange take last instrument level test (incoming test in satellite AIT) as reference. On B-side, one TxA LoS stability measurement was done on instrument level, after ATLID dismounting for laser electronic repair, and subsequent remounting on the platform.

Overall ATLID Laser Beam LoS error are well below the success criterion of +/-230μrad and don’t show any apparent trend, hence ATLID good health and unchanged performance can be concluded from this IPC for the related performance aspects.

2.2

IPC-2 for Activation of Laser beam steering mechanism

Objective of this IPC is the health check of each PLH beam steering mechanism, as a combined functional and optical check of BSM operation and emission beam displacement.

The Power Laser Head (PLH) includes a Beam-Steering Mechanism (BSM), controlled by BSM electronics (BSME), aiming at adjusting the emission line-of-sight to maintain emission / reception co-alignment in flight (bistatic Lidar design). The full angular range of beam steering function is approx. +/-450μrad in outer space.

The TxA IPC OGSE allows measurement of TxA laser beam spot position on TxA IPC OGSE CCD matrix detector. This beam spot measurement is used for IPC for ATLID Laser beam line of sight (section 2.1), with beam steering mechanism remaining in zero/default position.

The IPC for Activation of Laser beam steering mechanism consists in commanding an angular movement of the BSM and optical measurement of angular displacement of the laser beam, detectable by displacement of laser beam spot on the TxA IPC OGSE. BSM is commanded of 0.1mrad tilt (outer space) displacement on each X and Y axis. Such angular displacement results in beam spot displacement on TxA IPC OGSE of approx. 5 CCD pixels (pixel angular size = 22 μrad, centroid accuracy +/- 0.5 pixel = +/- 11 μrad).

For this IPC, the Laser beam is commanded from zero/default position by 0.67mrad in x-axis direction, then 0.67mrad in y-axis direction and then back to zero/default position. Actual beam angular displacement is calculated from laser beam spot centroid displacement (pixel) on the TxA IPC OGSE CCD detector.

The setup for this IPC with TxA IPC OGSE mounted on ATLID is as above, see Figure 9 (IPC OGSE control and measurements by dedicated software on notebook). Degradation of the beam steering would reveal electrical/mechanical problem of the BSM Piezosystem or control.

Figure 11.

Individual test results for first satellite IPC for activation of Laser Beam Steering mechanism - and optical measurement of laser beam angular displacement (μrad).

Figure 12.

Trend of test results for IPC for activation of Laser Beam Steering mechanism - in instrument AIT (grey color) and satellite AIT (orange color), measured angular beam displacement in μrad.

ATLID Laser Beam Steering mechanism activations show well reproducible measurements of beam spot displacement on TxA IPC OGSE detector. Results are well below the success criterion of +/30μrad and don’t show any apparent trend, hence ATLID good health and unchanged performance can be concluded from this IPC for the related performance aspects.

2.3

IPC-3 for Laser pulse energy knowledge stability

Objective of this test is the health check of emitting paths (TxA/PLH + E-Bex), for detecting any unexpected significant evolution of laser energy due to internal misalignment or contaminations or laser hardware degradation. Laser energy level is set at 35mJ +/-2mJ for ensuring nominal signal detection with satisfactory signal to noise ratio; the laser energy knowledge is needed to ensure radiometric stability performance of the instrument as each LIDAR echo acquisition is corrected from laser energy variation

The IPC for Laser pulse energy knowledge stability consists in parallel ATLID internal/external measurement of ATLID laser beam pulse energy. The ATLID laser pulse energy can be measured by PLH internal photodiode, whilst the external pulse energy measurement is done with TxA IPC OGSE. The TxA IPC OGSE allows measurement of TxA laser beam pulse energy, after installation of a power meter inside the OGSE sliding rack (Figure 9).

The setup for this IPC with TxA IPC OGSE mounted on ATLID is as above, see Figure 9 (IPC OGSE control and measurements by dedicated software on notebook).

Figure 13 shows the IPC results of ATLID laser pulse energy measurement by internal/external photodiodes, well correlated with time. The visible energy oscillations, of approx +/-0.5mJ with 2min period, are nominal behavior and are related to the temperature stabilization/oscillations of laser head internal subsystems.

Figure 13:

(left) ATLID external OGSE measurement of ATLID laser pulse energy (IPC for Laser pulse energy knowledge stability). (right) ATLID internal measurement of ATLID laser pulse energy (IPC for Laser pulse energy knowledge stability). PLH internal Photodiodes HK-TM for UV pulse energy (PD74). Also shown IR energy of master oscillator MO (PD71), IR energy of Amplifier (PD73) and green visible energy at second harmonic stage (PD75)

For the IPC for Laser pulse energy knowledge stability, we take the 5min average of measured pulse energies.

Figure 13 also shows for information further PLH internal photodiode measured laser energies, at the Master Oscillator (IR), amplifier (IR), second harmonic stage (Vis Green), beside the UV pulse energy after third harmonic stage (PD74).

The following Figure 14 shows the available measurement results and test evaluation for instrument and satellite level IPC repetitions. One can see the final “energy knowledge absolute stability”, that is calculated from:

Figure 14.

ATLID A-side Laser Pulse Energy measurement/trend, in instrument AIT (grey color) and satellite AIT (orange color)

The accuracy of the measurement has been estimated to +/-7% (driven by OGSE detector calibration and detector thermal sensitivity). Test Success Criterion is +/-10% for the Energy Knowledge absolute stability.

Figure 14 and Figure 15 show ATLID A/B-side Laser Pulse Energy measurement/trend. Instrument level tests are colored in grey and take first instrument level measurement as reference. Satellite level test are colored in orange and take same first instrument level test as reference. On B-side, one TxA Laser pulse energy measurement (#13) was done after ATLID dismounting from platform for laser electronics repair, and subsequent re-mounting on the platform.

Figure 15.

ATLID B-side Laser Pulse Energy measurement/trend, in instrument AIT (grey color) and satellite AIT (orange color)

Apparent step of Energy Knowledge absolute stability for ATLID A-side from measurement #9 to #10 is considered nominal behavior, due to specific laser amplifier phasing adjustment, done just before measurement#10, for energy recovery of the laser.

Overall ATLID Laser pulse energy knowledge stability is far better than the success criterion +/-10% and doesn’t show any apparent trend, hence ATLID good health and unchanged performance can be concluded from this IPC for the related performance aspects.

3.1

IPC-4 for Receive chain optical response check

The objective of this test is the health check of receiving paths of ATLID and detecting potential unexpected degradation of instrument response, for example due to strong internal misalignments between field-stop and fiber cores or obscuration of optical path e.g due to optics degradation (e.g. for cleanliness issues).

The IPC for Receive chain optical response check consists in illumination of the ATLID telescope aperture by OGSE laser light source, injected into the receive chain by optical fiber mounted on the receive chain (RxA) IPC OGSE cover. (Figure 16).

Figure 16:

Priciple of IPC-4 for Receive chain optical response check. Receive chain (RxA) illumination by OGSE external laser light source coupled by optical fiber into the receive IPC OGSE cover.

The illumination level at the output of the fiber is controlled through a calibrated photodiode. The distance between the fiber support on the cover and the M1 mirror of the telescope is fixed and repeatable. The angular repeatability of fiber and cover mounting is better than 10mrad with an NA = 0.21 optical fiber in order to limit theoretical angular impact on the coupling efficiency to less than 1%. Roughly 4x10^-5 of the fiber flux is geometrically coupled into 66μrad instrument field-of-view. Acquisitions of the 3 ATLID science channels (Mie-Co, Mie-X, Rayleigh) are then performed at the laser pulse repetition frequency (PRF) rate of 51Hz. Acquisitions of the averaged Coalignment Sensor (CAS) map are performed at the PRF/32 rate (fixed rate at Instrument Detection Electronics output).

The IPC for Receive chain optical response check consists of two measurement steps:

For the receive chain IPC measurements, a specific light tight receive chain (RxA) IPC OGSE cover has been designed that can be mounted on ATLID receive/telescope baffle, see Figure 17.

Figure 17:

Photo of the receive chain (RxA) IPC OGSE cover mounted on ATLID telescope baffle.

The test evaluation then calculates:

Figure 18 shows measurement/trend results for IPC for receive chain optical response check. Instrument level tests are colored in grey, satellite level tests are colored in orange. Both show the relative response error wrt same instrument level reference (as defined by instrument manufacturer).

Figure 18.

IPC for Receive chain optical response check measurement/trend results, in instrument AIT (grey color) and satellite AIT (orange color)

Figure 19 shows the test setup for the IPC for Receive chain optical response check in satellite AIT. OGSE laser is installed and handled within the protected compartment of black laser protection walls, with receive chain (RxA) IPC OGSE cover mounted on ATLID and connected to the controlling notebook.

Figure 19.

Photo of test setup for the IPC for Receive chain optical response check in satellite AIT.

Overall ATLID receive chain optical response stability is far better than the success criterion +/-20% or +/-30% and doesn’t show any apparent trend, hence ATLID good health and unchanged performance can be concluded from this IPC for the related performance aspects.

3.2

IPC-5 for Detection chain total noise in darkness

Objective of this test is the health check of the ATLID detection chain, for any unexpected degradation of detector or detection electronics.

The IPC for Detection chain total noise in darkness consists of measurement of ambient noise figures in darkness, read out noise and DSNU, and comparison with reference data from incoming instrument level reference tests. The read-out noise is a significant contributor to the noise budget and difficult to discriminate from the total noise in darkness. Therefore, specific Readout Noise Calibration mode (RONC) is implemented to limit contribution from other noise sources (such as clock induced charge or dark signal generation with temperature). It consists of reading quasi-empty pixels in order to estimate the read-out stage contribution to the total detection noise.

DSNU will also be recorded in dedicated DCC (Dark Current Calibration mode), but only for information and potential anomaly investigation (no quantitative success criterion).

IPC for Detection chain total noise in darkness uses the same light tight receive chain (RxA) IPC OGSE cover mounted on ATLID receive/telescope baffle, see Figure 15.

The IPC for Detection chain total noise in darkness consists in basically 2 measurement steps:

For the evaluation of all echo profiles, we calculate average (detection offset per sample) and standard deviation (readout noise for each sample) over all acquisitions. Success criteria are defined such that the so called RONC maps (average + standard deviation in LSB) shall not differ more than 20% from instrument level reference values.

Above Figure 20 and Figure 21 show the available test results, from instrument and satellite level IPC repetitions. Overall ATLID Detection chain read out noise in darkness is far better than the success criterion +/-20% and doesn’t show any apparent trend, hence ATLID good health and unchanged performance can be concluded from this IPC for the related performance aspects.

Figure 20.

IPC for Detection chain total noise in darkness measurement/trend results – ATLID A- and B-side detection chain average of readout noise (LSB) for 3 science channels, in instrument AIT (grey color) and satellite AIT (orange color)

Figure 21.

IPC for Detection chain total noise in darkness measurement/trend results – ATLID A- and B-side detection chain standard deviation of readout noise (LSB) for 3 science channels, in instrument AIT (grey color) and satellite AIT (orange color)