This document describes the development of a synthetic aperture radar system called IceSAR for an autonomous cryobot called VALKYRIE that is designed to explore the subsurface of Europa. IceSAR uses novel log-periodic folded slot array antennas and a radar system to provide forward-looking obstacle avoidance. Laboratory testing was conducted to characterize the antennas in ice and initial in situ testing on a glacier produced a radar image though left-right ambiguity remains. The system is intended to help study the potential ocean beneath Europa's icy surface.

1. SYNTHETIC APERTURE RADAR FOR AN AUTONOMOUS CRYOBOT FOR SUBSURFACE
EXPLORATION OF EUROPA
Omkar Pradhan1
, Srikumar Sandeep1
, Albin Gasiewski1
, Vickie Seigel2
and Bill Stone2
1- Electrical, Computer and Energy Engineering, University of Colorado-Boulder
2- Stone Aerospace Inc., Austin, Texas
ABSTRACT
In this paper a forward-looking synthetic aperture
radar (IceSAR) system is described for the ‘Very deep
Autonomous Laser-powered Kilowatt-class Yo-yoing
Robotic Ice explorer’ (VALKYRIE) project.
Fabrication and laboratory characterization of novel
log periodic antennas for IceSAR and system design
and in situ testing of the synthetic aperture radar
system are presented.
Index Terms— Cryobot, Forward-looking SAR,
Conformal Antennas
1. INTRODUCTION
The VALKYRIE project focuses on the design and
development of a field deployable autonomous ice-
penetrating cryobot to deploy realistic astrobiology
science payloads through substantially thick ice caps.
Technology concepts proved in VALKYRIE will lead
to surface lander mission concepts to Europa. The
need for radar sounding of Europan sub-surface is
motivated by the imaging results from the Galileo
mission [1]. Radar sounding for Europa in the
upcoming Europa fly-by mission has been discussed in
[2]. However studying the ocean of liquid water that
may lie beneath the icy crust is a significant
engineering challenge and in VALKYRIE these
challenges will be met with novel ice-penetrating
technology that will allow for carrying science
payloads. The SAR system described in this paper will
provide critical forward-looking obstacle avoidance
capabilities to the VALKYRIE ice-penetrating vehicle.
Testing and characterization of the antenna and radar
system was carried out at the Stone Aerospace facility
in Austin, Texas and on the Matanuska Glacier in
Alaska.
2. PROTOTYPE DESIGN
The development of a synthetic aperture radar system
consists of two parts. First is the design of an array of
four conformal log-periodic folded slot array (LPFSA)
antennas that form agile radiating elements. The
design adheres to severe physical constraints while
achieving the primary objective of operating within the
ultra-high frequency (UHF) band and providing a main
beam at the front of the cryobot for forward looking
obstacle avoidance. Second part involves the design
of the radar analog and digital system that includes RF
signal generation, transmit-receive isolation and digital
SAR data processing using a combination of ARM
processor and field programmable gate arrays (FPGA).
The SAR will provides end-fire beam angular
resolution by creating a synthetic aperture over ~10
meters of travel at a forward velocity of ~0.1 mm/s. A
forward looking SAR system has been previously
discussed in [3] and [4].
2.1. Antenna design
The type of antenna chosen for IceSAR system is a
Log Periodic Folded Slot Array (LPFSA) [5], which
makes use of an antenna structure complementary to
the conventional log periodic dipole array antenna[6].
A LPFSA antenna has a radially polarized electric
field vector, wide impedance bandwidth (0.7 to 1
GHz) and conformability to curved surfaces. An array
of four such antennas will be used so as to allow
resolution of left-right ambiguity in the radar image.

2. LPFSA antennas radiate with a null observed in the
end-fire direction because of opposing polarities of
the electric field vector on both sides of the antenna
plane. One way to avoid this null is to design resonant
cavity backings for each folded slot element of the
LPFSA antenna. However in the case of the
VALKYRIE cryobot geometrical constraints preclude
cavity depths of more than 𝜆/10 over over the
required band. The IceSAR antenna is thus designed to
make use of the envelope of melted glacial water (real
relative permittivity, 𝜖 𝑟 = 81 and conductivity,
𝜎~3.5 𝜇𝑆/𝑐𝑚) as a high permittivity dielectric inside
the resonant cavities. This allows for quarter
wavelength distances inside the cavities. Fabricated
resonant cavities are shown in Figure 1(a). One of the
fabricated antenna panels is also shown in Figure 1(b).
2.2. SAR system design
On the analog and digital hardware side, IceSAR uses
commercial off the shelf (COTS) components in a
compact assembly to achieve signal generation,
transmit/receive and data storage.
A functional block diagram of the radar system is
shown in Figure 2(a). In the transmit path the power
amplifier allows up to 40 dB of gain. The two Single
Pole 4 Throw (SPST) switches enable switching
between the antenna elements thus allowing for 16
possible transmit-receive antenna pairs. The digital
processing system leverages the sophisticated yet easy
to use FPGA platform Zedboard which is based on the
Xilinx Zynq 7000 all programmable system on chip
(SoC). The Zynq SoC uses an ARM+ FPGA
architecture core processor. Specifically the Zynq
7000 features a dual-core ARM® CortexTM –A9
processing system and 28 nm Xilinx programmable
logic in a single device. The Zedboard provides a
FMC interface to the Analog Devices FMCOMMS1-
EBZ radio transceiver card. The FMCOMMS1-EBZ
has a 512 MS/s 16 bit digital-analog converter and a
250 MS/s 14 bit analog-digital converter.
(a)
(b)
Figure 1: (a) Fabricated resonant cavities and
(b) CPW antenna panel.
(a)
(b)
(c)
Figure 2: (a) Functional block diagram of the
IceSAR system (b) Microwave components
assembled on the top tray of IceSAR box and (c)
Zedboard & FMCOMMS1 attached to the
bottom tray.

3. 3. TESTING AND RESULTS
The atypical environment in which the radar operates
makes it is very difficult if not impossible to emulate
an exact representative and at the same time controlled
environment for characterizing its antenna elements.
Nevertheless outdoor laboratory tests for antenna
characterization were carried out at the Stone
Aerospace Inc. facility in Austin, Texas. In situ testing
of the antennas and operation of the IceSAR system
was also carried out on the Matanuska Glacier in
Alaska.
3.1. Laboratory antenna characterization
Initial testing of the antenna array attached to the
Valkyrie melthead was carried out in ice at Stone
Aerospace’s facility in Austin, TX. To re-create real-
world operating conditions a 30 inches diameter by 70
inches tall (~one ton) cylinder of ice was used, into
which the antenna was gradually inserted over a period
of ~8 hours by boring using a water pump. Figure 3..
A Copper Mountain Planar 814/1 vector network
analyzer was used for both reflection coefficient as
well as front lobe radiation measurements of the
LPFSA antenna. A log-periodic dipole array antenna
(LPDA) was separately designed and characterized
and used as a reference probe antenna.
Figure 4 shows the measurements of antenna
reflection coefficient and radiation pattern
characterization and comparisons with simulated
results. Reflection coefficient measurements shown in
Figure 4(a) are obtained when the aft end of the
antenna has just submerged inside the ice block so that
all the resonant cavities are filled with surrounding
melt water. The thickness of the melt water layer
around the antennas is found to be ~1.5 cm. The HFSS
model used for simulating the structure was thus also
designed with a 1.5-cm cylindrical fresh water layer
around the antenna. The LPDA probe was translated
under the antenna being tested to measure front lobe
radiation. Measurements were recorded made at 10”
linear intervals.
3.2. In situ tests on Matanuska Glacier, Alaska
Figure 3: Antennas melting inside ice block at
Stone Aerospace Inc., Austin, Texas.
(a)
(b)
Figure 4: Measured (a) reflection coefficient and
(b) front lobe radiation.

4. In situ testing of the LPFSA antennas and IceSAR
radar system was carried out on the Matanuska
Glacier, Alaska near 61o
42’ 09.3” N latitude. and 147o
37’ 23.2”W longitude. Ice boring was achieved using
a hot water heater/pump (“Hotsy”) along with the
same melt head attached to the antenna as was used at
Stone Aerospace. The antenna structure was
suspended from its aft end by a gantry with a pulley
arrangement for for deliberate lowering down or
raising up of the antenna. A tape measure attached to a
fixed point on the gantry was used to accurately
measure the depth of the antenna. Figure 5 shows the
expected environment around the cryobot as it melts
through the ice. The older melt hole was bored in a
previous expedition and subsequently filled up with
soil. The glacier bed is at an unknown depth. Apart
from this other discontinuities in ice also act as
scatterers of the transmitted radiation.
Figure 6 shows in terms of normalized power the
reconstructed image formed by coherent aggregation
of pre-detected reflected signals from transmitted
radiation at every depth i.e. over the synthetic aperture
formed by the melting of the cryobot. The transmit
and receive antenna are the same for this image. It is
important to note however that the left-right ambiguity
inherent to forward looking radar systems is not
resolved in the reconstructed image. Ongoing work
will focus on using radar returns recorded from
different combinations of transmit and receive
antennas to resolve left-right ambiguity.
CONCLUSIONS
In this paper design and testing of a forward looking
synthetic aperture radar was for a ice-penetrating
cryobot vehicle was presented. As part of this work a
novel antenna design making opportunistic use of the
layer of fresh melt water around the vehicle for
achieving electrically small resonant cavities is
outlined along with detailed characterization and
testing methodologies. Testing and in situ operation of
the radar system including reconstruction of a first-
light image has been presented. Resolution of the left-
right ambiguity in radar image reconstruction is
currently on going
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Figure 5: Schematic of in situ tests on the
Matanuska Glacier, Alaska.
Figure 6: First-light image reconstructed from
coherent aggregation of radar echoes.