A.

ATLID measurement principle

Operating in the UV range at 355 nm, ATLID provides atmospheric echoes with a vertical resolution of about 100 m from ground to an altitude of 20 km and 500m from 20km to 40km altitude. The measurement geometry is displayed in figure 3. Thanks to a high spectral resolution filtering, the lidar is able to separate the relative contribution of aerosol (Mie) and molecular (Rayleigh) scattering, which gives access to aerosol optical depth. Co-polarised and cross-polarised components of the Mie scattering contribution are also separated and measured on dedicated channels.

Figure 3.

ATLID sampling and measurement principle

The measurement principle which was retained for ATLID uses the fact that interaction of light with molecules or aerosols leads to different spectra. Whereas the Brownian motion of molecules induces a wide broadening of the incident light spectrum, the single scattering with an aerosol does not affect the spectrum shape of the incident light. As a consequence, a simple means of separating the backscattering contributions consists in filtering the backscattered spectrum with a high spectral resolution filter centred on central wavelength, as depicted on figure 4.

Figure 4.

Mie / Rayleigh scattering contributions separation principle

Table 1 summarizes the observation requirements applicable to ATLID. The instrument shall allow retrieval of the pure Mie and molecular backscatter signals. The measurement accuracy is specified for a reference atmospheric scene consisting of thin cirrus at 1o km altitude. A telescope footprint smaller than 30 m is favored to minimize the multiple scattering effects and to reduce the solar background noise by reducing the telescope field of view.

TABLE I.

ATLID OBSERVATION REQUIREMENTS

B.

Instrument overview

ATLID is designed as a self-standing instrument reducing the mechanical coupling of instrument/platform interfaces and allowing better flexibility in the satellite integration sequence.

The lidar functional architecture is organised in four main functions, namely the transmitter, the emit/receive telescope, the receiver and the control unit called ATLID Control and Data Management (ACDM) unit.

The emitter includes the power laser head and its transmitter laser electronics, a beam steering mechanism for ensuring co-alignment of the emitting and receiving lines of sight over the mission life time, and a beam expanding optics. The laser is a highly stable single-mode laser emitting at 355 nm (tripled frequency of a Nd:YAG laser) and therefore requires a reference laser seeding the laser oscillator.

The receiving telescope is an afocal Cassegrain aiming at collecting the backscattered light and providing a large magnification ratio to reduce effect of internal misalignments.

The receiver implemented at the back of the telescope includes a background light filtering stage, a High Spectral Resolution filter and a co-alignment sensor. The signal is transported to the detectors by means of fibre couplers, which allows deporting the whole detection chain to the interface panel. The receiver also includes all the detection functions that range from the detector to the analog-to-digital convertor.

The control and data management unit ensures the following electrical functions: 1) synchronisation between laser emission and data acquisition; 2) data processing and data stretching toward the S/C; 3) mechanisms drive and thermal regulation functions; 4) TM/TC and commandability/observability management.

The instrument design relies on re-use of technologies developed for the ALADIN wind lidar for the Aeolus mission as the single-mode pulsed laser transmitter, memory CCD detector, receiver high spectral resolution spectral filter. Moreover, the design worked-out since beginning of feasibility phases now features further improvements and innovations which are implemented to match the ATLID mission demanding objectives:

The instrument, which overview is shown in figure 6, is designed, integrated and tested by Astrium SAS as prime contractor leading a European consortium in charge of developing the ATLID sub-systems.

Figure 5.

ATLID instrument block diagramme

Figure 6.

ATLID instrument overview

C.

Optical design

The optical design is based on a bistatic architecture. This architecture was selected with the objective to separate the emission and reception functions allowing a full pressurisation of the emission path. An afocal 620 mm diameter Cassegrain telescope is used in reception, with a high magnification ratio. The receiver field-of-view is thus kept below 75 μrad, minimising the shot noise associated with the acquisition of Earth background signal.

The design is based on an all-Silicon Carbide mirror and mounts, designed by EADS Astrium and manufactured by Mersen.

The receiver optical design performs a separation of polarisation (co-polarised and cross-polarised signals) and spectral components (Mie or Rayleigh scattering contributions) with the constant goal to limit the cross-talks between each of the three channels, namely the Mie co-polarisation, the Mie cross-polarisation and the Rayleigh channels. Several filtering stages (narrow-band interference filter, spatial filter and Fabry-Perot etalons) are required to achieve such purity and to reject the high amount of Earth background signal around the narrow laser wavelength.

The High Spectral Resolution Fabry-Perot etalon features narrow bandwidth of the order of 0.3pm, so that separation of the Mie and Rayleigh backscatter signals can be efficiently performed. The design developed by RUAG and SESO is inheriting from Aladin spectrometers (figure 8) and is based on a combination of polarizing beam splitters and a Fabry-Perot etalon.

Figure 7.

Telescope primary mirror made of SiC

Figure 8.

The spectral co-registration approach consists in periodically tuning the laser transmitter frequency to the high spectral resolution filter peak transmittance by sweeping the laser frequency over its tuning range and estimating from the signal distribution on Mie and Rayleigh channels the best frequency command.

D.

Low noise detection chain

The ATLID detection chain shall be able to measure single photon events to meet the worst case radiometric performance requirements. Therefore, a high response together with an extremely low noise are necessary to fulfill the signal acquisition requirement. As for ALADIN instrument, ATLID encompasses a memory CCD. The ATLID design performs fast sampling of the echo signal (1.5 MHz corresponding to 100 m vertical sampling distance) and on-chip storage of the echo samples which allows delayed read-out at very low pixel frequency (typically below 50 kHz). Combined with an innovative read-out stage, the detection chain provides an extremely low read-out noise (< 3e-rms per sample). Accumulation of several consecutive echoes on the chip is possible with the detector design, enhancing the acquisition chain radiometric performance. The detector is developed by E2V while the detection chain electronics is developed by CRISA.

E.

Transmitter requirements and design

The laser transmitter of ATLID instrument shall deliver high energy pulses at a repetition rate of 51 Hz corresponding to 140 m ground horizontal sampling. The Pulse Repetition Frequency has been lately changed allowing a relaxation of the Master Oscillator requirements while using the amplifier with the same operation mode as for Aladin development. More than 35mJ at 355 nm (tripled Nd:YAG wavelength) are required at laser output to meet the instrument radiometric performance. At the same time, high frequency purity (line-width of typically 50 MHz) and extreme stability (50 MHz on one month time scale) are mandatory in order to separate the Mie and Rayleigh scattering contributions by High Spectral Resolution technique.

This is achieved by a transmitter architecture based on three sub-systems:

Figure 9.

ATLID receiver layout

Figure 10.

ATLID power laser head

A significant evolution of the laser design with respect to the ALADIN transmitter lies in the fact that ATLID power laser head is sealed and pressurized. This improvement is believed to ensure more stable operating conditions to the sensitive components of the laser, and to isolate the laser internal space from surrounding contaminants over the ground and operational lifetime. Pressure is also deemed to improve tolerance to laser induced contamination, which is the degradation of an optical surface resulting from the interaction of molecular contamination with a high laser illumination level. Such a pressurised design has been extended to the whole emission path, including the beam expanding optics. The laser transmitter is developed under the prime leadership of Selex Galileo with sub-contractors Quantel for the development of the amplifier and laser diodes, TESAT for the Reference Head.

An Emission Beam Expander is used to further expand the laser beam and reduce the laser beam divergence to about 45 microradians so that it is coupled within the narrow receiver field of view.

The expansion of the beam brings also a significant reduction of the energy density on the last optics submitted to vacuum, thus beneficial for minimizing risk of laser induced damage and contamination. An expansion of about 7 is achieved through a 3 lens design bringing the beam to more than 100mm diameter. The Emission Beam Expander is developed by SODERN.

F.

Co-Alignment Sensor

The instrument is equipped with a Co-Alignment Sensor that is aimed at measuring the direction of the laser emission with respect to the receiver field of view. The measured information is fed back to a beam steering mechanism correcting for any mis-pointing of the laser. The control loop is operated at low speed as it corrects mainly the variation in the line of sight due to thermo-mechanical deformations along the orbit and lifetime of the instrument.

The Co-Alignment Sensor is a small camera measuring the integrated backscatter signal from ground to top of the atmosphere. It includes the following functions as rejection of the solar background, separation of the optical signal with the scientific channels, acquisition and sampling of the integrated signal with a CCD. The camera is developed by a consortium led by CRISA.

G.

Thermal control design

Another innovation is related to the thermal control of the power laser head. This sensitive active sub-system presents high heat dissipation (around 150 W) and requires stable interface temperature (0.5K). Mini loop heat pipes are used in ATLID design to efficiently evacuate the laser heat while offering a low stiffness mechanical interface; the flexible pipes which transport the ammonia from evaporators to the anti-sun side radiator allow a good mechanical decoupling of the laser with respect to its radiator, thus minimising the stress experienced by the laser optical bench. This new technology was successfully validated in flight during FOTON experiment, and a life test has been running for the last 2.5 years to demonstrate the lifetime of such devices. High conductance, low sensitivity to gravity orientation during ground tests are other decisive advantages which make the loop heat pipes preferable to standard heat pipe technology for ATLID application.