title: The Array of Long Baseline Antennas for Taking Radio Observations from the Sub-Antarctic
authors: H. C. Chiang T. Dyson E. Egan S. Eyono N. Ghazi J. Hickish J. M. Jauregui-Garcia V. Manukha T. Menard T. Moso J. Peterson L. Philip J. L. Sievers S. Tartakovsky
text: 
  
   
  
  H. C. Chiang et al.ALBATROS instrument
  
  
  
  
  
  
  ^1Department of Physics, McGill University, Montréal, Quebec H3A 2T8, Canada
  
  ^2School of Mathematics, Statistics, and Computer Science,
    University of KwaZulu–Natal, Durban 4000, South Africa
  
  ^3Department of Astronomy, University of California, Berkeley,
    California 94720, USA
  
  ^4Canadian Institute for Theoretical Astrophysics, Toronto, Ontario M5R 2M8, Canada 
  
  ^5South African National Space Agency, Hermanus 7200, South Africa
  
  ^6Department of Physics, Carnegie Mellon University,
    Pittsburgh, Pennsylvania 15213, USA
  
  ^7School of Chemistry and Physics,
    University of KwaZulu–Natal, Durban 4000, South Africa
  
  
  The Array of Long Baseline Antennas for Taking Radio
    Observations from the Sub-Antarctic
      H. C. Chiang^1,2,†, T. Dyson^1, E. Egan^1,
    S. Eyono^2, N. Ghazi^2, J. Hickish^3,
    J. M. Jáuregui–Garcia^4, V. Manukha^5, T. Ménard^1,
    T. Moso^2, J. Peterson^6, L. Philip^7,
    J. L. Sievers^1,7, S. Tartakovsky^1
      December 30, 2023
  ===========================================================================================================================================================================================================================
  
  
  
  ^†Corresponding author: hsin.chiang@mcgill.ca
  
  
  (to be inserted by publisher);
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  Measurements of redshifted 21-cm emission of neutral hydrogen at
  ≲30 MHz have the potential to probe the cosmic “dark ages,”
  a period of the universe's history that remains unobserved to date.
  Observations at these frequencies are exceptionally challenging
  because of bright Galactic foregrounds, ionospheric contamination, and
  terrestrial radio-frequency interference.  Very few sky maps exist at
  ≲30 MHz, and most have modest resolution.  We introduce the
  Array of Long Baseline Antennas for Taking Radio Observations from the
  Sub-Antarctic (), a new experiment that aims to image
  low-frequency Galactic emission with an order-of-magnitude improvement
  in resolution over existing data.  The  array will consist
  of antenna stations that operate autonomously, each recording baseband
  data that will be interferometrically combined offline.  The array
  will be installed on Marion Island and will ultimately comprise 10
  stations, with an operating frequency range of 1.2–125 MHz and
  maximum baseline lengths of ∼20 km.  We present the
   instrument design and discuss pathfinder observations that
  were taken from Marion Island during 2018–2019.
  
  
  
  
  
  
  § INTRODUCTION
  
  
  Measurements of redshifted 21 emission of neutral hydrogen
  across a wide range of radio frequencies have the potential to
  elucidate the universe's history from the cosmic “dark ages” up to
  the formation of large-scale
  structures <cit.>.  The dark ages, which
  occurred after recombination and correspond to a period when the
  universe was filled with neutral hydrogen, are unexplored to date and
  represent one of the final observational frontiers in cosmology.  The
  number of independent modes that are contained in the matter power
  spectrum in this epoch is orders of magnitude greater than the
  corresponding number for cosmic microwave background (CMB)
  measurements <cit.>.  Redshifted 21-cm
  measurements of the dark ages therefore hold the potential to
  significantly improve cosmological parameter constraints over those
  currently derived from the CMB; however, the required observational
  frequencies of ≲ 30 MHz are exceptionally difficult to
  access.  The primary experimental challenges include Galactic
  foreground emission, ionospheric interference, and terrestrial
  radio-frequency interference (RFI).
  
  
  
  Very few experiments have surveyed the radio sky at ≲ 30 MHz,
  and here, we highlight only a subset.  At the very lowest frequencies,
  the state of the art among ground-based measurements dates from the
  1950s, when Reber and Ellis caught brief glimpses of the 2.1 MHz sky
  at ∼ 5 resolution, using an array of
  192 dipoles <cit.>.  A few space-based missions
  have also performed measurements at similarly low frequency ranges;
  for example, the Radio Astronomy Explorer-2 operated at
  0.02513 with ∼ 10 resolution at
  4.7 <cit.>.  Subsequent ground-based
  experiments include the Dominion Radio Astrophysical Observatory
  (DRAO) surveys of the northern sky at 10 MHz and 22 MHz, with
  ∼2 and 1.1–1.7 resolution,
  respectively <cit.>.
  Low-frequency imaging experiments that are currently operational or
  under construction include the Ukranian T-shaped Radio
  telescope <cit.>, with 0.5 resolution at
  the center of its 8–32 MHz operating range, and the New extension in
  Nançay upgrading LOFAR <cit.>, which operates at
  10–85 MHz with 1.5 resolution in standalone mode, and
  improved resolution at >30 MHz when combined with LOFAR.  Existing
  measurements with finer resolution are mainly at higher frequencies;
  for example, the Owens Valley Long Wavelength Array mapped the sky
  with 15 resolution between 36.528 MHz and
  73.152 MHz <cit.>.  The dearth of information
  about the ≲ 30 MHz sky, particularly in the southern
  hemisphere, is illustrated by the Global Sky
  Model <cit.>, which incorporates the DRAO 10 MHz
  and 22 MHz data as the only two inputs at these low frequencies with
  sufficiently meaningful sky coverage and resolution.
  
  
  
  
  
  The first step toward laying the groundwork for
  possible measurements of the dark ages is obtaining an improved map of
  Galactic foregrounds at ≲ 30 MHz.  In addition to serving as
  a stepping stone for future cosmological constraints from the dark
  ages, maps at these frequencies can also shed new light on Galactic
  astrophysics.  Above ∼30 MHz, synchrotron emission from the
  Galaxy follows a power law with a frequency dependence of ∼ν^-0.7.  At lower frequencies, synchrotron self-absorption
  becomes non-negligible, and the spectrum transitions to ν^+5/2
  dependence.  Since self-absorption causes the optical depth along the
  line of sight to depend on frequency, observations at ≲
  30 MHz have the potential to probe the three-dimensional cosmic ray
  structure of the Galaxy <cit.>.  Low-frequency
  observations can also be used to image radio recombination lines
  (RRLs), which provide a means for studying cool, low-density regions
  of the interstellar medium (ISM).  These absorption lines arise from
  Rydberg atoms and are highly sensitive to the surrounding environment.
  The RRL spectrum and absorption profiles can therefore constrain the
  detailed properties of the ISM regions in which the Rydberg atoms are
  formed <cit.>.  Finally,
  observations at ≲30 MHz may provide new views of Earth's
  ionosphere, which absorbs and refracts at radio frequencies and
  becomes completely opaque below the plasma cutoff frequency.  The
  cutoff frequency and the levels of absorption and refraction are
  time-varying and spatially dependent.  Experiments that image the
  radio sky at multiple frequencies bracketing the plasma cutoff can
  therefore simultaneously image the ionosphere and provide spectral
  riometry data for space weather studies <cit.>.
  
  There are proposed efforts to perform new low-frequency measurements
  from space, where there is no contamination from the ionosphere, and
  the lunar farside can potentially block RFI from the
  Earth <cit.>.  Although the
  combination of ionosphere and RFI significantly impedes low-frequency
  radio observations from most locations on Earth, preliminary
  observations from Marion Island <cit.> suggest
  that such observations may still be accessible from carefully selected
  locations and with new technology developments.  In this paper, we
  present the Array of Long Baseline Antennas for Taking Radio
  Observations from the Sub-Antarctic (), a new experimental
  effort that aims to map the low-frequency sky using an array of
  autonomous antenna stations.  We describe the overall instrument
  design and preliminary measurements from engineering runs that were
  performed on Marion Island during 2018–2019.
  
  
  
  
  
  § ALBATROS OVERVIEW
  
  
  The primary requirements that drive the design for a low-frequency
  imaging experiment are 1) desired resolution, 2) low RFI, and 3) quiet
  ionospheric conditions.  As a benchmark, an interferometer operating
  at 30 MHz requires baseline lengths of ∼0.5–1 km 
  to achieve a resolution of ∼ 1.  This length scales
  inversely with frequency, and therefore ∼ 10-km lengths are
  required at few-MHz frequencies to improve substantially upon the resolutions
  achieved to date.  The experiment installation site must be remote to
  keep RFI to a minimum, and polar or near-polar latitudes generally
  have lower ionospheric plasma frequency cutoffs relative to other
  locations on Earth.  <ref> illustrates predictions from
  the International Reference Ionosphere
  model <cit.> for the minimum plasma frequency
  during the last solar minimum, which started in approximately 2007.
  Three locations are shown: Marion Island, the focus of this work;
  Dome C in Antarctica, which is another isolated location used for
  astronomical observations; and Hobart, Tasmania, where Reber performed
  his ∼2 MHz observations.  The model predictions illustrate that
  the ionospheric plasma cutoff frequency at Marion Island may drop as
  low as ∼1.5 MHz during solar minima.  Since we are currently
  experiencing another solar minimum <cit.>, the
  timing is opportune for new low-frequency observations.
  
  Marion Island is a research base that is located in the southern
  Indian Ocean at 46;54;45S, 37;44;37E and is operated by
  the South African National Antarctic Programme.  The island lies
  roughly 2000 from the nearest continental landmasses
  and has an exceptionally quiet RFI
  environment <cit.>.  As illustrated in
  <ref>, Marion has an area of 335 km^2 and can
  therefore support antenna installations with >10-km baseline
  lengths.  The main Marion base is located on the northeast side of the
  island, and there are eight rest huts along the coast (Cape Davis,
  Grey-headed, Kildalkey, Mixed Pickle, Repettos, Rooks, Swartkops,
  Watertunnel) and one in the interior (Katedraal) that can serve as
  existing infrastructure points for antenna installations.  The planned
   installation sites include the coastal huts, but exclude
  the main base and Katedraal for RFI and accessibility reasons,
  respectively.  <ref> also shows the locations of
  the  pathfinder antennas that are currently installed at the
   site and at the hydro shack.
  
  Using the eight coastal huts, the  site, and the hydro shack as
  the nominal  installation locations, the computed
  synthesized beam is shown in <ref>.  The beam
  width at 5 MHz is roughly 7, which represents over an
  order of magnitude improvement in resolution over other existing
  measurements.  One of the challenges in constructing an interferometer
  array on Marion Island is that the rugged terrain precludes the
  possibility of directly cabling and correlating antennas across large
  distances.  The final  antenna stations will therefore
  operate autonomously, recording baseband data over extended
  periods of time for subsequent offline correlation.  We have conducted
  two engineering runs: 1) a two-element, directly correlated pathfinder
  to qualitatively understand the sky signal, and 2) a single station to
  test the readout and power handling technology that are required for
  autonomous operation.
  
  
  
  
  
  § TWO-ELEMENT PATHFINDER
  
  
  
  
  The first exploratory  measurements were conducted with a
  two-element pathfinder that used direct correlation (without
  autonomous operation).  <ref> illustrates this
  pathfinder, which was installed at the  site (46;53;13S,
  37;49;10.7E) in April 2018.  The system block diagram is shown
  in <ref>, and each of the subsystems is
  described in detail below.
  
  
  
   §.§ Antenna
  
  
  We employ two dual-polarization Long Wavelength Array (LWA) dipole
  antennas <cit.>.  The LWA antennas have a long development
  history, are well characterized, and are simple to install and
  physically robust.  The antennas form an east–west baseline with a
  separation of 110m, and the polarizations are aligned with the
  cardinal directions.  Welded wire mesh screens, roughly 3 m on a side,
  are installed on the ground below the antennas.  The beam pattern is
  roughly omnidirectional with a solid angle that varies between 2.2 and
  2.7 steradians over 5–100 MHz.  Although the LWA antenna is not
  optimized for observations below 10 MHz, using this standard antenna
  allowed an initial test of the site and of the interferometer systems.
  For the final  array, we are exploring alternative antenna
  designs that will cover the full frequency range.
  
  
  
   §.§ Front-end active balun
  
  
  The LWA active-balun front-end electronics (FEE) circuit uses a
  Mini-Circuits GALI-74+ MMIC to amplify each dipole leg against ground,
  presenting each leg with a 50 impedance and providing a nominal
  gain of 25 dB. The two GALI-74+ outputs are differenced using a
  passive 180 hybrid coupler. The coupler output is filtered by
  a ∼150 MHz lowpass and receives an additional 12 dB of gain from
  a Mini-Circuits GALI-6+ MMIC. This last amplifier drives the output
  signal onto a 100-m 50 coaxial cable. Thus, neglecting mismatch
  loss between the dipoles (electrically small in the frequency range of
  interest) and the 100 input impedance, as well as the hybrid
  insertion loss (<1dB), the front-end electronics provide a gain of
  ∼37 dB <cit.>.  Each FEE circuit is powered
  by 16 V, which is fed on the coaxial cable through a bias tee.  The
  lengths of the coaxial cables are the same for both antennas, thus
  placing the phase center at zenith.
  
  
  
   §.§ Back-end electronics
  
  
  The back-end readout electronics are housed in a Faraday cage located
  100 m away from the antennas to mitigate possible self-generated RFI.
  Each of the four antenna signals is passed to a second-stage
  electronics chain consisting of an amplifier (Mini-Circuits ZX60-V63+),
  and a pair of high- and low-pass filters (Mini-Circuits ZFHP-1R2+ and
  SLP-90+) that together band-limit the signal to 1.2–81MHz.
  The amplifier has a nominal gain of 20 dB, and the high- and low-pass
  filters contribute nominal insertion losses of 0.2 dB and 0.14 dB,
  respectively.
  
  A Smart Network ADC Processor <cit.>
  board digitizes the RF signals at 250 Msamp/s and uses a Xilinx
  Kintex 7[<http://www.xilinx.com/products/silicon-devices/fpga/kintex-7.html>]
  FPGA to calculate full cross-correlations of the four inputs,
  producing four auto- and six cross-spectra as outputs, over 2048
  channels spanning the frequency range 0–125 MHz.  The channelization
  is performed with a
  CASPER[<https://casper.berkeley.edu/>]-based polyphase
  filter bank, and the spectra are accumulated over ∼4-s intervals.
  A Valon 5007 frequency synthesizer module provides the clock signal
  for the SNAP board.  A Raspberry Pi (RPi) 3B+ single board computer
  controls the SNAP board and receives the spectra via GPIO connections.
  The data rate of the averaged spectra is roughly 400 MB/day (with
  compression), and this low volume allows the spectra to be saved to
  the RPi on-board SD card.  An Adafruit Ultimate GPS
  module[<https://www.adafruit.com/product/746>], connected
  to an active external GPS antenna, provides absolute timing for the
  RPi.
  
  
  
   §.§ Power
  
  
  A bank of four 12-V, 200-Ah AGM batteries, connected in parallel,
  powers the two-element pathfinder system.  The batteries are manually
  charged with a Honda EU30is generator that is housed on site.  The
  total power draw of the two-element pathfinder is ∼45 W, and when
  fully charged, the battery bank can power the system for about a week.
  The raw battery voltage is passed to several DC/DC converters that
  supply regulated voltages to the SNAP board, FEE, amplifiers and the
  clock.
  
  
  
  § SINGLE AUTONOMOUS STATION PATHFINDER
  
  
  
  
  
  
  The full  configuration will comprise an array of 10
  autonomous antenna stations, each recording baseband over tunable
  frequency windows (with 10–20 MHz total bandwidth) within the full
  0–125 MHz operating range.  Baseband data will be periodically
  physically retrieved from each station and subsequently correlated
  offline.  The  stations, located at the eight coastal hut
  sites plus the hydro shack and  site, will be separated by
  baselines of ∼20 km as shown in
  <ref>. One fully autonomous
   station, shown in <ref>, was deployed in
  April 2019 at the hydro shack location (46;52.205;S,
  37;50.612;E) on Marion Island as a first step in testing the
  technology needed to establish the full array.  The system block
  diagram is shown in <ref>.  The antenna and
  front-end active balun in the single autonomous station are identical
  to those used in the two-element pathfinder (<ref> and
  <ref>), and the back-end electronics and power system are
  described in detail below.
  
  
  
   §.§ Back-end electronics
  
  
  The back-end readout electronics are housed in a Faraday cage located
  50 m away from the antennas. The analog signal chain consists of a
  pair of high- and low-pass filters (Mini-Circuits ZFHP-1R2+ and
  SLP-150+) that together band-limit the signal to 1.2–140 MHz, and the
  filters are followed by a Mini-Circuits ZFL-500+ amplifier.  In
  contrast to the two-element pathfinder, the low-pass cutoff is
  increased from 81 MHz to 140 MHz to capture downlink signals at
  137–138 MHz from the ORBCOMM satellite constellation.  For the final
   array, the ORBCOMM signals provide a convenient means for
  synchronization across the antenna stations, serving as a backup to
  the GPS timing discussed below.  Preliminary lab tests suggest that on
  time scales of ∼30 s, relative timing between different antenna
  stations can be measured to an accuracy of better than a few tenths of
  a nanosecond using a single satellite.  The corresponding phase error
  at 10 MHz is ≲1^∘.  With publicly available orbits and
  multiple satellites typically within the field of view, we expect that
  ORBCOMM baseband data saved at the same time as the astronomical data
  can provide offline synchronization of the  stations.
  Because the ORBCOMM and science data are recorded at the same time by
  the same system, improvements to the timing calibration can be applied
  in post-processing.
  
  As with the two-element pathfinder, a SNAP board digitizes each of the
  two RF signals at 250 Msamp/s. In the autonomous station
  configuration, the SNAP board ADCs are locked to a 10-MHz reference
  produced by a Trimble Thunderbolt E GPS-disciplined clock module.  The
  SNAP board FPGA computes two data products: 1) channelized baseband
  data for each polarization over tunable frequency windows within the
  0–125 MHz operating range, with the options of 1-, 2-, or 4-bit
  compression, and 2) auto- and cross-spectra from the two polarizations
  over the full 0–125 MHz span, accumulated over few-second intervals.
  The reduction in bit depth happens only after the SNAP board
  has channelized the baseband, and the cross-channel leakage (due to,
  e.g., RFI and slopes in RF power as seen by the ADCs) is therefore
  unaffected by the low bit depth.  An RPi 3B+ controls the SNAP board
  and and receives the auto- and cross-spectra via GPIO connections, and
  the spectra are saved to an on-board SD card.  The baseband data are
  passed from the SNAP board to the RPi via ethernet and written to
  external hard drives.  The introduction of gigabit ethernet with the
  RPi 3B+ model has enabled the high data throughput associated with
  writing baseband.  As a benchmark, 1-bit baseband recording of two
  polarizations over 10 MHz of bandwidth yields an approximate data rate
  of 5 MB/s, or 0.4 TB/day.
  
  
  
  
   §.§ Correlation
  
  
  
  
  The autonomous station can be configured to record 1-, 2-, or 4-bit
  baseband.  Signal fidelity improves with higher numbers of bits, but
  the penalty is increased data volume.  Here we discuss the prospects
  of employing 1-bit correlation for the final  array to keep
  the baseband data volume to a minimum.  While the basics of 1-bit
  correlation are well known in the limit where the signal level is much
  lower than the noise,  may be in a regime where the signal
  level is non-negligible.  The fundamental output of a 1-bit correlator
  for real data is
  
      x_ij≡ < Ẽ_i Ẽ_j  >.
  
  Here Ẽ_i is the quantized version of the underlying electric
  field E_i for antenna index i:
  
      Ẽ_i=
        
          1    if E_i > 0 
      
          -1    if E_i < 0.
  
  Complex data can be handled as the combination of real components.
  The output is nonlinear in the underlying true signal and noise
  levels; in the limit of perfectly correlated electric fields, x_ij
  saturates at unity.  Recovering the true sky signals requires
  inverting this nonlinear relation using the Van Vleck
  corrections <cit.>.  For a 2-level correlator, the
  relationship between x_ij and the sky signals
  is <cit.>
  
      < sin(π/2x_ij) > = V_ij/√(V_ij+N_i)√(V_ij+N_j),
  
  where V_ij is the true sky visibility, and V_ij+N_i and
  V_ij+N_j are the total, signal-plus-noise, power levels measured
  by antennas i and j, respectively.  Inverting this relation to
  obtain V_ij requires knowing the total power levels, which cannot
  be determined from 1-bit data alone.  However, as described in
  <ref>, the SNAP board calculates and records
  auto-spectrum data products, which provide power level information on
  few-second time scales.  Changes in the power levels occur on
  significantly longer time scales, so the true sky visibilities can be
  derived by combining cross-correlation data between different antenna
  stations and the auto-spectrum data from each individual station.
  
  To assess the loss of sensitivity as the signal level increases, we
  use <ref> to derive the correlator efficiency
  ϵ, which is the ratio of measured signal-to-noise ratio (SNR)
  to the ideal, infinite-precision SNR.  <ref>
  illustrates that the decrease in ϵ as a function of SNR is
  relatively shallow.  When the SNR is near zero, ϵ approaches
  2/π≈ 0.64, and if the signal level increases to two times
  the noise level, ϵ decreases by only a factor of ∼2.
  The final  array will have baselines that are long relative
  to the observing wavelength, and the correlated component of the
  electric field (which sets the relevant signal level) is likely a
  small fraction of the total received power.  This qualitative argument
  can by made by considering antennas in the uv plane in the flat-sky
  approximation.  Any individual antenna receives all power present in
  the uv plane (plus system noise), but the correlated contribution
  comes only from the portion of the uv plane that is encompassed by
  the primary beam at the antenna's uv-space coordinate.  Assuming
  that the sky can be described as a Gaussian random field, the maximum
  correlated signal between two antennas is determined by the aperture
  filling factor of that pair.  For the case of dipole antennas, the
  filling factor is the square of the wavelength divided by the baseline
  length.  Therefore, in the long-baseline limit, the correlated signal
  is small in comparison to the total power received, and the correlator
  efficiency is unlikely to decrease significantly from the ideal
  2/π value.  This efficiency behavior remains true even when the
  brightest radio sources are visible, since the primary beam
  encompasses more than a steradian solid angle (the sun is irrelevant
  since ionospheric effects restrict  to night-time
  observing).  Centaurus A <cit.> and M1 <cit.>
  affect the total power expected from the Global Sky
  Model <cit.> at only the ∼10% level.  Additionally, as
  will be shown in <ref>, preliminary  data
  demonstrate that the visibilities are only a few percent of the total
  power amplitude, thus indicating that diffuse emission dominates even
  the brightest sources over the large primary beam.
  
  
  
  
  
   §.§ Solar power system
  
  
  Although the first-deployed two-element pathfinder is located at a
  reasonable walking distance from the main base and uses a manually
  operated generator to periodically recharge batteries, the bulk of the
  antenna stations that will comprise the full array will be located at
  points farther removed from the main base and will therefore require
  fully autonomous energy sources.  For the autonomous station
  pathfinder, we developed a solar charging system to power the station,
  and we are investigating small wind turbines for future
   stations.
  
  The autonomous station is powered by two series-connected, 12 V, deep
  cycle, lead acid batteries, charged by an array of nine SunPower
  SPE-E-Flex-110 solar panels. These panels each have a standard test
  capacity of 110 W. Although only ∼50 W is required to run the
  station, the ∼1 kW charging capacity is intentionally oversized
  to account for the frequent overcast conditions on Marion.  The
  generous power margin allows the charge level to recover quickly on a
  short sunny day in winter in the event of charge loss over several
  consecutive overcast days.  The nine solar panels are distributed
  between three custom designed structures built from aluminum
  extrusion, with rigid metalized plastic panels backing the
  semi-flexible solar cells. The structures are oriented due north and
  are designed to incline the solar panels at a relatively steep angle
  to maximize performance under winter conditions, when sunlight hours
  are at their minimum. The solar panel mounts have been designed to
  withstand frequent gale-force winds on Marion. While the mounting
  structures themselves appear to be strong enough, the greatest
  challenge in deployment has been to find adequate anchoring in the
  volcanic ground.  Each group of three panels is wired in series, and
  the three series strings are connected in parallel to a Victron
  BlueSolar MPPT 50|35 charge controller, which optimizes power
  transfer from the solar array when charging is required, and also
  monitors charge level, reducing output current when the battery bank
  is fully charged.
  
  The power logging and control system runs on an Arduino, which logs
  information received from the Victron charge controller to an SD card,
  and switches power on and off to the readout electronics box. Logged
  power data will be used to refine power system requirements for future
  autonomous station deployments. The on/off control is necessary to
  prevent battery system damage from overly deep discharge. The system
  can also be configured to conserve battery power by running on a
  schedule between particular hours, typically overnight, when
  ionospheric conditions are more favorable.  The power logging and
  control system, along with the Victron charge controller and an EMI
  filter, are housed together in an aluminum box. The EMI filter reduces
  conducted emissions on the photovoltaic side of the solar charge
  controller, which could radiate from the solar array and connecting
  wires. (Data collected during the night are not at risk of solar
  charge controller EMI contamination.)
  
  
  
  
  
  
  
  § PRELIMINARY OBSERVATIONS
  
  
  The primary goal of the two-element pathfinder described in
  <ref> was to qualitatively assess the level of RFI and
  ionospheric contamination at low frequencies.
  <ref> illustrates auto- and cross-spectra
  from one of the co-aligned polarization pairs from the two antennas.
  The data are shown over a three-day period (June 18–21, 2018), and no
  calibration or cuts have been applied.  The rising and setting of the
  Galaxy is visible in all of the spectrum magnitudes, and repeatable
  fringes are visible in the phases of the cross-spectra.  The signal
  drop-off below ≲30 MHz in the autospectra is caused largely
  by the combined response of the LWA antenna and FEE, as illustrated in
  <ref>.  Although the LWA antenna is not
  optimized for the lowest  observing frequencies, as
  described in <ref>, it is sufficient for these initial,
  exploratory measurements.  The RFI lines at ≲20 MHz arise
  primarily from shortwave radio transmission reflecting off the
  ionosphere and are roughly a factor of two fainter in the
  cross-spectrum magnitude in comparison to the autospectrum.  During
  the night, when the ionospheric plasma cutoff frequency drops, the
  maximum frequency of the RFI contamination falls to ∼10 MHz.
  
  We can estimate the instrumental gain stability by comparing the total
  power between the different days shown in
  <ref>.  We find that the RMS gain
  fluctuations between 30 and 40 MHz (where the Galactic signal peaks in
  the data) are less than 1%.  The typical noise on ∼3-second time
  scales is roughly 0.04% of the power.  The low noise suggests that
  relative calibration, using autospectra recorded by each autonomous
  station, can yield sub-percent accuracy.  (Absolute calibration in the
  future will be derived by comparing autospectra against , which
  is co-located and absolutely calibrated, or the Global Sky Model.)  As
  a qualitative illustration of the sky signal repeatability on smaller
  scales, <ref> shows the phases of the
  cross-spectra binned in local sidereal time.  About 372 hours of data
  are averaged into this plot, and no RFI excision has been performed.
  A more detailed analysis of signal repeatability will be presented in
  a future paper, but the high signal-to-noise fringe pattern, which is
  even visible slightly below ∼10 MHz, demonstrates the proof of
  concept for building the expanded  array.
  
  We have been unable to retrieve data from the single autonomous
  station pathfinder on Marion (<ref>) because of
  COVID-19 restrictions and limited bandwidth to the island, although we
  have confirmed that the solar power system has successfully met the
  power demands of the antenna station.  To demonstrate baseband writing
  capability in the absence of Marion data, we instead present a short
  observation that was taken by a separate, single-element
   pathfinder station that operated from the McGill Arctic
  Research Station (MARS; 79;26;N, 90;46;W) on Axel Heiberg
  Island, Nunavut, during July 2019.  The RF signal chain of the MARS
  installation had the same architecture as that shown in
  <ref>, but the power was supplied by manually
  charged batteries, rather than a solar power system.  The MARS RFI
  environment is somewhat noisier than that of Marion Island, and
  because the data were taken during the height of Arctic summer, the
  ionospheric plasma cutoff frequency is higher than the typical cutoff
  reflected in the Marion data.  Despite the differences in the power
  configuration and RFI environment, the MARS installation is suitable
  for illustrating baseband data writing.
  
  
  
  
  
  We present short segments of baseband data that were recorded over two
  adjacent frequency windows, 5.3–12.6 MHz and 12.6–20.0 MHz, at
  separate times.  The baseband data were recorded with 4-bit
  quantization to preserve autospectrum information, which is not
  available with 1-bit quantization.  <ref>
  shows autospectra for one polarization of the  pathfinder
  installed at MARS, comparing the accumulated spectra that are directly
  computed by the SNAP board against spectra that are calculated from
  baseband data.  All of the waterfall plots have the same 61-kHz
  frequency resolution, set by the resolution of the spectra computed by
  the SNAP board.  The waterfall plots are shown with a logarithmic
  scale to highlight the qualitative agreement between the
  directly-accumulated and baseband spectra for bright and faint
  spectral features.  The bottom panels show the time-averaged spectra
  on a linear scale to illustrate differences in bright RFI features
  that saturate the baseband data.  The fraction of baseband data with
  values at the 4-bit extrema are overplotted with spectra, and there is
  agreement between the directly-accumulated and baseband spectra except
  when significant saturation is present.
  <ref> shows waterfall plots of phases
  computed from the cross-correlation between the two orthogonal
  polarizations of the  pathfinder at MARS, comparing the
  directly accumulated and baseband data.  Both sets of recorded data
  qualitatively agree, up to small differences caused by the 4-bit
  quantization.
  
  
  
  § SUMMARY AND FUTURE OUTLOOK
  
  
  We have presented the design of , a new interferometer that
  will image the radio sky at ≲30 MHz using an array of
  autonomous antenna stations installed on Marion Island.  With a
  two-element, directly correlated pathfinder, we have demonstrated that
  clear, repeatable sky signal is visible from Marion down to
  ≲10 MHz with no data processing or cuts.  We have constructed
  the first prototype autonomous  station powered by solar
  panels, and we have successfully tested the electronics and software
  for recording baseband data.
  
  With the proof of concept demonstrated from the pathfinder instruments
  presented here, we plan to improve the design of the future
   stations that will be installed at the coastal huts on
  Marion.  The LWA antenna and FEE are currently not optimized for the
  lowest  observing frequencies, and we are investigating
  possible modifications to the antenna and FEE designs to improve the
  low frequency response, as well as building in calibration circuitry.
  Because the future  stations will be located farther from
  the base and will be more difficult to access on a regular basis, each
  station will require larger total hard drive volume to store baseband
  data over extended periods of time.  We are developing a custom
  low-power hard drive bank with >100 TB total storage capacity that
  will employ a USB multiplexer to select and power only one hard drive
  at a time.  The autonomous station pathfinder presented here uses
  solar panels to charge the batteries, and we are investigating the use
  of small wind turbines as a possible alternative for future stations.
  We are also developing analysis tools to compute time-domain data from
  the recorded channelized baseband by inverting the polyphase filter
  bank while minimizing artifacts from quantization and saturation.
  
  In addition to Marion Island, we are also planning on installing a
  second  array at MARS in the high Arctic.  As described in
  <ref>, a single pathfinder antenna was installed in July 2019
  and observed for about three weeks to assess the RFI environment and
  ionospheric conditions.  When fully operational, the Marion and MARS
   installations will provide new views of the low-frequency
  sky across both hemispheres.
  
  
  
  § ACKNOWLEDGMENTS
  
  
  We gratefully acknowledge the National Research Foundation (grant
  number 110929), the South African National Antarctic Programme, the
  Natural Sciences and Engineering Research Council of Canada (grant
  number RGPNS-2019-534549), and the Polar Continental Shelf Program for
  providing funding and logistical support for our research program.
  This research was undertaken, in part, thanks to funding from the
  Canada 150 Program.  This research was enabled in part by support
  provided by SciNet(https://www.scinethpc.ca/) and Compute Canada
  (www.computecanada.ca).  The financial assistance of the South
  African SKA Project (SKA SA) towards this research is hereby
  acknowledged (<www.ska.ac.za>).  We additionally extend our
  sincere gratitude to the staff at SKA SA for hosting our lab tests.
  We thank the South African National Space Agency for their technical
  support, the crew of Ultimate Heli for safely delivering us and our
  cargo, and all of the Marion takeover and winter team members who
  provided invaluable advice and field help.  The authors also wish to
  thank members of the LWA and RAPID collaborations for useful
  discussions, and Raul Monsalve for helping make our first MARS
  campaign a success.
  
  apj
  
  
  