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SAMBA SAtellite for Measurements of Background Anisotropies

Yvan Blanc Centre National d'Etudes Spatiales, 18, avenue E.Belin, 31055, Toulouse, France

ABSTRACT

SAMBA is a proposal for a small space mission, whose goal is to provide mapping of the CMB fluctuations at angular resolutions from 20 arcmin to several degrees with a sensitivity and an angular resolution more than one order of magnitude than COBE. The purpose of this paper is neither to describe the scientific goals of SAMBA, neither details on detectors or other hardware but to briefly describe some particulars aspects and technical developments of the SAMBA mission.

I .INTRODUCTION

The SAMBA concept was born as an answer the call for new idea for small missions made by CNES in 1993 and was translated to answer the ESA/ AO for M3 missions. This concept merged with the COBRAS proposal gave the COBRAS/SAMBA mission. On the US side, the FIRE mission with similar goals but different solutions for cryogenics is being submitted to the MIDEX AO. The SAMBA project is presented to CNES by several scientific institutes with IAS (Institut d'Astrophysique Spatiale) as payload PI (Principal Investigator). It is now under phase A by CNES to be concluded in April 96, and completes for the first opportunity of a launch as early as mid 2000. The final choise will occur by mid 96.

2.SCIENTIFIC GOALS

The basic goal of SAMBA is to produce mapping of the CMB fluctuations in several channels ( 0.3 - 6mm range ) with a sensitivity per pixel full sky 2 years mission aT /T�lQ-6 and an angular resolution between 20 arcmin to 7 degrees. Secondary goal is to measure the Sunyaev-Zeldovich effect on several hundred clusters of galaxies and to obtain constraints on the fundamental cosmological parameters ( Qo, H, Qs ) .

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3.MISSION PROFILE

The scientific goals previously described will be satisfied by a space mission using very sensitive detectors like bolometers to survey the sky in the 0.3 - 6 mm range. The volume of the budget of the "small mission" program favors a cheap orbit. A preliminary mission analysis showed that a sun synchronous orbit at 825 km to 1000 km with a zenital line of sight offers (figure 1): - a complete sky coverage over a period of 6 months, - the orbit plan rotates as to the sun perpendicular to the line of sight, - Moon shadows zones of 3 by 40° appear near the ecliptic plane (space out 27°). Over one year period, a shift of 9° permit to observe the hiden zones of the before half-year.

Figure 1: SAMBA sky coverage (one year of observations integration time)

The low altitude of the orbit implies careful payload design from the straylight rejection and thermal stability. The SAMBA mission is compatible with several launchers (EUROCKOT, TAURUS, LLV ,PSLV ) .

4.MODEL PAYLOAD

The model payload (figure2) presently under study includes: - a telescope with a 80 cm primary mirror focusing the signal to be detected on the focal plane assembly called also bolometer box, - a system of baffles at respectively 150 K and 100 K, these baffles are also the sunshades of the telescope, - the associated subsystems necessary to the operationnal running of the instrument, like cooling system (figure 3) and associated electronic boxes.

The low temperature required by the bolometers (0.1 K) must be provided by an active precooling system 60K and 4K cryocoolers (RAL and MMS developments sponsored by ESA) coupled to an open-cycle He dilution refrigerator (CRTBT /Grenoble sponsored by CNES). A cooling dilution system power of 100 nW at O.lK has been demonstrated.

primary mirror

bolometer box

C-x

compresseu111 4K Joule Thomson

platform

• - - - - - -- - - - - - - - - - - _ J

SUN ,l, Figure 2: SAMBA model payload

cryocoolers

boxes

.--------- - - - - - - - ----...,

r-·-compresseurs de pr&relroidtssemenl

OOK

"H• 3H• He dllu• dllullon 0.1K

Figure 3: SAMBA cooling system

100K

300K

EARTH�

5 1

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The bolometer box (figure 4) includes the detectors, the 0.1 K dilution refrigerator stage, the light concentrators, the filters to select wavelengths, the different shields and boxes necessary for thermal reasons. At this time, seven channels have been selected to cover the range (857 to 53 GHz) with previous sensitivities per pixel over a full sky 2 years mission giving in figure 5.

SAMBA Bolometer Box ( BOB) JML - JD - YL l l . .\vnl 95

60 K Box 20 K

100

4 K 1 .6 K i

s CAN I < <i."f"rditm i 3,�� 1 I ! I i : ----+- - +..-----,-! .

Figure 4: SAMBA bolometers box

Channels I I 2 3 4 5 6 7

Central Frequency (GHz 857 I 545 353 217 150 90 53

Central WavelengUJ (mm) 0.35 1 0.55 0.85 1 .38 2.00 3 .33 1 5.66

Pixel diameter 1arcmrn) 7 . 8 I 7.8 7.8 1 2.7 1 8 .4 30 52 >---

i'l T Sensitivity per pixel I full sky 2 years mission 0.45 0.7 1 . 2 0 .85 0.85 0.9 0.66 (µK Ravlei £h-Je;ins)

Figure 5: SAMBA channels

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5.P A YLOAD PROGRESS REPORT OF TECHNOLOGY

Payload phase A studies have been achieved:

- the optical design of the telescope consists in an off-axis tilted Gregorian system with a parabolic primary mirror of 0.80 m. This size is restricted by the fairing envelope. A possible increase of this size will be studied during phase B,

- simulations demonstrate that the final sensitivity is not affected by the side lobe rejections constraints,

- general CAO establishments have been performed,

-mechanical studies (first mode, impact resistance, clamping system, efforts on the colfinger tip) confirm the design,

- thermal studies with thermal decoupling between the payload and the platform show a thermal environnement of telescope and bolometers box near 100 K.

Several actions of research and technologic program and parallel developments are in progress:

- the MOB program (Matrix Qf _!iolometers) funding by CNES consists in the development of 2 lines of 4 bolometers cooling at 0.1 K. The objective is to produce bolometers with a NEP � 10-17 w.../Hz (IAS development)

- the SYMBOL program (SY.stem aspects for Measurement by BOLometers) funding by CNES procures a complete testing bench for matrix of bolometers (like temperature fluctuations compensations,read out electronics . . . ) (IAS development)

- an other very crucial action is a common action funding by CNES and ESA is called "300 K- 0.1 K demonstrator". This action consists to assembly and spatialize the cryogenic architecture of SAMBA with a precooling 300 K to 4 K by mechanical cryocoolers developed by RAL and MMS­Bristol, and the final cooling 4K to O.lK with the dilution system developed by CRTBT I Grenoble. The other goal of this action is to validate the use of this architecture with bolometers. At this time an engeneering prototype model 1.6K to O.lK has been manufactured by CRTBT. The principle of the cryogenic architecture and the zero-g compatible O.lK stage have been succesfully tested in vibrations and thermal performances. The fully demonstration of this action (IAS development) is expected by end 1997.

The DIABOLO ground experiment consisting in a photometer with 2 bolometers (1 and 2 mm diameter) has validated the assembly of the O.lK cryogenic dilution with bolometers and original read-out electronics during ground observations (!RAM telescope . . . ) .

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6.THE SPACECRAFT

SAMBA has been studied in the context of the CNES Small Satellite Initiative concerning the development of a reconfigurable multimission platform called PROTEUS. This initiative concerns a new satellite product line with a total cost of 300 MF (z 50 MUSD) and a 2-3 years development phase. Three potential missions have been selected mid-95. These missions are for astronomy SAMBA and COROT and the Topex Poseidon Follow On mission for earth studies. SAMBA is fully compatible with the PROTEUS concept and the main caracteristics of the mission in this context are listed in figure 6. The satellite is 3-axis stabilized with a zenital field of view. The total mass is approximatively 500 kg and dimensions are compatible with a 2 m fairing diameter small launcher (Rockot,LLV,Taurus). The restitution of the LOS telescope is assumed with a star tracker aligned with it. Telemetry data collection and communications are assumed by a 2 Giga-octets on-board memory and a dedicated band-S ground station.

7.CONCLUSION

We have shown that an existing CMB measurements mission can be planned with a small budget (60 M$). Working in a low orbit, it will be able to output calibrated full­sky maps ranging in frequency from 75 to 850 GHz and in angular resolu ';on from 20 arcmin to 7°. Developp PROTEUS seems a strong recommendation of the French government to CNES and the first mission in this context will be the TPFO one wich flight is scheduled in 99. Scientists have an unique advisability to have SAMBA as the second PROTEUS mission with a launch mid 2000 (figure 7).

, . 1>K•s•ott 1995

TPFO

i

MISSION 2 {n. SAMBA J -

Ar1'£.._ H� IE.Sit 1997 .... , ...

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---------- . � Lancetnenl L•ncem«lt ;::; T?FO I , J n TPFO 1 r •u i:wua tot T�FO 1+ TPF0 2 + 1 ,.l,U T?F02 i'

AIT TPF0 1 I P1F ' S I L I L I I lr.;;;-"°2 . I 1- - - .....ll 1 c : :. ���I�� :. :. �I OPS TPF0 1 I I I I

Figure 7: SAMBA schedule

---�������SAMBA�������--

S c i e nt i f i c g o a l s

Measurements o f the CMB anisotropies (2.726 K } a t angular resolution from 2 0 arcmin t o 7°,

with a sensitivity per pixel .0.T/T� 1 0-6 , in the frequency range75 to 850 GHz. Detection of the Sunyaev-Zeldovich effect in 3 channels Constraints on the fundamental cosmological parameters (no,HJ1B)

P a y l o a d

Telescope - off-axis Gregorian (primary 0 80 cm, secondary 0 4 5 c m }

- passive cooling at 1 00 K (baffes a t 1 50K and 1 O O K Focal plane - bolometers cooled at 0 . 1 K

- wavelength channels 6 mm to 0.35 mm - precooling by mechanical cryocoolers at 60 K - cooling of the focal plane by cryocoolers at 4 K associated with a Helium dilution refrigerator at 0 . 1 K

Observation mode sky coverage by zenital pointing and rotation of the orbit plane Lifetime 1 .5 to 2 years (limited by the Helium tanks capacities}

S a t e l l i t e

Type 3-axis stabilization with a zenital l ine of sight (LOS} Mass 470 kg(platform 220 kg and payload 250 kg } Structure 3.2m heigh x 2m diameter Electric power 400 W maxi.by deployed solar panels

(payload:240 W nominal, 1 1 4 W launch} Pointing precision of 0.2° , stability 0.07° per second

restitution 0.02° by startracker calibrated with the payload LOS Communications dedicated S-band ground station, TC=4 Kbps, TM<300 Kbps Storage on board memory 480 Mbits/day

Launch >2000 (LLV,Taurus, Rockot...} Orbit 825 km sun synchronous orbit(820 to 1 000 km}

period of 1 0 1 .4 min

Figure 6: SAMBA mission characteristics

CT/fO, I · Y.8LAHC VI VI