3.1

Block diagram of the LIFT Concept

The LIFT instrument is composed (Fig. 1) of

Fig. 1.

Block Diagram of the LIFT interferometer

The following section describes in more details each of the LIFT sub-systems.

3.2

Subsystems description

The selected design of this instrument results from several trade-offs performed in parallel during the definition phase. The compliance with the observation requirements as well as the verification of the radiometric performances have been the main drivers of this trade-off, however the reduction of the costs of the instrument has been another parameter considered in this selection.

To be compliant with the asymmetric FOV two solutions have been envisaged. First, the most straightforward solution compliant with the 2 x 30 km field of view includes a scan mirror able to reach one after one the 15 position of the elevation field of view, an anamorphic telescope to generate a square field of view at the interferometer level and a single detector. The main advantage of this configuration is the simplicity of the detection chain, however the time allocated for the acquisition of one interferogram is reduced to less than 0.2 s which could be an issue for the radiometric performances. Moreover, the design and implementation of the anamorphic telescope could have induced some difficulty.

The second solution, selected for the current design of LIFT, is far less complex from an optical design point of view but it requires a more complex detector assembly. The detector is now a matrix of 15 x 15 pixels, each of them covering a 2 x 2 km square field of view. At the instrument level, the total field of view is considered as a square of 30 x 30 km, leading to a more regular telescope, while the time for each interferogram acquisition is close to 3 s.

In order to meet the noise and radiometric performance requirements, the diameter of the entrance pupil is fixed at 300 mm. This pupil is located on the pointing mirror to minimise its size and allow fast motion and good pointing accuracy. The Pointing Mirror Assembly - PMA - orients the instrument FOV towards either the limb, the deep space or the internal calibration blackbody.

A two-mirror afocal input telescope with a magnification factor of 4.15 is located between the pointing mirror and the interferometer. The exit pupil of the telescope is located on the apex of the retro-reflector by means of a field lens located on the intermediate image. This lens allows us to place the retro-reflector as far as we need for mechanical constraints. The intermediate image can be used to place a field stop to minimise stray-light

The core of the instrument is composed of a double-pendulum, cube-corner interferometer as shown in Fig. 2. The scanning mechanism is highly inspired from the ACE-FTS interferometer on-board Scisat-1 designed and manufactured by ABB. The advantage of this approach is to provide a mechanical-to-optical-path-difference factor of 4. This configuration is also less sensitive to vibration induced shear due to the fact that the interferometric mirrors are permanently aligned at each end of the balanced scanning arm. The LIFT interferometer offers an optical path difference coverage of ± 5 cm, corresponding to spectral sampling of 0.1 cm^-1, while the corner-cubes mechanical displacement is only ± 1.25 cm. The entrance pupil diameter along with the magnification factor of the telescope define a pupil size of 7.15 cm inside the interferometer. State-of-the art retro-reflectors developed for the IASI program are re-used in the LIFT design. The angle of incidence on the beamsplitter is 45° like in the ACE-FTS instrument. This angle of incidence may give rise to slightly higher polarisation effects but it considerably reduces the overall dimensions of the subsystem.

Fig. 2.

Core of the interferometer

At the output of the interferometer, the spectral separation is done with a single dichroic located after a parabolic focusing mirror. Focusing optics is implemented in each spectral channel and the F/# at the detector is 0.97.

The detection chain is composed of two arrays of detectors covering the LIFT spectral bands and working at 95K in a passive cooler. Two MCT photovoltaic detectors have been chosen to cover the spectral range (channel A from 4.7 pm to 9.5 pm, channel B from 9.5 to 13 pm). Each focal plane consists in 15 x15 pixels (to cover the 30km x 30km field of view). The pixel size is 0.175 mm leading to a total active area equal to 2.625x2.625mm².

The selection of the focal plane cooler system is based on improved performances of the MCT detectors with lower temperatures but also on the increase in costs and complexity of the active system compared to a passive solution. Moreover, LIFT being on the same orbit than IASI on Metop, the experience gained on IASI could be fully applied. Since using a recurrent system from IASI significantly reduces the development costs of LIFT, the passive concept is preferred. This sub-system uses a 3-staged radiator and a Cold Box (Fig. 3). The Cold Box structure contains the cold optics and the detectors. The radiator position & orientation are such that it avoids any IR radiance input from the Earth and from the rest of the spacecraft - platform and payload instrument. The Sunshield is designed to keep the radiator free from sun illumination. As it is exposed to Earth radiation, it is given a specific curved profile that prevents any reflected light to reach the cold radiator. This cold box is located on the [+X_S] side of the spacecraft.

Fig. 3.

Cold box compact layout

The last aspect of the instrument to be considered is the calibration target. In order to measure the gain and the offset, a two-point calibration is the minimum needed. For the cold calibration, deep space from 90 to 120 km is used. For the hot calibration, the concept is much more complex. Due to the large dimensions of the pupil (300 mm), a classical black body cavity cannot be envisaged since these devices offer acceptable performances when their length-to-aperture ratio is higher than one. Weight and envelop constraints avoid the use of such a large cavity. On the other hand, using a smaller source would require a dedicated afocal telescope between the black body and the pointing mirror, also inducing constraining implementation requirements. Therefore, a passive plate blackbody with a corner-cube pattern and a specular coating is the selected approach. Such plate blackbody could have a diameter of about 300 mm (Fig. 4) which covers the entire entrance pupil.

Fig. 4.

Plate blackbody

3.3

TIMELINE

The requirement for 90 % global coverage must be adapted to the calibration need. One calibration each 30s is enough for the deep space view (stability of the instrument) while detector drift can be calibrated less often (each 80 s in IASI)

The calibration sequence is different, depending of the pointed calibration source. According to the pointing mirror specification the Deep Space depointing is 300 ms and 400 ms are use to point back at the scene. The spectrometer last rotation is performed in the same time. For the limb view, the depointing time is 700 ms and the pointing back is 1000 ms. This depointing is more time demanding as the required angle is greater than for the deep space view. Table 1 summarises the times needed for the re-orientation of the pointing mirror.

Table 1:

Mirror Pointing Constraints

The acquisition of the interferograms during calibration is made at 1/10 of the nominal resolution. Two interferograms are acquired for a total of 646.2 ms including 75 ms for the direction change of the interferometer scanning mechanism between these two acquisitions (see Fig. 5)

Fig. 5:

Interferogram mechanism movement during acquisition and calibration

The following timeline (Table 2) is proposed as the baseline for the LIFT instrument. The scene acquisition takes 2.856 s, this acquisition is repeated 5 time to cover the 100 km of along track sampling interval. Between two acquisitions, the scanning mechanism of the spectrometer must change direction. This turn around motion is estimated to be shorter than 75 ms and must be repeated 4 times, the corresponding cumulated time is therefore 300 ms. The time between two calibrations is about 30 to 33 s between two deep space views and 66 s between two BB views. This sequence is sufficient to insure the right performances of the FTS. This time sequence is compliant with the requirement of 0.9 duty cycle.

Table 2:

Timeline for the LIFT instrument

4.1

NESR performances

The principal source of noise in the total NESR is the noise from the detector and its associated electronics. Fig. 6 shows that the NESR requirement is obtained within the spectral the band B where the detectivity is higher at 95K.

Fig. 6:

NESR performance with F_number of 0.97

These NESR values are obtained after a combination of the signals collected from the 15 horizontal pixels (corresponding to the horizontal resolution requirement) and a summation of 5 interferograms (corresponding to the along track resolution of 100 km). This latest summation is made on ground in order to discard cloud contaminated measurements.

In term of SNR (Fig. 7), SNR is greater than specifications with a maximum SNR equal to 2620 at σ = 1080 cm^-1.

Fig. 7:

SNR for a F_number of 0.97

4.2

ILS calculation

As described in Section 3, the detector is a 15 by 15 focal plane array. The pixel divergence inside the interferometer is square. The input afocal telescope maps the instrument instantaneous FOV to a pixel divergence of 2.46 mrad inside the interferometer. This divergence for the most off-axis pixel is small enough so that the resolving power of the FTS is preserved (Fig 8). The next picture gives the calculated ILS at 1750 cm⁻¹ for a divergence inside the interferometer that maximises the modulated signal at MPD.

Fig. 8:

Instrument line shapes for a square 15 by 15 focal plane array at 1750 cm⁻¹

Finally, for the corner pixel (worst case) of a this square focal plane array, the ILS FWHM is about 0.13 cm⁻¹ and the asymmetry of the secondary lobes is about 0.11%.

4.3

overall mechanical layout and Budgets

As shown in Fig. 9, the instrument is sandwiched between its optical bench and the platform. The mounting of the instrument uses three fixation feet. The overall dimensions of the instrument are 2000 mm x 1630 mm x 1077 mm taking into account the input baffle but not the electronics boxes to be placed on the spacecraft. The overall mass is estimated to be smaller than 196 kg, the electrical consumption is less than 235 W and the data rate, 507 kbps.

Fig. 9:

overall mechanical layout of the LIFT instrument