Orbital Radar for Callisto shallow Layers Exploration (ORACLE)

Announcement of Opportunity
JUICE
Orbital RAdar for Callisto shallow Layers
Exploration
Submitted by
Jordi Coll Ortega jordicoll.28@gmail.com
Carlo Girardello cargir-7@student.ltu.se
Pau Molas Roca paumolasroca@gmail.com
Muhammad Ansyar Raﬁ Putra m.ansyaraﬁ@gmail.com
Mattia Ricchi mattia.ricchi@gmail.com
Hamad Siddiqi hamadsiddiqi@gmail.com
Elisabeth Werner eliwerner11@gmail.com
Ivan Zankov ivan.zankov@gmail.com
Rymdcampus, 981 28 - Kiruna, Sweden
Institute of Space Physics, IRF
Kiruna, Sweden
19th of December 2017

Contents
1 Experiment Objectives and Description 1
1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Callisto’s icy crust . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Crystal orientation fabric . . . . . . . . . . . . . . . . . . . . 3
1.4 Scientiﬁc objectives and measurement requirements . . . . . . 4
1.5 Proposed investigation in context of other space missions . . 5
2 Instrument Capabilities 6
2.1 Measurement principles . . . . . . . . . . . . . . . . . . . . . 6
2.2 Performance requirements . . . . . . . . . . . . . . . . . . . . 7
2.3 Functionality requirements . . . . . . . . . . . . . . . . . . . . 8
2.4 Sensitivity to environmental parameters . . . . . . . . . . . . 10
3 Mission Operations and Instrument Interface Requirements 12
3.1 Orbit Requirements . . . . . . . . . . . . . . . . . . . . . . . 12
3.2 Accommodation, Attitude and Pointing Requirements . . . . 13
3.3 Flight Operation Concept . . . . . . . . . . . . . . . . . . . . 14
3.3.1 Initialization Mode . . . . . . . . . . . . . . . . . . . . 14
3.3.2 Measurement Mode . . . . . . . . . . . . . . . . . . . 14
3.3.3 Operative Mode . . . . . . . . . . . . . . . . . . . . . 14
3.3.4 Safe Mode . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.3.5 Standby Mode . . . . . . . . . . . . . . . . . . . . . . 14
3.3.6 Oﬀ Mode . . . . . . . . . . . . . . . . . . . . . . . . . 14
4 Technical Description 15
4.1 Overview - Functional Block Diagram . . . . . . . . . . . . . 15
4.2 Antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.2.1 Maturity of the design . . . . . . . . . . . . . . . . . . 16
4.3 Electronics Box . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.4 Ground Processing . . . . . . . . . . . . . . . . . . . . . . . . 18
4.5 Resource Budget . . . . . . . . . . . . . . . . . . . . . . . . . 18
i

4.5.1 Mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.5.2 Power Consumption . . . . . . . . . . . . . . . . . . . 18
4.5.3 Telemetry . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.6 System reliability and redundancy . . . . . . . . . . . . . . . 19
4.7 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5 Data reduction and scientiﬁc analysis plans 20
5.1 Data Processing and Dissemination Requirements . . . . . . . 20
5.2 Error sources . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.3 Minimum data set that meets the science goal . . . . . . . . . 21
6 Scientiﬁc Closure 22
7 Organization and Management Structure 24
Bibliography 26
A Attitude and accommodation 30
B Acronyms 31
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Chapter 1
Experiment Objectives and
Description
1.1 Introduction
The very first time mankind observed Jupiter and its moons was a long time
ago. On a dark and cold night back in January 1610, Galileo Galilei observed
through his telescope that there were three fixed stars close to Jupiter. As
a scientist, he kept on observing these three stars night after night until he
came to the conclusion that there were not fixed stars, they were Jupiter’s
moons. Even if he missed a moon, his discovery changed the path of history.
From then on, the interest for Jupiter and its moons developed expo-
nentially. In the last years a lot of space missions were focused on Jupiter
and its moons: Pioneer 10, Pioneer 11, Voyager 1, Voyager, Ulysses mis-
sions, Galileo, Juno and the almost-ready-to launch JUICE. The discoveries
made by past missions evolved mankind’s concept of life since the presence
of oceans under kilometers of ice could potentially hosts life.
Callisto is the geologically least evolved of the Galilean satellites [Gra+13].
Its surface has suffered a great number of impacts and by landforms that
suggests an high level of erosion and degradation. The internal structure of
Callisto remains a mystery that ORACLE aims to solve in part.
The results from Galileos mission indicate a lack of dynamics in its core
but the induced magnetic field measured suggests the existence of a liquid
sea under the outer icy crust that is produced by the interaction of Jupiters
magnetic field with it [Khu+98]. Different from its neighbour Ganymede,
Callisto does not show any sign of recent tectonism activity. In fact, on
Callisto, there are no mountains or other visible effects that can be related
to plate tectonic activity. This happened in early times due to a process
called sublimation degradation [Gra+13] that caused the rapid degradation
of every high terrain leaving nowadays only the craters formed by impacts.
Callisto has a really thin atmosphere which is mainly composed by CO2
1

that has been discovered thanks to the measurements made during the
Galileos mission. An interesting aspect of this atmosphere is its pressure
and its density. Pressure and density values should let the moon lose its at-
mosphere in a very short time but thanks to the slow sublimation of carbon
dioxide ice from the outer icy shell, a complete atmospheric loss is avoided
[Gra+13].
The mission duration on Callisto is estimated to be the one used for the
JUICE mission timeline which is presented in Figure 1.1.
Figure 1.1: JUICE mission timeline [Gra+13].
1.2 Callisto’s icy crust
Just like drilling ice cores can tell us something about climate change and
the past climate on Earth, studying the internal structure of icy moons in
the Jovian system can tell us much about the past and the evolution of
this remarkably different part of the solar system [Niend]. These studies
lead the way for comparative planetary research, the findings of which can
be extrapolated to the study of exoplanetary systems and the conditions of
life.
Ice have the quality of being, to a large extent, transparent to radio
waves in the MHz range [Pet+15]. When a planar radio wave hits a struc-
ture or an interface with differing dielectric permittivity, the reflected pulse
is offset to its initial state and may be subject to a loss in signal strength
[Bru+15]. Radio waves incident on individual objects of a length scale equal
to or larger than the wavelength utilized by the radar is also subject to re-
flection. Such subsurface scatterers might include metallic contaminants
in the ice or small water pockets, and are identified by isolated or diffuse
pulses [Bru+15]. A definitive solid/liquid interface would give an indis-
putably strong radar return [Bru+15] due to the high absorbance of water
to radio emission [Pet+15]. However, this is not an expected return within
the framework of this mission, as the likely depth of Callisto’s subsurface
ocean (∼ 100 km) lies much farther down than the expected maximum reach
of the radar (∼ 15 m [Heg+17]). The temperature, porosity, salinity and
impurity profile is what mainly determines the extent of radar attenuation
in the ice [Heg+17; BS11].
2

It has previously been implied that the ice-I 1 layer of Callisto is com-
pletely stable against convections, and has been so for its entire geological
lifetime [Rui01], a result which was later applied to mid-sized icy moons
and large Kuiper belt objects as well [McK02]. Convection in ice-I shells
was thus considered as the exception rather than the norm, and would only
occur if the viscosity of the ice had somehow been lowered. This indeed
supports the observed lack of geological activity today, but the mathemati-
cal foundation upon which this argument rests upon is erroneous [McK06].
Evidently, Newtonian processes should not be neglected, specifically creep
by volume diffusion in fine-grained ice (∼ 1 mm) under low stress [McK06;
BP05; BM07]. Such diffusion can lead to instabilities that may supply a
convection dynamo [BPZ04; BP05]. Considering the conditions at Callisto,
Mckinnon (2006) was able to show that such solid-state convection must
be occurring in the ice I-sheet, or has so for most of its geological lifetime
[McK02]. This result is consistent with an ice I-layer thickness of ∼ 100 km,
which lies within the limits of previous estimates [Sch02; ZKK00; Khu+98].
1.3 Crystal orientation fabric
The anisotropy of a sheet of polycrystalline ice depends on the preferred
orientation of the individual crystals. A single crystal is less susceptible
to deformation when stress is applied in a plane parallel to the symmetry
(c-)axis of the crystal [DAA83]. Hence, if the majority of the crystals that
make up the sheet have the same c-axis orientation, the crystal orientation
fabric (COF) is anisotropic and stronger than if the c-axis orientations had
been completely random (isotropic COF) [Gus+12]. As ice is subject to
strain such anisotropy tends to develop, which is why COF observed in the
ice at the Earth poles tends to grow more anisotropic with larger depth
[Wan+03; Wan+02].
Radio-echo reflections from internal layers within the ice on Earth have
given rise to much debate regarding its physical cause [Clo77]. Layers with
differing impurity content, density [REB69], conductivity (due to acidity)
[Mil81] or a change of direction in the COF [Wan+08; Har73] are possible
explanations. Density fluctuations is the most likely verdict for internal
reflections detected within the uppermost couple hundred of meters of ice at
Earth [Fuj+99]. The depth of some, deeper internal reflection layers in the
ice on Earth dates back to large volcanic eruptions [Mil81]. The increased
acidity in precipitation after such an event, which can continue for years on
end, is thought to be the cause of this class of internal reflection layers (the
conductivity changes along with the acidity) [Fuj+99]. If detected in the
1
Ice-I represents the very first form of water ice which has not been put under elevated
pressures or low temperatures. Indeed, this type of compound is stable in this form down
to 72 K and/or under a pressure of 210 MPa [YC15].
3

subsurface ice of Callisto, it would imply something about a possible history
of geological activity. This is a more suitable objective for a mission to Io
or Ganymede however, and is thus not the primary scientific objective of
ORACLE. For the chosen target, internal reflections due to differing COF is
the most likely case. The dielectric permittivity parallel and perpendicular
to the c-axis in a COF differs only by about 1% [FMM93], but is capable of
causing reflections up to −50 dB [BS11; FMM93]. It is necessary to utilize
multi-polarization radar sounding to separate this effect from that of density
or conductivity. By comparing the received power for different polarizations
the cause of internal reflection may be effectively constrained. Analogous
subsurface studies at Earth have utilized antennas that may be rotated 90
degrees, so as to produce linear polarization parallel and perpendicular to
the ground track [Dal10; Dal09; Mat+03]. A possible alternative to this
solution is the use of a circularly polarized, pulse-modulated radar, which is
exactly what ORACLE will offer.
Multi-polarization radar sounding will also enable the identification of
the COF type (single-pole or vertical girdle) [Mat+03].
Single-pole COF forms as a result of uniaxial compression due to di-
vergent ice flow, while vertical girdle COF is found in regions where the
ice flow is convergent, giving rise to uniaxial extension [Mat+03; Azu94].
This implies that regions harboring predominantly single-pole or vertical
girdle COF gives valuable information on the flow direction and may offer
definitive proof of past or ongoing convection.
1.4 Scientific objectives and measurement require-
ments
ORACLE will perform an in depth analysis of Callistos subsurface environ-
ment and its role in the Jupiter system. To do so, ORACLE will perform
radar sounding measurements to characterize the shallow layers of Callistos
icy crust. The sounder will characterize the subsurface structure based on
the dielectric permittivity of the ice, and as such may detect compositional
changes as well as water reservoirs and larger structures buried in the ice.
ORACLE will focus on the study of the past (and possibly current) ge-
ological activity of Callisto, through the identification of internal reflections
in the ice due to variations in the crystal orientation fabric (COF). Only the
strongest COF reflections is expected to be detectable with singly polarized
data [BS11], which is why a multi-polarization survey is absolutely key for a
mission aiming to study the subsurface structure of Callisto. This requires
the use of a dipole antenna with circularly polarized radar pulses. A vertical
depth above > 1 km and a resolution of 30 m will be sufficient to reach
the scientific goal [Eis+07; Mat+03].
4

1.5 Proposed investigation in context of other space
missions
ORACLE is not the first mission that has attempted to study the subsurface
of an another solar system body. Mars Express (2003) by ESA and Mars
Reconnaissance Orbiter (2005) by NASA are two missions which shared the
same objective as ORACLE. Together, they provide a strong flight heritage
for ORACLE.
Mars Express (2003) studied the subsurface structure of Mars with the
aid of MARSIS, a subsurface sounding radar altimeter with the goal of
finding frozen water in Mars subsurface, with stronger emphasis on pene-
trating ability than vertical resolution [Sch+99]. SHARAD was a subsurface
sounding radar mounted on the Mars Reconnaissance Orbiter mission, which
shared the same goal as MARSIS, but with a higher resolution at the cost
of a reduced penetrating ability.
Together, these two experiment managed to collect a large amount of
data which ultimately led to the discovery of frozen ice in Mars subsurface
[NAS09; ESA09]. The success of Mars Express and Mars Reconnaissance
ensures that if there exists water ice pockets under Callistos first kilometers
of crust, ORACLE will be able to detect it.
As radar sounding has not been previously performed on other icy plan-
ets/moons, the detection of internal reflections due to COF in the substruc-
ture of Callistos icy crust would mean the first detection of COF on another
celestial body besides our own. On Earth, COF searches at the Earth poles
have been conducted via extensive radar surveys and detailed studies of ice
cores under microscope [Tan+16; Dre+12; Dal10; Dal09; Eis+07; Wan+08;
Mat+03; Fuj+99; FMM93]. The depth at which COF tend to occur, the
approximate resolution needed to detect it and the received power it usually
produces has been effectively constrained from these searches. This lays the
perfect groundwork for detecting COF on Callisto.
5

Chapter 2
Instrument Capabilities
The proposed instrument is a Space-Based Sounding Radar at low frequency.
This kind of instrument, also included in the classification of monostatic
ground-penetrating radar, is an electromagnetic geophysical method used
for subsurface imaging and mapping.
The subsurface sounder system is based on a robust and mature technol-
ogy and it has already been used in some successful space missions. Based
on its relevance and similitudes with ORACLE objectives, it is possible to
stand out two of them:
• MARSIS instrument, in MARS Express ESA mission.
• SHARAD instrument, in MARS Reconnaissance Orbiter NASA mis-
sion.
2.1 Measurement principles
Broadly speaking, a radar sounder is a device that has the capability of
penetrating the volume of a target by transmitting electromagnetic waves
like shown in Figure 2.1. The resulting backscatter indicates variations in the
dielectric constant of the different subsurface layers and allows the analysis
through a few kilometres, mostly depending on the crust properties. As a
result of this process, a two-dimensional profile is generated as the radar is
moved along the ground surface.
In general, penetration depth increases with wavelength and also with
radiated power. Of course, once radar signals penetrate the surface, the
usual volumetric attenuations and reflections occur. Usually, the material
behaves as a low pass filter, which modifies the transmitted waves in ac-
cordance with the electrical properties of the propagating medium. Radar
sounding to appreciable depth is possible only in dry materials such as lu-
nar regolith or in very cold low-loss ice [Sko09], which fits in the science
requirements for the Ice-penetrating radar for JUICE spacecraft payload.
6

The challenge comes when it has to be chosen a frequency and band-
width that provides a proper trade-off between penetration and resolution
requirements, under the fixed constraints of available power and antenna
aperture.
Figure 2.1: EM wave diagram for a sounder radar[Sko09].
2.2 Performance requirements
In terms of performance, the main aspects that have been taking into con-
sideration in order to obtain proper measurements to fulfill the scientific
objectives are the penetration depth, the vertical resolution and the foot-
print of the radar beam over the surface.
Since the study of Crystal Orientation Fabric (COF) is one of the main
objectives, it leads the radar design with a required vertical resolution about
20 m. As it is shown in Table 2.1, where a comparison between past instru-
ments and ORACLE proposal is done, no other parameters are relevant
related with the study of COF. This is because these crystal formations
have only been measured in Earth before, with a ground-based ground-
penetrating radar, which is completely different to the necessary radar to
obtain similar samplings from an orbital altitude.
Table 2.1: Performance requirements comparison between heritage and pro-
posed instrument.
Parameter SHARAD MARSIS COF research JUICE IPR Estimations ORACLE
Penetration Depth 0.3 to 1 km up to 15 km Not relevant 3 to 9 km 3 to 5 km
Vertical resolution 10 to 20 m 50 to 150 m 21 m From 10 m to 1% of the depth ∼15 m
Along-track resolution 0.3 to 1 km 5 km Not relevant 2 km 2 km
Cross-track resolution 5 km 25 km Not relevant 10 km 10 km
7

Regarding to the penetration depth, there is no requirements to observe
COF since they can be found everywhere into the ice. The deeper the radar
can see, the ancient the crystals will be. Then, because of the expected
vertical resolution will be greater than the one used in past experiments, it
can be assumed that ORACLE will be capable to characterize the Crystal
Orientation Fabric in Callisto subsurface.
In relation to the instrument heritage, SHARAD and MARSIS were
mean to operate in a complementary performance. On the one hand, SHARAD
was designed to study the shallow subsurface of Mars up to a few hundred
meters with a very high resolution. On the contrary, MARSIS was designed
to scan the subsurface deeper but with worse resolution. As a result, it is
obvious that ORACLE shall be more like SHARAD in terms of required
performance than MARSIS.
Fundamentally, range resolution is determined by the bandwidth of the
received signal. Whether several features are located in some regions, a large
bandwidth is necessary to identify the different elements and their structure.
[BCH91].
In the end, the performance values are estimations that have been done
as much accurately as possible with the current understanding of Callisto
characteristics. It must be remembered that these estimations can be af-
fected by the unknown composition and shape of the subsurface geology of
the moon, as well as for the JUICE spacecraft operations and payloads. For
instance, the geometrical resolution in the across-track direction depends
on the orbiter altitude. Thus, a constant resolution cannot be expected in
real life since JUICE flybys around Callisto are based on non-circular orbits
around Jupiter [Dou+11].
2.3 Functionality requirements
In Table 2.2 are shown the radar parameters chosen to the operation of ORA-
CLE. It has been designed to fulfill the performance requirements mentioned
in the previous section.
Table 2.2: Radar parameters comparison between heritage and proposed
instrument.
Parameter SHARAD MARSIS JUICE IPR Estimations ORACLE
Central frequency 20 MHz 1.8, 3, 4, 5 MHz 5 to 50 MHz 50 MHz
Bandwidth 10 MHz 1 MHz 10 MHz 10 MHz
Pulse width 300 µs 250 µs - 300 µs
Pulse repetition frequency 200 Hz 130 Hz - 500 MHz
Antenna type dipole dipole dipole dipole
Antenna size 10 m 40 m ∼10 m 4 m
8

Once again, it can be notice that, as a result of the similarities in its pur-
pose, ORACLE is more like SHARAD than MARSIS. The main difference
is in the Pulse Repetition Frequency, which is mainly determined according
to the orbit altitude. Both SHARAD and MARSIS were operative up to
800 ∼ 1000 kilometres of altitude [Sko09]. While ORACLE will operate at
200 kilometres of altitude at minimum. [Dou+11].
The time that it takes for the signal to travel to a target and come
back can be estimated by twice the orbit altitude over the velocity of the
electromagnetic wave. Thus, with higher altitude, more time is required,
which means that the next pulse shall be send later if it is desired to not
overlap it with the backscattered signal from the ground. Therefore, the
Pulse Repetition Frequency will be lower.
In realistic spacecrafts operating at orbital altitudes, it is not feasible
to transmit a simple short pulse that has sufficient energy to generate use-
ful reflections from depth. Since the available power is also limited, it is
commonly used a technique called chirp pulse compression [Seu+03], which
allows a better performance in range capability without losing quality in the
vertical resolution.
For this reason, the relation to estimate the pulse width through the
inverse of the bandwidth [Sko01] is not fulfilled. In this case, pulse compres-
sion waveforms have much greater bandwidth than the reciprocal of their
pulse width.
The antenna type for a Space-Based sounder is always a dipole. Its size
is related to the Central frequency used in such a way that the higher the
frequency is, the smaller the antenna will be.
Finally, a single frequency sounder will be appropriate for JUICE because
it provides a good trade-off between the scientific goals and the complexity
of the system. A dual-frequency system is not recommended since it also
would increase the mass of the instrument [Dou+11].
The choice of the central frequency depends on several factors:
• The required penetration depth. The deeper, the lower the frequency.
• The complexity of the antenna construction and deploying once in the
spacecraft. The smaller the antenna is, the easier to deal with it.
• The radiation noise at Jupiter and other environmental considerations
such as the ice absorption.
9

2.4 Sensitivity to environmental parameters
In case of ground-penetrating radars, the radar signals can be significantly
affected by the propagation characteristics of the medium, which can be
non-isotropic, layered and high dielectric. This turns into the attenuation
responsible of establishing the penetration limit below the surface of the
measurements. Also, the orientation and the shapes of the subsurface layers
play an important role in scattering and suppressing the signal by complex
angles of refraction.
Concerning space-based subsurface sounders the main difficulty to con-
sider in terms of quality of the gathered data is the existence of surface
clutter, which is range-coincident with the desired return from the depth.
Clutter is defined as sources of unwanted reflections that occur within the
effective bandwidth and search window of the radar and present as spatially
coherent reflectors [Heg+17]. To deal with this issue, a proper understand-
ing of its source and impact on the radar is required.
Figure 2.2: Average radar loss profile for Callisto[Heg+17].
Figure 2.2 shows the radar loss estimated in a theoretical model for a
sounder radar operative at a central frequency of 9 MHz for Callisto. For the
modelling, it is considered that Callisto is characterized by a highly rough
surface and by a large concentration of impurities in the outer layers.
Due to the resemblance between the instrument wavelength and the
roughness of the surface, during the first kilometre, the backscattered power
of the loss is not negligible, up to the point that it could strongly affect the
analysis in that range.
Referring to ORACLE performance, it can be deducted that at 50 MHz
the clutter could still affect the scattered signal. However, since the esti-
mated depth of the measurements will reach up to 3 to 5 km, it is expected
10

that the results related with the other ranges will not be affected. Thus, the
current understanding of Callistos subsurface dielectric properties does not
suggest sufficiently strong contrasts to produce unambiguous radar returns.
The other possible main source of difficulties upon radar performance is
the background radiation that could interact with the instrument.
As it can be seen in Figure 2.3, the decametric radiation emitted by
Jupiter is one of the highest signals in the Solar System for frequencies be-
tween 1 to 40 MHz. These signals are produced by a resonance interaction
called the Cyclotron Maser Instability. The emission strength decrease on
the cyclotron frequency for the Jovian magnetic field lines, which corre-
sponds with 40 MHz. From that value, the radiation from the synchrotron
emission of electrons in Jupiters magnetic field lines is significantly weaker
[Rom+14].
Figure 2.3: Average decametric flux density of Jupiters radio emission in
the vicinity of its icy moons[Rom+14].
Due to the large distance from Jupiter, a sounder radar operating around
Callisto should not record critical radiation noise [TS12]. In any case, OR-
ACLE will operate at 50 MHz, thus, above the most affected frequency
range.
In a similar way, the Sun could be another source of background radiation
for ORACLE during the operative phase. Because of that, the Radar pay-
loads in JUICE spacecraft are arranged into an Observation Scenario Mode
characterized to operate during night side[Dou+11]. Therefore, neither the
Sun radiation should affect the backscatter readings.
11

Chapter 3
Mission Operations and
Instrument Interface
Requirements
3.1 Orbit Requirements
The JUICE spacecraft will perform a total of 12 orbit flybys around Callisto
at 200 km of altitude in non-circular orbit around Jupiter with an approx-
imate velocity of 2.1 km/s. These flybys will occur in two different phases
during the Jupiter Tour.
The first phase is expected to have a duration of 11 month, from January
2030 to December 2030, and will lie in a Jupiter equatorial orbit. The
purpose is to insert JUICE spacecraft into Jupiter orbit by a Ganymede
gravity assists and reduce the energy for transfer to Callisto.
On the other hand, the second phase of 260 days, from January 2031
to October 2031, will be used to raise the orbit inclination to 29 degrees
and bring it back to the equatorial plane with Callisto gravity assists. The
spacecraft will be in Callisto resonance and will therefore encounter Callisto
at almost the same positions. After having achieved the target inclination,
this will be reduced with the same strategy.
Due to the required spacecraft manoeuvres optimization, the available
surface coverage will be limited. However, it is considered enough to fulfill
the scientific objectives of the experiment.
12

Figure 3.1: Groundtrack simulation for JUICE Spacecraft flybys around
Callisto.
3.2 Accommodation, Attitude and Pointing Re-
quirements
The major concern in this section is centered on the antenna performance.
ORACLE will consist of a dipole antenna that needs to be placed out-
side the spacecraft. While performing the experiments, it is mandatory to
make the antenna see the ground and the nadir pointing altitude should be
maintained during the experiments. This is because a pure dipole have a
toroidal reception and radiation pattern where the axis of the toroid centers
about the dipole.
In addition, another important consideration is that the best option for
the reduction of the clutter in this kind of radar measurements is to have
the antenna in the cross-track direction. Otherwise, the antenna beamwidth
becomes very broad resulting in degraded spatial resolution.
All these requirements can be summarized like:
• The antenna shall be nadir-looking with a nadir pointing accuracy of
±5◦.
• The antenna shall be parallel to the ground.
• The antenna should be perpendicular to the flight direction.
Moreover, referring to attitude requirements, see Figure A.1 in Appendix
A, it is possible to state that:
• Roll: a deviation from parallelism to the ground shall be < 1◦.
• Yaw: a deviation in perpendicularity to the flight direction should be
< 10◦.
• Pitch: No critical issues identified.
13

3.3 Flight Operation Concept
The pre-selected sequences stored in the memory include several mode changes
required to perform the experiment effectively. The sequence can be started
by a command or another predefined signal, this makes the flight operation
simple and reduces the amount of commands to be sent.
It is important to remark that the IPR instrument will not operate con-
tinuously during the functioning time due to downlink limitations.
3.3.1 Initialization Mode
This is the only state where ORACLE could set fundamental settings and
configurations up. ORACLE will also go to initialization mode after SAFE
mode is turned off to reset all parameters.
3.3.2 Measurement Mode
During the flybys around Callisto, ORACLE will conduct the experiment at
the frequency of 50 MHz.
3.3.3 Operative Mode
Calibration or on-board processing will be conducted in this mode. After
the measurement during the flyby, ORACLE will process the raw data in
this mode.
3.3.4 Safe Mode
In an emergency situation, ORACLE will be put to SAFE mode, where the
instrument will not do any experiment and will not listen to any command
except wake up command.
3.3.5 Standby Mode
During the flight from Earth to Jupiter, ORACLE will be in standby mode.
This is an extremely low power consumption, but ORACLE will still main-
tain its settings and configuration.
3.3.6 Off Mode
In this state, the instrument does not perform any action neither consume
any power.
14

Chapter 4
Technical Description
4.1 Overview - Functional Block Diagram
Figure 4.1: Dipole antenna diagram for attitude and accommodation.
Figure 4.1 shows a general layout of all the elements of ORACLE and
both its inner connections and with the S/C platform.
The setup of ORACLE, which is really close to the one used in SHARAD,
has 2 main components: the antenna (ANT) and the Electronics Box (EB).
The antenna is connected to the Transmitter (TX) and the Receiver (RX),
allocated inside EB. Besides TX and RX, EB also consist of the Digital
Electronic Subsystem (DES), which generates the signal parameters. DES is
connected to On Board Data Handling for TC/TM and Solid State Recorder
for saving the data collected.
15

4.2 Antenna
The main element of ORACLE will be its antenna, which is the essential
element of any radar. A dipole antenna consisting of two ﬁberglass tubes
will be used. Each 4-meter long arm will act as a mechanical support struc-
ture for a metal wire that runs inside them and that represents the actual
active part of the antenna. These tubes are based on the Fiber Foldable
Tube (FFT) technology. They will be folded into four segments each, with-
out using hinges/springs or other mechanical parts for the deployment, but
relying only on the material elasticity to extend into the deployed position
once released. Therefore, during the cruise phase, the antenna will be kept
stowed by two hinges, and enclosed to protect it from the radiation. The
antenna installation on the spacecraft is similar to SHARAD installation.
The dipole is parallel to the y-axis (spacecrafts velocity vector), while the
Callisto surface is in the x-direction.
The render presented, Figure 4.2, is a probable conﬁguration of JUICE
spacecraft: the dipole antenna is placed perpendicularly to the solar arrays.
Figure 4.2: Dipole antenna diagram for attitude and accommodation.
4.2.1 Maturity of the design
The antenna has a Technology Readiness Level (TRL) of 9, since the design
is based on SHARAD antenna that has been successfully operated on MRO
spacecraft. There are 4 models to be built (Software Development Model,
Engineering Model, Structural and Thermal Model, ProtoFlight Model) be-
fore the launch in order to ensure a high reliability antenna performance.
16

4.3 Electronics Box
The Electronic Box (EB) of ORACLE is composed of two main blocks: The
Receiver and Digital Assembly (RDA), including the controller and Digital
Signal Processing (DSP) functions and the chirp generator (included in the
Digital Electronic Subsystem DES), and of the Rx module; The Transmitter
and FrontEnd (TFE) in charge of power amplification, antenna matching,
and Tx/Rx duplexing. RDA and TFE are, physically, two separate boxes.
They are mounted inside the mechanical structure of the EB, which is a
table-shaped structure. Its external sides act as a radiator for the passive
thermal control, with the two boxes mounted upside down on the inner side.
Several heaters, 10, will be included in order to actively thermally control de
instrument. Both RDS and TFE have their internal power converters, and
are powered directly by the spacecraft unregulated 28-V power bus. The
EB itself is located inside the S/C so there is no need to add a radiation
shielding.
Receiver and Transmitter modules
The receiver is in charge of amplifying and filtering the received signal, as
well as to implement the gain control function and to digitalize the signal.
On the other hand, the transmitter takes care of power amplification
and duplexing among other tasks.
Digital Electronic Subsystem
This component is the heart of ORACLE and concentrates many functions
including:
• command and control of the experiment,
• low-power radar pulses generation,
• science data processing and formatting,
• timing.
DES provides Command, Science and Power interface between ORACLE
and the S/C.
As the antenna, the Technology Readiness Level (TRL) of 9 as this module
is also space proven with SHARAD.
17

4.4 Ground Processing
The ground processing has the primary function of compressing the trans-
mitted chirp waveform and to implement the synthetic aperture to improve
the along-track ground resolution. The range compression is performed by
means of an algorithm, called Phase-Gain Algorithm (PGA) [Wah+94], that
adaptively compensates the phase distortions introduced by the hardware
(mainly the matching network). This algorithm is very well known from the
literature and considered reliable and already proven by SHARAD. Con-
cerning the synthetic aperture processing, given the unconventional nadir
looking geometry, three processing algorithms have been analyzed: omega-
k, specan, and chirp scaling or CSA [CW05]. The choice for ORACLE was
based on the following considerations: The range of cell migration is really
significant and requires an accurate correction, for which the CSA algorithm
shows the best performance; The CSA has focused correctly on the targets
in the simulations; The CSA has preserved and correctly compensated the
Doppler effect and then the phase history; The CSA has a greater compu-
tational burden but it is conceptually easy to implement. For all of these
reasons, the final decision has been to implement a CSA adapted to the
original nadir looking geometry of ORACLE.
The technology is adapted from SHARAD on MRO mission. SHARAD has
successfully completed its mission, then this technology is TRL-9.
4.5 Resource Budget
Taking intro account all the components, the mass of the instrument will
stay within the mission requirements. A compact box will be constructed,
the dimensions in Table 4.1 are without considering the antenna.
4.5.1 Mass
Being an adoption of SHARAD, ORACLE has similar mass budget to the
former. The electronic box is slightly lighter because the same performance
can be achieved using lighter components. The mass budget is shown in
Table 4.1.
4.5.2 Power Consumption
The power consumption is 5 W lower than SHARAD. The latter is possi-
ble since each element of ORACLE uses the latest low powered components
which can achieve the same performance as SHARAD. Performance and
18

reliability are not affected. Different power consumption are achieved de-
pending on the operation mode of ORACLE.
4.5.3 Telemetry
The data production rate for SHARAD can vary from as low as 300 kbps
to over 20 Mbps, depending on the pulse repetition frequency, presuming
strategy, and number of bits per sample. The anticipated data return of 10
to 15 Tb is the total data for all Callisto observations.
Table 4.1: Summary of Resource Budget.
Parameter ORACLE
Mass 10.2 kg
Size 37 x 25 x 13 cm
Data Rate 300 kbps
Average Radiated Power 20 W
Standby Power 12 W
Peak Power 20 W
4.6 System reliability and redundancy
Since the majority of components used are based on SHARAD and MAR-
SIS, the instrument ORACLE will have the necessary reliability in order to
accomplish the objectives of the mission during the whole duration of it.
4.7 Calibration
Calibration is a fundamental step in any science instrument in order to
provide valid data [Cro+11]. The essential calibration is end-to-end impulse
response of the system (to derive the reference chirp for range compression).
The calibration is divided into two parts: calibration of the system impulse
response of the electronics in the SEB (including the matching network),
performed on-ground,and in-flight correction of the relevant reference to
account for the antenna response [Wah+94]. For the ground part of the
characterization, transmit chirps have been acquired both at lab ambient
temperature (22◦C ± 2◦C) and at 20◦C steps over the operating temperature
range (in thermal-vacuum conditions, representative of the real operating
environment) and, in the same way, a theoretical chirp has been injected
into the receive chain to characterize its end-to-end response. The two have
been then combined analytically to generate the reference chirp waveform.
[Cro+11]
19

Chapter 5
Data reduction and scientific
analysis plans
The instrument shall interpret the chemical composition for various depths
of the moon to identify landmarks of interest. If any correlations are found
between the sets of geological data obtained from the moons surface, they
will be extrapolated to visualize past interior geological activity.
5.1 Data Processing and Dissemination Require-
ments
D5.2.1 The PI shall include technology for recording, analyzing, and re-
porting data according to the science teams documented request.
D5.2.2 The PI shall possess hardware and software elements of a calibre
necessary for meeting ORACLEs objectives until end of mission.
D5.2.3 The PI shall deliver preliminary instrument data approved by
ORACLEs SWT in accordance to the SWTs specified calibration parameters
given in the collaboration report.
D5.2.4 The PI shall stand by (when inactive) for any incoming requests
from scientific research projects relating to ORACLE and participate by
procuring to them its currently stored data, upon receiving the consent
from the program’ss SWT.
D5.2.5 The PI shall credit ESA where necessary in any and all data
published concerning the ORACLE mission.
D5.2.6 The PI shall gather and process initial data series return refined
data products via downlink for additional analysis and documentation.
20

5.2 Error sources
The optimal conditions for the instrument to take the measurements with
a nadir pointing are set to be with an accuracy of ±5o; which means that
the vertical resolution measurements, that is 10 m, should be within the
proposed accuracy.
The illumination conditions of the moon will not put any constraints on
the observations because of the use of an active remote sensing radar.
5.3 Minimum data set that meets the science goal
Almost all Callisto observations are targeted at most 1000 km and closest
200 km approach altitude. The uplinking-downlinking requirements will be
done daily using one ground station. The collected high-level data will be
provided to the users/community. Alongside the data generation, support
regarding the analysis of the expected results will also be provided. The
expected processed data rate is of about 300 kbps. A single frequency of
50 MHz has been selected with an estimated bandwidth of 10 MHz to map
out the targeted portions of the Callistos surface. The characterization of
the instrument discussed here will provide the minimum data set required
to meet the science goal.
21

Chapter 6
Scientific Closure
Ice penetrating radars (IPRs) has been described as the most promising
technique for the direct detection of solid/liquid interfaces in the subsurface,
and the existence of a subsurface ocean in particular [Rom+14]. This is
however not a feasible objective when considering the dimensions of Callisto,
as opposed to the much thinner ice I-shell of Europa. The objective of
ORACLE is no less interesting however, with the aim to provide shallow
depth (∼ 3 km) high-resolution data of the subsurface structure. With such
science products much can be inferred about the moon’s geological history
and current state.
With ORACLE the subsurface will be imaged in great detail without
actual physical contact between the instrument and the ice (as in the case
of a drill), thus eliminating the risk of biological contamination. The focus
on providing high resolution radargrams at shallow depths in the ice opens
up the possibility of using higher frequencies well away from Jupiter’s deca-
metric radio emission cut-off limit at 40 MHz [Rom+14], which originates
from cyclotron resonance processes in Jupiter’s atmosphere [WL79]. This
source of radio noise is nevertheless confined to a small spatial region in the
sky as seen from Callisto, which means that the depth resolution will not
be source limited [Rom+14].
By operating the radar at 50 Mhz we could attain a resolution of ∼ 10
m across 3 km in vertical depth, while a 30 m resolution is possible across 9
km for a main operating frequency at 9 MHz. This proves that the proposed
instrument is well suited to detect internal reflections due to COF for a main
frequency within the interval ∼ 10 − 50 MHz.
The results from previous radar sounding studies give an indication of
what the expected results from this mission would look like (see Figure 6.1).
It is clear that layers with differing COF are present in the ice, and that a
vertical resolution of 42 m is sufficient to detect it.
22

Figure 6.1: Radar echoes for polarization perpendicular (b) and parallel (c)
to the ground track, at a vertical resolution of 42 m. Layers with diﬀerent
received power Pr for the diﬀerent polarizations are easily seen at a vertical
depth of 800-1000 m and 1200-1400 m. Adapted from [Mat+03].
23

Chapter 7
Organization and
Management Structure
ORACLE will be designed, fabricated, tested and calibrated jointly by the
several institutions involved. IRF will be the leading institution, responsible
for the antenna and electronics design and assembly, and its scientists will
be responsible of analyzing the gathered data.
All the investigators have a long and successful experience in space
projects.
A well established management setup has been chosen since it allows an
accurate and continuous monitoring of the progress.
Figure 7.1: Team schematic configuration.
The key members of this investigation are:
Principal Investigator
The PI will ensure that the proposed investigation is successfully carried
out in accordance within the project schedule and cost. He will supervise all
experiment definition, instrument design and development, and support for
mission operations. He is responsible for supplying the experiment, support
equipment, documentation, and being hardware Co-I. The ORACLE PI
represents the single point formal interface with the ESA JUICE Project
Office for scientific and general matters
24

Co-Principal Investigator
The Co-PI will assume overall responsibility for coordinating the OR-
ACLE development, as well as being a responsible for supplying required
software on time and to the agreed specifications. In addition, the Co-PI
will interface with the responsible engineers at the CO-I institutions
Experiment Manager
He/she will coordinate the hardware and software efforts of all segments
of ORACLE. The EM represents the single point formal interface with the
ESA JUICE Project Office for technical matters.
25

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Appendix A
Attitude and accommodation
Figure A.1: Dipole antenna diagram for attitude and accommodation.
30

Appendix B
Acronyms
List of abbreviations used.
ANT - Antenna
COF - Crystal Orientation Fabric
DES - Digital Electronic Subsystem
DSP - Digital Signal Processing
EB - Electronics Box
FFT - Fiber Foldable Tube
PGA - Phase-Gain Algorithm
RDA - Receiver and Digital Assembly
TFE - Transmitter and FrontEnd
TRL - Technology Readiness Levels
31

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