Help please condense
The Euclid mission architecture is strongly driven by the science requirements and programmatic constraints. Major
issues of the sky survey are its speed, depth, precision, and imaging quality while the main programmatic constraint is
the mission duration. The survey speed is guaranteed by the combination of a large field of view, about 0.54 deg2
, and an
optimised survey strategy. Ensuring the high image quality leads to demanding requirements on the pointing and thermoelastic
stability. The survey depth leads to a minimum telescope aperture, dedicated baffling design, low temperature
optics and detectors, a cold telescope for low near-infrared background and on-board data processing for the noise
reduction of the near-infrared detectors.
A large amplitude orbit around the second Sun-Earth Lagrange point (SEL2) has been selected because it imposes
minimum constraints on the observations and allows scanning of the sky outside the galactic latitude b ±30 degree band
around the Milky Way within the mission duration. The Euclid spacecraft will be launched from the Guiana Space
Centre, Kourou, on board a Soyuz ST 2.1-B. The launch date and time determine the ellipticity and size of the
operational orbit and influence the Sun-Spacecraft-Earth angle plus the daily visibility from the ground station. The
launch is possible in most of the days of the year with minor restrictions to avoid eclipses during transfer and in the
operational orbit, and by the angle between the Sun and telescope aperture during transfer. Once in operational orbit the
spacecraft performs a step-and-stare scanning of the sky.
2.1 Survey design
Euclid aims to cover very large areas with great stability, thermal stresses must be minimised and this impacts on
operations. In fact, the allowed pointing range is limited to observe orthogonally to the Sun, in a range of -3º towards to
+10º from the orthogonal. In practice Euclid can scan parts of a circle on the sky along the ecliptic meridian; the
visibility at ecliptic equator is ~ one week per semester (the target can be seen six months later along the same circle).
This visibility period increases with the distance from the equator, up to two small circles at the ecliptic poles, which
have perennial visibility.
The elementary observation sequence of a field is composed of four frames of the 0.54 deg2 common area, observed with
a dither step in-between. During each frame the visual instrument (VIS) and the Near Infrared Spectro-Photometer
(NISP) spectrometer carry out exposures of the sky simultaneously. Subsequently, because of the disturbing vibration
from filter wheel rotation, VIS closes its shutter during the remaining exposures while NISP photometric imaging is
performed. At the end of the last frame, a slew towards the next field is performed. A significant part of the mission is
ECSURV Summary of Survey(s) Status and perspectives
Ref:
Version 1.0
Date 29/03/2016
Page 7/19
3. Wide Survey
Because of the large amount in number, time and repeats of calibrations, the strategy has to fulfil calibrations
first, then use the remaining time to observe the wide areas. At present there is a single standard sequence
for each wide field, as given in the figure (recall there is no longer room for blue grim exposures in the wide).
The wide coverage achieved year by year can be easily seen in the picture below [ECTile, J. Dinis],
where horizontal bars show what is observed in a given year (two bars for each year, since one can observe
either in the leading or trailing direction wrt the orbit). In each bar in the lower panel the smaller dark areas
are the times reserved for calibrations (J. Amiaux, I. Tereno), while the wider ones are color coded with the
corresponding regions of the wide on the sky.
The presented document is Proprietary information of the Euclid Consortium.
This document shall be used and disclosed by the receiving Party and its related entities (e.g. contractors and subcontractors) only for the purposes
of fulfilling the receiving Party's responsibilities under the Euclid Project and that the identified and marked technical data shall not be disclosed or
retransferred to any other entity without prior written permission of the document preparer.
Current&state&(example&survey)&
BeYer&space3;me&coherence&
Much&lower&presence&of&bubbles&
(s;ll&work3in3progress)&
How:&&added&patch&adjacency&to&patch&scoring&
&&&&&&&&&&&backward3;ling&(backward&in&;me)&
&&&&&&&&&&&patch&compression&
Mission Operation Concept
Document part B:
Reference Survey
Ref.
Version:
Date:
Page:
EUCL-EC-RP-8-001
6.8
11/09/2015
48/88
The presented document is Proprietary information of the Euclid Consortium.
This document shall be used and disclosed by the receiving Party and its related entities (e.g. contractors and subcontractors) only for the purposes
of fulfilling the receiving Party's responsibilities under the Euclid Project and that the identified and marked technical data shall not be disclosed or
retransferred to any other entity without prior written permission of the document preparer.
o The Solar Panel Solar Aspect Angle (SPSAA) is defined as the angle between the
spacecraft +XSC axis and the direction to the centre of the solar disk.
o not exceed variations of the Solar Panel Solar Aspect Angle (SPAA) of up to 10 (TBC)
degrees
5.1.1. Elementary Observation Implementation
The elementary observation sequence over a field is composed of four frames observed with a dither
step in between. At the end of the last frame, the FoV slews towards either:
x The next Survey Field of View
x A calibration field (frequency see calibration sequence). The calibration field can be a high
density star field located in the Galactic plane or the current observed field, observed stabilised
with calibration source on for flat field calibration (see VIS and NISP IOCD [AD8] and [AD9]).
The Field of View duration is (without margin on integration time):
• 4 x 973 s + 3 x 60 s + 290 s = 4072 s + 290 s = 4362 s
The exposure time (including read out overheads) in the VIS and NISP are given in VIS and NISP
Performance report ([AD6] and [AD7]) based on current Space Segment definition (see [AD1]):
x VIS exposure time = 565 s
x NISP Spectroscopy exposure time = 565 s
x NISP Photometry exposure time:
o Y = 121 s
o J = 116 s
o H = 81 s
Figure 5-4: Nominal Field Observation Sequence.
To Next Field Dither 01 Dither 02 Dither 03 Dither 04 Δt < 290 s Dither Step 01 Dither Step 02 Dither Step 03 Slew
60 s 60 s 60 s 290 s
VIS
NISP
Shutter
10 s
NISP
565 s
VIS
565 s
Shutter
10 s
FWA
10 s
Y
121 s
FWA
10 s
J
116 s
FWA
10 s
H
81 s
GWA
10 s
Dither = 973 s
Stab
10 s
Stab
10 s
Stab
10 s
Nominal Science Observation Sequence = 4362 s
Figure 1 Euclid standard observing sequence
devoted to instrument calibrations and sample characterisation. The former class needs specific observations, ~6 months
in total, on given targets (spectro-photometric standards, repeated fields for stability and flats). The latter needs repeated
observation for depth and have different dispersion angles for the same objects or observations on well-known
astronomical fields, ~6 months in total. Interleaved with calibrations, much time will be devoted to the Euclid Deep
Fields (EDFs), which will be two magnitudes deeper than the wide survey and cover a minimum total area of 40 deg2
.
Because of the need of repeated observations, long observability plays a key role here and therefore possible locations
are forced to be close to the ecliptic poles.
The wide area has to solve a number of demanding constraints related to the visibility: the interspersed set of
calibrations, the zodiacal background (which increases by factors in going from ecliptic poles to the ecliptic equator and,
moreover, is also time dependent), galactic dust extinction and scattered light. The resulting Euclid sky coverage has to
exclude the ecliptic plane and the galactic plane and bulge.
Even though there are many solutions for the sky coverage, it is not obvious to find a solution satisfying all visibility,
operational, and programmatic constraints. An optimal solution was found for the MPDR by the Euclid Consortium
Survey group using a novel software, ECTile, developed exclusively for this purpose. The algorithm takes into account
all the constraints and optimises possible sequences of pointing’s on the sky. In practice the time reserved for
calibrations is set as an input and the best coverage is found by using the unallocated days to observe the not yet covered
sky, weighted with a merit function.
The solution, shown in Figure 2 with colour coding according to the epochs, is close to the theoretically maximum
achievable (a straight line in the second plot). Towards the end of the survey most of the visible sky has been observed
previously, so that no new areas have very limited visibility, causing only small increases in the growth curve.
Figure 2 Left panel: area covered by the wide survey (ecliptic coordinates, colour coding follows the epoch of observation).
The empty regions reflect the ecliptic equator and the galaxy plane Right panel: growth curve, the increase of the area
covered by the wide survey as a function of time.
It must be recalled that the above reference survey is a proof of feasibility, the final survey will be delivered after launch
and in-orbit performance verification.
3. SPACECRAFT DESIGN
The spacecraft can be subdivided in three main parts: a Service Module, a Payload Module, including the telescope, and
the Scientific Instruments. They are separately described in the following sections.
3.1 Service Module
The Service Module (SVM) comprises the spacecraft subsystems supporting the payload operation, hosts the warm
electronics of the payload, and provides structural interfaces to the Payload Module (PLM) and the launch vehicle. The
Sunshield, part of the SVM, protects the PLM from illumination by the sun and supports the photovoltaic assembly
supplying electrical power to the spacecraft. The overall spacecraft envelope, compatible with the Soyuz ST fairing, fits
within a diameter of 3.74 m and a height of 4.8 m, see Figure 3.
ECSURV Summary of Survey(s) Status and perspectives
Ref:
Version 1.0
Date 29/03/2016
Page 7/19
3. Wide Survey
Because of the large amount in number, time and repeats of calibrations, the strategy has to fulfil calibrations
first, then use the remaining time to observe the wide areas. At present there is a single standard sequence
for each wide field, as given in the figure (recall there is no longer room for blue grim exposures in the wide).
The wide coverage achieved year by year can be easily seen in the picture below [ECTile, J. Dinis],
where horizontal bars show what is observed in a given year (two bars for each year, since one can observe
either in the leading or trailing direction wrt the orbit). In each bar in the lower panel the smaller dark areas
are the times reserved for calibrations (J. Amiaux, I. Tereno), while the wider ones are color coded with the
corresponding regions of the wide on the sky.
The presented document is Proprietary information of the Euclid Consortium.
This document shall be used and disclosed by the receiving Party and its related entities (e.g. contractors and subcontractors) only for the purposes
of fulfilling the receiving Party's responsibilities under the Euclid Project and that the identified and marked technical data shall not be disclosed or
retransferred to any other entity without prior written permission of the document preparer.
Current&state&(example&survey)&
BeYer&space3;me&coherence&
Much&lower&presence&of&bubbles&
(s;ll&work3in3progress)&
How:&&added&patch&adjacency&to&patch&scoring&
&&&&&&&&&&&backward3;ling&(backward&in&;me)&
&&&&&&&&&&&patch&compression&
Mission Operation Concept
Document part B:
Reference Survey
Ref.
Version:
Date:
Page:
EUCL-EC-RP-8-001
6.8
11/09/2015
48/88
The presented document is Proprietary information of the Euclid Consortium.
This document shall be used and disclosed by the receiving Party and its related entities (e.g. contractors and subcontractors) only for the purposes
of fulfilling the receiving Party's responsibilities under the Euclid Project and that the identified and marked technical data shall not be disclosed or
retransferred to any other entity without prior written permission of the document preparer.
o The Solar Panel Solar Aspect Angle (SPSAA) is defined as the angle between the
spacecraft +XSC axis and the direction to the centre of the solar disk.
o not exceed variations of the Solar Panel Solar Aspect Angle (SPAA) of up to 10 (TBC)
degrees
5.1.1. Elementary Observation Implementation
The elementary observation sequence over a field is composed of four frames observed with a dither
step in between. At the end of the last frame, the FoV slews towards either:
x The next Survey Field of View
x A calibration field (frequency see calibration sequence). The calibration field can be a high
density star field located in the Galactic plane or the current observed field, observed stabilised
with calibration source on for flat field calibration (see VIS and NISP IOCD [AD8] and [AD9]).
The Field of View duration is (without margin on integration time):
• 4 x 973 s + 3 x 60 s + 290 s = 4072 s + 290 s = 4362 s
The exposure time (including read out overheads) in the VIS and NISP are given in VIS and NISP
Performance report ([AD6] and [AD7]) based on current Space Segment definition (see [AD1]):
x VIS exposure time = 565 s
x NISP Spectroscopy exposure time = 565 s
x NISP Photometry exposure time:
o Y = 121 s
o J = 116 s
o H = 81 s
Figure 5-4: Nominal Field Observation Sequence.
To Next Field Dither 01 Dither 02 Dither 03 Dither 04 Δt < 290 s Dither Step 01 Dither Step 02 Dither Step 03 Slew
60 s 60 s 60 s 290 s
VIS
NISP
Shutter
10 s
NISP
565 s
VIS
565 s
Shutter
10 s
FWA
10 s
Y
121 s
FWA
10 s
J
116 s
FWA
10 s
H
81 s
GWA
10 s
Dither = 973 s
Stab
10 s
Stab
10 s
Stab
10 s
Nominal Science Observation Sequence = 4362 s
ECSURV Summary of Survey(s) Status and perspectives
Ref:
Version 1.0
Date 29/03/2016
Page 8/19
This is a result of a further optimisation
wrt the reference for mission PDR,
shows from the other figure that with
4400 sec exposures we are able to
cover the wanted area in the wanted
time (all the calibrations and EDFS are
incorporated, as well as the 1 day per
month of orbit maintenance). The lower
curve refers to the non observed fraction
because of lack of visibility of new areas
at that particular epoch (of course one
will use those dead times -if any- wisely,
e.g. reobserve some fields, but at
present these are too variable and
sparse to have now a precise plan for
them).
However, this fantastic result, close
to theoretical achievable maximum, can
only worsen in practice because of a
number of factors. There is a science issue related to the coupling of visibility vs “good” sky areas: because
of the choice of fixed exposures, the S/N will vary quite a bit in the Euclid fields, according to the local
background. This is due to the zodiacal effect (mainly a function of the ecliptic latitude but also time
dependent) but also to the straylight from the plane of the galaxy and other star rich areas. Therefore we can
observe 15000 sq degs but are all of them useful? The answer is no but this has to be quantified better by
the end to end study but it leaves open an interesting possibilities:
WIDE Possibility #1: see if an increase
in overlap fraction between nearby fields
would improve results (e.g. photometric
dispersion) at some cost in covered area/
time.
WIDE Possibility #2: to have a
quantised approach to in part compensate
for the different levels of S/N given by fixed
exposure time and varying backgrounds. I.e.
to divide the whole areas in blocks to be
observed with, say two or three different
approaches in which the whole sequence or
only VIS/NISP spectral exposure times are
varied with respect to a fixed global one. The
figure reflects the goal of a continuous
variation of T_exp so to have S/N~constant,
which is not possible.
A quantised version would still aim to
decrease the strong gradient expected for an
global T_exp but the tradeoff with the
needed increase of calibrations must be
assessed.
WIDE Possibility #3: instead of
observing 15,000 sq degs once, observe
(15-M)x 103 sq degs once and observe twice
(M/2)x 103 sq degs or trice (M/3) x 103 sq
degs so to have two (or three) passes in
selected areas: this would on only give
confidence on the overall results and their
repeatability (repeatability is currently limited
to a few tens of sq degs in the basic
reference) but also increase the S/N in those
areas. This approach therefore can be used
to observe areas which are interesting from a
general point of view because have other
The presented document is Proprietary information of the Euclid Consortium.
This document shall be used and disclosed by the receiving Party and its related entities (e.g. contractors and subcontractors) only for the purposes
of fulfilling the receiving Party's responsibilities under the Euclid Project and that the identified and marked technical data shall not be disclosed or
retransferred to any other entity without prior written permission of the document preparer.
Weirdo W#N
first/second year: go faster,
last: go slower
Low
|β|
fixed
T_exp
variable
T_exp 30
35 32 28 25
29 31 30
High
|β|
tech time same, t_exp varies (but keep still in backgr noise limit)
(
))
))
for fixed S/N:
T_exp ~ Zodi
Not allowed by instrument teams
(calibs, stability, CTI reconstr.)
Low
|β|
std exp
expose 2x, 3x,
Weirdo W#N+2
latest epochs: less area but with better sampling and in selected
areas already observed from the ground (some close to the ecliptic
plane). Better for legacy and other Xchecks
Replace worst 1000 sq deg with 300-500
(double or triple exposures because of
larger backgr. on “good/interesting” areas
(some cost to FoM but better science
overall - ?large slews?)
Current&state&–&performance&plot&
Survey&performance&improved&a&bit&(lower&idle3;me)&
Figure 3 Euclid spacecraft overview
Mechanical and Thermal Architecture
The SVM (Figure 4) is an irregular hexagonal base built around a central cone that provides the interfaces with the
launcher and with the PLM and encloses the Hydr
Figure 4 SVM overview (the central cone aperture is sealed by MLI belonging to the SVM, not shown here
Figure 5 SVM equipment accommodation
The thermal control is based on a passive design using radiators, multilayer insulation (MLI) and heaters operated in
Pulse Width Modulation. The design drivers are the short-term temperature stability of the PLM conductive and radiative
interface under the maximum commanded Solar Aspect Angle (SAA) change, and minimal (<25 mW) heat flux into the
coldest NISP radiator. High performance Kapton MLI is installed on the on SVM top floor, PLM bottom and Sun Shield
rear side to minimise the heat flux and thermal disturbances onto the PLM.
Electrical and Data Handling Architecture
The spacecraft provides 28V regulated power to equipment and instruments electronics through protected lines
individually commandable provided by the on-board power conditioning and distribution unit (PCDU). The PCDU also
provides power to the heaters, to the pyro actuators and controls the charge and discharge of the battery. The battery is
used only during the launch phase and is design to provide up to 419 W of power and a total energy of 300 Whr. In the
other phases of the mission the sun shield three panels provide a power between 2430 and 1780 Watt depending from the
spacecraft orientation and ageing of the panels. See budgets in Table 2.
One centralised on-board computer (Command and Data Management Unit, CDMU) provides spacecraft and AOCS
command, control and data processing. The CDMU is a modular unit including standard core boards plus dedicated I/O
boards to interface AOCS and spacecraft units and devices. The Processor Module is based on a general-purpose space
qualified microprocessor (LEON-FT) with minimum computational power of 40 MIPS and 5 MFLOPS. Two processor
modules are comprised in a single-failure tolerant unit.
The number of scientific exposures and high-resolution images generate a high science data volume and require large on
board memory capable of hosting the 850 Gbit of daily generated. The on-board Mass Memory Unit (MMU) has a
capacity of 4Tbit EoL sufficient to store 72 hrs of scientific data and 20 days of spacecraft housekeeping. The MMU
stores instruments data and housekeeping and other ancillary data in named files organized in a two level folders’
structure.
The commands and telemetries are distributed and collected mostly via two standard Mil-Std-1553 buses, one dedicated
to the spacecraft equipment and another to the instruments and mass memory, although some spacecraft equipment have
dedicated connections. The instruments deliver high volume scientific data via high speed SpaceWire links directly into
the mass memory. The platform bus handles non-packet remote terminals (RT) and the FGS, and is characterised by
cyclic communication frames at 10 Hz, linked to the AOCS control cycle. The transfer layer protocol of the science bus
is based on a cyclical communication frame at 60 Hz, maximising the efficiency of data transfer per communication
frame.
Figure 6 CDMS interfaces
Files stored in the mass memory are downloaded using the standard CCSDS File Delivery Protocol (CFDP) using the
reliable transfer with acknowledges for the downlink and the simple unreliable transfer for uplink. Both the X and K
band communication link can be used for the file transfer. The baseline configuration expect the directives of the CFDP
to be transmitted via X-band to ensure visibility of the file downlink in progress by Ground also in case of adverse
weather conditions, while data are downloaded via K-band to maximise the data rate. Any other combination is however
possible.
Telecommunications
The telecommunications architecture includes two independent sections: an X-band section used for telecommands,
monitoring and ranging and a K-band section dedicated to high rate telemetry (Figure 7).
The X-band section supports uplink of telecommands at two different rates (4 kbit/s and 16 kbit/s), downlink of real time
housekeeping TM at two different information rates (2 kbit/s and 26 kbit/s), and standard ranging. The X-band section
uses two X-band transponders, with receivers operated in hot redundancy and cross-coupled with CDMU TC decoders
and transmitters operated in cold redundancy and cross-coupled with CDMU TM encoders. Three X-band LGA’s with
hemispherical coverage are used. Two of them (LGA-1 and -2), placed on opposite sides of the spacecraft and working
in opposite circular polarisations, provide the omnidirectional coverage. LGA-3, mounted to the HGA support structure
and sharing its pointing mechanism, supports high rate telecommand. The receivers are cross-coupled with LGA-1 or
LGA-3, and LGA-2.
Figure 7 TT&C block diagrams, X-band (top) and K-band (bottom)
The K-Band section supports downlink of recorded science and housekeeping telemetry at two different data rates:
nominal at 73.85 Mbit/s and reduced at 36.92 M
between different sky zones). After each slew manoeuvre the wheels are controlled to slow down until friction stops
them. Keeping the reaction wheels at rest during operation ensures noise-free science exposures by eliminating the
micro-vibration associated to reaction wheel actuation.
The micro-propulsion employed for fine attitude control is based on cold-gas Nitrogen thrusters in a configuration of two
branches with six thrusters each. Four high-pressure tanks provide storage of 70 kg Nitrogen, sufficient for 7 years
operation with nearly 100% margin.
Orbit control and attitude control in non-science modes are actuated by two redundant branches of ten 20N hydrazine
thrusters. In each branch, two thrusters, one on either side of the spacecraft, provide torque-free thrust for the Trajectory
Control Manoeuvres on the way to SEL2, monthly Station Keeping Manoeuvres at SEL2, and disposal at end of life. The
other eight thrusters provide force-free torques for angular momentum and attitude control in non-science modes.
Hydrazine storage is provided by one central tank with 137.5 kg propellant mass capacity with 10% volume margin over
and above the prescribed delta-v margins.
System budgets
The mass budget in Table 1 shows the breakdown at module level based on detailed estimates validated by the
subsystem suppliers. A lightweight adapter of the same design used on Gaia is employed.
Table 1 System mass budget: the SVM and the PLM masses include the instrument units located in each module, reported
below each item
Current mass [kg]
SVM 920.6
Payload warm units 98.0
PLM 848.3
Payload cold units 193.0
Dry mass reserve 113.9
Propellant 199.2
Launch vehicle adapter 78.0
Total launch mass 2160.0
Table 2 System power budget. The sizing case is shown (maximum payload power demand, communications on;
end of life; maximum voltage).
Power [W]
SVM (@ 28 VDC) 774
PLM (@ 28 VDC) 88
Instruments (@ 28 VDC) 392
System losses (3%) 38
System margin (20%) 258
Array power demand (at PCDU input and @
29.5 VDC)
1360
Table 3 RF link budget. Cebreros ground station, 1.77 million km, elevation above horizon 5° (X band) and 20° (K band).
For the LGA items, degrees in parentheses indicate angle from centre of pattern.
Antenna Bit
rate
[kbit/s]
Nom.
Margin
[dB]
X band
TC (90°) LGA1/2 4.0 4.1
TC (5°) LGA3 16.0 7.3
TM (90°) LGA1/2 2.0 5.8
TM (5°) LGA3 26.0 8.7
K band
TM (low rate) HGA 73.85 4.8
TM (high rate) HGA 36.93 7.6
Table 4 Pointing budgets (99.7% confidence level)
Requirement
[arcsec]
Performance
[arcsec]
APE (X/Y axis) 7.5 6.25
APE (Z axis) 22.5 12.27
RPE (X/Y axis) 75⋅10-3 70.5⋅10-3
RPE (Z axis) 1.5 0.275
3.2 Payload Module
The Euclid PLM is designed around a three mirrors anastigmatic Korsch Silicon Carbide (SiC) telescope feeding the two
instruments, VIS and NISP, see schematic in Figure 8. The light separation between the two instruments is performed by
a dichroic plate located at exit pupil of the telescope. The PLM is in charge of providing mechanical and thermal
interfaces to the instruments (radiating areas and heating lines). Whereas NISP is a stand-alone instrument with interface
bipods, VIS is delivered in several separate parts: a focal plane assembly (FPA) connected to proximity electronics,
readout shutter unit and calibration unit, with dedicated mechanical and thermal interfaces with PLM.
The secondary mirror (M2) is mounted on a mechanism (M2M) for 3-DOF adjustment to compensate for launch and
cool-down effects. In addition, the PLM hosts the FGS, used as pointing reference by the AOCS. All these detectors are
mounted on the structure carrying the VIS focal plane, in order to ensure precise co-alignment.
Except proximity electronics of the focal planes and FGS, all electronics are placed on the SVM to minimise thermal
disturbances to the PLM.
issues of the sky survey are its speed, depth, precision, and imaging quality while the main programmatic constraint is
the mission duration. The survey speed is guaranteed by the combination of a large field of view, about 0.54 deg2
, and an
optimised survey strategy. Ensuring the high image quality leads to demanding requirements on the pointing and thermoelastic
stability. The survey depth leads to a minimum telescope aperture, dedicated baffling design, low temperature
optics and detectors, a cold telescope for low near-infrared background and on-board data processing for the noise
reduction of the near-infrared detectors.
A large amplitude orbit around the second Sun-Earth Lagrange point (SEL2) has been selected because it imposes
minimum constraints on the observations and allows scanning of the sky outside the galactic latitude b ±30 degree band
around the Milky Way within the mission duration. The Euclid spacecraft will be launched from the Guiana Space
Centre, Kourou, on board a Soyuz ST 2.1-B. The launch date and time determine the ellipticity and size of the
operational orbit and influence the Sun-Spacecraft-Earth angle plus the daily visibility from the ground station. The
launch is possible in most of the days of the year with minor restrictions to avoid eclipses during transfer and in the
operational orbit, and by the angle between the Sun and telescope aperture during transfer. Once in operational orbit the
spacecraft performs a step-and-stare scanning of the sky.
2.1 Survey design
Euclid aims to cover very large areas with great stability, thermal stresses must be minimised and this impacts on
operations. In fact, the allowed pointing range is limited to observe orthogonally to the Sun, in a range of -3º towards to
+10º from the orthogonal. In practice Euclid can scan parts of a circle on the sky along the ecliptic meridian; the
visibility at ecliptic equator is ~ one week per semester (the target can be seen six months later along the same circle).
This visibility period increases with the distance from the equator, up to two small circles at the ecliptic poles, which
have perennial visibility.
The elementary observation sequence of a field is composed of four frames of the 0.54 deg2 common area, observed with
a dither step in-between. During each frame the visual instrument (VIS) and the Near Infrared Spectro-Photometer
(NISP) spectrometer carry out exposures of the sky simultaneously. Subsequently, because of the disturbing vibration
from filter wheel rotation, VIS closes its shutter during the remaining exposures while NISP photometric imaging is
performed. At the end of the last frame, a slew towards the next field is performed. A significant part of the mission is
ECSURV Summary of Survey(s) Status and perspectives
Ref:
Version 1.0
Date 29/03/2016
Page 7/19
3. Wide Survey
Because of the large amount in number, time and repeats of calibrations, the strategy has to fulfil calibrations
first, then use the remaining time to observe the wide areas. At present there is a single standard sequence
for each wide field, as given in the figure (recall there is no longer room for blue grim exposures in the wide).
The wide coverage achieved year by year can be easily seen in the picture below [ECTile, J. Dinis],
where horizontal bars show what is observed in a given year (two bars for each year, since one can observe
either in the leading or trailing direction wrt the orbit). In each bar in the lower panel the smaller dark areas
are the times reserved for calibrations (J. Amiaux, I. Tereno), while the wider ones are color coded with the
corresponding regions of the wide on the sky.
The presented document is Proprietary information of the Euclid Consortium.
This document shall be used and disclosed by the receiving Party and its related entities (e.g. contractors and subcontractors) only for the purposes
of fulfilling the receiving Party's responsibilities under the Euclid Project and that the identified and marked technical data shall not be disclosed or
retransferred to any other entity without prior written permission of the document preparer.
Current&state&(example&survey)&
BeYer&space3;me&coherence&
Much&lower&presence&of&bubbles&
(s;ll&work3in3progress)&
How:&&added&patch&adjacency&to&patch&scoring&
&&&&&&&&&&&backward3;ling&(backward&in&;me)&
&&&&&&&&&&&patch&compression&
Mission Operation Concept
Document part B:
Reference Survey
Ref.
Version:
Date:
Page:
EUCL-EC-RP-8-001
6.8
11/09/2015
48/88
The presented document is Proprietary information of the Euclid Consortium.
This document shall be used and disclosed by the receiving Party and its related entities (e.g. contractors and subcontractors) only for the purposes
of fulfilling the receiving Party's responsibilities under the Euclid Project and that the identified and marked technical data shall not be disclosed or
retransferred to any other entity without prior written permission of the document preparer.
o The Solar Panel Solar Aspect Angle (SPSAA) is defined as the angle between the
spacecraft +XSC axis and the direction to the centre of the solar disk.
o not exceed variations of the Solar Panel Solar Aspect Angle (SPAA) of up to 10 (TBC)
degrees
5.1.1. Elementary Observation Implementation
The elementary observation sequence over a field is composed of four frames observed with a dither
step in between. At the end of the last frame, the FoV slews towards either:
x The next Survey Field of View
x A calibration field (frequency see calibration sequence). The calibration field can be a high
density star field located in the Galactic plane or the current observed field, observed stabilised
with calibration source on for flat field calibration (see VIS and NISP IOCD [AD8] and [AD9]).
The Field of View duration is (without margin on integration time):
• 4 x 973 s + 3 x 60 s + 290 s = 4072 s + 290 s = 4362 s
The exposure time (including read out overheads) in the VIS and NISP are given in VIS and NISP
Performance report ([AD6] and [AD7]) based on current Space Segment definition (see [AD1]):
x VIS exposure time = 565 s
x NISP Spectroscopy exposure time = 565 s
x NISP Photometry exposure time:
o Y = 121 s
o J = 116 s
o H = 81 s
Figure 5-4: Nominal Field Observation Sequence.
To Next Field Dither 01 Dither 02 Dither 03 Dither 04 Δt < 290 s Dither Step 01 Dither Step 02 Dither Step 03 Slew
60 s 60 s 60 s 290 s
VIS
NISP
Shutter
10 s
NISP
565 s
VIS
565 s
Shutter
10 s
FWA
10 s
Y
121 s
FWA
10 s
J
116 s
FWA
10 s
H
81 s
GWA
10 s
Dither = 973 s
Stab
10 s
Stab
10 s
Stab
10 s
Nominal Science Observation Sequence = 4362 s
Figure 1 Euclid standard observing sequence
devoted to instrument calibrations and sample characterisation. The former class needs specific observations, ~6 months
in total, on given targets (spectro-photometric standards, repeated fields for stability and flats). The latter needs repeated
observation for depth and have different dispersion angles for the same objects or observations on well-known
astronomical fields, ~6 months in total. Interleaved with calibrations, much time will be devoted to the Euclid Deep
Fields (EDFs), which will be two magnitudes deeper than the wide survey and cover a minimum total area of 40 deg2
.
Because of the need of repeated observations, long observability plays a key role here and therefore possible locations
are forced to be close to the ecliptic poles.
The wide area has to solve a number of demanding constraints related to the visibility: the interspersed set of
calibrations, the zodiacal background (which increases by factors in going from ecliptic poles to the ecliptic equator and,
moreover, is also time dependent), galactic dust extinction and scattered light. The resulting Euclid sky coverage has to
exclude the ecliptic plane and the galactic plane and bulge.
Even though there are many solutions for the sky coverage, it is not obvious to find a solution satisfying all visibility,
operational, and programmatic constraints. An optimal solution was found for the MPDR by the Euclid Consortium
Survey group using a novel software, ECTile, developed exclusively for this purpose. The algorithm takes into account
all the constraints and optimises possible sequences of pointing’s on the sky. In practice the time reserved for
calibrations is set as an input and the best coverage is found by using the unallocated days to observe the not yet covered
sky, weighted with a merit function.
The solution, shown in Figure 2 with colour coding according to the epochs, is close to the theoretically maximum
achievable (a straight line in the second plot). Towards the end of the survey most of the visible sky has been observed
previously, so that no new areas have very limited visibility, causing only small increases in the growth curve.
Figure 2 Left panel: area covered by the wide survey (ecliptic coordinates, colour coding follows the epoch of observation).
The empty regions reflect the ecliptic equator and the galaxy plane Right panel: growth curve, the increase of the area
covered by the wide survey as a function of time.
It must be recalled that the above reference survey is a proof of feasibility, the final survey will be delivered after launch
and in-orbit performance verification.
3. SPACECRAFT DESIGN
The spacecraft can be subdivided in three main parts: a Service Module, a Payload Module, including the telescope, and
the Scientific Instruments. They are separately described in the following sections.
3.1 Service Module
The Service Module (SVM) comprises the spacecraft subsystems supporting the payload operation, hosts the warm
electronics of the payload, and provides structural interfaces to the Payload Module (PLM) and the launch vehicle. The
Sunshield, part of the SVM, protects the PLM from illumination by the sun and supports the photovoltaic assembly
supplying electrical power to the spacecraft. The overall spacecraft envelope, compatible with the Soyuz ST fairing, fits
within a diameter of 3.74 m and a height of 4.8 m, see Figure 3.
ECSURV Summary of Survey(s) Status and perspectives
Ref:
Version 1.0
Date 29/03/2016
Page 7/19
3. Wide Survey
Because of the large amount in number, time and repeats of calibrations, the strategy has to fulfil calibrations
first, then use the remaining time to observe the wide areas. At present there is a single standard sequence
for each wide field, as given in the figure (recall there is no longer room for blue grim exposures in the wide).
The wide coverage achieved year by year can be easily seen in the picture below [ECTile, J. Dinis],
where horizontal bars show what is observed in a given year (two bars for each year, since one can observe
either in the leading or trailing direction wrt the orbit). In each bar in the lower panel the smaller dark areas
are the times reserved for calibrations (J. Amiaux, I. Tereno), while the wider ones are color coded with the
corresponding regions of the wide on the sky.
The presented document is Proprietary information of the Euclid Consortium.
This document shall be used and disclosed by the receiving Party and its related entities (e.g. contractors and subcontractors) only for the purposes
of fulfilling the receiving Party's responsibilities under the Euclid Project and that the identified and marked technical data shall not be disclosed or
retransferred to any other entity without prior written permission of the document preparer.
Current&state&(example&survey)&
BeYer&space3;me&coherence&
Much&lower&presence&of&bubbles&
(s;ll&work3in3progress)&
How:&&added&patch&adjacency&to&patch&scoring&
&&&&&&&&&&&backward3;ling&(backward&in&;me)&
&&&&&&&&&&&patch&compression&
Mission Operation Concept
Document part B:
Reference Survey
Ref.
Version:
Date:
Page:
EUCL-EC-RP-8-001
6.8
11/09/2015
48/88
The presented document is Proprietary information of the Euclid Consortium.
This document shall be used and disclosed by the receiving Party and its related entities (e.g. contractors and subcontractors) only for the purposes
of fulfilling the receiving Party's responsibilities under the Euclid Project and that the identified and marked technical data shall not be disclosed or
retransferred to any other entity without prior written permission of the document preparer.
o The Solar Panel Solar Aspect Angle (SPSAA) is defined as the angle between the
spacecraft +XSC axis and the direction to the centre of the solar disk.
o not exceed variations of the Solar Panel Solar Aspect Angle (SPAA) of up to 10 (TBC)
degrees
5.1.1. Elementary Observation Implementation
The elementary observation sequence over a field is composed of four frames observed with a dither
step in between. At the end of the last frame, the FoV slews towards either:
x The next Survey Field of View
x A calibration field (frequency see calibration sequence). The calibration field can be a high
density star field located in the Galactic plane or the current observed field, observed stabilised
with calibration source on for flat field calibration (see VIS and NISP IOCD [AD8] and [AD9]).
The Field of View duration is (without margin on integration time):
• 4 x 973 s + 3 x 60 s + 290 s = 4072 s + 290 s = 4362 s
The exposure time (including read out overheads) in the VIS and NISP are given in VIS and NISP
Performance report ([AD6] and [AD7]) based on current Space Segment definition (see [AD1]):
x VIS exposure time = 565 s
x NISP Spectroscopy exposure time = 565 s
x NISP Photometry exposure time:
o Y = 121 s
o J = 116 s
o H = 81 s
Figure 5-4: Nominal Field Observation Sequence.
To Next Field Dither 01 Dither 02 Dither 03 Dither 04 Δt < 290 s Dither Step 01 Dither Step 02 Dither Step 03 Slew
60 s 60 s 60 s 290 s
VIS
NISP
Shutter
10 s
NISP
565 s
VIS
565 s
Shutter
10 s
FWA
10 s
Y
121 s
FWA
10 s
J
116 s
FWA
10 s
H
81 s
GWA
10 s
Dither = 973 s
Stab
10 s
Stab
10 s
Stab
10 s
Nominal Science Observation Sequence = 4362 s
ECSURV Summary of Survey(s) Status and perspectives
Ref:
Version 1.0
Date 29/03/2016
Page 8/19
This is a result of a further optimisation
wrt the reference for mission PDR,
shows from the other figure that with
4400 sec exposures we are able to
cover the wanted area in the wanted
time (all the calibrations and EDFS are
incorporated, as well as the 1 day per
month of orbit maintenance). The lower
curve refers to the non observed fraction
because of lack of visibility of new areas
at that particular epoch (of course one
will use those dead times -if any- wisely,
e.g. reobserve some fields, but at
present these are too variable and
sparse to have now a precise plan for
them).
However, this fantastic result, close
to theoretical achievable maximum, can
only worsen in practice because of a
number of factors. There is a science issue related to the coupling of visibility vs “good” sky areas: because
of the choice of fixed exposures, the S/N will vary quite a bit in the Euclid fields, according to the local
background. This is due to the zodiacal effect (mainly a function of the ecliptic latitude but also time
dependent) but also to the straylight from the plane of the galaxy and other star rich areas. Therefore we can
observe 15000 sq degs but are all of them useful? The answer is no but this has to be quantified better by
the end to end study but it leaves open an interesting possibilities:
WIDE Possibility #1: see if an increase
in overlap fraction between nearby fields
would improve results (e.g. photometric
dispersion) at some cost in covered area/
time.
WIDE Possibility #2: to have a
quantised approach to in part compensate
for the different levels of S/N given by fixed
exposure time and varying backgrounds. I.e.
to divide the whole areas in blocks to be
observed with, say two or three different
approaches in which the whole sequence or
only VIS/NISP spectral exposure times are
varied with respect to a fixed global one. The
figure reflects the goal of a continuous
variation of T_exp so to have S/N~constant,
which is not possible.
A quantised version would still aim to
decrease the strong gradient expected for an
global T_exp but the tradeoff with the
needed increase of calibrations must be
assessed.
WIDE Possibility #3: instead of
observing 15,000 sq degs once, observe
(15-M)x 103 sq degs once and observe twice
(M/2)x 103 sq degs or trice (M/3) x 103 sq
degs so to have two (or three) passes in
selected areas: this would on only give
confidence on the overall results and their
repeatability (repeatability is currently limited
to a few tens of sq degs in the basic
reference) but also increase the S/N in those
areas. This approach therefore can be used
to observe areas which are interesting from a
general point of view because have other
The presented document is Proprietary information of the Euclid Consortium.
This document shall be used and disclosed by the receiving Party and its related entities (e.g. contractors and subcontractors) only for the purposes
of fulfilling the receiving Party's responsibilities under the Euclid Project and that the identified and marked technical data shall not be disclosed or
retransferred to any other entity without prior written permission of the document preparer.
Weirdo W#N
first/second year: go faster,
last: go slower
Low
|β|
fixed
T_exp
variable
T_exp 30
35 32 28 25
29 31 30
High
|β|
tech time same, t_exp varies (but keep still in backgr noise limit)
(
))
))
for fixed S/N:
T_exp ~ Zodi
Not allowed by instrument teams
(calibs, stability, CTI reconstr.)
Low
|β|
std exp
expose 2x, 3x,
Weirdo W#N+2
latest epochs: less area but with better sampling and in selected
areas already observed from the ground (some close to the ecliptic
plane). Better for legacy and other Xchecks
Replace worst 1000 sq deg with 300-500
(double or triple exposures because of
larger backgr. on “good/interesting” areas
(some cost to FoM but better science
overall - ?large slews?)
Current&state&–&performance&plot&
Survey&performance&improved&a&bit&(lower&idle3;me)&
Figure 3 Euclid spacecraft overview
Mechanical and Thermal Architecture
The SVM (Figure 4) is an irregular hexagonal base built around a central cone that provides the interfaces with the
launcher and with the PLM and encloses the Hydr
Figure 4 SVM overview (the central cone aperture is sealed by MLI belonging to the SVM, not shown here
Figure 5 SVM equipment accommodation
The thermal control is based on a passive design using radiators, multilayer insulation (MLI) and heaters operated in
Pulse Width Modulation. The design drivers are the short-term temperature stability of the PLM conductive and radiative
interface under the maximum commanded Solar Aspect Angle (SAA) change, and minimal (<25 mW) heat flux into the
coldest NISP radiator. High performance Kapton MLI is installed on the on SVM top floor, PLM bottom and Sun Shield
rear side to minimise the heat flux and thermal disturbances onto the PLM.
Electrical and Data Handling Architecture
The spacecraft provides 28V regulated power to equipment and instruments electronics through protected lines
individually commandable provided by the on-board power conditioning and distribution unit (PCDU). The PCDU also
provides power to the heaters, to the pyro actuators and controls the charge and discharge of the battery. The battery is
used only during the launch phase and is design to provide up to 419 W of power and a total energy of 300 Whr. In the
other phases of the mission the sun shield three panels provide a power between 2430 and 1780 Watt depending from the
spacecraft orientation and ageing of the panels. See budgets in Table 2.
One centralised on-board computer (Command and Data Management Unit, CDMU) provides spacecraft and AOCS
command, control and data processing. The CDMU is a modular unit including standard core boards plus dedicated I/O
boards to interface AOCS and spacecraft units and devices. The Processor Module is based on a general-purpose space
qualified microprocessor (LEON-FT) with minimum computational power of 40 MIPS and 5 MFLOPS. Two processor
modules are comprised in a single-failure tolerant unit.
The number of scientific exposures and high-resolution images generate a high science data volume and require large on
board memory capable of hosting the 850 Gbit of daily generated. The on-board Mass Memory Unit (MMU) has a
capacity of 4Tbit EoL sufficient to store 72 hrs of scientific data and 20 days of spacecraft housekeeping. The MMU
stores instruments data and housekeeping and other ancillary data in named files organized in a two level folders’
structure.
The commands and telemetries are distributed and collected mostly via two standard Mil-Std-1553 buses, one dedicated
to the spacecraft equipment and another to the instruments and mass memory, although some spacecraft equipment have
dedicated connections. The instruments deliver high volume scientific data via high speed SpaceWire links directly into
the mass memory. The platform bus handles non-packet remote terminals (RT) and the FGS, and is characterised by
cyclic communication frames at 10 Hz, linked to the AOCS control cycle. The transfer layer protocol of the science bus
is based on a cyclical communication frame at 60 Hz, maximising the efficiency of data transfer per communication
frame.
Figure 6 CDMS interfaces
Files stored in the mass memory are downloaded using the standard CCSDS File Delivery Protocol (CFDP) using the
reliable transfer with acknowledges for the downlink and the simple unreliable transfer for uplink. Both the X and K
band communication link can be used for the file transfer. The baseline configuration expect the directives of the CFDP
to be transmitted via X-band to ensure visibility of the file downlink in progress by Ground also in case of adverse
weather conditions, while data are downloaded via K-band to maximise the data rate. Any other combination is however
possible.
Telecommunications
The telecommunications architecture includes two independent sections: an X-band section used for telecommands,
monitoring and ranging and a K-band section dedicated to high rate telemetry (Figure 7).
The X-band section supports uplink of telecommands at two different rates (4 kbit/s and 16 kbit/s), downlink of real time
housekeeping TM at two different information rates (2 kbit/s and 26 kbit/s), and standard ranging. The X-band section
uses two X-band transponders, with receivers operated in hot redundancy and cross-coupled with CDMU TC decoders
and transmitters operated in cold redundancy and cross-coupled with CDMU TM encoders. Three X-band LGA’s with
hemispherical coverage are used. Two of them (LGA-1 and -2), placed on opposite sides of the spacecraft and working
in opposite circular polarisations, provide the omnidirectional coverage. LGA-3, mounted to the HGA support structure
and sharing its pointing mechanism, supports high rate telecommand. The receivers are cross-coupled with LGA-1 or
LGA-3, and LGA-2.
Figure 7 TT&C block diagrams, X-band (top) and K-band (bottom)
The K-Band section supports downlink of recorded science and housekeeping telemetry at two different data rates:
nominal at 73.85 Mbit/s and reduced at 36.92 M
between different sky zones). After each slew manoeuvre the wheels are controlled to slow down until friction stops
them. Keeping the reaction wheels at rest during operation ensures noise-free science exposures by eliminating the
micro-vibration associated to reaction wheel actuation.
The micro-propulsion employed for fine attitude control is based on cold-gas Nitrogen thrusters in a configuration of two
branches with six thrusters each. Four high-pressure tanks provide storage of 70 kg Nitrogen, sufficient for 7 years
operation with nearly 100% margin.
Orbit control and attitude control in non-science modes are actuated by two redundant branches of ten 20N hydrazine
thrusters. In each branch, two thrusters, one on either side of the spacecraft, provide torque-free thrust for the Trajectory
Control Manoeuvres on the way to SEL2, monthly Station Keeping Manoeuvres at SEL2, and disposal at end of life. The
other eight thrusters provide force-free torques for angular momentum and attitude control in non-science modes.
Hydrazine storage is provided by one central tank with 137.5 kg propellant mass capacity with 10% volume margin over
and above the prescribed delta-v margins.
System budgets
The mass budget in Table 1 shows the breakdown at module level based on detailed estimates validated by the
subsystem suppliers. A lightweight adapter of the same design used on Gaia is employed.
Table 1 System mass budget: the SVM and the PLM masses include the instrument units located in each module, reported
below each item
Current mass [kg]
SVM 920.6
Payload warm units 98.0
PLM 848.3
Payload cold units 193.0
Dry mass reserve 113.9
Propellant 199.2
Launch vehicle adapter 78.0
Total launch mass 2160.0
Table 2 System power budget. The sizing case is shown (maximum payload power demand, communications on;
end of life; maximum voltage).
Power [W]
SVM (@ 28 VDC) 774
PLM (@ 28 VDC) 88
Instruments (@ 28 VDC) 392
System losses (3%) 38
System margin (20%) 258
Array power demand (at PCDU input and @
29.5 VDC)
1360
Table 3 RF link budget. Cebreros ground station, 1.77 million km, elevation above horizon 5° (X band) and 20° (K band).
For the LGA items, degrees in parentheses indicate angle from centre of pattern.
Antenna Bit
rate
[kbit/s]
Nom.
Margin
[dB]
X band
TC (90°) LGA1/2 4.0 4.1
TC (5°) LGA3 16.0 7.3
TM (90°) LGA1/2 2.0 5.8
TM (5°) LGA3 26.0 8.7
K band
TM (low rate) HGA 73.85 4.8
TM (high rate) HGA 36.93 7.6
Table 4 Pointing budgets (99.7% confidence level)
Requirement
[arcsec]
Performance
[arcsec]
APE (X/Y axis) 7.5 6.25
APE (Z axis) 22.5 12.27
RPE (X/Y axis) 75⋅10-3 70.5⋅10-3
RPE (Z axis) 1.5 0.275
3.2 Payload Module
The Euclid PLM is designed around a three mirrors anastigmatic Korsch Silicon Carbide (SiC) telescope feeding the two
instruments, VIS and NISP, see schematic in Figure 8. The light separation between the two instruments is performed by
a dichroic plate located at exit pupil of the telescope. The PLM is in charge of providing mechanical and thermal
interfaces to the instruments (radiating areas and heating lines). Whereas NISP is a stand-alone instrument with interface
bipods, VIS is delivered in several separate parts: a focal plane assembly (FPA) connected to proximity electronics,
readout shutter unit and calibration unit, with dedicated mechanical and thermal interfaces with PLM.
The secondary mirror (M2) is mounted on a mechanism (M2M) for 3-DOF adjustment to compensate for launch and
cool-down effects. In addition, the PLM hosts the FGS, used as pointing reference by the AOCS. All these detectors are
mounted on the structure carrying the VIS focal plane, in order to ensure precise co-alignment.
Except proximity electronics of the focal planes and FGS, all electronics are placed on the SVM to minimise thermal
disturbances to the PLM.
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