Planetary Spectroscopy Interferometer A low cost mission to explore - - PowerPoint PPT Presentation
Planetary Spectroscopy Interferometer A low cost mission to explore planetary atmospheres and surfaces of our solar system Design study and Science Objectives by: Bruce Swinyard - UCL/RAL Space Leigh Fletcher, Neil Bowles Oxford University
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Design study and Science Objectives by: Bruce Swinyard - UCL/RAL Space Leigh Fletcher, Neil Bowles – Oxford University Stuart Eves – SSTL Brian Ellison – RAL Space Bastien Rouquie – SSTL (Intern summer 2012) Craig Underwood – University of Surrey PSI
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Ø Heterodyne THz interferometer for planetary science Ø Low Earth orbit – passively cooled Ø “Room temperature” technology – Schottky diode mixers Ø 4 receivers with ~60 cm antennae to ensure phase reconstruction and efficient of u-v plane filling Ø THz spectral range – precise frequencies TBD Ø 10 m maximum baseline Ø At the 179 µm water line (1667 GHz) à 4.5 arcsec resolution Ø Complementary to ALMA (400-3000 µm or 100-750 GHz.) Ø We undertook a short study in Summer 2012 to identified critical issues
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PSI - Conceptual Design
Key point: High TRL technology and low cost spacecraft solution key to delivering cost efficient science.
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Ø Dynamics and Composition of Middle Atmospheres of all the planets in our solar system – Middle atmospheres are underexplored, winds have never been measured. – Planetary water cycles, origin of oxygen compounds in upper atmospheres. – Stratospheric circulation Ø Temperature variability of cool rock/ice bodies in the solar system. – Thermal light curves and rotational variability of TNOs, KBOs, asteroids, Pluto system, moons. Ø Water coma/tail mapping of cometary atmospheres. Ø Astronomical Targets – major star formation regions only poorly mapped in key species Ø Possible option for terrestrial limb sounding of mesosphere and thermosphere
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Key Science Drivers
Spatial resolution similar to MIR on SOFIA 2.5 m telescope Spectral resolution sufficient to see wind induced doppler shift
Key point: PSI offers a higher spatial resolution at terahertz wavelengths than previously achieved, and at wavelengths inaccessible to ALMA.
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Ø A 3.1 THz channel would have 2.4” spatial resolution, compared to 4.5” for the 1.7 THz channel. Ø Assuming a 4.5" resolution, this spatially resolves Jupiter (average 48" size); and allows 4-5 spots across Saturn's 20" disc for similar purposes. Ø You still only get disc-averaged spectra of Uranus (3-4"), Neptune (2") and Titan (0.9"). Ø At opposition, Mars can be up to 25" in size, so you'd get 5-6 spots across the disc. Ø Venus can be up to 66" at opposition, 14-15 FOVs across. However, PSI will be unable to view Venus due to the need for a sunshield.
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Planetary Spatial Scale
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Key point: Although horizontal resolution on these planets is limited, heterodyne spectral resolution permits excellent vertical coverage, resolving structures throughout planetary atmosphere.
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Ø Key species: CH4 (temperature sounding), H2O, CO, HCN and isotopes.
– CH4 lines get stronger with higher frequencies. – CO peaks near 50 cm-1 (1498 GHz) – HCN peaks near 45 cm-1 (1350 GHz) – HCl, HI, HBr, HF all have lines in the sub-mm – H2O strongest lines are: 100.5 cm-1, (3013 GHz) 88.0 cm-1 (2640 GHz), 92.5 cm-1 (2273 GHz), 104.6 cm-1 (3135 GHz), 55.7 cm-1 (1669 GHz),
Ø Goal – cover three critical bands
– 1667 GHz H2O (55.6 cm-1), 4.5 arcsec spatial resolution, 13000 km on Jupiter. Weak CH4 nearby. – 2200 GHz H2O (and CH4 at 73.2 cm-1) – 2500 GHz OH (and CH4 near 83.6 cm-1) – 3100 GHz (CH4, 104 cm-1, H2O at 100.5 cm-1, 2.4” spatial resolution) or 3400 GHz (CH4, 115 cm-1) 5
Frequency Band Choice
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Key point: Some design freedom here –
i)Tunable system with sub-harmonic mixing – all three bands ii) Two bands, one near 1.7 THz, one near 3.1 THz. iii) Single band at 2.5 THz Science/technical trade off to be done – assumed 2.5 THz single band for study PSI
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Receivers
Based on operation at 2.5 THz:
Ø Room temperature or passively cooled Schottky diode mixer (RAL). Ø Quantum Cascade Laser (QCL) as Local Oscillator (LO), offers good potential > 2 THz but current technology requires
Ø Translate incoming frequency, in THz, into a lower Intermediate Frequency (IF), in the GHz range. Ø Alternative design: use sub-harmonic mixer and more standard LO design. Ø Overall system sensitivity at 150 K ~2500 K and at 250 K ~4500 K
Key development areas over a 5-year development timescale: Ø Improvement in LO efficiency, power output and higher operating temperature . Use of QCL extremely promising,
particularly for a passively cooled system Ø Improvements in room temperature and passively cooled Schottky diode performance. Ø System integration (mixer, LO, IF) for reduction in mass and power. Ø Requirement for sideband separation, either to reduce noise or spectral contamination. Quasi-optical filtering and/or sideband separating mixers. Ø Enhancement of backend spectrometer – broader bandwidth, lower mass and power.
Example pictures of RAL fabricated air-bridge planar Schottky diodes – balanced diode (left) and fully integrated structure, i.e. including filtering (right). Anode sizes are in the region of 1 to 2 µm. Quantum Cascade Lasers integrated in a micro machined waveguide - Sandia Labs.
Key Point: Technical Readiness Level
MLS, Odin, SWAS TRL 8
gas laser LO) for ~ 3 years (EOS Aura mission) TRL 8
(development required for J-E mission)
– a factor 7-10x worse than Herschel
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Ø Choice of antenna configuration is complicated Ø Simple view is as the S/C rotates we take data and auto-correlate between antenna positions to get visibility fringes Ø Gives good u-v coverage but data rate restriction may prevent implementation Ø Alternatively we correlate on board Ø Distribute the antenna slightly differently to attempt even coverage of Fourier components Ø Many (many) ways of doing this –
Ø With 4 antenna may be overkill – here’s another configuration
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Spacecraft Layout and u-v coverage
Transmit digitised integrated signal Digitised IF Digitised IF Correlate Visibility function 5.00
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PSI Ø On-board correlation easier because it reduces the problem of transmitting data, created especially by the rotation of the spacecraft. Ø Two options under consideration:
Ø 1. Intermediate frequency (IF) from the mixers is transmitted to the central hub via co-axial cables, where either they are fed into a digital correlator. Ø 2. Use the Wide Band Spectrometer (Star Dundee/Astrium) to digitise and spectrally deconvolve the signals providing digitised amplitude and phase. The digitised information is transmitted to a central spatial digital correlator. 8
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On Board Correlation Options
LO IF Amp and filter LO IF Amp and filter LO IF Amp and filter LO IF Amp and filter Transmitted phase lock Digital Spectral Correlator Digital Spectral Correlator Digital Spectral Correlator Digital Spectral CorrelatorDigitised Amplitude and Phase
N(N-1) Cross correlation productts
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Orbit and Mission Concept
800 km circular sun-synchronous
degrees, and a “dawn-dusk” local time of ascending node. Solar panels and sun-shield continuously pointed toward the Sun. In practice, observations of a particular target object will be conducted over a period of time when the Earth orbits between the target and the Sun
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Ø Central hub with four deployed booms Ø Remote antenna attached at launch or free flying and attached in orbit? Ø Need to balance centre of mass – restricts configuration
Ø Single sun-shield to protect receivers and booms Ø Where to put the solar panels and comms? Ø Or multiple smaller shields protecting each receiver Ø Keeping the booms thermally stable? PSI
Satellite configuration issues
Launch configurations – pre-attached to booms or free flyers
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PSI Thermal control Ø Temperature required for current QCL technology is ~150 K Ø Mixers are better colder but will operate at room temperature Ø Thermal environment provided by passive radiators and deployed sun-shield Ø Two possibilities (see previous): Ø over the whole structure Ø only over the booms and the detectors (preferred from communications point of view) Ø TDS-1 drag sail concept (Cranfield) looks like a possible solution but some issues to address Ø Reflectivity of the sail (multiple layers) Ø Robustness to deal with manoeuvres Ø RF transparency
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PSI Ø Pointing requirement is TBD but likely to be sub-arcmin Ø Better than currently achieved on SSTL UoSat series but sensors and reaction wheels should be enough Sensors (SSTL) Ø Star camera x 2 - Altair or Rigel Ø Sun sensor x 2 - 2 axis DMC Mechanisms (SSTL) Ø Wheel x 2 – several options 10SP-M or 100SP-O or 200P-SP-ST Ø Magnetorquer x 2 - MTR5 or MTR30
Rigel MTR30
Attitude and Orbit Control System
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Booms
Ø Baseline needed for PSI: 5 m boom length. Ø Phase control: Ø The receivers must be on the same plane to have the same phase. Ø Booms must be stable so the difference remains small Ø Small, fixed, differences can be corrected by the correlator. Ø Vibration and flexure compensation using PZT actuators and sensors Ø Lab scale demonstration of principle has been achieved by a number of groups. Ø CFEsat boom (SSTL)
Ø Compact stowage: cylindrical envelope 450 x 150 mm Ø Deployed length > 3.6 m: achieved within 5 minutes. Ø Deployed frequency (with 3 kg payload) > 1 Hz. Ø Stowed frequency > 150 Hz. Ø Deployed alignment to be maintained within +/- 3°. Ø Deploy and support a 10 mm diameter payload harness. Ø Minimise mass (target < 4.5kg). Ø Minimise deployment power Ø Provides indication of deployed length to +/- 10 mm.
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Power, Mass and Bus Choice
Ø Power required ~ 500 W Ø 28.5% efficient triple-junction GaAs cells assumed for the solar panel giving a solar panel area ~ 2.5 m² Ø Need to establish the orbit average power requirements during the operational mode, and the orbit duty cycle for the instrument and the data downlink Ø Need to establish several components of the mass budget Ø Indicative mass ~ 175-250 kg Platform generically on the order
present
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Operations Concept
Sun-synchronous
Ø Number and Location of ground stations needs to be appropriate to provide several
Ø For Sun-synch orbit latitude (only) dictates number of contact passes Ø UK ~ 6 per day – Svalbard 12 per day Ø Contact time for one pass over a ground station ~ 7 - 10 min ~ 90minutes a day if Svalbard used Ø Max data rate looking like 400 MBits/s Ø Need to track the ground station during a pass Ø Steerable antenna needs to deal with rotating spacecraft Ø Implementation presents a challenge (!) Ø Could point whole spacecraft Ø This would limit operations but avoids antenna problem
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PSI Ø The basic concept of a low cost heterodyne interferometer looks feasible and can be built entirely using UK technology Ø Such a mission will deliver outstanding scientific return not only on Solar System objects but lalso on Galactic sources with spatial resolution 3x greater than Herschel and observes species inaccessible to ALMA Ø Our short study has highlighted some critical items for further study: Ø Detailed interferometer layout, correlation operations, pointing and data rate requirements Ø Temperature requirements for the receivers – especially stability requirements Ø Demonstration receiver system needs to be breadboarded Ø Can we achieve the required temperatures with low cost spacecraft? Ø Can we achieve the required pointing requirements (APE, RPE, PRE) with a low cost spacecraft? Ø How to downlink the data - antenna implementation on rotationg spacecraft and issue? Ø We have done enough to show that the concept is not stupid..... Ø ....full Phase A study now required.
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Summary
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