| Metadata | Details |
|---|
| Publication Date | 2021-09-22 |
| Journal | Space Science Reviews |
| Authors | Christopher S. Edwards, P. R. Christensen, G. Mehall, Saadat Anwar, Eman Al Tunaiji |
| Institutions | Goddard Space Flight Center, Northern Arizona University |
| Citations | 36 |
| Analysis | Full AI Review Included |
- Mission and Function: The Emirates Mars InfraRed Spectrometer (EMIRS) is a flight-heritage FTIR instrument for the Emirates Mars Mission (EMM), designed to remotely characterize the Martian lower atmosphere and surface.
- Key Measurements: EMIRS determines the geographic and diurnal variability of key constituents (water ice, water vapor, dust) and atmospheric temperature profiles up to 60 km altitude on sub-seasonal timescales.
- Core Optics: The instrument features a 17.78 cm diameter Cassegrain telescope feeding a Michelson interferometer. The beamsplitter is a 60 mm diameter, 1 mm thick Chemical Vapor Deposited (CVD) diamond etched with an Antireflection Microstructure (ARM).
- Performance Metrics: The instrument achieves a Noise Equivalent Spectral Radiance (NESR) of <3.0 E-8 W cm-2 sr-1/cm-1 (in 5 cm-1 mode) and an absolute integrated radiance error of <2% (200 K to 340 K scene temperatures).
- Thermal Control: High thermal stability (<0.1 °C per minute drift) is maintained through conductive isolation and active operational heaters (5.7 W) to ensure consistent radiometric calibration between internal target and space observations.
- Detector System: EMIRS uses an uncooled 3x3 array of deuterated L-alanine doped triglycine sulfate (DLaTGS) pyroelectric detectors, offering high longevity and low power consumption compared to mechanically cooled systems.
| Parameter | Value | Unit | Context |
|---|
| Instrument Type | FTIR Spectrometer | N/A | Michelson Interferometer |
| Spectral Range | 1666 to 100 | cm-1 | (6-100 ”m) |
| Spectral Sampling | 5 & 10 | cm-1 | Selectable modes |
| Telescope Aperture | 17.78 | cm | Cassegrain design (f/3.3) |
| Instantaneous FOV (FWHM) | 5.1 x 4.6 | mrad | Elevation x Azimuth |
| Detector Material | DLaTGS Pyroelectric | N/A | Uncooled, 3x3 array |
| Detector D* (Average) | 9.4 x 108 | cm Hz1/2 W-1 | At 10 Hz, 22 °C |
| Beamsplitter Material | CVD Diamond | N/A | 60 mm diameter, 1 mm thick |
| Beamsplitter Feature | ARM | N/A | Antireflection Microstructure |
| Metrology Laser Wavelength | 0.846 | ”m | Self-apodized (25 °C) |
| Metrology Sampling Frequency | 40 | kHz | Servo positional control |
| Infrared Signal Sampling | 625 | Hz | Detector readout rate |
| Michelson Mirror Travel (Max) | ±0.686 | mm | Dual voice-coil linear motor |
| NESR (5 cm-1 mode) | <2.2 E-8 | W cm-2 sr-1/cm-1 | At 10 cm-1 sampling |
| Absolute Radiometric Accuracy | ±1.5 | % | Level 3 requirement |
| Calibration Target Emissivity | 0.98 ± 0.005 | N/A | Internal v-groove blackbody |
| Operational Temperature | +10 to +40 | °C | Performance in specification |
| Operational Heater Power | 5.7 | W | Thermostatically limited, always-on |
| Mass (Total) | 14.715 | kg | Including interface adapter plate |
- Interferometer Servo Control: The moving mirror assembly uses a dual voice-coil linear motor. Servo control is enhanced by sampling the metrology interferometer (0.846 ”m laser) at 40 kHz, enabling a rate estimator algorithm (5 kHz) to predict zero crossings and trigger infrared sampling (625 Hz), thereby rejecting vibrational noise.
- CVD Diamond Beamsplitter Optimization: A 60 mm CVD diamond substrate was used for its thermal and mechanical stability. To maximize throughput, the diamond was etched with an Antireflection Microstructure (ARM) on one side and coated with germanium on the other to achieve the required 50% reflection/transmission split.
- Two-Point Radiometric Calibration: Absolute calibration is achieved by bracketting scene observations with measurements of two known targets: deep space (Rspace) and an internal v-groove blackbody (Rcal). This full-aperture calibration method eliminates uncertainties associated with fore-optics degradation.
- Internal Calibration Target Characterization: The internal v-groove blackbody target properties (emissivity 0.98 ± 0.005) were derived during Thermal Vacuum (TVAC) testing by comparing its measured calibrated radiance against two external, precision Bench Calibration Units (BCUs).
- Thermal Stability Management: Instrument temperature stability (<0.1 °C/minute) is maintained by isolating the instrument chassis and using operational heaters (5.7 W) to keep the temperature within the optimal 10 °C to 40 °C range, minimizing temperature-dependent fixed mirror tilt (<10 arcseconds).
- Retrieval Algorithm: Atmospheric parameters are retrieved using a sequential, iterative constrained linear inversion routine. The forward model includes full multiple-scattering treatment (discrete-ordinates formulation) for aerosols and correlated-k approximation for CO2 gas absorption.
The technologies developed for EMIRS, particularly in high-stability FTIR, space-hardened optics, and advanced thermal control, are applicable to several high-value engineering sectors:
- High-Performance Spectroscopy: Manufacturing of next-generation FTIR spectrometers requiring extended spectral range (far-infrared) and high throughput, leveraging CVD diamond beamsplitters for superior performance and durability.
- Space and Aerospace Remote Sensing: Design and qualification of robust, low-power thermal infrared instruments for Earth observation, climate monitoring, and planetary science missions, where mass and power constraints are critical.
- Precision Metrology and Motion Control: Application of dual voice-coil linear motors and high-frequency metrology systems (40 kHz sampling) in industrial machinery requiring ultra-precise, non-contact motion control in vacuum or harsh environments.
- Thermal Management in Electronics: Implementation of passive (conductive isolation) and active (thermostatically controlled heaters) thermal stabilization techniques for sensitive electronics and optical assemblies operating under fluctuating thermal loads.
- Optical Materials Engineering: Utilization of Antireflection Microstructure (ARM) etching techniques on high-index materials like diamond to maximize optical throughput in demanding mid-to-far infrared applications.
- 2019 - AGUFM
- 2014 - 2014 6th Workshop on Hyperspectral Image and Signal Processing: Evolution in Remote Sensing (WHISPERS) [Crossref]
- 1979 - The Principles of Interferometric Spectroscopy