mirax payload description and requirement document · 2010. 10. 12. · payload description and...

INAF ISTITUTO DI ASTROFISICA SPAZIALE E FISICA COSMICA

Roma - Bologna

MIRAX

Payload Description and

Requirement

Document

Prepared LA, AA, MF, FF, CL

Approved

Reference MIRAX-IASF-PL-RQ-001

Issue 1 Revision 1

Date 21 May 2010

Payload Description and Requirement Document

Doc: MIRAX-IASF-PL-RQ-001 Issue:1, Revision: 1

Date: 21 May 2010 Pag. I

DISTRIBUTION LIST

DATE Name Institution



Date: 21 May 2010 Pag. II

DOCUMENT CHANGE RECORD

DATE ISSUE REV. DESCRIPTION AUTHOR

25/02/10 1 0 Draft MF

21/05/10 1 1 First Release LA, AA, MF, FF, CL



Date: 21 May 2010 Pag. III

Table of contents

INTRODUCTION ................................................................................................................................. 1

1.1 SCOPE ...................................................................................................................................... 1

REFERENCES .................................................................................................................................... 1

1.2 LIST OF ABBREVIATIONS ....................................................................................................... 1 1.3 APPLICABLE DOCUMENTS .................................................................................................... 2 1.4 REFERENCE DOCUMENTS ..................................................................................................... 2

FUNCTIONAL SPECIFICATION SUMMARY ...................................................................................... 2

PAYLOAD REQUIREMENTS AND CONSTRAINTS ........................................................................... 3

1.5 SYSTEM DESIGN REQUIREMENTS ........................................................................................ 3 1. STUDY APPROACH ........................................................................................................ 3 2. MISSION ANALYSIS ........................................................................................................ 3 3. COST ............................................................................................................................... 3

1.6 OVERALL SCIENCE REQUIREMENTS .................................................................................... 3 1.7 SPACE SEGMENT REQUIREMENTS....................................................................................... 5

4. SPACE SEGMENT LIFETIME .......................................................................................... 5 5. BUS .................................................................................................................................. 5 6. OPERATIONAL ORBIT .................................................................................................... 5 7. ATTITUDE CONTROL SYSTEM ...................................................................................... 5 8. PAYLOAD ........................................................................................................................ 7

1.7.1.1 Overview of Paylod Design Concept ........................................................................... 7 1.7.1.2 The All Sky Monitor ................................................................................................... 13

1.7.1.2.1 ASM - Experiment Requirements ......................................................................... 13 1.7.1.2.1.1 Field of View ...............................................................................................................13

1.7.1.2.1.2 Number of Units ..........................................................................................................13

1.7.1.2.1.3 Geometric Area ............................................................................................................13

1.7.1.2.1.4 Effective Area ..............................................................................................................13

1.7.1.2.1.5 Angular Resolution ......................................................................................................14

1.7.1.2.1.6 Energy Range ...............................................................................................................14

1.7.1.2.1.7 Energy Resolution ........................................................................................................14

1.7.1.2.1.8 Time Resolution ...........................................................................................................14

1.7.1.2.1.9 Absolute Time Accuracy .............................................................................................14

1.7.1.2.1.10 Dead Time..................................................................................................................15

1.7.1.2.1.11 Sensitivity ..................................................................................................................15

1.7.1.2.1.12 Weight ........................................................................................................................15

1.7.1.2.1.13 Power lines .................................................................................................................15

1.7.1.2.1.14 Power Budget .............................................................................................................15

1.7.1.2.1.15 Dimensions ................................................................................................................15

1.7.1.2.1.16 Telemetry ...................................................................................................................15

1.7.1.2.1.17 Thermal Ranges .........................................................................................................15

1.7.1.2.1.18 Electrostatic grid and a membrane cover ...................................................................16

1.7.1.2.1.19 Transparency of the Electrostatic Grid ......................................................................16

1.7.1.2.1.20 Transparency of the membrane cover ........................................................................16

1.7.1.2.2 Silicon Detectors .................................................................................................. 16



Date: 21 May 2010 Pag. IV

1.7.1.2.2.1 Spatial resolution .........................................................................................................16

1.7.1.2.2.2 Active Area ..................................................................................................................16

1.7.1.2.2.3 Energy Resolution ........................................................................................................16

1.7.1.2.2.4 Time Resolution ...........................................................................................................17

1.7.1.2.3 Coded Mask ......................................................................................................... 17 1.7.1.2.3.1 Mask Size .....................................................................................................................17

1.7.1.2.3.2 Mask Material and Thickness ......................................................................................17

1.7.1.2.3.3 Opacity of empty elements ..........................................................................................17

1.7.1.2.3.4 Mask Code and Open Fraction ....................................................................................17

1.7.1.2.3.5 Mask Element ..............................................................................................................17

1.7.1.2.4 Collimator ............................................................................................................. 17 1.7.1.2.5 Front-End Electronics (FEE) ................................................................................. 18 1.7.1.2.6 Back-End Electronics (BEE) ................................................................................. 18

1.7.1.2.6.1 Local OBT Management..............................................................................................18

1.7.1.2.6.2 Observation Data Acquisition ......................................................................................19

1.7.1.2.6.3 The Synchronous Phase ...............................................................................................19

1.7.1.2.6.3.1 The Trigger Filtering Phase ..................................................................................19

1.7.1.2.6.4 The Front-End Freeing .................................................................................................20

1.7.1.2.6.5 The Asynchronous Phase .............................................................................................20

1.7.1.2.6.5.1 The Event Pre-Processing .....................................................................................20

1.7.1.2.6.6 The Data Packet Preparation ........................................................................................20

1.7.1.2.6.7 Pedestal Data Acquisition ............................................................................................21

1.7.1.2.7 Sun Avoidance ..................................................................................................... 21 1.7.1.2.8 Attitude reconstruction .......................................................................................... 21

1.7.1.3 The Soft Gamma-ray Spectrometer (SGS) ................................................................ 21 1.7.1.3.1 Main requirements: .............................................................................................. 21

1.7.1.3.1.1 SGS system architecture .............................................................................................22

1.7.1.3.1.2 Detection units description ..........................................................................................22

1.7.1.3.1.3 Collimators...................................................................................................................24

1.7.1.3.1.4 Front End / Back-End Electronics (FEE/BEE) ............................................................25

1.7.1.3.1.5 System generating events for gain stability control and calibration ............................26

1.7.1.3.1.6 Thermal Ranges ...........................................................................................................27

1.7.1.4 PAYLOAD DATA HANDLING ................................................................................... 27 1.7.1.4.1 The MIRAX P/L configuration ............................................................................... 27 1.7.1.4.2 DH general specifications ..................................................................................... 27 1.7.1.4.3 Operating modes and mode transitions ................................................................ 28 1.7.1.4.4 Software description ............................................................................................. 30 1.7.1.4.5 Scientific data processing ..................................................................................... 30

1.7.1.4.5.1 Observation Scientific Tasks .......................................................................................30

1.7.1.4.5.1.1 Burst triggering .....................................................................................................30

1.7.1.4.5.1.2 Burst data acquisition ...........................................................................................30

1.7.1.4.5.1.3 Burst images acquisition and processing ..............................................................31

1.7.1.4.5.2 Calibration Scientific Tasks: ........................................................................................31

1.7.1.4.5.2.1 Pedestal calculation ..............................................................................................31

1.7.1.4.5.2.2 FEE Electrical Calibrations ..................................................................................31

1.7.1.4.6 On-board Time management ................................................................................ 31 1.7.1.4.7 TM/TC interface .................................................................................................... 32

1.7.1.4.7.1 Telecommands .............................................................................................................32

1.7.1.4.7.2 Telemetry .....................................................................................................................32



Date: 21 May 2010 Pag. V

1.7.1.4.7.3 Housekeeping Telemetry .............................................................................................32

1.7.1.4.7.4 Science Telemetry ........................................................................................................32

1.7.1.4.7.4.1 ASM Event-by-event data telemetry ....................................................................32

1.7.1.4.7.4.1 SGS Data collection modes ..................................................................................32

1.7.1.4.8 Telemetry Budget (TBC) ....................................................................................... 33 1.7.1.5 Power Supply Unit (PSU) .......................................................................................... 33

1.7.1.5.1 ASM Low Voltage (FEE) ....................................................................................... 33 1.7.1.5.2 ASM Medium Voltage (Detector) .......................................................................... 33 1.7.1.5.3 ASM High Voltage (Detector) ............................................................................... 33 1.7.1.5.4 SGS Low Voltage ................................................................................................. 33 1.7.1.5.5 SGS High Voltage ................................................................................................ 34

1.7.1.6 Payload Reference Frame ........................................................................................ 34 1.7.1.7 Payload Accomodation ............................................................................................. 34

1.7.1.7.1 ASM Accommodation ........................................................................................... 35 1.7.1.7.2 SGS Accommodation ........................................................................................... 35 1.7.1.7.3 Mounting and coalignement.................................................................................. 35 1.7.1.7.4 Thermal environment............................................................................................ 35 1.7.1.7.5 Dissipation ............................................................................................................ 35

9. SPACECRAFT ............................................................................................................... 35 1.7.1.8 Fast Communication channel .................................................................................... 35 1.7.1.9 Data downlink capability ............................................................................................ 35 1.7.1.10 Data storage capability ............................................................................................. 35 1.7.1.11 Compatibility with the ASI Ground Station of Malindi ................................................. 35

1.8 GROUND SEGMENT REQUIREMENTS ................................................................................. 36 10. GROUND SEGMENT RESPONSIBILITIES .................................................................... 36

1.9 ASSEMBLY, INTEGRATION AND VERIFICATION REQUIREMENTS ................................... 36



Date: 21 May 2010 Pag. 1

INTRODUCTION

1.1 SCOPE

Scope of the present document is to provide an overview of the proposed instrumentation for the MIRAX

payload. The current baseline is one possible option, to be fully evaluated with respect to the available mass,

power and telemetry budget. The system is modular, and it can be adjusted to fit tighter requirements. Where

available, numbers and specifications for the current option are provided.

REFERENCES

1.2 LIST OF ABBREVIATIONS

[AD] Applicable Document [RD] Referring Document ADC Analog Digital Converter AGN Active Galactic Nuclei AIV Assembling, Integration and Verification AO Announcement of Opportunity ASI Agenzia Spaziale Italiana ASIC Application Specific Integrated Circuit ASM All Sky Monitor BE Back-End BEB Back-end Electronics Blocks BEE Back-End Electronics CE Control Electronics DH Data Handling DPP Data Packet Preparation DSP Digital Signal Processor EM Engineering Model EPP Event Pre-Processing FE Front-End FEB Front end Electronics Board FEE Front-End Electronics FEF Front-End Freeing FIFO First In First Out FPGA Field Programmable Gate Array GS Ground Segment GSE Ground Support Equipment HK Housekeeping HM Health Monitoring HV High Voltage HW Hardware IASF Istituto di Astrofisica Spaziale e Fisica Cosmica INAF Istituto Nazionale di Astrofisica INFN Istituto Nazionale Fisica Nucleare




LUT Look Up Table MGSE Mechanical Ground Support Equipment MOC Mission Operation Center OBT OnBoard Time P/L Payload PA Product Assurance PDHU Payload Data Handling Unit PEB Power Electronics Box PPS Pulse Per Second PSU Power Supply Unit SGS Soft Gamma-ray Spectrometer S/C Spacecraft SDD Silicon Drift Detectors STA Silicon drift detectors Tile Assembly STAFEE STA Front End Electronis SW Software TBC To Be Confirmed TBD To Be Decided TE Test Equipment TFP Time Filtering Phase TLC Telecommand TM Telemetry ToO Target of Opportunity TT&C Telecommanding , Tracking and Communications WBS Work Breakdown Structure

1.3 APPLICABLE DOCUMENTS

TBD

1.4 REFERENCE DOCUMENTS

TBD

FUNCTIONAL SPECIFICATION SUMMARY

The mission should be capable to perform timing and spectra from Gamma ray Bursts from 1 keV to 10 MeV

and to monitor celestial X-ray sources between 2 and 50 keV using sound technology and, mostly, already

available in the critical parts, in order to limit the developing time and the cost.

- The payload should be designed to be ready to be launched on schedule. - The payload should be designed to be made within the allocated budget. - The payload should be designed to meet the specification on volumes, mass and power - The payload should be capable to fulfil the scientific requirements.




PAYLOAD REQUIREMENTS AND CONSTRAINTS

1.5 SYSTEM DESIGN REQUIREMENTS

1. STUDY APPROACH

The MIRAX scientific payload is an ambitious design bringing to space for the first time ever large area,

position-sensitive Silicon Drift Detectors. Together with high energy phoswich detectors they open up the

possibility of wide-band imaging and spectroscopy studies of GRBs and transient sources.

2. MISSION ANALYSIS

The orbit of the Lattes satellite is an equatorial LEO, the best for X-ray Astronomy and for an efficient

downlink to Alcantara (and possibly Malindi). The mission is dual, envisioning an Astronomy payload and a

Earth observation payload. The optimization of the observing time is given by a zenith-pointing strategy,

allowing to scan simultaneously the Earth and the open sky.

3. COST

The mission is intended to be cost-effective.

1.6 OVERALL SCIENCE REQUIREMENTS

The MIRAX payload will have the main scientific objectives of measuring the wide-band energy spectrum of

gamma ray bursts and monitoring X-ray sources simultaneously over a large portion of the sky.

Concerning GRBs, the primary aim of MIRAX is to achieve scientific goals of fundamental importance not

fulfilled by GRB experiments flying presently or in the next future (e.g., Swift, Konus/WIND, GLAST/GBM).

These can be summarized as follows:

- detection and study of transient X-ray absorption column / features for tens of medium/bright GRBs per

year; these measurements are of paramount importance for the understanding of the properties of the Circum-

Burst Matter (CBM) and hence the nature of GRB progenitors (still a fundamental open issue in the field);

- measure the spectral shape below 10 keV, which is a fundamental test for models of GRB prompt

emission (still to be settled despite the considerable amount of observations);

- provide a substantial increase (with respect, e.g., to Swift) in the detection rate of X-Ray Flashes (XRF),

a sub-class of soft / ultra-soft events which could constitute the bulk of the GRB population and still have to be

explored satisfactorily, and of high redshift (z > 5) GRBs, which are of fundamental importance for the study of

evolutionary effects, the tracing of star formation rate and ISM evolution, the study of pop. III stars;

- the broad band, from a few keV to ~1 MeV, obtained by the combination of the ASM and the SGS, will

allow an accurate determination of spectral peak energy, which is a fundamental quantity for the test and study

of spectrum-energy correlations and the possible use of GRBs as cosmological probes, together with a good

detection efficiency for short/hard GRBs, whose difference with long GRB in terms of progenitors and emission

physics is a central issue in the field;

- the fast and accurate location capabilities of the ASM will allow the prompt multi-wavelength follow-

up of GRBs with ground and space telescopes, thus leading to the identification of optical counterparts / host

galaxies and the estimate of the redshift, a fundamental measurement for the scientific goals listed above.

In addition to this GRB studies, the scientific objectives of the X-ray all-sky monitor capabilities include:




- long-term monitoring of the flux of hundreds of persistent or transient sources, study of variability,

periodicity, quasi-periodicity and super-orbital periods;

- spectral monitoring of Galactic black hole and neutron star transients - detection and localization within a few arcmin of Soft Gamma Repeaters (SGR) and many other classes

of galactic X-Ray Transients (XRT), like, e.g., galactic low and high mass X-ray binaries in outburst, cataclismic

variables, accreting ms pulsars, etc., also triggering follow-up observations by ground and space observatories,

with a fast alert to the world-wide community;

- to perform an all-sky survey in the 2 – 50 keV complementary to that by eROSITA at lower energies.

In order to achieve the above goals, the instrument should satisfy the following requirements.

• Energy passband from ~1-3 keV up to ~5 MeV. The lower threshold is fundamental for the study of

transient X-ray absorption features and the substantial increase in the detection and study of XRFs and high-z

GRBs. The wide band is of key importance for the identification estimate of fundamental spectral parameters

like the low energy photon index and the peak energy Ep (GRB physics and cosmology) and to increase the

detection efficiency for short GRB.

• Energy resolution 50 keV is enough for accurate measurement of the spectral peak energy.

• Source location accuracy of a few arcmin in order to trigger and allow follow-up observations of the

detected transients by other telescopes (including eROSITA and ART), which is essential for fulfilling the

scientific objectives for GRBs, for the all-sky monitor functionality and to perform a sensitive X-ray all-sky

survey.

• Field of View (FOV) of at least 2 sr combined with a sensitivity of ~ 500 mCrab (5σ, 1s) in the 2-50

keV energy band, in order to allow detection and localization of ~150 GRBs and XRFs per year (including ~2-4

events at z > 6), the sensitive spectroscopy of ~2/3 of them and the use of the instrument as an all-sky monitor

for galactic transients (SGRs and other XRTs).

• An average effective area of ~700 cm2 in 50 – 200 keV, ~500 cm

2 in 200 – 500 keV and 300 cm

2 in 0.5

– 1 MeV within a FOV of 2 sr, to allow sensitive broad band spectroscopy of GRBs (GRB physics and

cosmology) and increase the overall trigger efficiency, especially for short (and spectrally hard) GRBs.

• On board data handling electronics allowing GRB trigger, discrimination of false triggers and fast source

position reconstruction, in order to allow prompt alert distribution and follow-up observations.

Parameter Requirement

Sensitivity to sources < 5 mCrab in 50 ks

Sensitivity to GRBs < 2.5 ph/cm2/s in 1s

< 0.4 ph/cm2/s in 60 s Timing resolution 10 µs Spectral resolution < 500 eV FWHM Angular resolution < 6 arcminutes Field of view >3 steradians

Table 1: ASM Overall Scientific Requirements




1.7 SPACE SEGMENT REQUIREMENTS

4. SPACE SEGMENT LIFETIME

Parameter Requirement Goal

Space segment lifetime 2 years 4 years

Table 2: Space segment lifetime

5. BUS

Table 3: Main characteristics of the MMP Platform .

6. OPERATIONAL ORBIT

Table 4: Operational Orbit.

7. ATTITUDE CONTROL SYSTEM

It is assumed that the Lattes satellite will drift « like an airplane » always keeping the same face of the payload

cube at the Nadir. This will allow the Earth-observing EQUARS payload to operate continuously its Earth-

observation duty, while MIRAX will see the open sky always drifting above the Zenith at a speed of 4º/minute

(1 full round of 360º every orbit of ∼100 minutes). This condition is outlined in the following figure, showing the

relative directions of the various systems.

Parameter Value

Mass ~500 kg (PMM total), ~100 kg (MIRAX payload)

Power ~240 W (total), ~90 W (MIRAX payload)

Telemetry Downlink S-band, ~1.5 Mbps for 15 passes/day (Alcantara station only)

Total TM/orbit 0.9-1.0 Gbit (Alcantara station only)

Ground Station(s) Alcantara (Brazil) and Malindi (Kenya) - TBC

Parameter Value

Altitude 600 km

Inclination < 15 degrees




Figure 1: A schematic view of the Lattes satellite and key orientations.

Despite the above strategy, the PMM platform foreseen for the Lattes mission is intrinsically capable of fine

attitude control and reconstruction, as outlined by the summary of characteristics listed in the following table.

While the attitude and position reconstruction performance are of high relevance also in a drifting attitude

strategy, the pointing stability properties may still be considered for occasional use to operate Target of

Opportunity observations. This pointing strategy is still TBC.

Parameter Value Attitude Absolute Measurement Accuracy 0.005o (3 σ) Absolute Pointing Error 0.05o (3 σ) Absolute Pointing Stability 0.001o/s Satellite Position Accuracy GPS Sky Accessible Full Sky Forbidden directions None

Table 5: AOCS




8. PAYLOAD

1.7.1.1 Overview of Paylod Design Concept

The MIRAX payload will be composed of two independent instruments, aimed at the following

scientific objectives: a) detection, localization and wide-band spectral measurements of Gamma Ray Bursts

(GRBs) and b) All Sky Monitoring, i.e., long-term monitoring and discovery of sources or source variability.

The experiment will then be composed of a set of coded-mask X-ray imagers (All Sky Monitor, ASM) and a set

of scintillator spectrometers (Soft Gamma-ray Spectrometer, SGS).

The primary scientific objectives of the experiment are the measurement of the wide-band (2 keV – 5

MeV) energy spectrum of GRBs, with emphasis on the low-energy part of the spectrum (down to 2 keV, goal to

1 keV, see Fig. 7), and the monitoring of sources in the largest fraction of the sky, with priority on the Galactic

plane. The best instrumental approach to the science requirements is to have separate detectors for the low and

high part of the energy band. The low energy (∼2-50 keV) is best covered with Silicon detectors, whereas the

high energy portion of the spectrum requires high-Z detectors, such as inorganic scintillators. Considering the

zenith-pointing strategy, the sky will drift in front of the field of view of the experiment. For this reason, the

field of view of the experiment will be largest in the direction orthogonal to the direction of motion of the

spacecraft, while in the other direction it can be smaller, being covered through the satellite motion. The overall

concept is illustrated in the following sketch.

Figure 2: An hypothesis of allocation of the ASM (in orange color) and SGS (in magenta color) experiments onboard the MIRAX payload and Lattes satellite.




Based on the first-order geometrical constraints on the MIRAX payload, for the ASM experiment we

envisage a set of three pairs of coded-mask X-ray cameras, each one with asymmetric 2D position capability.

The fine imaging coding direction, with angular resolution in the range of ∼5 arcminutes, will be oriented at 90º

in each pair of cameras, in order to guarantee an overall 2D arcmin-localization capability by intersecting the

information gathered from the two units. Both X-ray cameras will have also the capability to coarsely identify

the second coordinate of sources, with an angular resolution in the range of a few degrees. This will help in

improving the signal-to-noise ratio for the individual detection and to decrease the confusion limit.

Each X-ray camera will be composed of a matrix of Silicon Drift Detectors, each one approximately 40-

50 cm2, to form an X-ray sensitive plane with a total geometric area of about ∼6-700 cm

2 per unit. The field of

view will be limited by a wide collimator, to an opening of approximately ∼4 steradian (TBV) covered by a

coded mask, with a code reflecting the 2D asymmetric position capability of the detectors. The mask will be at a

distance of approximately ∼12-15 cm from the detector and it will have a size ∼1.5 times that of the detector

plane. This will result in a fully coded field of view (FCFOV) of ∼0.7-0.8 steradians and a partially coded field

of view (PCFOV) of ∼3 steradians, for each of the cameras. The X-rays cameras will cover the nominal energy

range 2-40 keV. In Fig. 3 we show a pictorial view of the X-ray cameras and coded mask.

Figure 3: Pictorial view of the X-ray imager and coded mask.

With the aim of covering the largest portion of the sky, a possible configuration of the ASM units is as

shown in Fig. 2, with the 3 pairs of detector misaligned by 60º to each other in the direction orthogonal to the

spacecraft motion. In Fig. 4 we show a cross section of the same sketch, where the field of view of the individual

units is shown. In this configuration the total FCFOV approaches 2.5 steradian at any time, while the PCFOV

extends the field of view to approximately 6 steradian, with the central unit adding partially coded area to some

of the directions covered in full coding by the other units. With this configuration, considering the spacecraft

zenith-pointing configuration, each direction will cross the field of view at the “speed” of 4º/minute. A single

source at the zenith will then have a “transit time” across the FCFOV of approximately 700 s.

This design is still only indicative and it is mostly intended to show the approach. It will likely require

substantial revision to comply with the telemetry and power budgets.




Figure 4: Cross-section view of the payload configuration shown in Fig. 2.

Figure 5: The sky accessible to the ASM experiment after every orbit (90 minutes). Thick lines represent the center of the field of view of the three subsystems.




The basic element of the X-ray cameras are linear Si drift detectors developed at INFN-Trieste as an

heritage of the work carried out for the LHC/ALICE experiment. The baseline monolithic Si detector is

approximately squared, ∼50 cm2, ∼450 µm thick (∼800-1000 µm is under study). It is read-out through a set of

512 anodes to which the charge generated in the photo-electric absorption of an X-ray photon is drifted. In the

current baseline, each anode collects charge from a “drift channel” pitch-wide (currently, 294 µm) and 35 mm

long. The charge read-out is self-triggered. Upon trigger, the read-out ASIC will read-out the charge cluster (1 to

5 anodes) and all the un-fired anodes to be used for an estimate of the common mode noise, to be subtracted to

the signal (an option is being studied where the read-out ASIC provides a single estimate of the common mode

noise, to simplify the signal processing). An estimate of the pedestals level will also be performed every TBD

orbits by a synchronous read out of all channels. Analog-to-digital conversion and signal processing will be

carried out in the back-end electronics before sending the event to the experiment CPU for telemetry packing,

together with the address and timing information. The X-ray cameras will require power supplies of different

types from DC/DC converter units. A low voltage (∼ +/-3.5 V) will power the front-end electronics. An

intermediate voltage (∼ +/-50-200 V) will power the final part of the voltage divider and the charge collection

region. A high voltage (∼ -700-2500 V) is instead required to build the drift electric field and to fully deplete the

Silicon bulk. In Fig. 6 we show a picture of one detector and a sketch of the principle of operation.

Figure 6: Left: a picture of the Silicon Drift Detector. The sample shown has dimensions 7.56 cm x 8.26 cm. Center: principle of operation of the SDD. Right: structure of the electric potential, showing

the charge collection process.




Figure 7: Energy spectrum obtained at room temperature with a lab prototype of the SDD camera exposed to an X-ray tube with lines at 2.04 keV, 4.08, 6.12 and 8.16 keV. The dim line on the left of

the 2.04 keV line is Al fluorescence, at 1.5 keV. Energy resolution is approximately 270 eV here.

The SGS will be in charge of providing detection trigger and spectral information for GRBs in the

nominal energy range 30 keV – 5 MeV. An extension of the dynamical range up to 50 MeV is also being

considered. It will be composed by 8 units each one being an independent detector in which the active part is

made by scintillating material with PMT readout and electronics. This will allow for an efficient read-out and

improve the particle background rejection. The baseline design includes two set of 4 units each, passively

collimated to reduce the background and slightly misaligned (~20°) in order to optimize the response to the same

field of view as the X-ray imagers. The individual detectors will be made of NaI and CsI scintillating crystals,

optically coupled in phoswich configuration and read-out by a single photomultiplier, using the pulse shape

Figure 8: Effectiveness of the Pulse Shape Analysis (PSA) for the phoswich detectors of the BeppoSAX/PDS instrument.




information to discriminate the signal. This technique, exploits the different time constant of two

optically coupled scintillators, allowing to discriminate between photons releasing their full energy in the top

scintillator and those releasing energy in both detectors. It was successfully adopted for the PDS instrument

onboard BeppoSAX, providing excellent and still unique results, and will allow a reduction of the BKG by a

factor of ~10 up to ~200 keV with respect to the use of a single crystal device (e.g., BATSE, Fermi/GBM). The

dimension of each crystal will be: geometrical area ∼ 14x14 cm2 (12x12 cm

2 active area) and thickness 4 cm (1

scm NaI + 3 cm CsI). Each of the 2 collimators will be made of Pb, 0.3 cm thick, with a total height of 10 cm (4

cm from the bottom to the top of the crystals plus 6 cm above them). The read-out and signal processing for this

instrument will be of “standard type” (pre-amp, amp, discriminator, sample & hold, ADC, ..), using discrete

electronics. Power supplies will require ~100 mW for each detection unit .

Figure 9: Sketch of a detection unit (left) and 1 of the 2 assemblies (right) of the the Soft Gamma-ray Spectrometer (SGS) .

As a baseline, the X-ray cameras will operate in photon-by-photon transmission mode at any time.

Instead, the high energy detectors will integrate light curves and spectra with pre-set time intervals. Both units,

ASM and SGS, will have the autonomous capability to trigger transient events. When a triggering condition

occurs in the ASM, photon-by-photon information will be saved for the SGS. Vice-versa, when a trigger comes

from the SGS, the onboard imaging is started in the ASM, while the standard photon-by-photon transmission

goes unaffected. In case the telemetry rate will become an issue for the MIRAX payload, an integrated

information may be considered also for (part of) the ASM, including energy-selected light curves, images and

spectra, with photon-by-photon information reduced to the trigger condition.




1.7.1.2 The All Sky Monitor

The ASM is the instrument devoted to the detection, localization and X-ray spectral study of Gamma Ray Bursts

and to the monitoring (flux and position) of persistent and transient sources. It is composed of 6 instruments,

each one composed of Linear Silicon Drift Detectors, a Collimator and an asymmetric 2D coded masks and read-

out electronics. In the following sections we will outline the requirements of the full instrument as well as the

requirements of the individual subsystems.

1.7.1.2.1 ASM - Experiment Requirements

1.7.1.2.1.1 Field of View

The experiment field of view will be >1 steradian fully coded (goal 2 sr) and >3 steradian partially coded (goal 4

sr) at any time. A coverage of >50% of the full sky shall be reached after any satellite orbit (goal 100%).

In the current baseline the individual detector is 30 cm x 22 cm and the mask is 44 cm x 32 cm. The mask

detector distance is 12 cm. With this geometry, each unit has a 0.8 sr fully coded field of view and a 3.4 sr

partially coded field of view. The fully coded angle is 60º in the X direction (perpendicular to the spacecraft

travel direction) and 45º in the Y direction (parallel to the spacecraft travel direction) . The zero-response angle

is 144º in the X direction and 132º in the Y direction. This baseline is most likely subject to significant changes

due to telemetry constraints.

1.7.1.2.1.2 Number of Units

Due to the intrinsic asymmetric angular resolution of the instrument (order of arcmin in one direction and few

degrees in the orthogonal direction), each ASM will be composed of 2 units, oriented at 90º to each other,

observing the same field of view. The complete ASM will composed of 3 pairs of instruments.

1.7.1.2.1.3 Geometric Area

In the current baseline the geometric area of each unit is 660 cm2 (30 cm x 22 cm). Any direction in the Fully

Coded field of view is observed with 1320 cm2 of geometric area.

1.7.1.2.1.4 Effective Area

With the current baseline of geometric area, assuming a coded mask with a 50% open fraction, the resulting

effective area of one ASM unit is as shown in Fig. 10, peaking at approximately 270 cm2.




Figure 10: the effective area of one ASM unit

1.7.1.2.1.5 Angular Resolution

The angular resolution of the fine-coded direction of each ASM unit, aimed at localizing the events, will be

better than 10 arcminutes (goal 5 arcminutes).

The angular resolution of the coarse-coded direction, aimed only at reducing the confusion limit, will be

better than 6º (goal 3º).

1.7.1.2.1.6 Energy Range

The energy range of the experiment is defined as the minimum and maximum energy of an incoming photon for

which the effective area of the experiment will be higher than 10% of its maximum value. It will be defined by a

Lower Energy Bound and an Upper Energy Bound. The requirement for the Lower Energy Bound is 2 keV (goal

1.5 keV). The requirement for the Upper Energy Bound is 30 keV (goal 40 keV).

1.7.1.2.1.7 Energy Resolution

The requirement for the energy resolution is < 700 eV full width at half maximum (FWHM) over the full energy

range and at any position of the detectors. The goal is 500 eV.

1.7.1.2.1.8 Time Resolution

The requirement for the event time-tagging resolution is 1 µs.

1.7.1.2.1.9 Absolute Time Accuracy

The requirement for the absolute time accuracy is 2 µs at 3σ with respect to UTC. It is assumed that the absolute

time reference is provided through a GPS receiver.




1.7.1.2.1.10 Dead Time

The dead time will apply to the processing of the events recorded by each individual read-out ASIC. Different

ASICs will be independent one another. The dead-time for the complete processing of each event will be smaller

than 10 µs.

1.7.1.2.1.11 Sensitivity

Each module of the X-ray monitor shall have a 5-σ on-axis sensitivity better than 5 mCrab for a net source

integration time of 50 ks on an extragalactic field.

1.7.1.2.1.12 Weight

The weight of each unit X-ray shall be less than 10 kg, including: detection plane, mask, collimator, front-end

electronics and harness.

1.7.1.2.1.13 Power lines

The ASM subsystem requires 3 power lines: low, medium and high voltage. All the lines will have an

independent ground return.

1.7.1.2.1.14 Power Budget

The power requested by each unit shall be lower than 10 Watts, at the secondary.

1.7.1.2.1.15 Dimensions

Overall dimensions of any of the X-ray cameras will be included in a volume 60 cm x 60 cm x 40 cm.

1.7.1.2.1.16 Telemetry

Any photon is potentially read-out with up to 5 anodes, due to the charge diffusion during drift.

Assuming the following figures:

Energy: 10 bits

Time: 48 bits

Address: 16 bits

each event will require a minimum of 128 bit (= 6 x energy + 48 + 16), including the amplitude of the common

mode noise information and 5 maximum fired channels.

The overall telemetry budget will depend on the configuration of the instrument and the mode of

operation (photon by photon at any time, or only upon trigger). A preliminary estimate of the expected count rate

is 1.8 cts/cm2/s from the Crab and 3 cts/cm

2/s/sr from the diffuse X-ray background.

The expected count rate under reasonable assumptions on the geometry is in the range between 4000 and 5000

cts/s in each camera. This corresponds to about 600 kbit/s only for the photon-by-photon transmission in each

camera, to which the relevant housekeeping data should be added. An onboard telemetry compression, by a

factor of 5 or more, is then required unless changes in the experiment design will lower the telemetry budget.

1.7.1.2.1.17 Thermal Ranges

In absence of a detailed mechanical design, the temperature requirements are related to the Si detector

and front-end ASIC.




Lower Limit (ºC) Upper Limit (ºC) Non-operative -40 +50 Operative -10 +15

Table 6. Thermal ranges

Table 7. Operational requirement

1.7.1.2.1.18 Electrostatic grid and a membrane cover

The surface of the Si detector will be at negative HV (about 2000 V). It is to be established if this can cause the

following problems, or if the presence of the mask can provide an electromagnetic shielding to it. The interaction

between the negative polarized detector and the ionospheric plasma can provide two effects :

• Current as high as TBD µA for TBD km orbit can be present

• The impact of energetic ions with the window will produce electrons which are accelerated back and impacting with the surrounding materials could in turn produce unwanted secondary X-ray background.

At this regard, following the experience of BeppoSAX, ASCA etc., it may be necessary to insert either an

electrostatic grid (-28 V TBC) and/or a membrane cover. A thin membrane aluminized on both sides will

definitively shield the window.

1.7.1.2.1.19 Transparency of the Electrostatic Grid

TBD

1.7.1.2.1.20 Transparency of the membrane cover

TBD

1.7.1.2.2 Silicon Detectors

1.7.1.2.2.1 Spatial resolution

The spatial resolution of the Si detectors is asymmetric and position dependent. The fine position resolution will

be better than 100 µm FWHM. The coarse position resolution will be better than 5 mm FWHM.

1.7.1.2.2.2 Active Area

The detector active area is defined as the region from where an absorbed photon produces a readable signal. In

the baseline (ALICE-like) design the ratio between the active and geometric area is 83%. This value will be

optimized.

1.7.1.2.2.3 Energy Resolution

The energy resolution in this type of detector is known to be position and energy (TBV) dependent. In fact, the

charge cloud is drifted to the anodes and it spreads as much as the interaction point is far from the anodes. Since

the charge is subdivided over N anodes (typically 1 near the anode and 3-4 at maximum distance), the signal-to-

Maximum Operational Thermal Gradient

oC

Spatial




noise ratio is determined by the noise of N anodes. The FWHM energy resolution is requested to be better than

400 eV for events collected on a single anode and better than 800 eV for multiplicity up to 5 anodes.

1.7.1.2.2.4 Time Resolution

The intrinsic time resolution of the Si detector is determined by the maximum time needed to drift a charge from

the point of interaction to the anode. This time depends on the intensity of the electric field and the exact

dimension of the drift channel. The maximum drift time, determining the minimum time resolution, will be

smaller than 5 µs. The uncertainty introduced by the system OBT shall always be smaller than the intrinsic time

jitter of the camera. The 1 µs requirement give in the previous section is coherent with this choice.

1.7.1.2.3 Coded Mask

1.7.1.2.3.1 Mask Size

The size of the coded mask will be in the range between 1.5-2 times the size of the Silicon detection plane and

will cover the complete field of view.

1.7.1.2.3.2 Mask Material and Thickness

The material and thickness of the mask will be chosen in order to satisfy the following constraints:

a) no less than 90% (TBC) of opacity at the upper bound of the energy range b) a thickness- pixel size ratio not larger than 0.5 (TBC) to avoid vignetting effects in the imaging c) fluorescence lines outside the nominal energy range of the experiment (this may be achieved also with a

graded shielding)

d) allowing a stiff mechanical structure. Materials under study are Steel, possibly Gold-coated, or Tungsten.

1.7.1.2.3.3 Opacity of empty elements

The lower bound of the experiment energy range is requested to be 2 keV, goal 1 keV. This implies a

requirement to the maximum opacity of the empty elements of the mask, that is requested to be less than 20%

(TBV) at 2 keV (TBV).

1.7.1.2.3.4 Mask Code and Open Fraction

The mask code will be chosen is such a way to optimize the experiment sensitivity to both persistent (and

multiple) sources and the GRBs. Similarly, the open fraction will be chosen to optimize both the sensitivity and

to reduce the telemetry load at its minimum. In case the open fraction will be chosen to be 50% the code will be

chosen as a Hadamard sequence.

1.7.1.2.3.5 Mask Element

The size of the mask element will be at least 1.2 times (TBV) the position resolution of the detector, in each of

the two directions.

1.7.1.2.4 Collimator

The field of view of the ASM will be reduced by a large collimator. The aim of such collimator is to reduce the

contribution by the diffuse X-ray background arriving from the angles not seen through the coded mask. The

material of the collimator is TBD, and it may use different materials for the mechanical and shielding purposes

(e.g., Aluminium for structure and Tungsten coating for shielding radiation). It is required to provide a minimum

absorption of 95% (TBC, for perpendicular crossing) at the upper bound of the operational energy range of the




experiment. The collimator structure will provide support for the coded mask. (The materials, size, thickness and

cover of the collimator will be defined by simulations).

1.7.1.2.5 Front-End Electronics (FEE)

The charge generated at the anodes from the absorption of an X-ray photon in the Silicon will be read-out

through custom analog signal processing chains integrated in self-triggering ASIC chips. The connection

between the detector output pads and the ASIC inputs will be done through wire-bonding.

Prototypes of an ASIC are being developed at University of Pavia. Each ASIC will read-out 32 channels (TBV).

The ASIC bias and control signals are provided from the Back-end electronics, that is also in charge to read-out

the ASIC outputs. The ASIC will be able to operate in electrical calibration (TBC), by using reference charge

injections.

The Front-end Electronics Board (FEB) will include the ASICs and all the needed electronic components to

operate it properly.

1.7.1.2.6 Back-End Electronics (BEE)

The Back-End Electronics (BEE) is a set of electronic boards connecting the detector assembly with the Data

Handling (DH) and the Power Electronics Box (PEB).

The detector plane is composed of 4-8 independent SDD Tile Assemblies (STA), each consisting of 4-2

SDD detector tiles connected together and equipped with the relevant front-end electronics (STAFEE). Each

STAFEE contains a total of 64-32 readout ASICs (16 ASICs x 4-2 SDD tiles) and each ASIC is connected to 32

readout channels.

Following the same approach, the BEE is structured in a set of 4-8 independent Back End Blocks (BEB),

each managing a single STA.

The single BEB is in charge of:

• distributing and filtering the power supply required to the low voltage STAFEE;

• supplying the STA with the high voltage and medium voltage needed;

• implementing the latch-up protection circuit for each readout ASIC;

• managing the Observation Data Acquisition;

• managing the Pedestal Data Acquisition;

• managing the FEE Electrical Calibration;

• implementing a duplicate of the On-Board Time (OBT) using the Pulse Per Second (PPS) signal and the 1MHz clock provided by the DH;

• providing the HKs to the DH for active monitoring and telemetries purposes;

• integrating the scientific ratemeters;

• loading the STAFEE configuration provided by the DH;

• executing the Write and Read commands sent by the DH;

• managing all the digital interfaces with the DH and the power interfaces with the PEB.

1.7.1.2.6.1 Local OBT Management

The local OBT is implemented by the BEB as a duplicate of the master OBT resident in the DH. The principal

aim of the local OBT is to provide the event time-tags with the resolution of 1µs and with the accuracy of 2µs at

3 sigma respect to the UTC.




The local OBT is composed of a 28-bits counter for the seconds, a 20-bits counter for the microseconds

and a 8-bits register for the OBT error. All these items are functionally dependent on the PPS signal and on the

1MHz clock signal provided by the DH.

The seconds counter is incremented by the PPS signal; the microseconds counter is incremented by the

1MHz clock signal and reset by the PSS signal; the OBT error register is updated, on each PPS signal, with the

new difference (evaluated with sign) between the current microsecond counts and the ideal 1 milion counts.

It's worth noticing that Inside the BEB init procedure managed by the DH, a special task is dedicated to

align the local OBT with the master one.

1.7.1.2.6.2 Observation Data Acquisition

The Observation Data Acquisition (ODA) is the operative mode of the BEB dedicated to the nominal acquisition

of the scientific events.

In this configuration, the readout ASICs work in auto-triggering mode.

The acquisition pipeline implemented by the BEB is composed of the following two phases:

1. Synchronous phase:

• Trigger Filtering Phase (TFP)

• Front End Freeing (FEF) 2. Asynchronous phase:

• Event Pre-Processing (EPP)

• Data Packet Preparation (DPP) The Synchronous phase determines the BEB contribution to the observation deadtime.

Assuming an average event rate per STA (e.g., 2-4 Si tiles) equal to 600-1200 Hz, the deadtime introduced by

the Synchronous phase shall be lower than1% (TBC).

No deadtime is added by the Asynchronous phase.

1.7.1.2.6.3 The Synchronous Phase

1.7.1.2.6.3.1 The Trigger Filtering Phase

The TFP carried out by the BEB is aimed at validating the trigger signals provided by the STAFEE and latching

the event time-tag.

The TFP is activated by the STAFEE global trigger determined as the OR of all the readout ASIC trigger

lines. In consequence of a fired global trigger, the 48 ASIC trigger lines are sampled by the BEB as well as the

local OBT. The latter is stored in memory in order to be inserted in the event packet at the end of the processing

pipeline.

The TPF is composed of two parts which are executed serially. In the first part, the TPF verifies that the

triggered readout ASICs are all connected to the same SDD tile. If that condition is not verified the event is

rejected and the ASICs are reset otherwise the second part of the filtering is executed. The algorithm of the

second part is based on a set of 3 Look-Up Tables (LUTs) sized 64kx1 bits each and stored in RAM memory.

Each LUT is associated to one SDD tile. The TFP logic builds the LUT 16-bits address using the 16 ASIC

trigger states belonging to the fired SDD tile. The TFP logic builds the LUT 16-bits address using the 16 ASIC

trigger states. Then, the TFP logic reads the LUT associated to the fired SDD tile and interprets the relevant

output deciding about the validity of the current trigger. In case of a positive response, the FEF is activated

otherwise the event is rejected and the ASICs are reset.

The TPF LUT is programmable by means of memory patch telecommands (TC).




1.7.1.2.6.4 The Front-End Freeing

The FEF phase is devoted to the event readout and A/D conversion. The FEF is started by a successful TFP.

In this case, the first action performed by the BEB is the activation of the clock distribution towards the STAFEE

in order to allow, at first, the acquisition of the address of the First Fired Channel (FFC) from each triggered

ASIC and, then, the multiplexing and downloading of the analog signals.

The FEF is carried out applying a sparse readout approach. In this sense, all the channels belonging to

the triggered ASICs are acquired and A/D converted. The not triggered ASICs are acquired only if contiguous to

triggered ASIC with the FFC address positioned at the nearest border. The closeness definition is depending on a

programmable parameter.

Three ADCs, one for each SDD tile, are mounted on the STAFEE very closely to the readout ASICs. In

terms of energy deposit in the detector, the ADC input ranges are equal to 0keV – 100keV. The BEB is in charge

of commanding the A/D conversion and acquiring the resulting converted data.

After the FEF phase completion, the readout ASICs are reset by the BEB and the clock distribution is

inhibited.

1.7.1.2.6.5 The Asynchronous Phase

1.7.1.2.6.5.1 The Event Pre-Processing

The event pre-processing is aimed at preparing the data to the download to the DH.

The pipeline carried out by each BEB is composed of the following steps:

• pedestal subtraction

• common noise subtraction

• simplified cluster ID Once ready, the data are sent to the DH where the processing will be completed.

The EPP is an asynchronous phase respect to the FEF, meaning that no deadtime is added (e.g. proper

buffers).

Pedestal subtraction: the pedestal of each readout channel is computed during the PDA phase and kept in a

dedicated memory region. Before going to the next EPP step, the channel pedestal value is subtracted from each

channel amplitude acquired during the FEF phase.

Common noise subtraction: the common noise is a noise component common to all the channels connected to

the same readout ASIC entailing an undesired global baseline shift. For each ASIC involved in the event, the

common noise is computed considering a programmable number (possible values: 16 or 8) of channels selected

among the not-triggered and not disabled ones. If a sufficient number of such channels is not available a dummy

event cluster is set and the next EPP stage is by-passed. In the opposite case, the common noise is computed as

the mean value of the selected data and, before going to the next EPP step, the resulting value is subtracted from

the channel amplitudes acquired during the FEF phase.

Simplified Cluster ID: the readout channels in the same detector which collect the charge released by a

particle/photon form a “cluster”. The simplified cluster ID is carried out searching the channel with the

maximum charge deposit and extracting a sequence of channels centered on this one. The number of channels to

be considered in the cluster reconstruction is programmable (typical value: 5).

1.7.1.2.6.6 The Data Packet Preparation

At the end of the EPP, the DPP is performed in order to prepare the event packet to be transmitted to the DH.

The event packet format provides information on:

� the 48-bit time-tag;

� the 16-bit address

� the 10-bit charge content of each of the elements of the cluster

� the 10-bit value of the common mode




After the event data collection completion and the packet preparation, the event packet is serialized and

transmitted to the DH through a dedicated digital bus.

1.7.1.2.6.7 Pedestal Data Acquisition

The Pedestal Data Acquisition (PDA) is the operative mode of the BEB dedicated to the measurement of the

pedestal values of each detector channel.

In this configuration, the readout ASICs work forced by an external trigger.

The procedure carried out by the BEB is composed of the following main steps:

� generation of a number TBD of external trigger signals;

� after each trigger, activation of the FEF for all the STA channels;

� at the end of each FEF phase, generation of a STA reset signal, execution of the DPP (the EPP is by-

passed) and transmission of pedestal data to the Data Handling in order to be processed.

At the end of the procedure, the Data Handling updates the pedestal tables stored in each BEB in dedicated

memory area.

1.7.1.2.7 Sun Avoidance

The impact of the Sun in the field of view of the ASM will be evaluated and either an operation or a data

handling strategy defined.

1.7.1.2.8 Attitude reconstruction

The ASM shall receive attitude information from the spacecraft with a frequency >10Hz and an uncertainty




1.7.1.3.1.1 SGS system architecture

The SGS instrument will be functionally composed by:

• 8 detection units

• 2 detector group each one consisting of 4 detection units

• 1 passive collimator for each group

• Front-End / Back-End Electronics (FEE and BEE, divided in 8 TBC units)

• A system generating events for gain stability control and calibration (2 TBC, units)

The detection units will be arranged as shown in Figures 2 and 11, so that the instrument, exploiting the satellite

motion and pointing, perform a scan of the sky.

The detecting group will be surmounted by a passive collimator, allowing to reduce the diffuse background in

the FOV.

1.7.1.3.1.2 Detection units description

A detection unit will be of the phoswich type, with the following characteristics:

- Materials: NaI(Tl) + CsI(Na)

- Size phoswich unit (including frame) 140 x 140 mm

- Size NaI(Tl) crystal 120 x 120 x 10 mm (144 cm2)

- Size CsI(Na) crystal 120 x 120 x 30 mm (144 cm2)

- Material of entrance window Al 0.3 mm (TBC)

- Material of case Al 10 mm

- PMT number 1 PMT diam 127 mm (diam), 111 mm (active area)

- PMT type R877-100 Hamamatsu (TBC)

- Light pipe quartz (10 mm thickness TBC)

- Power 100 mW (voltage divider)

- Total active area (8 units) 1152 cm2

- Total weight (8 units without collimator) ~ 40 kg

- Total effective area (cm2) (see Figure 13)

tot tot (NaI ph) NaI NaI ph CsI

a) @ 100 keV 1136 1133 1134 1131 2

b) @ 300 keV 1119 840 521 242 598

c) @ 1 MeV 825 617 222 15 602

- Nominal energy range (keV) 20 - 5000 keV

- Energy resolution ~ 8 % FWHM @ 661 keV

- Timing resolution ~ 1 µsec




Figure 11 Sketch of a SGS detection unit element including scintillator, PMT, and FEE

µ-metal

NaI Crystal

CsI Crystal

Light guide

PMT

V divid.




1.7.1.3.1.3 Collimators

Each assembly of 4 units will be surmounted by a passive collimator, which will both define the FOV and shield

the scintillator crystals. Each collimator will have the following characteristics:

FOV: ~2.1sr

Collimator material: Pb 0.3 cm thick

Collimator dimensions: 56cm x 28cm

Collimator height: 10cm

Figure 12 Sketch of a SGS detectors units assembly




Figure 13: Effective area of the SGS as a function of energy

1.7.1.3.1.4 Front End / Back-End Electronics (FEE/BEE)

The system FEE/BEE will be made of 8 identical electronic chain, one for each detection unit with the following

function and characteristics:

- FEE/BEE functions: o PMT polarisation o PMT signal analysis with the following step

� Signal buffering and amplification � Signal discrimination above the electronic noise level

� Time tag of the discriminated signal above the noise (with 1 µsec precision) � Analog to digital conversion of the maximum signal amplitude in two ranges of different

precision:

• Low energy range between 20 and 500 keV (with 12 bit)

• High energy range between 20 and 10000 keV (with 12 bit) � Determination of the pulse shape with the measure of the duration in time of a fraction of

the signal (for example from 20% to 90% of the maximum) with a precision of TBD nsec

o Digital data generation consisting in a word of (TBC): � 3 bit for detection unit identification � 10 bit for signal amplitude (12 bit ADC, 1 bit range) � 9 bit for signal duration TBC (shaping) (step of some tens of nsec) � 48 bit for time signal tagging (TBC)




The above operation could be done or with a standard classical analog circuit (as in the SAX-PDS system) or

with a system based on continuous digitisation of the signal and its processing with a dedicated logical system.

o TC implementation. The TC should include:

� Threshold setting

� Gain setting

� PMT HV setting

� TBD

o Automatic gain control system as a feedback of the events generated for calibration by a dedicated system (see later)

o I/F for data transmission and receiving

- FEE/BEE further requirements: o The FEE/BEE of each detection unit should be able to sustain a rate of 20.000 event/sec o The FEE/BEE Dead Time should be less than 0.5 % during no burst observations and about 5 ÷

10 % during strong burst observations

o The FEE/BEE should evaluate the dead time of each unit o The FEE/BEE Power should be ~ TBD W for PMT power supply distributor and TBD W for the

electronics chain dealing with PMT signal

o The FEE/BEE of each detection unit should be able to sustain without degradation an energy release on the scintillator of TBD GeV without spurious retriggering (use of a baseline restorer

TBD).

- FEE/BEE mechanical arrangement: o The FEE/BEE of each detector unit should be arranged on the bottom of the PMT assembly o Connections to/from FEE/BEE should include

� HV power supply

� LV power supply

� TC data line

� Digital data line

� Survival lines (TBD)

� System clock for data tagging (TBD)

o Provision should be made to avoid HT discharge in all the operation conditions

1.7.1.3.1.5 System generating events for gain stability control and calibration

The system will be based on signals generated by a determined energy deposit in the detection units. This

constant energy release will be used as a continuous feedback inside the FEE towards the High Voltage fine

regulation of the PSU to stabilise the gain of the detector and to produce a calibration spectra.

The system can be made for example of:

- one (or two) low energy calibration unit where a small plastic scintillator with PMT or Si-PMT readout contain

melded inside tracks of Am-241 isotope. The signal from α-decay will be used as marker.




- one (or two) high Energy calibration unit where a medium energy source like the isotope Na-22 is deposited on the surface of a scintillator with PMT os Si-PMT readout has. The 511 gamma of the source will be used

as a marker of the 511 keV gamma-ray in the opposite direction (and of the eventual 1275 keV ray)

1.7.1.3.1.6 Thermal Ranges

Qualified thermal range (TBC)

Lower Limit (ºC) Upper Limit (ºC) Non-operative -15 +35 Operative -10 +30

Table 8. Thermal range

Operational requirement (TBC)

Maximum Operational Thermal Gradient

0.2oC

Maximum Thermal time gradient 0.5°C/hour

Table 9. Operational requirement

1.7.1.4 PAYLOAD DATA HANDLING

1.7.1.4.1 The MIRAX P/L configuration

The instrument is made of the following main parts:

� All Sky Monitor (ASM) composed of 6 scientific S/Ss based on Silicon Drift Detectors (SDD), colimators and coded masks.

� Soft Gamma-rays Spectrometer (SGS) composed of 8 scientific S/Ss based on phoswich type detectors and collimators.

� Back-End Electronics (BEE) to provide the interface electronics between the scientific S/Ss and the DH and the PSU.

• Data Handling (DH) to monitor and control the instrument and to process science data.

• Power Supply Unit (PSU) to filter, regulate and distribute the power supplies needed by the DH, the BEEs, the detector assemblies and the PSU itself.

1.7.1.4.2 DH general specifications

The DH shall be in charge of managing the following interfaces:




• one command bidirectional serial interface with each BEE to:

• configure the corresponding Back-End and Front-End unit;

• manage the digital HK periodic acquisition (already conditioned and converted inside the BEEs)

• read back the configuration parameters

• one high speed serial interface (~10Mbps) with each BEE to retrieve science data

• manage a dedicated interrupt line to trigger the science data retrieval of the already pre-processed data from the BEE;

• Manage and filter all the regulated secondary voltages internally needed by the unit and provided by the PSU;

• manage a high speed data interface with the spacecraft (Spacewire like) . This link is used to get TC from the Spacecraft and to send both Science and HK telemetry packets.

• Manage the Pulse Per Second (PPS) synchronization signal line from the S/C in order to perform the event time-tagging with the required accuracy (2µs at 3sigma);

• Distribuite the PPS signal and the 1MHz clock to the BEEs in order to allow the implementation of the local event time-tagging system.

The DH unit shall be designed to implement three main functions: the science data processing, the data-handling

and the internal power supply.

The Science Data Processing Function is responsible of the following main tasks:

• Interface the BEEs through a dedicated link to receive the scientific data

• Acquire the scientific ratemeters (integrated by the BEEs), generate and send telemetry packets

• Format scientific event-by-event data into telemetry packets and send them to the S/C

• Real-time execution of the foreseen scientific algorithms producing the post-processed data. Generate a stream of post-processed science telemetry to be transferred to the S/C .

The Data Handling Function is responsible of the following tasks:

• Receive the telecommands from S/C.

• Receive the attitude data from the S/C sampled with a frequency higher than 10 Hz and an accuracy better than 0.5 arcmin on each axis

• Generate and send HK telemetry to the S/C.

• Execute the telecommands and generate related telemetries

• Manage the Operative Modes (Boot, Maintenance, Standby, Observation and Test);

• Perform the Instrument Control Function, i.e. the active monitoring of Payload safety critical parameters in order to implement a nearly real-time reaction to avoid damages;

• Perform science data processing and compression.

A third block is responsible of the internal power supply management and filtering.

1.7.1.4.3 Operating modes and mode transitions

DH shall control and manage the operating modes and their transitions according to the following diagram.




Figure 14 Operating modes and transitions

BOOT is the start-up mode at power on. A limited SW application, called Boot Software runs from the PROM

in order to perform all the checks and the initialization of instrument resources and items. Report of its activities

is stored into memory and delivered in telemetry.

MAINTENANCE is reserved by DH to support the in-orbit maintenance program.

This mode can be reached on request when the DH is in BOOT mode and it is dedicated to memory management

operations like code and data load/download in/from EEPROM and RAM.

STANDBY Nominally, at the end of the bootstrap phase, the DH moves in STANDBY mode to start the

instrument monitoring and control. The Application Software runs from the RAM and performs the data

handling function.

This mode is dedicated to:

• power ON the instrument items

• process any incoming S/C TC and generate the related TM

• handle the PPS

• collect, monitor and generate the housekeeping TM

• configure the instrument and the science data processing.

OBSERVATION From the point of view of DH , all instrument calibration and scientific modes are managed

into an unique OBSERVATION mode that shall be preventively configured while in STANDBY mode.

Configurations specify parameters, tables and typology of the new planned observation, that the DH shall

effectively apply entering the OBSERVATION mode so that the instrument can issue its science data and the

DH can start the science data processing.




TEST This mode is introduced to implement a merge of STANDBY and OBSERVATION modes. DH, when in

TEST mode, allows the fully comandability and observability of the instrument in order to speed up activities

and testing procedures.

1.7.1.4.4 Software description

According to the operating modes above described, the software running on the DH shall be composed of the

three components:

� Boot software : it is installed in the PROM and performs start-up operations.

� Maintenance software : it is installed in the PROM and supports the maintenance of the software

and of the instrument configurations.

� Application Software : it refers to the software that operates the Payload.

It implements both the data-handling functions and the scientific data processing.

1.7.1.4.5 Scientific data processing

The scientific data processing is structured on observation scientific tasks and calibration scientific tasks.

1.7.1.4.5.1 Observation Scientific Tasks

The observation scientific tasks are all related to the Prompt Acquisition of GRBs and consist of the following

main tasks.

1.7.1.4.5.1.1 Burst triggering

The burst trigger tasks is a continuous check on whether the burst trigger criteria are satisfied. It is based on the

comparison between foreground and background counts, independently for ASM and SGS. The trigger check is

performed on different timescales and segmenting the two experiments in geometrical part and energy ranges.

The ASM will be divided as follows (TBC):

- 4 energy ranges - 16 independent detector parts

The trigger condition will be checked over any combination of the above and on timescales of 1, 4, 16, 64, 256,

1024 ms and 4, 16, 64 and 128 s, to be compared to background rates on several options of timescales (e.g.,

from seconds to several minutes). The resulting burst search will be performed in parallel on all timescales,

shifting the reference time by ¼ of the checked timescale. A logic will combine the individual triggers to

generate a formal ASM burst trigger.

The SGS will be divided as follows (TBC):

- 4 energy ranges - 8 independent detector parts

The trigger condition will be checked over any combination of the above and on timescales of 1, 4, 16, 64, 256,

1024 ms and 4, 16, 64 and 128 s, to be compared to background rates on several options of timescales (e.g.,

from seconds to several minutes). The resulting burst search will be performed in parallel on all timescales,

shifting the reference time by ¼ of the checked timescale. A logic will combine the individual triggers to

generate a formal SGS burst trigger.

1.7.1.4.5.1.2 Burst data acquisition




Event by event data for the SGS is stored in a cyclic buffer. A formal ASM or SGS Burst Trigger starts the

collection of the burst SGS event-by-event data to be transmitted on ground. A logic for the determination of the

duration of the data download is TBD.

1.7.1.4.5.1.3 Burst images acquisition and processing

At any time the DH will integrate attitude-corrected cyclic images for the ASM units. The integration time shall

be programmable between 1 and 256 s. A formal ASM trigger will start the onboard deconvolution of the ASM

images and on-board Burst coordinates determination, through comparison between backgrounds and foreground

images.

1.7.1.4.5.2 Calibration Scientific Tasks:

1.7.1.4.5.2.1 Pedestal calculation

This processing is aimed at computing for each ASM channel the mean and rms values of the incoherent noise.

This task is performed processing the Pedestal data acquired by the ASM BEBs.

1.7.1.4.5.2.2 FEE Electrical Calibrations

The FEE electrical calibration are aimed to establish the detector health status and to measure the gains of the

different electrical data acquisition chains.

1.7.1.4.6 On-board Time management

To guarantee the precise time tagging in every condition, we assume that the S/C will be able to provide a Pulse

Per Second (PPS) with a very high accuracy determined by a GPS receiver, to synchronize the DH on-board time

counter every second. The PPS signal edge shall have the required accuracy (i.e. less than 2µs at 3sigma).

Furthermore, the DH shall be endowed with an on-board stable Cristal Oscillator (XO) with a stability of few

ppm (ppm= part per million) on the entire temperature drifts range and aging effects included.

The above approach does not adjust the XO frequency, but only re-synchronizes the On-Board Time with the UT

and makes possible a measurement of the current frequency drift to be applied on-ground, during post processing

to correct the event time-tags.

The presented solution, with respect to a very accurate on-board clock (e.g. a GPS receiver dedicated to the

payload), has for the Payload side advantages in terms of cost, mass, space allocation and power consumption.

The OBT is composed of a 28-bits counter for the seconds, a 20-bits counter for the microseconds and a 8-bits

register for the OBT error. All these items are functionally dependent on the PPS signal and on the 1MHz clock

signal provided by the DH.

The seconds counter is incremented by the PPS signal; the microseconds counter is incremented by the 1MHz

clock signal and reset by the PSS signal; the OBT error register is updated, on each PPS signal, with the new

difference (evaluated with sign) between the current microsecond counts and the ideal 1 million counts.

The OBT will be initialized during the boot-strap phase or by TC request sampling the GPS time information

provided by the S/C.

The DH shall distribute the PSS signal and the 1MHz clock to the BEEs in order to allow local OBT

implementation to time-tag the events.




1.7.1.4.7 TM/TC interface

1.7.1.4.7.1 Telecommands

Once the DH is ON, the instrument is operated with commands included in this tentative list:

• Set operative mode

• Scientific observation set-up command to select observation mode either Calibration or Observation Data Processing

• Switch on/off scientific S/Ss

• Observation HW set-up commands

• Calibration set-up command

• Acquisition HW set-up command

• Observation SW set-up command

• Monitoring set-up command

• Memory management commands

1.7.1.4.7.2 Telemetry

The DH generates the telemetry packets included in this preliminary list:

� Housekeeping Report

� Telecommand Verification Reports

� Event Reports: Boot Report , Error Reports, Monitoring exception report

� Memory management reports

� Science Data Reports carrying event-by-event data or post-processed data

1.7.1.4.7.3 Housekeeping Telemetry

Starting from the activation of the Application SW, the DH generates at 16 Hz (TBC) the housekeeping

telemetry to report the health and status of the payload and to allow for a continuous surveillance.

Analog and digital data of currents, temperatures, voltages, ratemeters are collected every second for a maximum

of TBD parameters with 16bits of accuracy.

1.7.1.4.7.4 Science Telemetry

Science telemetry delivers the results of instrument observations.

1.7.1.4.7.4.1 ASM Event-by-event data telemetry

Science raw data telemetry contains in its format all the information provided at DH-BEE interface, without

further processing at DH level. For each ASM event, this telemetry provides information on:

• the address of the central channel of the cluster

• the energy content of each channel into the cluster and the value of the common mode estimate

• the time tag

1.7.1.4.7.4.1 SGS Data collection modes

The SGS transmission mode can be either in event-by-event or in ratemeter mode. The selection between

the two will be performed by telecommand. In the case of event by event transmission, the maximum number of




bits are the following: 10 bits (energy) + 9 bits (pulse shape) + 3 bits (detection unit identifier) + 16 bits (time of

occurrence). Total bit number/event become: 38 bits.

1.7.1.4.8 Telemetry Budget (TBC)

Here the telemetry budget assumes a likely unrealistic case of 128 bit/event and 5000 cts/s/unit. Once the overall

maximum telemetry budget is available, the ASM design must be modified in order to fit into the telemetry

budget. Viable options are:

- Reduction of the area of ASM units - Reduction of the field of view - Reduction of the mask open fraction

ASM Science

Telemetry

SGS Science

(Burst and

Ratemeters)

Telemetry

Housekeeping

and Scientific

Ratemeters

Raw

Telemetry

Budget

Notes

3800 kbit/s 40 kbit/s (TBC) 10 kbit/s 3850 kbits/s The maximum

compression factor is

required

Tab. -10. Telemetry Budget

1.7.1.5 Power Supply Unit (PSU)

1.7.1.5.1 ASM Low Voltage (FEE)

A high-quality low voltage line is reque

mirax payload description and requirement document · 2010. 10. 12. · payload description and...

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