science with a rover-based low-frequency dipole array a multi-configuration rover-borne dipole array...

Science with a Rover-based Low-frequency

Dipole Array A multi-configuration rover-borne dipole array

for

Low-frequency RadioAstronomy from the Moon

Ettore Carretti

INAF-IRA Bologna

On behalf of the Radio WP group

in collaboration with CIRANO (PoliMi)E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007

Why =3-30 MHz astrophysics?

• Almost unexplored frequency band

– next years (2009+) the ground-based array LOFAR will explore the 30-80 and 120-240 MHz bands;

– < 30 MHz almost impossible from ground (ionosphere)

– Only strongest sources and very large scales maps (HPMW = 2°-10°)

TO OPEN A NEW ASTROPHYSICS WINDOW !TO OPEN A NEW ASTROPHYSICS WINDOW !

E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007

Extragalactic astrophysics

• Cosmology

• Physics of Acceleration of high energy particles in the Universe

• Origin and amplification of magnetic fields in the Universe

• Physics of Black Holes and Duty cycles

• Most distant and oldest radio emitting plasma in the Universe

• New Physics … (coherent emission, serendipity)


Galactic astrophysics

• Diffuse Emission from the Galaxy

• Magnetic field in the Galaxy

• Acceleration and diffusion of high energy particles in the Galaxy

• Mapping HII in the Galaxy

• Supernova Remnants in the Galaxy

• Physics of Micro-Quasars

• New Physics … (coherent emission, serendipity)


What is possible from ground?

2.1 MHz

Ellis 1982

3.7 MHz

Ellis 1982

8.3 MHz

Ellis 1982 Roger et al. 1999

22 MHz

= 2°-3°

LOW RESOLUTION !


Resolution improvement by >10X...

2.1 MHz

Ellis 1982

Haslam et al. 1982

• subdegree resolution (at least) down to 3 MHz


Low frequency: strong signal

• Low frequencies dominated by synchrotron

• Synch spectrum: negative slope => strong signal at low frequency

– Galactic diffuse emission >10000s K (30MHz) even at high lat.

– > 1 MK (2-3MHz)

• It is not required a huge collecting area to map the sky.

Haslam et al. 1982

408 MHz

Roger et al. 1999

22 MHz

= 2°-3°

2.1 MHz

Ellis 1982E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007

Instrument• Need for high resolution => large telescope aperture

• No need for high sensitivity (strong signal and first survey experiment)

• => interferometric small antenna array– many separated antenna, whose signals are 2-by-2 correlated– N antennas => N(N-1)/2 independent pairs– resolution = /Dmax (Dmax = maximum baseline – pair distance)

• 10 Km equivalent aperture: [MHz][m] [arcmin] ---------------------------------------------------- 100 3.3 1.0 30 10 3.5 10 33 10 3 100 35


Array• each antenna pair is sensitive to the angular scale /D

• several baselines D are necessary to cover from largest to smallest angular scales

• D from “a few”-m to 10 Km (even 3 Km would be OK)

• Array “shape”


Antennas• Which antenna?

• Mirrors not suitable:– need D >>

– 3 MHz => = 100m

– it’d be D = HUGE !?!

• Dipole antenna: “Natural” choice for a low frequency antenna– ./2 electric dipoles not suitable because of lengths and very narrow

frequency band

– magnetic dipoles: using coils, much higher efficiency with same length

– length: 10s-cm

– double dipoles: both polarizations

– Large F.o.V.: 90° => optimal for surveys, i.e. full sky mapping

– lightweight: just wireE. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007

Antenna (2)• Receiver: simple and well developed technologyReceiver: simple and well developed technology

• Dipole antennas

• Receiver: sky signal dominated conditions (10-kK to 1-MK)– amplifier sensitivity is not an issue (state-of-the-art technology not

necessary);

– commercial components can be used;

– standard Medium/Short-Wave telecommunication components (well tested in space-enviroment and space-qualified);

– standard electronics

• No “critical” technology to be developed


Experiment• Main requirement:

• Mass < 60-70 Kg

10-15 antennas: not enough to realize all the baselines from 3-5m throughout 10km

• solution: reconfigurable array

• => Rover-Borne antennas (new approach w.r.t. ESA or EADS studies) PROs:

– it allows to deploy antennas from the lander to the wanted position with the wanted attitude.

– flexibility– it allows to re-deploy the antennas in different configurations of larger and

larger size

LIGHTWEIGHT!!!LIGHTWEIGHT!!!


Rover-Borne Stations• Station = Station =

telescope (dipole + electronics) + rovertelescope (dipole + electronics) + rover

• Mass station: 5-6 kg

10 stations stay within mass requirements


A “walking” experiment

• reconfigurable array:

• first the 10 antennas realize the most compact core and observe the sky with this configuration;

• ... and so on for a number of steps until the 10km baselines are set so to realize the full array

• 6 steps:

Dmax = 30m, 100m, 300m, 1km, 3km, 10km

• then: antennas are moved to longer distances to realize a larger configuration and observe the sky;


Where• Far side of the Moon:

Earth is shielded => RFI fully mitigated

• Lunar latitude:

within the range 30°- 60°


Operations• observing frequencies:

– 3, 6, 12, 25, 50, 100 MHz

– 3, 4.5, 7, 10, 15, 22, 30 MHz

• lunar night observations – Sun is too strong !

• observations all lunar night long, for optimal angular frequency space coverage

• 1 array configuration a lunar night

• 1 lunar day (= 1 month) observing cycle: – 6 months to realize Dmax = 30m, 100m, 300m, 1km, 3km, 10km


Operations (2)• Station data must be correlated for each station pair

• Data must be collected by one processing station. Two possible solutions:

– one master station (central one);– orbiter.

• Communications to Earth: only once a month (simple and cheap mission control)


Science with WARP• Walking Array Radio Pathfinder (WARP):

– subdegree Galactic diffuse emission– relativistic electron age, confinement and diffusion processes, magnetic field

– New SNR: oldest visible at low freq only (high res to separate them from diffuse em.)

– Micro quasars (e.g. SS433): no lobes. Why? old, or p+?

– Extragalactic sources: all 3C catalogue:• spectrum, lowest energy e-, evolution history;• free-free absorption: W-IGM• synchrotron self-absorption: direct measurement of B, ne

– galaxy-cluster halos and relics

•... but ... most important.... it is an unexplored window: hunt for new sources and astrophysical (and physical) processes fully unknown so far.


Conclusions• To open a NEW window in astrophysics.To open a NEW window in astrophysics.

• High interest of the scientific community (e.g. ESA and EADS studies for experiments in this frequency band).

• Instrument: a simple, easy, well developed technology: No “critical” developments requiredNo “critical” developments required

• New approach based on a Rover-borne experiment: FLEXIBILITYFLEXIBILITY

• Safe against failure of one/more stations

• Many-stations array realized with “few” stations

• Lightweight payload.E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007

Conclusions (2)• ESA studies for a lunar experiment in such band

• WARP: unique opportunity for the italian community (astrophysics + space engineering) to earn experience and be candidate to lead the European mission.

• A rover colony getting around on the lunar surface: high impact for the Italian System

– italian space industry;– italian engineer research;– italian astrophysics research;– visibility;– italian space technology capabilities.


science with a rover-based low-frequency dipole array a multi-configuration rover-borne dipole array...

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