PSI PSI 1 1

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 Stuart Eves – SSTL

Brian Ellison – RAL Space Bastien Rouquie – SSTL (Intern summer 2012)

Craig Underwood – University of Surrey PSI RAS 12th October 2012

PSI 2

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

2 PSI

PSI - Conceptual Design

Key point: High TRL technology and low cost spacecraft solution

key to delivering cost efficient science.

RAS 12th October 2012

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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

J. Hurley et al., Planetary and Space Science 58 (2010)

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. RAS 12th October 2012

PSI 4

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

RAS 12th October 2012

Key point: Although horizontal resolution on these planets is limited, heterodyne spectral resolution permits excellent vertical coverage, resolving

structures throughout planetary atmosphere.

PSI 5

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

RAS 12th October 2012

Key point: Some design freedom here – options are:

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 operation at < 100 K (Leeds University). 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 •  Heritage from past missions: UARS, MLS, EOS, MLS, Odin, SWAS TRL 8 •  2.5 THz receiver in operation in Earth orbit (using gas laser LO) for ~ 3 years (EOS Aura mission) TRL 8 •  > 1THz solid state LO for Herschel HIFI TRL 7 (development required for J-E mission) •  Sensitivity equal to or exceeding SWAS and ODIN – a factor 7-10x worse than Herschel

RAS 12th October 2012

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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 – optimum is Reuleaux triangle (Keto 1997)

Ø  With 4 antenna may be overkill – here’s another configuration

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Spacecraft Layout and u-v coverage

Transmit digitised

integratedsignal

Digitised IF

Digitised IF

Correlate

Visibility function

5.00

3.72

5.56

6.42

2.54

8.66

10.00

RAS 12th October 2012

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.

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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 Correlator

Digitised Amplitude and Phase

of IF

Digital Cross Correlator

N(N-1) Cross correlation productts

RAS 12th October 2012

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Orbit and Mission Concept

800 km circular sun-synchronous orbit with an inclination of 98.6 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 options?

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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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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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 of SSTL UoSat-12 indicated at present

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Operations Concept

Sun-synchronous orbit coverage

Ø Number and Location of ground stations needs to be appropriate to provide several opportunities to communicate in one day

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

RAS 12th October 2012

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

RAS 12th October 2012