Design and Initial Performance of the Prototype for the BEACON
Instrument for Detection of Ultrahigh Energy Particles⋆
D. Southalla,∗ , C. Deaconua , V. Decoeneb , E. Oberlaa , A. Zeollab , J. Alvarez-Muñizc ,A. Cummingsb , Z. Curtis-Ginsberga , A. Hendrickb , K. Hughesa , R. Krebsb , A. Ludwiga,d ,K. Mulreye,f , S. Prohirag , W. Rodrigues de Carvalho, Jr.e , A. Rodriguezh , A. Romero-Wolfd,i ,H. Schoorlemmerj,f , A. G. Vieregga , S. A. Wisselb,h,∗∗ and E. Zasca Dept. of Physics, Enrico Fermi Institute, Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637,

arXiv:2206.09660v3 [astro-ph.IM] 29 Mar 2023

b Dept. of Physics, Dept. of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802,
c Instituto Galego de Física de Altas Enerxías IGFAE, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain,d Jet Propulsion Laboratory, Pasadena, CA 91109, USA,e Department of Astrophysics/IMAPP, Radboud University, Nijmegen, The Netherlands,f Nikhef, Science Park Amsterdam, Amsterdam, The Netherlands,
g Dept. of Physics, Center for Cosmology and AstroParticle Physics, The Ohio State University, Columbus, OH 43210,
h Physics Dept., California Polytechnic State University, San Luis Obispo, CA 93407,
i Dept. of Astronomy, California Institute of Technology, Pasadena, CA 91109, USA,j Department of High Energy Physic/IMAPP, Radboud University, Nijmegen, The Netherlands,

ARTICLE INFOABSTRACT

Keywords:
Ultrahigh Energy Neutrinos
Ultrahigh Energy Cosmic Rays
Instrumentation

The Beamforming Elevated Array for COsmic Neutrinos (BEACON) is a planned neutrino telescope
designed to detect radio emission from upgoing air showers generated by ultrahigh energy tau neutrino
interactions in the Earth. This detection mechanism provides a measurement of the tau flux of cosmic
neutrinos. We have installed an 8-channel prototype instrument at high elevation at Barcroft Field
Station, which has been running since 2018, and consists of 4 dual-polarized antennas sensitive
between 30-80 MHz, whose signals are filtered, amplified, digitized, and saved to disk using a custom
data acquisition system (DAQ). The BEACON prototype is at high elevation to maximize effective
volume and uses a directional beamforming trigger to improve rejection of anthropogenic backgroundnoise at the trigger level. Here we discuss the design, construction, and calibration of the BEACON
prototype instrument. We also discuss the radio frequency environment observed by the instrument,
and categorize the types of events seen by the instrument, including a likely cosmic ray candidate
event.

1. Introduction
Cosmic rays have been measured at energies in excess
of 100 EeV [1–3], above the GZK cutoff [4, 5]. These
measurements may imply the existence of cosmogenic ultrahigh energy (UHE) neutrinos produced by interactionsof UHE cosmic rays with cosmic microwave background(CMB) photons [6], or may be consistent with the end of
the cosmic ray spectrum [7,8]. In either case, measurements⋆

This work is supported by NSF Awards # 2033500, 1752922,
1607555, PHY-2012980 & DGE-1746045 as well as the Sloan Foundation,
the RSCA, the Bill and Linda Frost Fund at the California Polytechnic
State University, and NASA (support through JPL and Caltech as well asAward # 80NSSC18K0231). This work has received financial support fromXunta de Galicia (Centro singular de investigación de Galicia accreditation
2019-2022), by European Union ERDF, by the “María de Maeztu” Units of
Excellence program MDM-2016-0692, the Spanish Research State Agency
and from Ministerio de Ciencia e Innovación PID2019-105544GB-I00 andRED2018-102661-T (RENATA). We thank the NSF-funded White Mountain Research Station for their support and in particular Steven DeVanzo,
Jeremiah Eanes, and all of the staff at Barcroft and Crooked Creek Stations.
Computing resources were provided by the University of Chicago Research
Computing Center.
∗ dsouthall@uchicago.edu∗∗ wissel@psu.edu
ORCID (s): 0000-0002-7577-6095 ( D. Southall); 0000-0003-0569-6978 (S. A. Wissel)

D. Southall et al.: Preprint submitted to Elsevier

or constraints of the neutrino flux at UHE will improve ourunderstanding of cosmic ray accelerators and their cosmological distribution (see. e.g. References [9] and [10] forrecent reviews). In addition to the predicted cosmogenic
neutrino flux, recent discoveries of a diffuse flux of astrophysical neutrinos [11–13] and a candidate for an extragalactic source of neutrinos [14] create strong motivationfor expanding the capabilities of UHE neutrino detectors.
Though only electron neutrinos and muon neutrinos are expected to be produced at the sources, flavor oscillations over
the astrophysical length scales should result in an observed
flavor ratio flux at Earth of 1:1:1 [15–17] and any deviations
from this could indicate new physics [18]. An exclusive
measurement of the tau neutrino flux would yield both flux
and flavor ratio information for testing both cosmogenic andastrophysical neutrino models [19, 20]. Studying these UHE
particles also provides a measurement of interaction crosssections at center-of-mass energies not achievable by current
or planned collider experiments, and has the potential toreveal new physics [21–24].
The Beamforming Elevated Array for COsmic Neutrinos(BEACON) concept consists of mountaintop phased radio
antennas that are designed for measuring the flux of tau neutrinos above 100 PeV [25]. At these energies, tau neutrinos
Page 1 of 23

The BEACON Prototype Instrument

Cosmic Ray
Extensive Air Shower

Radio Signals

Elevation > 2km

Cosmic Ray
Source Direction

Neutrino
Source Direction

Tau Decay
StationTrigger Array
Pointing Array
L~𝓞(100 m)

Tau Exit⟨𝑨𝜴⟩

Figure 1: A schematic overview of the BEACON concept, adapted from [25]. Tau neutrinos interacting in the Earth can produce
a tau lepton that escapes into the atmosphere, producing an upgoing air shower upon decay. Radio emission from the air shower
may be detected by mountaintop radio stations, each consisting of a small antenna array used for triggering and reconstruction.
BEACON stations are also sensitive to emission from cosmic ray-induced air showers, which will come from above the horizon,
and may be used for detector characterization.

interacting with the Earth via a charged current interaction
can produce a tau lepton boosted enough such that it may
escape the Earth and decay in the atmosphere [26–28]. The
tau lepton decay creates an upgoing extensive air showerthat will produce an impulsive radio signal. The primaryemission mechanism is geomagnetic radiation, a result of
the deflection of charges by the Earth’s magnetic field,with contributions from Askaryan radiation [29]. Air shower
radio signatures have been extensively studied by numerous
radio experiments (see e.g. References [30–36] and References [37, 38] for recent reviews) and have been modelled
at accelerator experiments [39, 40]. The probability that a
tau lepton will exit the Earth peaks near and below the horizon [41]. This process is shown schematically in Figure 1.
There are several detector concepts around the world
targeting the tau neutrino flux using this Earth-skimming
technique, including particle detectors [42, 43], imaging
Cherenkov and fluorescence telescopes [44–46], and radioarrays both on and near mountains [34, 47–50] and on
balloons [51, 52]. See Reference [53] for a recent review.
The BEACON concept is distinct for using phased array
triggering on a high-elevation mountain. At high elevation,
each BEACON station views a large area over which a tau
lepton can emerge. The combination of a large prominence
and a steerable phased array trigger capable of triggering on
events from hundreds of kilometers away provides an optimized detector design for neutrino searches near the horizon. A full-scale BEACON array would consist of (1000)
independent stations, creating a global network of low-costhigh-elevation mountaintop radio arrays designed to search
for these signals.
D. Southall et al.: Preprint submitted to Elsevier

Phased array, or interferometric, triggering and reconstruction also offers additional benefits to the BEACON
design [54]. Directional beams are formed by delaying andsumming signals from individual antennas. The trigger isthen formed on the coherent sum of the signals from eachantenna, which has a higher signal-to-noise ratio (SNR) than
the signal from each antenna for true plane-wave air shower
signals, thereby lowering the energy threshold of the detector
to 100 PeV [25] compared to triggering on individual antenna channels. Additionally, the trigger thresholds on eachbeam can be dynamically adjusted in response to changes in
the local noise environment. These characteristics enhance
the trigger’s capability to reject anthropogenic radio frequency interference (RFI), which can help maintain sensitivity to the expected diffuse flux while in noisy environments.
A full-scale BEACON array would consist of manystations in various mountain ranges and countries. Thoughsome sites might have existing infrastructure that can beleveraged, it is not a requirement for a BEACON site. Stations should therefore be capable of operating autonomously
in remote environments at sites with little to no infrastructure. This means the system must be low-power andoperate off-grid using either solar or wind energy. Stations
should also be minimally capable of transmitting monitoringand house-keeping data off-site, with full data transmission
desirable to remove the need for retrieval of hard disks.
Finally, such an array must be easy to deploy, robust to
weather and wildlife, and cost-effective.
Development towards the BEACON experiment has
been focused on building a prototype. The goals of the
prototype study are to evaluate the performance of anPage 2 of 23

The BEACON Prototype Instrument

Barcroft
Observatory Dome

15V DC power supply
Acopian L15MC610AC34G45

8x active
short-dipoles

Electronics Faraday Housing

Feed 8:1 or 4:1 transformer
into 50ohm LNA. Gain 27dB.
Local power regulation at feed

LegendRF signal - coax
DC Power
Digital link

Bank of Lightning
Arresters

8x RF + DC
LMR400 coax

RF Receiver
Board*35dB amplification*Power Limiters*BiasTee to
8 chantennas

8-ch 7-bit
digitizer &FPGA trigger
500 MSPS

Bandpass:
30-80MHzsupply on/off+15V

Power Reg. &Distribution

Microcontroller[LPC4088]

BeagleBone Black
Industrial SBC
32 GB
SD cardstorage

Barcroft
Solar
Inverter+5V supply
For microcontroller

Ethernet
SwitchNetwork

TimingGPS

Controller + Power Broker BoardGPS
antennaFigure 2: Schematic of the BEACON prototype instrument system.

interferometric trigger used in this context, and to use theobserved cosmic ray flux to measure the in-situ expected
performance of the full-scale array. As shown in Figure 1, theprototype is also sensitive to extensive air showers initiatedby downgoing cosmic rays.
Though the prototype instrument is not large enoughto detect tau neutrinos, we expect to detect cosmic ray air
showers with it. Cosmic ray air showers come from above
the horizon, whereas signals from tau neutrinos would comefrom below the horizon. The observed rate of cosmic ray
events in the prototype instrument presents a in-situ validation of the threshold of the instrument. The threshold is an
important factor in determining the expected sensitivity to
tau neutrinos, allowing us to better predict the sensitivity of
the full-scale BEACON experiment in a data-driven way.
In this paper, we describe the prototype, its goals, andcurrent performance. Section 2 gives an overview of the
BEACON prototype’s design, hardware, and implementation. Section 3 discusses the performance of the array andthe phased array trigger. We also present a study of common
sources of RFI backgrounds at the prototype site. We also
discuss a cosmic-ray-like impulsive event triggered by an
RF-only trigger at high-elevation in a noisy environment. In
Section 4, we place these results in a broader context anddiscuss future work.

2. The BEACON Prototype Instrument
In 2018, we installed an 8-channel prototype instrument
consisting of four dual-polarized antennas and an instrumentthat amplifies, conditions, and records triggered events. The
system diagram is shown in Figure 2. The design describedhere is robust to weather and operating conditions experienced at this remote site, and scalable to larger future
D. Southall et al.: Preprint submitted to Elsevier

deployments. This section describes the instrument and its
site.

2.1. White Mountain Site
The prototype is located at an altitude of 3.8 km in
the White Mountains of California, near White MountainResearch Center’s (WMRC) Barcroft Field Station. The
experiment looks east from the site, overlooking the Fish
Lake Valley with the valley floor having an altitude of
1.5 km. Figure 3 shows the local topography at the site. The
antenna locations are shown in red (and also photographedin Figure 4), and important structures like Barcroft Field
Station and the Observatory Dome are shown in gray. The
Observatory Dome is an enclosed structure with power andnetwork access where our data acquisition system (DAQ)
electronics are housed.
The site provides significant infrastructure that is advantageous for BEACON, including road access, a solar-battery
hybrid power system, internet access via a microwave relay
to Owens Valley Station (which is also operated by WMRC),
room and board during deployment, and remote support for
the BEACON prototype from WMRC staff. There are engineering challenges presented by the site that have influenced
the design of the prototype instrument: it is only accessible inthe summer months, sees wind speeds in excess of 130 km/h,
and is located on steep and rocky terrain. Additionally, thepermit for the site restricts erecting permanent structures(e.g. concrete foundations) under the current agreement with
the United States Forest Service.

2.2. Antennas and Mechanical Design
As the radio emission from air showers is broadband,several bands ranging from 30 to 1200 MHz can be used
to detect them [25, 55]. Prior to initial deployment of the
BEACON prototype, a site survey of RFI was conducted
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The BEACON Prototype Instrument

Elev. [m]Antennas
MSL Contours:
Calibration Sites
50m
Structures
10m
Road
Wilderness
Magnetic
North 12.4°

Figure 3: Left: A map of the immediate surroundings of the BEACON prototype at the White Mountain Research Station.
Electronics are housed in the Observatory Dome. A scale bar is provided for the local terrain, as well as the direction of magnetic
North. Top Right: A map showing the BEACON prototype’s location within California, USA. Bottom Right: A map showing
elevation profile of the region visible to the BEACON prototype. A cone extended 100 km from the site and spanning ±60◦ ofEast has been added for reference to illustrate the direction the BEACON prototype faces.

to help make a choice of band [56]. The antennas chosen
for the first implementation of the BEACON prototype wereinverted-V cross dipole antennas also used as part of theLong Wavelength Array (LWA) experiment at the Owens
Valley Radio Observatory [57]. These antennas were chosen
for their sensitivity to 30-80 MHz frequencies, as well as
their active balun that includes conversion to a coaxial cableline and amplification of 35 dB [34, 58].
Later modeling using antenna simulation packages NEC[59] and XFdtd [60] suggested that the effect of the groundwhen looking near the horizon was too severe for a nonelevated antenna design. Following this study it was determined that elevating the antennas off the ground wasnecessary to avoid ground interference; an antenna elevation
height of ∼3.96 m was chosen as a compromise between performance and deployment difficulty. Ground effects are stillpresent in the beam patterns, as shown in Figure 5; however,
the ground planes used in the simulation are smooth planes
rather than the rocky terrain seen at the site. Additional
interference contributions are mitigated by avoiding any
metal near the antennas in the support system.
We designed a custom short-dipole antenna with 2x76.2
cm (2x30 in) long tines with an active balun that could be
mounted inside of a small enclosure on top of the mast,

D. Southall et al.: Preprint submitted to Elsevier

providing a low-profile and low-mass device capable of surviving the extreme environment. Although these antennas
have a small effective height at the low-edge of the band,they provide a nearly omni-directional beam pattern acrossthe band. Our BEACON active balun, shown in Figure 6,
consists of a 4:1 transformer that is fed into a 50 Ω lownoise amplifier (LNA), which is followed by a second stageof amplification. The transformer not only boosts the input
impedance as seen by the antenna, but also isolates thecommon mode ground of the amplifier and coaxial cablefrom the dipole. To maintain a precision voltage to the onboard amplifiers, the balun is locally regulated to 3.0 V. The
board draws < 45 mA with a total gain of 35 dB. Similar
short dipole designs have been studied and utilized in the
LOPES and CODALEMA experiments [61, 62].
Figure 7 shows that galactic noise is visible in the vertically polarized channel. Because of the small differencein period of the solar and sidereal days, we stacked theroot-mean-squared (RMS) fluctuations in the noise over thecourse of a month at two periods of the year. When thegalaxy rises above the horizon, the RMS noise is slightly
elevated and the peak is correlated with the rising galactic
center. We also note that this effect is not visible in thehorizontal polarization, because the galactic center peaks in
the South where there is a null in the HPol beam pattern.
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The BEACON Prototype Instrument

Figure 4: Top: The BEACON prototype array consists offour crossed dipoles each with a custom active feed. The
antennas are positioned on a sloped rocky terrain; the HPol
(VPol) dipoles are oriented such that their physical extent andgain nulls align in the North-South (Up-Down) direction for
maximal sensitivity towards the horizon in the East. Bottom:
Close-up view of Antenna 3 shows the antenna masts withtwo dipoles and active feeds fed with LMR240 connecting to
LMR400 at the base of the antenna. The GPS patch antenna isused for the RTK-based calibration system. The antenna masts
are protected against high winds while minimally impacting the
local environment using ∼33 kg rubber bases, wooden struts,
and six guy-lines. All four antennas are elevated ∼3.96 m above
the ground and pointed toward the horizon to the East.

D. Southall et al.: Preprint submitted to Elsevier

While the effect in VPol is weak, there is a clear phaseshift correlated with the elevation of the galactic center at
two different times of year. To be sensitive to this faint but
pervasive galactic noise is a key goal of a transient radio
detector like BEACON [63, 64].
The short dipoles are mounted directly onto a wooden
masthead in a cross pattern for sensitivity in both horizontal
and vertical polarizations; these are referred to as HPol and
VPol antennas respectively. A second HPol antenna could
be a future addition for full angular sensitivity. However,
the array’s location on a mountainside reduces the need forsensitivity in directions parallel to the mountainside where
the effective area is already significantly reduced. The BEACON prototype is located on a North-South aligned ridge,
so the HPol antennas are oriented North-South with the gain
being maximized along the East-West axis, orthogonal to theridge. Because the Earth’s magnetic field points close to theNorth, this orientation of the array aligns the center of thearray’s sensitivity with the direction that air shower radio⃗
emission is expected to be strongest (given by 𝑣⃗ × 𝐵).
Coaxial cables (∼107 m of LMR400 and ∼6 m of
LMR240 in series) connect the antenna preamplifiers to the
DAQ, carrying both the amplified signal and DC power tothe preamplifiers, which have internal bias tees. These cables
are jumpered across the wooden masthead to the steel mast
along with grounding cables, where they are guided to theground before being run uphill towards the DAQ. The cables
are sheathed when on the ground to reduce damage fromweathering as well as the local wildlife.
Reliably elevating the antennas required a number of
iterations, especially as the array location within Inyo National Forest precluded any permanent structures, requiring amast support design that avoids drilling or pouring concrete.
The first iteration of this design secured the base of the mastwith a commercially available ∼33 kg rubber base, as well as
3 guy-lines tied to local rocks for each mast. This design was
improved in follow-up deployments in 2020 and 2021, which
addressed issues with failed wooden mastheads and fallen
masts. These issues were caused by the extreme weather
at the prototype site, with gusts of up to 130 km/h, heavy
snow and ice build-up, static discharges, lightning strikes,
and exposure to the sun. The improved supports included ≥6 guy-lines per mast using higher-grade rope, and 4 wooden
struts per mast. The struts are cut to length on-site such thatthey can be wedged securely into the local terrain (Figure 4).
This upgraded design is robust to animals climbing or
pulling on it, is readily adaptable to varied terrain and has
proven to be capable of withstanding winter conditions. In
places with fewer restrictions, drilling into the ground would
add additional stability. Though the wood used was highquality pressure-treated cedar, it still showed significant
weathering after just a single year, so improvements in RFsafe alternative materials to wood, such as fiberglass, for themasthead may be warranted for future deployment, while
steel struts could be an option for usage away from theantennas.

Page 5 of 23

The BEACON Prototype Instrument

Figure 5: The realized gain of the crossed dipoles simulated with XFdtd. The HPol antenna gain is shown on the left, while
the VPol is shown on the right. The full width of each dipole is 1.56 m, and they are elevated ∼3.96 m above the ground overa ∼3.05 m sign post. The antennas are simulated with a 200 Ω characteristic impedance to model the 4:1 transformer. The
simulated antenna sits in the center of a 150 m ground flat plane tilted by 30◦ in elevation and 10◦ from North to South. Thisconfiguration models the two antennas lower on the hill. An azimuthal angle (shown on the bottom) of 0◦ corresponds to due
East and an elevation angle (shown on the top) of 90◦ looks directly up. The HPol beam pattern develops modes at a frequencies
determined by the interference of ground reflections with the main lobe, while the VPol pattern appears to be mainly affected by
the presence of the steel pole.

2.3. Radio Frequency (RF) Signal Chain and Data
Acquisition System (DAQ)
The BEACON prototype uses a custom DAQ housed ina Faraday enclosure, shown in Figure 8. At the input, signals
pass through a lightning arrester bank to prevent static
discharge from damaging the system. Afterwards, signals
then pass through an RF receiver board, which provides
D. Southall et al.: Preprint submitted to Elsevier

35 dB of amplification, a DC bias for the antenna feeds,
filtering, and power limiting. Filters include both 30-80 MHz
band-definition filters as well as FM notch filters, which arenecessary due to the proximity of the FM broadcasts. Typicalnoise spectra for three generations of BEACON antennas areshown in Figure 9. Figure 10 shows the combined gain of thefull RF signal chain.
Page 6 of 23

The BEACON Prototype Instrument

In addition to being connected to the Barcroft network, theArchive Machine is also connected to a backup cellular
network, which allows communication with the DAQ system when the normal connection from Barcroft is down, arelatively common occurrence particularly in winter. Also
at Barcroft is a Raspberry Pi with a software-defined radio
tuned to listen to aircraft ADS-B transmissions, which isused alongside data provided by The OpenSky Network [65]for the purpose of tracking nearby commercial airplanes.
The use of ADS-B transmissions to correlate airplane locations with above-horizon RF signals in the 30-80 MHz bandhas been demonstrated previously by other experiments [49].

2.4. Trigger System
Figure 6: Short dipole antenna feed. The active dipole feeds
each incorporate a 4:1 transformer into a 50 Ω LNA. The
Polycase enclosure helps protect the front-end board fromweather. The antenna elements are connected directly tothe front-end board, with each extending outward throughgrommets in the enclosure.

Signals then reach the digitizer and beamforming trigger
board, which incorporates 8 channels of 500 MSPS 7-bit
digitization and a control FPGA responsible for triggeringand buffering up to 2048 samples per channel for readout
once triggered. Typically, only 1024 samples are read out per
event to increase readout speed and reduce dead time anddata volume. Tunable attenuators allow for gain matching
between channels and tuning the dynamic range of thedigitizer. A timing GPS is used to provide a reliable pulse
per second (PPS) to the digitizer board, which records thenumber of clock cycles when the PPS is received, allowing
for precise time tagging for each trigger.
The digitizer and beamforming trigger board is controlled and read out via SPI using a BeagleBone Black(BBB) single board computer (SBC), running Debian Linux.
Software on the SBC manages configuration of the trigger,
readout of event data and metadata, housekeeping, and transfer of the data off of the DAQ. The BBB is connected to
the Barcroft network via Ethernet. Also on the network is a
microcontroller which allows for remote power cycles of theentire system.
The DAQ system is powered by a 15 V DC supply,
plugged into the Observatory Dome power system. The
present power draw of the DAQ is ∼40 W, dominated by thedigitizer and trigger board. Power at the Observatory Domeis provided by a solar-battery hybrid system deployed by
WMRC. The typical power system capacity is considerably
greater than the daily usage (∼1 kWh), resulting in nearlycomplete live time, except under extended extreme cloud
cover or excessive snow lasting > 5 days.
Data is sent from the DAQ system to our Archive Machine computer located nearby at Barcroft Field Station. The
Archive Machine archives data before it is transferred to
the University of Chicago and provides local monitoring.
D. Southall et al.: Preprint submitted to Elsevier

The combination of an FPGA and streaming digitizer on
the digitization and trigger board allows for flexible triggering capabilities. Currently, a beamforming (i.e. phased array)
trigger is implemented, similar to the one deployed as partof the Askaryan Radio Array (ARA) at the South Pole [66].
This trigger uses a pre-calculated table of expected arrival
time differences between the antennas to delay signals before
summing them. Each set of time delays corresponds to abeam sensitive to a particular direction, and is most sensitive
to signals arriving from the specific direction where thedelays result in coherently summed signals. This coherent
sum will increase an incoming signal’s voltage by a factor
of 𝑁antenna , while√thermal noise will add incoherently andonly increase as 𝑁antenna , resulting in a net SNR increase√of 𝑁antenna [54].
The delayed and summed waveforms are further processed in the DAQ with a “power sum”. This is done by firstsquaring the combined signal to obtain a proxy for power,
before summing the combined power signal in 16 sample(32 ns) bins every 8 samples (16 ns) such that each bin has
some overlap with the previous bin. The power sum of acoherently summed signal will increase the power SNR bya factor of 𝑁antenna . Hereafter we refer to the beam SNR
calculated by aligning voltage waveforms and summing as
the “beam voltage SNR” and the power sum performed in thetriggering hardware as the “beam power SNR”. Currently,
the time delays for each beam are pre-calculated assuming
plane wave signals, however near-field time delays could be
implemented in the FPGA if desired.
The trigger rates in each beam are continuously monitored and the thresholds are adjusted to meet user-definedgoals. In this way, trigger thresholds are dynamically set to
be noise riding, managed by the SBC such that a global
trigger rate of 10 Hz is maintained. With a target rate of
10 Hz and temporary system outages accounted for, weconservatively estimate our dead time to be ∼1.5-2% overthe span of time used in the analysis discussed in Section 3.
The thresholds for each beam are adjusted automaticallyand in nearly real time so beams with consistently loudsources of RFI do not dominate the trigger. The rates of theindividual beams can be further refined by the user. This
directional trigger is essential in RFI-rich environments andhas allowed the BEACON prototype to maintain relatively
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The BEACON Prototype Instrument

September 2021

February 2022

0.3
0.0
0.3
0.6
0.0

60
30
0
30
Noise Values 60
Sun90
Galaxy0.8
1.0

Elevation (deg)Noise RMS (dB)

0.6

0.2

0.4

0.6

Fraction, Solar Day

0.8

1.0 0.0

0.2

0.4

0.6

Fraction, Solar Day

Figure 7: The fluctuations in the root-mean-squared (RMS) noise observed in VPol channel 5 during September 2021 (left) andFebruary 2022 (right). Superimposed on each plot are the range in elevation of the Sun and galactic core over the sampled time.
The RMS rises along with the galactic center, such that when the galaxy is visible in the antennas, the noise increases. The phaseof the RMS variations follows that of the galactic center throughout the year, rather than the sun.

1. a “side-swipe” veto, which can actively veto events
where the amplitude on one antenna is significantly
larger than others;
2. a saturation veto for high-power events which areclipped significantly;
3. a rear-facing veto which would avoid triggering whenan event hits both western antennas first;

Figure 8: Picture of the DAQ. The yellow region in the top
right contains the SBC, GPS clock, and power distribution.
The red region on the left contains second stage amplification
and band-pass filtering. The bottom right blue section is thecustom digitizer and beamforming trigger board.

low thresholds in the majority of beams despite prevalent
RFI from certain directions.
Currently we use 20 beams, distributed as shown in theupper panel of Figure 11. These beams were optimized fortriggering on above-horizontal events in the region expected
to be populated by cosmic rays. A full-scale BEACON
station would be targeting the near-horizon region where tau
neutrino events are expected. Moreover, the total number ofbeams would be expanded to uniformly fill the aperture. The
trigger logic for the original implementation with ARA is
described in more detail in Reference [66].
The trigger implementation allows for additional calculations to be performed in order to form noise-rejectionvetoes to improve performance. Some examples of vetoesthat have been considered:

D. Southall et al.: Preprint submitted to Elsevier

4. and a “band ratio” veto which compares relative power
seen through 2 finite impulse response (FIR) filtersin the low and high portions of the band to reduce
triggers from narrow band events.
Beams pointed at specific known sources of RFI canalso be used as a veto. As shown later in Figure 13, regularanthropogenic sources can be localized to well within the
beam width. A veto could be implemented that disallows
events from a certain beam direction if it also triggers asideband.
The performance of the phased trigger can be seenin Figure 11, which illustrates each beam’s definition anddynamic thresholds. Thresholds are computed as the powersum over a 16 sample (32 ns) window in each beam andare shown here referenced to the RMS noise in a beam,
which is monitored continuously by the DAQ. In the middle
and bottom panels of Figure 11, the power SNR thresholds
are compared to the voltage SNR thresholds in the beams.
The translation between the power thresholds used in thetriggering hardware and the voltage thresholds shown on theleft are computed from simulations of cosmic rays modeledwith ZHAireS and propagated through the√ prototype signal
chain [67]. The translation is 𝑉SNR = 1.8 𝑃SNR − 0.38. We
make this comparison because prior simulation studies usedbeam voltage SNR thresholds for modeling the tau neutrino
sensitivity [25], while the trigger hardware uses thresholdson the beam power SNR. The thresholds achieved on the
instrument approach the nominal thresholds assumed in the
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PSD (dB, arb)

40
30
20
10
0
40
30
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82
110 8090 7830

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2018
2019
2021
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Figure 9: Top: Time averaged spectra for 3 generations ofdipole antennas corresponding to the same HPol channel.
The time covered by each is set to be 50 runs, resulting inaverages covering 391 hours for 2018, 133 hours for 2019,
and 153 hours for 2021. These times are sufficiently long foreach generation that the differing time windows do not havea significant impact on the structure of the spectra. Bottom:
The same except VPol antennas. The spectra are presented as
Power Spectral Density (PSD) in arbitrarily offset dB units (aconversion between ADU and volts has not been performed).
The variation in baseline power is a result of differing antennaconstruction and amplification which affects both signal andnoise levels and is generally not representative of performancedifferences in SNR. The 2018 traces correspond to LWA antennas, which were significantly lower to the ground and weregenerally a different infrastructure. Comparing 2018 to otheryears it is clear that our VPol channel has significantly reduced
cross-polarization power, as noticeable by the disappearanceof television (TV) band noise in the VPol channel (with TV
contributions ranging from ∼53 to 60 MHz, discussed further in
Section 3.2). The antenna element lengths were increased from
2x68.6 cm to 2x76.2 cm for the 2021 model, which has resulted
in additional pickup in the high-end of the band, noticeableparticularly in the VPol antenna which may be experiencing
additional coupling with the steel mast due to closer proximity.
As the trigger operates primarily using HPol antennas this has
not negatively impacted performance of the trigger.

simulation studies (5𝜎 in voltage) [25]. While the thresholds
are often in the range assumed by the simulations, there arealso periods of time dominated by loud RFI in the field ofview.
Comparing the beam map to the thresholds, we can seethat there is some variation in the thresholds in each beam,corresponding to the observed rates in those beams. Somebeams, like beams 0 and 4, point near a source of RFI
below the horizon and maintain a higher mean thresholdcompared to other beams at the same elevation. Other beams,
like beams 5, 9, and 17, point well above the horizon but
may be triggering on sidelobes from RFI below the horizon.
The impact of sidelobes is expected to be reduced with
the increased number of antennas in a full-scale BEACON
station trigger array.
D. Southall et al.: Preprint submitted to Elsevier

Gain (dB)

PSD (dB, arb)

The BEACON Prototype Instrument

40

50

25

50

60

70

80

70

Antenna 0H
Antenna 0V
Antenna 1H
Antenna 1V
Antenna 2H
Antenna 2V
Antenna 3H
Antenna 3V

50
30
10
100

75

Frequency (MHz)

100

125

150

Figure 10: The system gain for each channel, including theactive feeds with a gain of 45 dB, cable losses through LMR400
and LMR240, bandpass and notch filters (Mini-circuits SHP50, SLP-90, and NSBP-108), and second stage amplifier board
with a gain of 40 dB.

3. Instrument Performance and Data Analysis
3.1. Antenna Position Calibration and Direction
Reconstruction
BEACON uses interferometry of waveforms from eachof its 8 channels to determine the source direction of signals.
Accurate source direction reconstruction (both at the trigger
level and in analysis) requires precise knowledge of the arraytiming, including the location of each antenna and signal
cable lengths. A calibrated array can use pointing for RFI
rejection of permanent sources or airplanes, as well as forcharacterization of the polarization and source properties of
the initiating radio source. The typical method of calibrating
an array consists of the following process:1. Perform initial position measurements using a Global
Positioning System (GPS) system.
2. Measure cable delays using a vector network analyzer
or time-domain reflectometer (TDR)
3. Send radio pulses at the array from a known location(also measured with comparable system to antennas),
recording the pulses through the DAQ for later analysis
4. Perform a 𝜒 2 minimization targeted at matching predicted arrival time differences in each channel with theactual measured delays through the DAQ for various
pulser locations or sources. A typical 𝜒 2 takes theform:]2𝑛sources 𝑛baselines [∑ ∑ 𝑡geometry,i,j − 𝑡measured,i,j
2
𝜒 =
(1)
[ (
)]2
𝜎 𝑡measured,i,j𝑗𝑖where the antenna positions (geometry of the array)
are adjusted at each iteration until minimization has
been obtained.
The number of degrees of freedom (DoF) for the minimization parameters is equal to 3 axes of movement + 1
Page 9 of 23

The BEACON Prototype Instrument

1.0
80
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Elevation (deg)Normalized Power

60

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40

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2.5

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Beam Voltage Threshold

90

Figure 11: Top: The current beam map, with gray-scale color map corresponding to the normalized maximum power perceived inany beam for a mock signal arriving from each point on the map; maximal sensitivity/power is achieved in the nominal directionsof each beam. Each beam is labeled and circled with radius set to 3 dB below that beam’s max power. Middle: The thresholds
for each beam during a quieter run. The measured power SNR, referenced to the instantaneous noise from the DAQ, is shown on
the right axis. Beam voltage SNR is shown on the left axis and is computed from cosmic ray simulations as described in the text.
Colors of each line correspond to the same colors used in the top plot, with beams near the horizontal being solid red lines, andabove horizontal beams being dashed blue and green lines. The near-horizon beams generally exhibit a higher power threshold, as
expected from anthropogenic noise. Some above-horizon beams point to prominent sidelobes of below-horizon RFI, and will also
show elevated thresholds. Bottom: The long-term distribution of thresholds in each beam over the ∼112 day period discussed in
Section 3.

cable delay per antenna, leading to 16 total DoF. The numberof distinct measurements provided by 𝑁site pulsing sites isgiven by DoF = 𝑁site ⋅ 𝐶(𝑁antenna , 2) where 𝐶(𝑛, 𝑟) is thechoose operator, which determines the number of arrival
D. Southall et al.: Preprint submitted to Elsevier

time differences (baselines) that can be calculated whencomparing 2 antennas from a set of 𝑁antenna . Minimization
was performed independently for each polarization, allowing
for variations in phase centers between HPol and VPol
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The BEACON Prototype Instrument

antennas. Pulsing locations are chosen to be far from thearray such that variations in timing from uncertainties intheir locations are negligible and do not add additional DoF
to the minimization.
Initial position measurements of the antenna masts weremade with the Real Time Kinematic (RTK) technique thatcompares GPS positioning of two nearby GPS antennas- resulting in cm level precision by correcting for the
propagation conditions in the local atmosphere [68]. Eachantenna mast includes a dual-band GPS patch antenna,
which may be connected to a GPS receiver on demand.
We use a u-blox C099-F9P application board (ZED-F9P
GPS [69]) to measure the position of each antenna, with corrections provided by a UNAVCO GPS station permanently
installed ∼30 m away from the BEACON site at 37.58915N,
118.23844W [70].
Pulsing data was taken over the course of 3 days during acalibration campaign in 2021, during which pulsing data wastaken for 6 separate sites in both polarizations. The transmitter included a high-voltage pulser (FID technologies FPM
10-1PNP) driving a biconical antenna (Aaronia BicoLOG
30100E) at known rates, with varying fixed attenuators.
These pulsing data are used in the fit described in Equation 1.
The resulting errors on the phase centers are estimated to beless than 5% of the shortest relevant wavelength.
The source direction is reconstructed using interferometry [71]. Cross correlations are calculated for each pair ofantennas in separate polarizations. The cross correlation fora given antenna pair is expected to peak at a time delayconsistent with the arrival direction of the signal. A “correlation map” is formed by sampling these cross correlationsat delays expected for each direction. Each direction in themap corresponds to the average correlation value from eachbaseline when sampled at the expected time delay for that
direction. The expected time delays depend on the geometryof the array and source direction on the map and are often
calculated assuming a plane wave for distant sources. The
peak value of the map corresponds to the direction which
has sampled each baseline’s cross correlation nearest the
maximum. Each baseline is weighted equally and is normalized such that identical signals result in a maximum crosscorrelation value of 1; a map generated with identical signals
in each channel would also result in a peak value of 1. Realsignals vary slightly across antennas, so the optimal map
value depends on each event and is typically < 1.
A perfect impulse would have a single peak in a crosscorrelation, resulting in a single ring of possible arrival
directions on the sky for each baseline due to the symmetryaround the axis connecting those 2 antennas. By averaging
maps of all 6 baselines, the degeneracy of these rings is broken, with all baselines overlapping only in a single location
for an impulsive plane-wave in a properly-calibrated array.
This requires a sufficient number of baselines to fully breakdegeneracy, or ambiguities in pointing can occur. Though
the BEACON prototype has a sufficient number of antennas
to accurately point to most impulsive RF sources, narrowband signals result in highly periodic cross correlations
D. Southall et al.: Preprint submitted to Elsevier

which in turn produce a series of concentric rings on themaps per baseline, increasing the degeneracy of potential
source directions. This problem can be exacerbated by thepresence of unrelated continuous wave (CW) noise, coincident signals from other RFI, or by the source signal itself
being insufficiently impulsive.
After minimization, signals from the mountainside pulsers
show maximal reconstruction offsets of ∼1◦ , with the majority of sites showing offsets <0.5◦ . The accuracy is discussed
further in Section 3.2 when presenting airplane reconstructions, which provide an external source of signals withknown directions and show a systematic offset of 1-2◦ . The
mountainside pulsers provide a limited range of elevation angles for the calibration minimization, which could contributeto the observed reconstruction error. Additionally, the cabledelays can have a degenerative effect with antenna position
within the minimization for adjusting baseline timings,
which also could be the source of the discrepancy. In future
efforts we aim to address these issues with a drone pulser(Section 4), which would provide a significant increase inangular range used for calibration.
The precision of the prototype to reconstructing the arrival direction of stationary radio signals was experimentally
determined by reconstructing arrival directions of belowhorizon RFI sources, the majority of which arrive from a
few very loud stationary emitters. A 2D Gaussian was fit to
7 of the most prevalent sources, with an average 90% integral
area for the fits of < 0.1 sq. degrees (see Figure 13).

3.2. Characterization of RFI
The BEACON prototype instrument is positioned near
the border of California and Nevada and looks East over
the Nevada desert. This region is populated by a series ofsmall towns with agricultural and mining industries, militarybases, and power infrastructure like the Crescent Dunes
Solar Project. The site is also just south of a common commercial air flight path. Though all of these anthropogenic
sources are tens to hundreds of kilometers away from thesite, many sources are visible to BEACON due to its highelevation and sensitivity to radio signals at signal strengths
near thermal levels.
In this Section, we discuss several ways this anthropogenic activity appears in the data taken with the prototype
instrument. The vast majority of anthropogenic signals in thedata can be easily separated from cosmic-ray and neutrinosignals due to signal shape, polarization, spatial and temporal clustering, and other event characteristics.
Static Sources: The most common category of events
come from towns and infrastructure. These events cluster
spatially, are expected to be localized to a single beam, andtherefore can be cut based on their direction. The signal
shapes observed from different RFI sources can vary significantly, however signals from a single source are generally
very consistent.
Continuous Wave Sources and the Television Band: CW
signals are narrow band, arriving at the array with very little
temporal variation. Because of this they are often not directly
Page 11 of 23

The BEACON Prototype Instrument

N

0

E
22

E-W Slope

40.1

m

55.

49.5 m1m

130.8 m5m

46.

3

26.0 m2
Figure 12: A top-down view of the array layout in local
East-North-Up (ENU) coordinates. Positions correspond to
calibrated HPol phase centers. Baseline distances have beenlabeled for each antenna pair. The slope the antennas are
situated on is rugged and generally amorphous, however theapproximate downhill slope across the array in the East-Westdirection is 22◦ . Relative to the lowest antenna (mast 0),
the heights of 1, 2, and 3 are approximately 15.9 m, 4.0 m,
and 13.7 m respectively. The size of each antenna has beenmagnified 5× compared to baselines for visibility.

responsible for activating the trigger (which is designed for
temporally impulsive signals); however, they are commonly
visible in the spectrum of triggered broadband signals. We
typically remove these from the data in offline analysis via
notch filters and the sine subtraction technique described in
Section 3.3.1. The nominal band of the BEACON antennas
overlaps with common radio communication frequencies, aswell as the low-VHF television (TV) broadcasting range.
Signals from the KHSV TV station in Las Vegas are pervasive in all HPol data, despite the transmitter being over
300 km away and lacking a direct line of sight to BEACON.
A notch filter is currently used in analysis exclusively in
HPol channels to combat this signal. Figure 14 shows the
spectra of HPol and VPol antennas over the course of a few
hours. Bright horizontal lines in these plots correspond to
CW sources. The TV band is visible in HPol from ∼53 to
60 MHz. Intermittent short bursts of activity can be seen at
42 and 48 MHz, which are associated with communication
systems for the California Highway Patrol and the Los
Angeles Department of Water and Power, respectively.
Periodic Noise Sources: An excess of events have been
observed to arrive at the BEACON prototype with time
D. Southall et al.: Preprint submitted to Elsevier

differences corresponding to multiples of 1/(60 Hz). Thesesignals can be associated with arcing or similar discharges
from power infrastructure, which operates at 60 Hz in the
US. A 55 kV high-voltage transmission line connecting
Nevada with the Owens Valley runs within the field-ofview of the prototype, with several substations. When there
is snow on the valley floor, this class of signal largely
is suppressed, perhaps because the snow is acting like aninsulator to prevent arcing.
Similar to CW, this category is a subset of static sources
and can be removed with directional cuts. However, since
it may be advantageous to keep those directions in somesearches, an algorithm was developed to demonstrate removal based on timing alone. We define a temporal test
statistic (TS) which gives a measure of the relative abundance of temporally nearby events with trigger times consistent with a period of 𝑇 . For each event, 𝑖, the differencein trigger times is calculated for a range of nearby events,
indexed by 𝑗, within a specified time window 𝑤. To allow for
multiples of the period, we calculate the absolute difference
to the nearest multiple of 𝑇 using:
)
|(
𝑇|
𝑇
% 𝑇 − || ,𝑑𝑖,𝑗 = || 𝑡𝑖 − 𝑡𝑗 +
2
2|
|

(2)

where % refers to the floored modulo operation (𝑎%𝑛 =𝑎 − 𝑛⌊ 𝑎𝑛 ⌋), resulting in 𝑑𝑖,𝑗 being near zero for times close to
an integer multiple of 𝑇 . Within each window 𝑤 containing
𝑁𝑤 (𝑖) events, we construct a histogram with 20 bins ranging
from 0 to 𝑇 ∕2. The top right portion of Figure 15 shows
an example histogram, with red highlighting the bin, 𝑐𝑖,0 ,containing the 𝑑𝑖,𝑗 most consistent with a periodicity of 𝑇
and green highlighting the 50% of bins least consistent with
𝑇 . The test statistic for that event (TS𝑖 ) is defined as thedifference between the red region and the mean of the green
region, given by:
1 ∑TS𝑖 = 𝑐𝑖,0 −𝑐
10 𝑘=10 𝑖,𝑘19(3)Arrival times from a uniform distribution would result inno significant difference in counts between the red and green
regions, resulting in a TS near 0, while a set of perfectly
periodic events would all lie within 𝑐𝑖,0 , resulting a TS of
1. Datasets contaminated with periodic noise sources are
in-between these two extremes, resulting in a distributionof TS that is broadened when compared to uniform arrival
times. Periodic events can thus be highlighted from withina contaminated set of data by cutting on high TS values. In
Figure 15 we show how this algorithm can separate events
observed arriving at a regular 60 Hz rate in data taken from
September 2021. This figure also shows how the baselinetiming of the periodic events fluctuates with time as the
60 Hz drifts, which is handled by choosing a value for 𝑤
that is short relative to the fluctuations. Depending on thedesired efficiency, this algorithm will not flag all events
arriving with a periodicity of 𝑇 , but can isolate a clean
subset of those events, which can then be used to motivate
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The BEACON Prototype Instrument

RFI Source 0
RFI Source 1
RFI Source 2
RFI Source 3
RFI Source 4
RFI Source 5
RFI Source 6
Horizon10
60

40

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105
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5

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101
2

1

Azimuth (deg)0Figure 13: Top: Reconstruction direction of events from one week in September 2021. Seven of the most populated RFI sources
have been highlighted. These sources are fit with a two-dimensional Gaussian after isolating the events in each region. Bottom
Left: Isolated events from RFI Source 3 (arbitrarily chosen as an example). Bottom Right: 2D Gaussian fit (color map), withoutline of the 90% integral area of the fit plotted on top. Note that the color scale is logarithmic and represents counts for all
3 plots. The average fit 90% integral area for all 7 sources was < 0.1 sq. degrees. The approximate location of the horizon has
been indicated at an elevation angle of -1.5◦ .

further targeted cuts based on template matching, direction,and signal properties to further improve the efficiency forremoving this form of RFI.
Airplanes: One of the few above horizon sources of
RFI is airplanes. As part of the above-horizon impulsive
events search discussed in Section 3.3.2, over 1000 RF triggered events were associated temporally and spatially withairplanes, corresponding to > 100 individual airplanes, anapproximate observation rate of (1) airplane per day. Airplane signals have been identified by other experiments [49];though several potential sources have been described, no
definitive cause for these signals has yet been determined.
The signals differ greatly in shape between airplanes andare not present for the majority of airplanes passing by thesite. Because of this we believe that the airplanes are not thesource of these signals but rather serve as reflectors to signals
originating on the ground. Therefore, we do not expect tosee signals from all airplane tracks in our band. An example
airplane-associated series of events is shown in Figure 16,
which also shows the self-reported trajectory of the airplanesuperimposed [65]. Apparent in this figure is a systematic
offset in reconstruction direction for airplanes. This offset
is approximately 1◦ in HPol and 2◦ in VPol (where eachpolarization is calibrated independently). This offset is small
D. Southall et al.: Preprint submitted to Elsevier

and does not significantly impact the results of this analysis,
however understanding and fixing it is a priority for future
analysis (see Section 4).

3.3. Above-Horizon Impulsive Events
We have categorized impulsive, above-horizon events inthe prototype instrument data, identifying a variety of eventclasses [72]. We are especially interested in above-horizon
impulsive events because they contain a sample of cosmic
ray air shower events, which can be used to determine thenominal sensitivity of the prototype instrument.
We expect a few cosmic ray events per day to trigger
the prototype given nominal beam voltage SNR thresholdsof 5𝜎 [67]. Cosmic ray candidates will appear as isolated
above-horizon events that are not identifiable as RFI eventsand do not cluster spatially or temporally. Additionally,
cosmic ray events are expected to be impulsive signals that
correlate well with cosmic ray templates from simulations.
They will also have a polarization angle correlated with thesource direction and the direction of radio emission from air
showers in the local Earth’s magnetic field (i.e. ∼ 𝑣×
⃗ 𝐵⃗ [37]).
Here we describe our classification process for abovehorizon impulsive events, and show a likely cosmic ray
candidate event from the data. We present an analysis of∼112 days of data taken from the beginning of September
Page 13 of 23

The BEACON Prototype Instrument30

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30

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35

Figure 14: Spectrograms of the HPol (top) and VPol (bottom)
channels of antenna 0 generated using untriggered events
recorded once per second during a run in October 2021. Several
features are highlighted in the spectrograms, including examples of CW noise, the TV broadcasting band, and intermittent
RFI at 42 and 48 MHz associated with radio communications.
The color map is presented in arbitrarily offset dB units (aconversion between ADU and volts has not been performed).
Strong CW sources are capable of saturating the system, which
results in aliasing that is apparent as a coincident broadband
increase in power within the spectrogram.

to the end of December 2021, consisting of ∼100 million
RF-triggered events.

3.3.1. Identifying Above-Horizon Impulsive Events
We first filter the data to remove both known frequencies of anthropogenic noise with static notch filters (at 27,
88.5, 107, 118, and 126 MHz in both polarizations, andadditionally from 52.5 to 60.25 MHz in HPol channels,
which removes RFI associated with the TV band). We use amethod called sine subtraction filtering [73], where we filter
the signals by fitting sine waves in the time domain withfloating phase and amplitude, and remove any frequencies
with amplitude above a threshold set in the analysis. Thismethod preserves causality in the data. We then remove thegroup delay added by the RF signal chain in the data torecover the original phase of the incident signals.
We then create a correlation map for each event andidentify the most likely incident arrival direction for eachby selecting the location of the peak cross-correlation value
from one of three maps: 1) HPol channels only, 2) VPolchannels only, and 3) the average of the two polarized maps.
We choose to use the peak location from the map thathas the maximum peak-to-sidelobe ratio multiplied by thenormalized map peak value. The peak-to-sidelobe ratio is
the ratio of the main peak to the second brightest peak in
D. Southall et al.: Preprint submitted to Elsevier

the correlation map. The normalized map peak value is theratio between the peak value and the optimal possible map
value for that event, which would be obtained if a particular direction perfectly sampled the peak of each baseline’s
cross correlation. Normalizing map peak values in this way
counteracts the trend for low SNR events to have lower
correlation values and thus lower map peak values. For each
map we mask out the direction of the mountainside itself(defined as the area below a simple plane fit to the antenna
locations).
We then separate above-horizon from below-horizon
events, which removes the vast majority of triggered events,
which are dominated by static below-horizontal RFI sources(see Section 3.2). We keep events in our sample that havean arrival direction between [−90◦ , 90◦ ] in azimuth (East =0◦ , North = 90◦ ), and [10◦ , 90◦ ] in elevation as shown in
Figure 17. The lower bound in elevation of 10◦ above thehorizontal is chosen to be far from the true horizon (whichis ∼1.5◦ below the horizontal), to create a cleaner sample ofdowngoing events. The azimuthal cut restricts the search to
the direction that the array is most sensitive to, which is to
the East, since it is on an East-facing slope. Sources of RFI
are finely resolved, suggesting that clustering could remove
backgrounds in future searches.
We then develop a series of cut parameters to select for
impulsive, isolated events that correlate with a cosmic ray
template. We intentionally keep these cuts loose so any one
cut is not overly restrictive, in order to investigate a variety
of classes of events of interest above the horizon, while
keeping a high fraction of triggered cosmic ray events in theremaining event sample. After all cuts, the data set is reduced
to 5,440 events. We list the cuts below, and in Table 1, alongwith the numbers and fraction of events that survive eachcut. The cuts are defined as:
Time Delay Clustering Cut: Remove events that are inruns with more than 10 events that have the same measured
arrival time delays between antenna pairs (with an absolute
tolerance of 2.5 ns per baseline). Runs in the data set are 1
to 3 hours long. This cut is used to remove events that comefrom the same direction.
Peak-to-Sidelobe Ratio: Remove events for which theratio of the main peak to the second brightest peak in the
HPol and VPol correlation maps sums to less than 2.15.
A peak-to-sidelobe ratio near 1 indicates two peaks withcomparable brightness. This cut removes events where it islikely that the event could be mis-reconstructed, i.e. the main
peak is indistinguishable from the sidelobes.
Impulsivity: Remove events that have summed HPol and
VPol impulsivities () below 0.3.  is a metric for measuring
the impulsiveness of a signal [74], defined here as  = 2𝐴 −1, where 𝐴 is the average of the cumulative distribution of
fractional power contained within a 400 ns window centeredon the peak of the Hilbert envelope of the aligned andaveraged waveforms for a particular polarization.
Cosmic Ray Template Correlation: Remove events forwhich neither polarization obtains a normalized correlation
value of 0.4 with a simplified cosmic ray template. The
Page 14 of 23

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Angledeg)1501530456075Elevation

90

=

North

deg,

0

=

East

(From

80604020020406080100Azimuth

Figure 15: Left: The arrival time of RF-triggered events within a run from September 2021, with sub-second timing plotted on
the y-axis; events not flagged are shown in the top left, with events flagged by the algorithm to be consistent with an arrival
rate of 60 Hz (with corresponding periodicity of 𝑇 = 1∕60 s) shown in the bottom left. Insets show striations in the bottom plot
consistent with the expected periodicity. These flagged events represent ∼20% of the total events in the 1 hour span shown. Top
Right: Histograms showing the portion of events arriving at an interval consistent with 𝑇 for the highest test statistic (TS) event.
The TS is the difference in counts in the red region to the mean of the green region. Histograms created using window 𝑤 = 20 s.
Bottom Right: A histogram of all TS values for this run. An example cut has been applied near the limit of the TS as calculated
for uniformly distributed trigger times, beyond which events are highly likely to be consistent with 𝑇 . The events flagged will
be used to motivate further targeted cuts based on direction, template matching, and signal properties, to further improve theefficiency for removing this form of RFI.

Figure 16: Left: The stacked correlation map of 52 events corresponding to a single airplane track, with a colorscale corresponding
to maximum correlation map value obtained from any event’s individual map generated using all 8 channels. The track of thecorresponding airplane using ADS-B data obtained from The OpenSky Network [65] is shown with the black line, and spans ∼3.5
minutes. The expected location of the airplane at the time of each triggered event in the map is shown with the black dots,
and the measured location of the peak correlation value of each triggered event is shown with the blue dots. Upper Right Inset:
Scatter plots showing the reconstruction offset observed for all airplanes when observed using either HPol (blue) or VPol (red)antennas, with a corresponding 2D Gaussian fit for each. This plot demonstrates an observed systematic offset of approximately
1◦ in HPol and 2◦ in VPol (each polarization is calibrated independently). This offset does not show significant angular or temporaldependence and is likely a result of the calibration. Additionally, the 90% integral area of the Gaussian fit observed for these
events is larger than the < 0.1 sq. degrees observed for static sources in Section 3.1. The original calibration was performed using
mountainside pulsing and showed maximal reconstruction offsets of ∼1◦ . Future calibration campaigns using calibration sources
mounted on drones would allow us to better constrain antenna positions by providing a large range of elevation angles for fitting
and validation. Calibration is discussed further in Section 3.1.

template used was a bipolar impulse with duration andamplitudes motivated by an off-axis angle of 1.37◦ for aslightly upgoing air shower [75]. This signal is then convolved with the appropriate channel-dependent responsesof the prototype instrument, before undergoing the same
filtration and cleaning as the waveforms, to create a templatefor correlation.
Likely Mis-Reconstructions of Known Below-Horizon
Sources: To remove events that mis-reconstruct above horizon due to prominent sidelobes of below horizon sources, a
set of bright below horizon sources were identified. Eventsare cut if their best below-horizon reconstruction direction
is associated with a known RFI source.
D. Southall et al.: Preprint submitted to Elsevier

Signal Amplitude Differences: Remove events that havesignificant peak-to-peak (P2P) voltage differences between
HPol channels, where the Max(P2PH ) is 95 adu or more
above the Min(P2PH ). This removes events where a subsetof channels is significantly brighter than the rest (indicativeof local noise at the array or electronics issues), and small
sample of events where one channel is not functioning
properly.
Combined Normalized Map Peak Value: Remove events
that do not achieve a threshold percentage of their optimalachievable map value, using a combination of VPol and HPol
maps. We remove events where 0.768𝑚H + 0.640𝑚V − 0.960

Page 15 of 23

The BEACON Prototype Instrument
100

Normalized
Counts

Before Cuts
If Last Cut
Remaining 36 Events

10 2

Cut Regions
5911-73399
Horizon

10 4
10 6
20

0

Elevation (deg)

20

150

100

50

60

80

Normalized
Counts

100

40

10 2
10 4
10 6
0

50

100

Azimuth (deg)150

Figure 17: Arrival directions of the received radio signal at the
BEACON prototype for the full data set (black), the data set
remaining after all other cuts have been applied (blue), and the
36 remaining events discussed in Section 3.3.2 (yellow). The
reconstructed elevation (azimuth) for each event are shown in
the top (bottom). Regions shown in red are excluded by the
cut value placed at the red line. For reference, the parametervalues for the likely cosmic ray candidate event (discussed in
Section 3.3.2) is shown with the yellow vertical line (Event
5911-73399). The approximate location of the horizon is shownon the top plot at an elevation angle of -1.5◦ .

Normalized
Counts

Before Cuts
If Last Cut
Remaining 36 Events

10 1

Cut Regions
5911-73399

10 3
10 5
0.00

0.25

0.50

0.75

1.00

1.25

0.5

0.6

0.7

0.8

Impulsivity H + V

1.50

Normalized
Counts

101
10 1
10 3
10 5
0.2

0.3

0.4

0.9

CR Template Correlation, Max(H,V)

1.0

Figure 18: Representative distributions of the impulsive character of the full data set (black), the data set remaining after
all other cuts have been applied (blue), and the 36 remaining
events discussed in Section 3.3.2 (yellow). The red line andregion represents the cuts on both the combined impulsivity in
HPol and VPol channels and a correlation with a CR template.
These cuts require the signal to be impulsive but are loose
enough to allow for a variety of signal classes to classify abovehorizon events. For comparison, the parameter values for the
likely cosmic ray candidate event (discussed in Section 3.3.2)
is shown with the yellow vertical line (Event 5911-73399).

is less than 0, where the normalized map peak value of theabove-horizontal region in each polarization is 𝑚.
Combined Peak-To-Peak / (2 ⋅ Standard Deviation): Remove events where the signal amplitude (calculated as peakto-peak divided by 2) is not sufficiently above the standard
deviation of the observed ADC counts in that waveform.
D. Southall et al.: Preprint submitted to Elsevier

Note that the standard deviation is calculated on the entire
waveform, which includes the signal, so this metric is distinct from the SNR. We remove events where the parameter
0.878𝑟H + 0.479𝑟V − 5.267 is less than 0, where 𝑟 is the ratioof half of the peak-to-peak over the standard deviation ineach polarization.
We show histograms of event distributions for a representative set of cut variables targeting impulsive events in
Figure 18, specifically for impulsivity and the correlation
with a cosmic-ray template as defined above. The cuts inthese two metrics were relatively loose, allowing us to investigate the varied signals we observe with the prototype.
We highlight one event in particular that has a high value inboth of these metrics.

3.3.2. Remaining Above-Horizon Events
The remaining 5,440 events were inspected by hand.
We found that three broad categories of events remained,as shown in Table 1. This hand-categorization of impulsive events that appear to come from above the horizon
is important for understanding the RFI environment of the
BEACON prototype site, to inform future design decisions
and future analyses of the data. Events were categorized into
three broad categories: likely mis-reconstructions of RFI
that originates from below the horizon, events associatedwith airplanes, and other impulsive above-horizon events.
Likely Mis-reconstructions of below-horizon events andevents with amplifier instability: The largest category is
events that are likely to be mis-reconstructions of belowhorizon sources of RFI and events that exhibit instability
in the amplifier chain, constituting 75% of the data set thatpassed all cuts described in Section 3.3.1. The vast majorityof triggered events originate from below the horizon, as
shown in Table 1, and if the correlation map peaks on a true
sidelobe of the signal, it is possible for such events to appear
to come from above the horizontal and pass the elevation cut
applied to the data. Manual inspection of the correlation map
can identify these events. Additionally, events are identified
that have features in the data that are a result of instability inthe amplifiers used in the electronics chain as well as events
containing multiple impulses which can create false crosscorrelations above the horizon.
Events associated with airplane tracks: The next largest
category is events that were associated temporally andspatially with over 100 known airplane trajectories fromThe OpenSky Network [65], which contains an extensive
database of ADS-B airplane data that most airplanes are
required to transmit [76,77]. An example airplane track seenin the data is shown in Figure 16. 64 individual airplanes
were associated with at least four triggered events, and six
airplanes caused 50 or more triggered events. Additional
events created other temporally clustered trajectories acrossthe sky, but with no known corresponding airplane track inthe database; these events have also been tagged as likely
airplane events. This category of events constitutes 24% of
the sample.

Page 16 of 23

The BEACON Prototype Instrument
Cut Name

Number of Events

Fraction Cut

Fraction Cut

Remaining

Sequentially

if Applied First

Full Data Set

96,483,288

Elevation

1,830,144

0.98

0.98

Azimuth

1,145,593

0.37

0.0075Time Delay Clustering, HPol

1,116,064

0.026

0.95

Time Delay Clustering, VPol

1,104,002

0.011

0.85

Peak-to-Sidelobe Ratio

201,926

0.82

0.065

Impulsivity

57,669

0.71

0.029

Cosmic Ray Template Correlation

42,184

0.27

0.028

Associated with Below-Horizon Sources

38,274

0.093

0.79

Signal Amplitude Differences

15,8090.59

0.0038Combined Normalized Map Peak Value

7,894

0.50

0.23

Combined Peak-to-Peak/(2 ⋅ Standard Deviation)

5,440

0.31

0.044

Number of Events

Fraction of Events

4,081

0.75

Events associated with airplanes

1,323

0.24

Remaining above-horizon events

36

0.0066

Hand-inspection breakdown of the 5,440 passing events:
Likely mis-reconstructions from below the horizon
and Events with unstable amplifiers

Table 1
Summary of analysis cuts. There are two stages of analysis: application of a variety of cuts (above the double line in the table)
and a hand-inspection of events that pass those cuts (below the double line in the table). The cut parameters and cut values usedin the first stage of the analysis are described in Section 3.3.1. The table shows the number of events remaining after each cut
is applied sequentially, the fraction of events rejected by each cut when applied sequentially, and the fraction of events that are
rejected if a given cut is applied first in the analysis. The categorization of events by subsequent hand-inspection of the passing
5,440 events is also shown.

Remaining events: Of the 5,440 events which passed thecuts aimed to identify impulsive above-horizon signals, only
36 (less than 1%) were not associated with airplane tracks
and were not categorized as likely mis-reconstructions of
below-horizon events or events with unstable electronics;
parameter distributions of these events are included in Figures 17 and 18. The events are uniform in azimuth and show
some structure in the elevation angle. The structure could beconsistent with either sidelobes from below-horizon sources
or cosmic rays, which are expected to be highly inclined
on average for the BEACON geometry. Understanding this
distribution will be the subject of future analyses (Section 3.3.3). One event of interest from this sample is shownin Figure 19 (event 5911-73399), and is a likely cosmic ray
event. The remaining events are of as yet unknown origin
and will be the subject of future study; these events are impulsive and above horizon and may include a combination ofunidentified backgrounds and additional cosmic ray events.
The candidate cosmic ray event has the third highest
impulsivity of all 5,440 events that pass cuts, and the highest
among the 36 remaining events. Further inspection of thetwo events with higher impulsivity categorized them as
D. Southall et al.: Preprint submitted to Elsevier

a likely mis-reconstruction of a below-horizon event anda likely airplane event. The candidate event also has thehighest SNR (beam voltage SNR of 91 𝜎 in HPol; 58 𝜎 in
VPol for the processed waveforms), peak-to-sidelobe ratio(> 1.7 for each polarization), and template correlation values (> 0.83 for each polarization) among the 36 remaining
events. This event also does not occur during a time ofsignificant lightning activity.
Figure 20 shows the waveform from the event of interest
alongside an event waveform generated with the cosmic ray
simulation [67] and compares the observed linear polarization angle and arrival direction with simulated distributions.
The tangent of the polarization angle is calculated as the ratioof the maximum of the aligned and averaged de-dispersed
waveforms in VPol to HPol when upsampled and symmetricfiltering is applied across polarizations (such that VPol is filtered with the TV notch filter as well, ensuring similar poweris lost in both averaged waveforms and a representative ratio
is preserved). In this way the measured polarization angle of∼28◦ is consistent with the purely geometric expectation of∼30◦ , with an uncertainty in the polarization measurementof ∼2◦ , corresponding to the ∼10% observed variance inPage 17 of 23

The BEACON Prototype Instrument

gain matching among channels. The geomagnetic expectation is for a signal arriving from the appropriate arrival
direction and local magnetic field for this event.

3.3.3. Future Work
The categorization of impulsive above-horizon events inthe prototype instrument data set is a critical step in defining cuts for future cosmic ray searches. While informative
for this analysis, the hand-inspection of events after cuts
are applied indicates that additional automated cuts would
need to be made to perform a true cosmic ray search. For
example, the structure in the spatial distribution of events
seen above the horizon (e.g. the elevation distribution in
Figure 17) indicates that a set of clustering cuts to remove
events associated with below-horizon sources would be effective. We are planning further analyses that will leverage
our understanding of the prototype system and local RFI
sources to perform a cosmic ray search with the data. These
searches will benefit from search metrics that are efficientat removing backgrounds and identifying cosmic ray eventswith low SNR. Cuts that take advantage of the directional
and temporal clustering in RFI sources may be sensitive to
weaker signals; however, confidence in identification can be
improved when clustering is combined with cuts that emphasize the impulsive characteristics of cosmic-ray signals.
Additionally, as discussed in Section 3.3.2, the polarization
angle of air shower events is a predictable metric dependent
on local magnetic field and signal arrival direction. In theimpulsive above horizon search presented here we use this
only as a check on our final candidate, however in future
searches polarization angle will serve as a powerful metricfor automated cuts. Combining the results of that searchwith input from the cosmic ray simulation will lead to an
updated sensitivity estimate to tau neutrinos of the full-scale
BEACON array.

4. Conclusions
The BEACON prototype instrument has been in operation since 2018. The current station design is robust andwith its custom antennas and phased array trigger represents
important first steps towards a scalable implementation of
the full BEACON array.
We have used data from the prototype instrument toverify the performance of the array and understand the
RFI environment at the BEACON prototype site. We havedeveloped analysis techniques to identify above horizon RFI
sources such as airplanes, and to isolate events consistent
with the expected properties of a cosmic ray. The resultsof this analysis have already validated the phased trigger’s
ability to maintain sensitivity to above horizon events using asmall number of antennas in a noise-dominated environment
like the Californian and Nevada deserts. While the RFI
rates at the current prototype site are higher than would be
beneficial for a larger instrument, the environment provides
an important stress test of the trigger’s capabilities.
The next stage for the BEACON prototype is to develop a
full cosmic ray search trained on simulated data and building
D. Southall et al.: Preprint submitted to Elsevier

on the background studies presented here and on techniquesfrom other autonomous searches for radio signals from air
showers. The dominant source of backgrounds come frombelow the horizon and are well clustered both spatially andtemporally, suggesting that they may be removable as has
been done in prior searches [30, 34, 78]. We plan to conduct
a template search based on simulated radio emission fromcosmic rays and exploiting clustering cuts.
Since anthropogenic noise predominantly comes frombelow the horizon and constitutes the main source of background, there will be a need for more background rejection
power in a search for upgoing tau neutrinos compared todowngoing cosmic rays. This ultimately could translate toa loss in analysis efficiency for a given background rate.
However, we may be able to further exploit differences in thecharacteristics of the signals – their spectra, their isotropy,
and impulse response – relative to the backgrounds. We canalso further tune the beamforming trigger to down-weightor directionally mask regular sources of RFI at a given site.
These studies will be important to pursue in future works.
The concept for each BEACON station includes moreantennas and longer baselines than the prototype and could
therefore achieve lower thresholds and improved background rejection. Measurements of correlation map characteristics, SNR, pointing resolution, and trigger thresholds all
benefit from the additional antennas and longer baselines of afull station, enabling better separation of below-horizon andabove-horizon events. Additionally, multiple stations withdiffering views of overlapping effective volumes can be used
to veto anthropogenic noise, compared to air shower signals,
which are highly beamed.
Finally, we note that we are exploring hardware upgrades
for the prototype. Antenna position calibration and trigger
validation using an RF source mounted on a drone canenable a more complete calibration of the in-situ antenna
beam patterns [79]. This drone pulser will also be used tofurther understand the observed elevation offset in abovehorizon events from airplanes (see Figure 16), and determine whether this offset is intrinsic to the hardware, current
calibration, or in our interpretation of the airplane database,which is important for trusting above-horizon reconstruction
accuracy in future analysis. While not cost-effective fora full-scale detector, adding scintillators to the prototype
can improve cosmic ray identification and validation at the
prototype stage. Signals from scintillator detectors could bedigitized alongside the existing RF channels and serve tovalidate RF-only triggered events [80]. Finally, an updated
DAQ is being designed, which because it is modular andflexible, can form the basis of autonomous stations withmore antennas. This will allow us to scale the BEACON
detector to the hundreds or thousands of stations needed todetect the tau neutrino flux.

Page 18 of 23

Filtered Waveforms (adu/128)
Offset by Antenna Number

The BEACON Prototype Instrument

3

3

2

2

1

1

0

0

HPol
0

100

200

300

400

Time (ns)

VPol

500

0

100

0

30

0

30

200

300

Time (ns)

400

500

60

60

45

45

30

30

15

15

0

0

15

Local
30 Mountainside
90
45
0

45

Azimuth (deg)

HPol

90

PSD
(dB, arb)

75

20
0
20

15

Local
30 Mountainside
90
45
0

45

VPol

Azimuth (deg)Filtered PSD(dB, arb)

Elevation (deg)

40
75

90

80

150

80

150

40
20
0
20

Frequency (MHz)Figure 19: Event display for a likely cosmic ray event (Event 5911-73399). Top: Waveforms from each of the 8 channels, normalized
and offset such that the y-scale indicates the antenna number for each waveform. This event has an averaged single-channel
voltage SNR of 42.5 in HPol and 38.6 in VPol. Bottom Left: HPol and VPol correlation maps. The colorscale of each map isindividually normalized, and the region of the maps pointing into the local mountainside is masked out. Bottom Right: The PowerSpectral Density (PSD) before and after filtering. The data has been filtered as described in Section 3.3.1.

Sample Simulated Cosmic Ray Event
Run 5911 Event 73399

200

0.25

Relative Frequency

5911-73399
Polarization = 27.6

0.20
0.15
0.10
0.05
0.000

10

20

30

40

Polarization Angle (deg)50Time (ns)

60
50
40
30
20
10
0 60

300

400

500

5911-73399

60

30

0

Azimuth (deg)30

0.01500.01250.01000.00750.00500.00250.0000Relative Frequency

100

Elevation (deg)Filtered Waveform
Antenna 2H (adu)

100
75
50
25
0
25
50
75
100 0

60

Figure 20: Top: The waveform for Event 5911-73399 from Antenna 2H superimposed with a sample simulated cosmic ray signal
with realistic thermal noise levels [67], which has been convolved with the system response of the same channel. Both waveforms
have been filtered as described in Section 3.3.1. Bottom Left: The distribution of expected observed linear polarization angles fortriggered simulated events. The polarization angle of the cosmic ray candidate event is shown with a yellow line. The measured
polarization angle of ∼28◦ is consistent with the purely geometric expectation of ∼30◦ , calculated assuming a geomagnetic signal
arriving from the appropriate arrival direction and local magnetic field. Bottom Right: The distribution of expected azimuth andelevation for simulated events compared to the candidate cosmic ray event (in yellow).

D. Southall et al.: Preprint submitted to Elsevier

Page 19 of 23

The BEACON Prototype Instrument

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