The novel airborne Gimballed Limb Observer for Radiance Imaging of
the Atmosphere (GLORIA) measures infrared emission of atmospheric
trace constituents. GLORIA comprises a cooled imaging Fourier
transform spectrometer, which is operated in unpressurized aircraft
compartments at ambient temperature. The whole spectrometer is
pointed by the gimbal towards the atmospheric target. In order to
reach the required sensitivity for atmospheric emission measurements,
the spectrometer optics needs to operate at a temperature below
220
The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA;
Friedl-Vallon et al., 2014) is a cooled Fourier transform infrared (FTIR)
spectrometer which has been designed to fly on board the German research
aircraft HALO (Krautstrunk and Giez, 2012) or the Russian M55 Geophysica
(Myasichev Design Bureau (MDB), 2002). It measures the infrared emission of atmospheric species in the
spectral range from 780 to 1400
Low temperature operation, as already proven in the previous balloon and aircraft borne spectrometers SIRIS (Brasunas et al., 1988), MIPAS-B2 (Friedl-Vallon et al., 2004), MIPAS-STR (Piesch et al., 1996) and CRISTA-NF (Kullmann et al., 2004), enables the detection of the characteristic infrared emission spectral features of atmospheric species at good signal to noise ratio.
The spectrometer is mounted in a three-axis gimbal to provide pointing
stability and agility to enable different atmospheric observation
modes (Friedl-Vallon et al., 2014): in chemistry mode (CM) with high
spectral resolution, the line of sight is nearly perpendicular to the
flight direction; whereas in dynamics mode (DM) with high spatial
resolution, the instrument scans the horizon stepwise with gimbal yaw angle
ranging from 45 to 132
The instrument is operated in unpressurized compartments and observes the atmospheric radiation through an opening on the side of the instrument bay. An installation in the pressurized cabin would require a window outside the calibration path of the instrument. Consequently, the instrument is exposed to the air flow and to variable ambient conditions depending on the flight profile, the aircraft velocity, and the meteorological situation. Overall instrument operation reliability and performance are influenced by environmental parameters such as temperature, humidity, vibration and pressure. A high-speed data acquisition system collects the scientific data as well as a large number of housekeeping data including the ambient conditions and the status of the GLORIA instrument.
The gimballed movement of the instrument requires a lightweight and compact mechanical design of the spectrometer which incorporates an integrated cooling system. In addition, a mechanically rigid structure for the sensitive optics of the interferometer is necessary in order to gain robustness against vibrations which are stimulated aerodynamically at the opening of the instrument bay and by the aircraft itself. The cooling system has to meet the needs of campaign operation with long flights and short stopovers at airports with limited infrastructure.
Section 2 describes the mechanical and thermal requirements of the airborne GLORIA spectrometer. The selected concept and its implementation are presented. The efforts which were made to achieve a stiff and thermally insulated optic module are emphasized. Furthermore the dedicated cooling system and its operation are shown. Sect. 3 follows a description of the environmental conditions during flights on both the M55 Geophysica aircraft and the G550 HALO aircraft based on sensor data. Finally the performance of the mechanical and thermal system is discussed in Sect. 4.
The mechanical structure of the spectrometer houses the optical
components comprising a 9
GLORIA uses time-equidistant sampling with post processing to account for velocity variations (Brault, 1996). In the ideal case, this technique can cope with velocity variations approaching 100 %. In reality, the optical paths of the infrared radiation and of the reference will not be completely identical and residual errors will arise. Based on unpublished experiences with MIPAS-STR, the goal for the velocity stability of the optical path difference measured by the reference laser system is 5 % RMS which provides a reasonable trade-off between technical limitation and theoretical requirements.
In addition, the design of the spectrometer shall be compact and
lightweight to allow its integration in the gimbal. Integrated local
electronics are used on every gimbal frame as well as on the
spectrometer itself in order to minimize the number of cables running
through the gimbal axes. Furthermore, the pointing stabilization
requires stiffness of the overall system – including the gimbal and
the spectrometer – to enable pointing stability within 0.7 arcmin
(1
The mechanical systems have to cope with low environmental temperatures (down
to 200
Mechanical and thermal requirements for the GLORIA spectrometer.
These vibration values are given for the attachment points of the whole instrument at the carrier. However, these values are used as requirements for the spectrometer, too. In reality, the vibrations may be damped or increased by the structure of the gimbal. Additionally, vibrations can be caused by airflow.
Additional constraints are imposed by the flight certification process; static load strengths must be demonstrated by calculations and testing. The requirements are different for HALO (Wernsdorfer and Witte, 2008) and for Geophysica (MDB, 2002). In Geophysica the instrument is inside of the fuselage and has to be handled as a built-in component. The choice of materials shall conform to aviation safety regulations.
The optical components have to be cooled below 220
For proper operation of the spectrometer in the harsh environment, care has to be taken to keep the cooled interferometer volume dry and clean. The outer surface should be kept free of condensate to reduce contamination and avoid disturbances at critical components such as on the entrance window or on electrical connectors. The requirements are summarized in Table 1.
The cooling system has to be located close to the interferometer optics –
within the gimbal to avoid insulated or pressurized hoses from one gimbal
level to the other, which may impact the pointing stabilization performances.
Compressor or Stirling coolers for the required working temperature and
environment are bulky, heavy and generate vibrations. Their integration in
the system is difficult. Therefore, a non-electrical cooling system with
a reservoir of coolant was considered. It allows direct contact between the
coolant reservoir and the interferometer on a large area and therefore
enables good spatial temperature uniformity. The working duration should achieve
at least 24 h to cover flights with two legs and short stopovers without
maintenance. The pitch agility of the gimbal, covering 112
Experiences from the precursor instruments MIPAS-B2 and MIPAS-STR have shown that operations during scientific measurement campaigns lead to additional requirements for the use of coolants: the coolant has to be refillable independently of the filling level or temperature of the instrument. This is required to handle delays in launch schedules. A coolant commonly available has to be chosen for operational considerations. The instrument's servicing ports have to be accessible through small hatches in the aircrafts fuselage to allow servicing without dismounting the cowling of the instrument bay. Finally, the capability to stabilize the temperature of the optic module during flight, on-ground and in the laboratory in a reproducible way is important for measurements and tests.
HALO with belly pod and opening for GLORIA (top) and belly pod with fairing dismounted showing the GLORIA instrument (bottom). The entrance window of the spectrometer is behind the opening in the gimbal's shield.
GLORIA Spectrometer ready for installation in the gimbal. Outer shell with electronic boxes (black), the entrance window (with cover), and the detector module (left side). The interferometer optics with cooling system is housed inside.
Exploded view of the GLORIA spectrometer showing components of the housing: pitch level plate, cover plates (grey), insulation (yellow), and local electronics (orange). Also shown is the optic module (cyan) with detector (red) and the cooling system (dark blue). External cables between electronic modules are not shown.
The GLORIA Instrument consists of the spectrometer in the three-axis gimbal, the blackbody calibration system, and the power electronics which are attached by a mounting frame to the hard-points in the fuselage of the aircraft by silicone vibration isolators. The central control computer can be positioned in the cabin – for flights on HALO – or in the bay – for flights on Geophysica.
Figure 1 shows the GLORIA instrument on the HALO aircraft. The gimbal has a shield with an opening in front of the entrance window of the spectrometer. An elongated opening in the belly pod allows observation of the horizon with different gimbal yaw angles.
The spectrometer dismounted from the gimbal is shown in Fig. 2. The
total mass of the spectrometer is 48
The spectrometer structure can be split into the three distinct parts shown in Fig. 3: the housing, the optic module, and the cooling system. The housing is built upon the gimbal pitch plate and forms, with additional cover plates, a protection shell containing the insulation. Both the cover plates and the gimbal pitch plate also serve as a support to electronic units. The optic module is a sealed compartment containing all optical components, including the entrance window and the detector.
A cooling system based on dry ice has been chosen. Contamination of
the interferometer volume with gaseous carbon dioxide has to be
avoided because it is an atmospheric trace species measured by
GLORIA. Therefore, the cooling system exhaust has to be diverted and
the interferometer volume has to be properly sealed. For work in
laboratories and during standby, an additional feedthrough with the
possibility to inject cryogenic liquid nitrogen from a storage vessel
for controlled cooling was added. This second coolant increases
flexibility for ground operation. It is not suited for flights because
the cooling system is not designed to store a liquid at
70
Besides the cooling system, the use of vacuum insulation panels (VIP) is an important part of the thermal design. The highly efficient panels ensure low heat input and, therefore, allow a small coolant reservoir for the required holding time. With small insulation thickness due to the exceptionally low thermal conductivity, the volume required for the insulation is maintained small. The spatial temperature gradients in the optical system are reduced by using an all-aluminium structure with direct contact to the cooling tank and the use of the mentioned insulation. An insulation feedthrough assembly was designed to reduce local heat input along cables and cooling system tubes.
The entrance port for the incoming radiation breaks the thermal containment of the instrument. An assembly consisting of two air-spaced Germanium windows was preferred over a shutter system in order to meet the requirements in mass and size. This solution also enables the system to perform measurements in a humid environment at low altitudes or even on ground, supported by a heater for the outer entrance window to prevent condensation. Hygroscopic breathers allow pressure compensation between the closed cooled interferometer and the outside environment while protecting the optics from humidity and other contaminants.
The optical and electronic units are based on a modular design, which allows parallel maintenance of the subsystems and easy replacement during development or operation. It also facilitates improvement to specific subsystems. The reproducibility of the positioning for the optical components is ensured by dowel pin and hole or slot and key configurations. Electrical connections between interchangeable parts are realized by docking connectors.
Several PT1000 platinum resistance temperature sensors, four triaxial accelerometers, and two pressure sensors are mounted at key locations in and on the spectrometer. Together with the sensors on the gimbal and support structure, which also include a microphone, these sensors enable a detailed instrument characterization.
The base structure of the housing is the pitch level plate, working as a stiff chassis which holds further housings components as well as the optic module and the cooling system. The components are shown in Fig. 3. Both ends of the pitch level plate are mounted to the gimbals' pitch bearings and motor drive. The position of the pitch plate is such that the pitch rotation axis coincides with the spectrometer's center of gravity.
The housing cover consists of thin-walled plates with stiffening ribs machined out of high-strength aviation grade aluminium. The housing has only openings for the detector, for the entrance window, and for the insulation feedthrough assembly. The box protects the optic module and the VIP insulation.
The combined optic module and cooler are fixed to the pitch level plate by three glass-fibre reinforced plastic (GFRP) spacers. They allow a stiff connection while providing thermal insulation. A high degree of stiffness is important for this connection as it is critical for the line of sight stabilization performance. The position and orientation of the plates compensate different thermal expansion between optic module and pitch level plate.
The thermal insulation used to insulate the optic module and the
coolant tank is based on VIP, which are made of evacuated micro porous
fumed silicon dioxide (Fricke et al., 2006). The silicon dioxide core
of the panels is sealed with a gas tight aluminized Polyester
foil. Rectangular panels are arranged in two overlapping layers of
a thickness of 10 to 15
The insulation feedthrough assembly, illustrated in Fig. 4 and shown with the cooling tank in Fig. 7, is the interfacing device between the cooled components and the outside environment. It provides a thermally insulated pass-through for all tubes and cables while minimizing heat input from outside to the cooler and optic module.
The insulation feedthrough assembly consists of two plates bonded together with a GFRP spacer tube. The warm-end plate is fixed to the pitch level plate and the cold side plate acts as a heat sink cooled by the cold coolant gas exhaust. This design considerably reduces the heat load entering through the outer service connections. The tubes passing through the feedthrough include those for feeding coolant into the cooler system, purging exhaust gas and flushing various interior cavities.
The insulation feedthrough assembly mounted on the pitch level plate (partly shown in gray). The heat input by tubes and cables between bottom part (ambient temperature) and top part (optic working temperature) is minimized.
The electrical feedthrough to the optic module consists of two cables and a custom designed combination of multilayer flex cable and printed circuit boards (PCBs). In the feedthrough, the cables pass around the cooler exhaust tube and the flex cable sidewards down to the pitch level plate. At this location, MICRO-D electrical docking connectors, which are environmentally sealed, provide interface to the outer electronic modules. The intermediary PCB of the flex cable assembly is mounted on the heat sink to further reduce the heat load from outside introduced by the large number of copper wires. The whole feedthrough system is covered by an insulation foam housing and the internal hollow spaces are filled with styrofoam pellets to minimize convection.
The spectrometer's internal spaces have to be pressure-balanced to the
environment to avoid forces on the housing and the optic module. Two
independent internal hollow spaces are found inside the spectrometer
housing: the optic module free space and the open room between the
housing and the optic module, including the interstices between
insulation panels. Incoming air has to be dry and clean to protect the
optics. Therefore, two hygroscopic breathers filled with molecular
sieves and integrated particle filters are connected to the
feedthrough assembly. This system allows ventilation, leaving
a residual pressure difference of a few hPa while limiting the input
of humidity,
Exploded view of the optic module illustrating the main opto-mechanical (cyan) and optical components of the interferometer, the entrance window, the imaging optic and the detector system (red).
The optic module comprises of the entrance window assembly, the optical components of the Michelson interferometer, the imaging lens system, and the detector. The optic module also holds a reference laser system for measuring the optical path difference. The optic module is described in detail below in this section and shown in an exploded view in Fig. 5.
The main structure of the optic module, the central body, is milled out of a massive block of fine-grained aluminium which exhibits low thermal distortions related to inner tension. The optic setup is modular and the subsystems are tightly mounted to the central body. The compact box-like platform makes the interferometer a system which shows high eigenfrequencies and is robust to external vibrations.
The optical components, along the path of the incoming radiation, are the entrance window assembly, the beam splitter unit (BSU), the fixed cube corner, the linear scanner with its moving cube corner, the adjustable infrared objective, and the detector. The supports and fixations for all optical components are made of the same aluminium alloy used for the central body, which leads to uniform thermal contraction during cooling and minimizes misalignments.
The infrared radiation enters the optic module via an insulating
double window assembly which hermetically seals the central
body. The diameters of the two AR-coated germanium optical windows are
100
The BSU consists (i) of a beam splitter with 104 mm diameter which is tilted 45
The fixed cube corner is mounted onto the central body. In order to minimize shear errors, adjustment in the directions perpendicular to the incoming beam is necessary. The position of the fixed cube corner at operating temperature is determined through interferometric measurements. The cube corner positioning then can be reliably adjusted by the use of gauge blocks.
Complete scanner with slide (left) and detail of slide (right). The preloaded dovetail guidance with high contact area and a leadscrew for the feed motion was chosen to get a stiff and rigid design.
The custom made cube corners are gold-coated Zerodur facets fixed by
contact bonding (Haisma and Spierings, 2002) with a clear aperture of
72
The reference laser is guided through the interferometer parallel to the infrared beam. It is folded from and back to the reference laser source and detection unit with folding mirrors above and below the imaging optic.
The imaging optic is an air-spaced infrared achromat with a focal
length of 72
The detector system, shown in red in Fig. 5, is a custom production by
AIM, Heilbronn, Germany. It comprises of an FPA detector within a Dewar,
the detector front-end electronics and a separate split Stirling
cryogenic cooler connected by a helium transfer tube. Together with
the imaging optic it forms the detector unit. The high-speed
HgCdTe (mercury cadmium telluride) large focal plane array (LFPA) detector with 256 by 256 detector
elements is sensitive in the mid and long-wave infrared spectral range
between 7 and 12.8
The opto-mechanical setup and especially the scanner has to endure the vibrations under the harsh environmental conditions during flight while holding true to the requirement for velocity variation under 5 % RMS.
The scanner assembly, shown in Fig. 6, was designed to maintain a stiff coupling between the moving cube corner and the central body. A rigid scanner construction was achieved with a base having high torsion and bending eigenfrequencies. For the guidance of the slide, a preloaded dovetail slide was chosen. Due to its large contact area, it is very stable and largely immune to external loads and vibrations when compared to ball-guided slides (Endemann, 1999) or guide slide bearings with small contact areas as used in high resolution laboratory spectrometers (Hase et al., 2013). These advantages are gained at the cost of higher friction; therefore, the material combination and lubrication have to be chosen carefully. For precise guidance of the slide in the dovetail, a lateral pretension is necessary, which is also important to compensate residual thermal expansion. The pretension is achieved by a flexure and supporting compression springs. This simple mechanical solution is flexible in one direction while providing the necessary stiffness along the other directions.
Guidance and carriage are manufactured out of the same aluminium alloy used for all components of the optic module. The guiding surfaces are ground to get a good surface quality in terms of shape and roughness in order to provide a uniform movement with reduced friction. The surfaces are electroless nickel plated. Pads made out of polytetrafluoroethylene (PTFE) are used on the side of the carriage. The combination of PTFE with nickel leads to low friction with low slip-stick effect. The prefabricated parts were cryogenically cycled before finishing in order to reduce thermal distortion.
Coolant tank internal view with insulation feedthrough
assembly (top right) mounted on the pitch level plate. The injection
pipe is used to spray liquid
For the movement of the slide a leadscrew with 1
In order to fulfil the thermal and logistical requirements described in
Sect. 2.1 we decided to cool the optic module by a reservoir filled with dry
ice. Charging solid
Cooling without dry-ice filling is also possible by injection of small
amounts of liquid
Figure 7 shows the coolant tank construction. A small polyethylene
pipe with holes in different zones of the coolant tank is used for
filling carbon dioxide. The positions of the holes are optimized to
fill the tank volume homogenously with
The small holes in the injection pipe act as spray nozzles and in
these sections the polyethylene pipe is led outside the PTFE drainage
tube. The injection pipe path and size, as well as the number of
nozzles, their orientation and their opening have been empirically
optimized in several tests to maximize the filled
The coolant tank has a total volume of 2.2
A separate inlet port on the bottom of the cooling tank enables access with an injection lance of a vacuum insulated transfer hose to spray liquid nitrogen in the tank. For safety reasons, two independent pressure relief valves and a bursting disc (not shown on Fig. 7) protect the cooler from overpressure.
Functional thermal setup of the GLORIA
spectrometer. Injection of liquid
A functional schematic drawing which shows the GLORIA thermal setup and cooling system with its different cooling possibilities is shown in Fig. 8. During dry-ice filling or for controlled cooling, the exhaust port at the pitch level plate hose has to be fully open to release the exhaust gas. After dry-ice charging, a smaller hose leading to the aircraft outlet is connected to the main exhaust port.
Controlled cooling is achieved by dispersing liquid nitrogen or liquid
carbon dioxide inside the coolant tank. The pressure drop and the
evaporation of the liquid produce the desired cooling effect. This
procedure can be used to cool down or to maintain the optic module at
constant temperature. It is used in the laboratory or during ground
operations while on campaign. The liquid nitrogen is taken out of
cryogenic storage vessels by a siphon and is transferred to the
L
The optic module is precooled before it is charged with dry ice. A custom
built pressure regulator followed by a non-insulated transfer line
provides the liquid
Spectrometer temperature and pressure values during
laboratory testing of dry-ice filling (about 30
Figure 9 shows the measurement of pressure and temperature during dry-ice filling and the following warming-up process. In this particular
test, the spectrometer was not in operation and thus all internal heat
sources were switched off, leaving only heat input through the
housing. The pressure in the coolant tank rises up to 1600
Operational characteristics of the GLORIA cooling system.
Ambient temperatures during typical operation of GLORIA on
the Geophysica (left side) and HALO aircraft (right side). The
evolution of ambient and instrument temperature is illustrated along
flight altitude profile (OAT
Table 2 summarizes the operational characteristics of the GLORIA cooling
system for both liquid
GLORIA has performed more than 100 flight hours on two different carriers. In this section results from three exemplary flights are presented: flight 1 on 11 December 2011 on the Geophysica in the polar winter and flights 8 and 19 on HALO at mid-latitudes on 28 August and on 25 September 2012.
Heat load on the cooling system.
Figure 10 shows temperature and altitude data of one flight with the
Geophysica and one flight with HALO. The flight characteristics of
both carriers differ in terms of altitude, duration, and velocity. The outside air temperature (OAT) during cruise varies from
190 to 230
Ambient conditions of GLORIA during flight 8 on HALO on 28
August 2012:
Spectrogram of vibration and acoustic pressure measurements of the pitch level plate for different gimbal yaw angles averaged over the whole flight 8 on HALO on 28 August 2012. Only discrete gimbal yaw angles are illustrated, as only those angles are used during the measurements.
Figure 11 displays the conditions observed during flight 8. Figure 11a
illustrates flight altitude, the gimbal yaw angle, and the belly pod air
temperature. The gimbal coordinate system is relative to the aircraft
orientation; the gimbal yaw angle is varied during operation between
Figure 11b shows the outside static air pressure given by aircraft
measurements, which goes down to 130
Figure 11c presents the acoustic pressure in the belly pod and the vibrations
measured at the pitch level plate. Shown are the RMS values for time periods
of 1.2
Figure 12 shows the spectrograms of the vibrations measured at the
pitch level plate – the base of the spectrometer – and the acoustic
pressure in the belly pod. The sampling rate of the sensors is
1
In order to avoid the strong vibrations at 90
A key indicator of the spectrometer function quality is the measurement of the moving mirror velocity using the reference laser system (Learner et al., 1996; Kimmig, 2001). It gives a good insight on the stiffness and damping characteristics of the system and on the impact of the vibrations on the IR measurement. The velocity measured with the reference laser system is directly proportional to the velocity of the moving cube corner and should ideally be constant. Unwanted velocity fluctuations are generated by fluctuations in the cube corner movement, but also by relative movements of the optical parts to each other. Such relative movements can e.g., be caused by independent vibrations of the optical components and the structure.
Vibration measurement at pitch level plate during flight 8 on
HALO during chemistry mode measurement at gimbal yaw angle of
approximately 86
In order to quantify the stiffness of the spectrometer the inflight
vibrations and the effects to the reference signal velocity are shown
in Figs. 13 and 14. These pictures show the vibrations on the pitch
level plate and the velocity signal recorded by the reference laser
system over one interferogram at 15:18 UTC during flight 8 while the
spectrometer is operated in CM. The environmental conditions remained
stable at the time as shown in Fig. 11 in the previous section. The
gimbal yaw angle varied only slightly in the range of 86.0 to
86.6
Spectrum of the velocity signal from the interferometer measured with the reference laser during the same time period which is illustrated in Fig. 13. The largest peak is caused by the strongest vibration identified in Fig. 13.
Figure 13 shows the power spectral density (PSD) of the vibration
magnitude in the direction of scanner motion measured on the pitch
level plate. This plot is derived from a single spectrum. Furthermore
the integrated value of the PSD, the cumulative spectral power (CSP),
is displayed. The peaks in the PSD and steps in the CSP curve show
very clearly that the largest power contribution of the vibrations is
found around 50 and 220
The velocity signal detected by the reference laser for the same time
interval shows a standard deviation of 9 %, exceeding the target
value of 5 % given in Table 1. Figure 14 shows the amplitude
spectrum of the velocity. The largest contribution to the velocity
fluctuations is again around 217
The main part of the velocity fluctuations in the interferometer is caused by
forced vibrations of the optical components induced by the strong acoustic
and mechanical excitations around 220
Conditions observed during flight 19 on HALO. The plots show
The temperatures measured inside and outside the spectrometer and the flight altitude are shown in Fig. 15 for flight 19 on the HALO aircraft. The same flight was already discussed in Sect. 3.1. In Fig. 15b the temperatures inside the optic module and the coolant tank are shown. The sensor on the coolant tank, identified as “coolant tank” is the farthest away from the central body and shows the lowest temperature of the cooling system. The sensor “coolant tank at central body” is located at the interface between the central body and the coolant tank. It shows the highest temperature of the cooling system and the lowest temperature of the central block as expected. A third sensor, positioned in the IR objective, shows the highest measured temperatures inside the optic module with the exception of the entrance and detector windows. The temperature of the latter is influenced by the environmental pressure, window heating and indirect heating by electronic components. Both window temperatures are shown in Fig. 15d.
The cooler had been filled with dry ice approximately 2
The inflight temperature of the optic module remains below
220
The belly pod and the cooling system pressure are shown in
Fig. 15c. The pressure in the unpressurized coolant tank follows the
aircrafts ambient pressure. Due to pressure decrease with increasing
altitude the sublimation temperature of the dry ice drops. This leads
to a higher sublimation rate and thus to a higher overpressure of
50
The temperature drifts of the entrance and detector windows are
required to be below 2
The inner window temperature drift is with 1
The outer window temperature is strongly affected by the environmental
conditions and the outside airflow. This leads to temperature drifts that exceed the
requirements. For instance between 10:00 and 11:00 UTC a drift of
The detector window temperature shown in Fig. 15d varies smoothly without
sudden changes during flight. The drift of 5
Remote sensing of atmospheric trace constituents with high spectral and spatial resolution requires a compact and rigid interferometer with permanently cooled optics. The chosen design of the instrument led to the successful operation on two different research aircrafts in unpressurized compartments under harsh and variable ambient conditions.
Especially the scanner with linear dovetail slide contributed to achieve a rigid optical system. The dove-tail slide suppresses relative motion of the cube corner and its guidance induced by external vibrations.
As the emission of the trace constituents is only measurable with
cooled optics, we designed a cooling system mounted directly to the
optic module. It is based on dry ice injected as liquid carbon
dioxide. Alternatively, the cooling system also works with liquid
nitrogen. The application of vacuum insulation panels contributed to
the compact design. The cooling system and the insulation feedthrough
assembly enable operability of the instrument in a wide range of
ambient temperature and humidity while the dry ice capacity ensures
a hold time of 24 h. Inflight measurements show that the thermal
requirements could be fulfilled as the temperature drift could be kept
below 1
The use of liquid
Since the first scientific campaigns with GLORIA were successful, GLORIA will further participate in dedicated campaigns in 2016 and the years following (Riese et al., 2014). For future flights, the vibration analysis presented in Sec. 3.2 has resulted in modification plans for the HALO belly pod, e.g., a spoiler in front of the opening. Additional changes to the gimbal's shield in order to reduce the aerodynamical excitation and to increase the stiffness of the gimbal are carried out. The central body and the housing of the scanner will be replaced by a monolithic housing to increase the stiffness of the optic module and to reduce the influences of the vibrations. Further improvements concentrate on the thermal fluctuations of the entrance and the detector window. Different strategies, e.g., changing the design of the detector unit to improve the insulation using VIP, integrating a cooling shield around the detector dewar to stabilize the window temperature, and an enhanced strategy to consider the entrance window temperature during calibration are currently being investigated.
We would like to thank the head of the prototyping facility at KIT IMK-ASF, A. Streili, for his commitment during the development of the GLORIA spectrometer. We thank the whole DLR flight operations team at Oberpfaffenhofen for the excellent flights. We are grateful to the DLR certification team for their valuable cooperation and support during testing and certification. We acknowledge the close collaboration with our partners at MDB, which has allowed the unique operation of our spectrometers in the lower stratosphere. The excellent infrastructure at Kiruna airport and the uncomplicated support by the airport staff have made Kiruna an unrivaled campaign site for us.
We acknowledge support by the Deutsche Forschungsgemeinschaft and Open Access Publishing Fund of the Karlsruhe Institute of Technology. The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association. Edited by: J. Notholt