POEMMA-Balloon Radio Pathfinder

• POEMMA Balloon with Radio (PBR) is a balloon-borne, multi-messenger observatory that integrates optical and radio instruments to detect ultra-high-energy cosmic rays, high-altitude air showers, and neutrinos.
• The mission features a modified Schmidt telescope with hybrid focal planes and dual sinuous antennas, enabling simultaneous observations in fluorescence, Cherenkov, and low-frequency radio bands.
• It acts as a pathfinder to validate innovative detection strategies, refine subsystem performance, and advance multi-detector synchronization for future space-based studies.

Searching arXiv for recent PBR papers and related POEMMA pathfinder literature. arXiv search query: "POEMMA Balloon with Radio PBR" POEMMA-Balloon with Radio (PBR) is a balloon-borne, multi-messenger, multi-detector pathfinder for the proposed Probe Of Extreme Multi-Messenger Astrophysics (POEMMA) mission. In the published PBR design literature, it is a compact, sub-orbital implementation of the POEMMA concept: a wide field-of-view Schmidt telescope with a hybrid optical focal surface and a dedicated radio instrument, intended to observe Ultra-High-Energy Cosmic Rays via fluorescence, High-Altitude Horizontal Air Showers with optical Cherenkov and radio, and very-high-energy neutrinos through Target-of-Opportunity follow-up of astrophysical transients (Olinto, 2023, Eser et al., 4 Sep 2025).

1. Programmatic emergence and mission profile

In the 2023 POEMMA roadmap update, PBR appeared only as a programmatic milestone: the “proposed POEMMA-Balloon-Radio (PBR) mission on a super-pressure balloon,” paired with Terzina as one of two projects expected to “further qualify POEMMA subsystems” and raise technical readiness (Olinto, 2023). That early mention contained no payload description, no radio specifications, and no performance projections. Subsequent PBR papers from 2024–2026 turned that roadmap placeholder into a detailed mission concept, consistently describing PBR as a precursor to POEMMA and a direct successor to EUSO-SPB2 (Battisti et al., 2024, Eser et al., 4 Sep 2025).

The mission profile in those later papers is centered on a NASA Super Pressure Balloon launched from Wanaka, New Zealand, flying at about $33\,\mathrm{km}$ altitude over the Southern Ocean. The published campaign duration varies by paper: “more than 20 days,” “as long as 50 days,” and, in some subsystem and earlier overview papers, “up to 100 days” (Eser et al., 4 Sep 2025, Wiencke et al., 17 Sep 2025, Battisti et al., 2024). This suggests an evolving planning baseline during mission maturation rather than a change in the scientific concept. The same evolution appears in schedule statements: the 2023 roadmap expected launch in 2026, while the more mature subsystem papers place the mission in Spring 2027 (Olinto, 2023, Battisti, 15 Nov 2025).

Across these publications, PBR is described not as a single-instrument balloon experiment but as a “multi-messenger, multi-detector” or “multi-hybrid” observatory. Its explicit pathfinder function is to validate the fluorescence detection strategy from above, to obtain the first simultaneous optical Cherenkov and radio observations of high-altitude horizontal air showers, and to demonstrate Target-of-Opportunity neutrino operations in a near-space environment (Battisti et al., 2024, Adams et al., 27 Jan 2026).

2. Observatory architecture and pointing geometry

The core payload is a modified Schmidt telescope with a $1.1\,\mathrm{m}$ entrance pupil and a segmented spherical primary mirror, coupled to a hybrid focal surface and a co-boresighted radio system. Published optical-mechanical descriptions give a $0.95\,\mathrm{m}^2$ aperture hybrid Schmidt telescope with a $3.81\,\mathrm{m}^2$ segmented mirror, while mission overviews describe the same observatory more generally as a $1.1\,\mathrm{m}$ aperture Schmidt telescope similar to the POEMMA design (Mayotte et al., 7 Sep 2025, Cafagna, 20 Nov 2025). The focal surface is split between a Fluorescence Camera and a Cherenkov Camera; underneath the telescope sits a radio instrument made of two sinuous antennas, and the payload also carries an X/γ detector and an infrared cloud camera (Mayotte et al., 7 Sep 2025, Eser et al., 4 Sep 2025).

Pointing flexibility is central to the mission. The gondola provides 360° azimuthal rotation, and the telescope assembly tilts between nadir and above-horizon configurations. Depending on the reference frame used in individual subsystem papers, the elevation capability is described as “from nadir up to 13° above the horizon” or, in the tilt-system paper, as a range from $+15^\circ$ to $-90^\circ$ relative to local horizontal, with $-90^\circ$ corresponding to nadir (Wiencke et al., 17 Sep 2025, Cafagna, 20 Nov 2025). The same tilt paper gives the limb geometry explicitly: at $33\,\mathrm{km}$ altitude, the Earth’s limb is about $5.8^\circ$ below horizontal, while a mission overview quotes a nadir angle of $1.1\,\mathrm{m}$0 for the limb as seen from the balloon (Wiencke et al., 17 Sep 2025, Battisti et al., 2024).

These pointing states map directly onto the science modes. Nadir favors UHECR fluorescence observations; just above the limb is used for HAHAs and for recording star images to monitor optical focus in situ; a few degrees below the limb is the neutrino-search configuration for Earth-skimming $1.1\,\mathrm{m}$1-induced upward-going air showers (Wiencke et al., 17 Sep 2025, Eser et al., 4 Sep 2025). The payload literature repeatedly emphasizes that the single large integrated telescope and radio antenna assembly must preserve co-pointing across these modes, because optical, radio, and auxiliary instruments are intended to observe the same atmospheric volume (Wiencke et al., 17 Sep 2025, Mayotte et al., 7 Sep 2025).

3. Detector suite

The Fluorescence Camera is the UV instrument for observing nitrogen fluorescence from extensive air showers. The mature FC design is a $1.1\,\mathrm{m}$2 camera made of four Photo Detection Modules, each a $1.1\,\mathrm{m}$3 matrix of 64-channel MAPMTs, for $1.1\,\mathrm{m}$4 pixels per PDM and $1.1\,\mathrm{m}$5 pixels in total (Cafagna, 20 Nov 2025). It uses Hamamatsu R11265 MAPMTs and SPACIROC-3 ASICs to perform single photoelectron counting on each pixel and charge integration on groups of 8 pixels, with a $1.1\,\mathrm{m}$6 or $1.1\,\mathrm{m}$7 acquisition gate and double-pulse resolution of order $1.1\,\mathrm{m}$8 (Battisti et al., 2024, Cafagna, 20 Nov 2025). A field flattener lens and a BG3 filter are mounted in front of each PDM; the wavelength range is described as $1.1\,\mathrm{m}$9 in one FC paper and $0.95\,\mathrm{m}^2$0 in another (Battisti et al., 2024, Cafagna, 20 Nov 2025).

The Cherenkov Camera is the fast optical instrument for HAHAs and upward-going neutrino-induced showers. Its baseline published design is a $0.95\,\mathrm{m}^2$1-pixel SiPM camera with total field of view $0.95\,\mathrm{m}^2$2, pixel angular scale $0.95\,\mathrm{m}^2$3, spectral sensitivity from $0.95\,\mathrm{m}^2$4 to $0.95\,\mathrm{m}^2$5, and $0.95\,\mathrm{m}^2$6 integration time (Scotti et al., 19 Nov 2025). The CC uses a bi-focal optical design: an “Optical Accordion” or bifocalizer splits light from a distant source into two spots separated by about $0.95\,\mathrm{m}^2$7, corresponding to about two $0.95\,\mathrm{m}^2$8 pixels, so that a valid event can require the expected two-spot topology (Adams et al., 27 Jan 2026, Mayotte et al., 7 Sep 2025). The front-end electronics are under active development around the MIZAR ASIC, a 64-channel, $0.95\,\mathrm{m}^2$9, low-power chip with 256 analog memory cells per channel and programmable 7–12 bit ADC conversion; an alternative Radioroc-based path is also described in the camera literature (Bertaina et al., 15 Nov 2025, Adams et al., 27 Jan 2026).

The Radio Instrument provides the low-frequency EAS channel. Here the published design record is still visibly evolving. Mission overviews and design papers describe two dual-polarized sinuous antennas mounted below the telescope, derived from the PUEO low-frequency instrument, with overlapping optical-radio boresight and a $3.81\,\mathrm{m}^2$0 field around boresight (Eser et al., 4 Sep 2025, Mayotte et al., 7 Sep 2025). The quoted radio band varies between $3.81\,\mathrm{m}^2$1, $3.81\,\mathrm{m}^2$2, and $3.81\,\mathrm{m}^2$3 across the literature (Battisti et al., 2024, Eser et al., 4 Sep 2025, Adams et al., 27 Jan 2026). This suggests refinement of the RI definition during development, but the overall concept is stable: two broadband sinuous antennas, dual polarization, low-noise amplification, RFSoC-based digitization, CC-external triggering as the primary mode, and a radio-only self-trigger intended mainly for daytime operation (Eser et al., 4 Sep 2025, Adams et al., 27 Jan 2026).

The X/γ detector adds a hard-photon channel specifically motivated by early-stage HAHA development. In its dedicated instrument paper, it consists of four scintillator-based sub-detectors using CsI(Tl) or NaI(Tl) crystals with SiPM readout, covering from $3.81\,\mathrm{m}^2$4 to $3.81\,\mathrm{m}^2$5 in overlapping bands: $3.81\,\mathrm{m}^2$6, $3.81\,\mathrm{m}^2$7, and two $3.81\,\mathrm{m}^2$8 channels (Battisti, 15 Nov 2025). The X channel has a $3.81\,\mathrm{m}^2$9 FoV and a beryllium entrance window; the higher-energy channels have $1.1\,\mathrm{m}$0 FoV and aluminium windows. A plastic scintillator anti-coincidence system surrounds the detector, and the entire instrument is aligned with the Cherenkov and Fluorescence cameras so that coincident optical and X/γ detections can be associated offline (Battisti, 15 Nov 2025).

4. Science program and predicted measurement regime

PBR’s first science goal is UHECR fluorescence from sub-orbital altitude. The published FC simulations consistently place the relevant threshold in the EeV range. One overview quotes $1.1\,\mathrm{m}$1 for nadir pointing and $1.1\,\mathrm{m}$2 for limb pointing, with event rates of $1.1\,\mathrm{m}$3 and $1.1\,\mathrm{m}$4 events per observation hour, respectively (Eser et al., 4 Sep 2025). A later summary gives $1.1\,\mathrm{m}$5 and $1.1\,\mathrm{m}$6 events per hour for nadir and limb, respectively (Adams et al., 27 Jan 2026). Both descriptions agree on the qualitative trade-off: nadir gives lower threshold, while limb geometry increases the high-energy aperture. The explicit programmatic significance is not primarily statistics but validation of fluorescence detection from above for a POEMMA-like mission (Cafagna, 20 Nov 2025).

The second science goal is the study of High-Altitude Horizontal Air Showers. The PBR overview defines HAHAs as nearly horizontal showers developing in the rarefied atmosphere above $1.1\,\mathrm{m}$7, with primary energies exceeding the cosmic-ray knee, $1.1\,\mathrm{m}$8 (Eser et al., 4 Sep 2025). In published HAHA performance estimates, the optical threshold is around $1.1\,\mathrm{m}$9, and the expected rate is $+15^\circ$0 events per hour for a 30-day flight with 20% duty cycle, implying $+15^\circ$1 events over the effective nighttime exposure (Eser et al., 4 Sep 2025). A later mission summary instead expresses the same scale as roughly $+15^\circ$2 event per minute and $+15^\circ$3 events in a 20-day flight (Adams et al., 27 Jan 2026). Instrument-specific papers refine the geometry: at the horizon ($+15^\circ$4) PBR is sensitive to primaries above $+15^\circ$5, at $+15^\circ$6 to horizontal events down to $+15^\circ$7, and showers above $+15^\circ$8 are expected above $+15^\circ$9 at a rate of order $-90^\circ$0 event per night (Battisti, 15 Nov 2025). The same literature emphasizes that PBR is intended to deliver the first simultaneous optical Cherenkov and radio observations of this class of shower (Adams et al., 27 Jan 2026).

The third science goal is the search for very-high-energy neutrinos, especially in Target-of-Opportunity mode. The mission literature repeatedly adopts the Earth-skimming $-90^\circ$1 channel: $-90^\circ$2, followed by $-90^\circ$3 emergence and atmospheric decay to an upward-going air shower (Eser et al., 4 Sep 2025, Reno et al., 18 Sep 2025). One mission overview states that Cherenkov detection of upward-going showers begins from $-90^\circ$4, while the RI contributes for $-90^\circ$5 (Battisti et al., 2024). A ToO-focused study on KM3-230213A-like events treats PBR as optimized for $-90^\circ$6, and computes 1000-s and >20-day horizon-range sensitivities in that band (Olinto et al., 3 Jul 2025). The main PBR overview is explicit that the mission is not expected to set competitive diffuse cosmogenic neutrino limits; its neutrino role is technique validation and transient follow-up (Eser et al., 4 Sep 2025).

A recurring scientific theme is complementarity across channels. The X/γ detector paper argues that coincidence between a Cherenkov flash and an X/γ burst would isolate the early high-energy electromagnetic component of a HAHA (Battisti, 15 Nov 2025). The EUSO-SPB2 results paper adds a further rationale: optical Cherenkov alone suffers an energy-geometry degeneracy, and “the upcoming POEMMA Balloon with Radio mission aims to break this degeneracy by measuring showers in low frequency radio” (Filippatos, 5 Sep 2025).

5. Triggering, synchronization, and data processing

PBR’s data system is designed around cross-instrument synchronization and hierarchical triggering. The data processing paper describes a payload with more than 8500 channels, integrating FC, CC, RI, and X–Gamma detectors through a customized Eurocard chassis with redundant CPUs, Zynq-based timing and trigger boards, differential GPS receivers, housekeeping electronics, and five SATA disks for storage (Scotti et al., 19 Nov 2025). The central timing architecture uses a 1 PPS reference from GPS and a GTU clock distributed to all sub-detectors, with event fragments tagged by 1 PPS counter, GTU counter, local trigger counter, global trigger counter, and dead/live time counters (Scotti et al., 19 Nov 2025). The stated timing goal is that “the time of the run should be the same for each sub-detector within one microsecond.”

The trigger logic is explicitly hierarchical. Local first-level triggers are generated independently inside the FC, CC, RI, and X/γ systems; the CC Trigger & Sync board then applies global coincidence logic and dispatches trigger accept signals back to the relevant sub-systems (Scotti et al., 19 Nov 2025). This architecture supports both joint acquisition and independent acquisition. The RI is specifically described as supporting a combination of external triggers from optical instruments and internal radio-frequency triggers based on waveform thresholds and pattern recognition (Scotti et al., 19 Nov 2025, Adams et al., 27 Jan 2026).

Operationally, the DP software handles Day, Night, and Day–Night–Day transitions, power sequencing, housekeeping, shutter state, environmental monitoring, and science-data prioritization under severe telemetry limits (Scotti et al., 19 Nov 2025). The telemetry stack combines very low rate Iridium, TDRSS up to about $-90^\circ$7, and Starlink maritime terminals with quoted download rates up to $-90^\circ$8 (Scotti et al., 19 Nov 2025). The same paper emphasizes that storage and transmission are priority-driven, because long balloon flights cannot downlink all raw data continuously. This operational philosophy inherits directly from EUSO-SPB2, but extends it to a mixed optical–radio payload.

Calibration and health monitoring are embedded in the design. The later PBR payload review describes GPS-locked Health LEDs for both FC and CC, star-field observations for optical focus and pointing, and forced radio triggers for noise studies (Adams et al., 27 Jan 2026). The tilt-system paper adds a PLC-controlled shutter that closes during the day to protect the $-90^\circ$9 entrance pupil and reopens for nighttime science observations (Wiencke et al., 17 Sep 2025).

6. Development lineage, simulation ecosystem, and interpretive issues

PBR belongs to a staged POEMMA pathfinder sequence. Before PBR, Mini-EUSO provided orbital UV-background measurements and EUSO-SPB2 flew a fluorescence telescope and a Cherenkov telescope on a super-pressure balloon (Olinto, 2023). EUSO-SPB2 was terminated after less than 37 hours because of a balloon failure, but it nevertheless commissioned the payload, validated trigger and telemetry concepts, and recorded PeV-scale Cherenkov candidate events consistent with simulations (Jr. et al., 27 May 2025, Filippatos, 5 Sep 2025). Later EUSO-SPB2 analyses present those results explicitly as “lessons learned” that will directly inform PBR, especially in pointing strategy, threshold setting, and the case for adding a complementary radio channel (Filippatos, 5 Sep 2025).

PBR performance studies are already embedded in a broader software ecosystem. The $-90^\circ$0SpaceSim paper presents an end-to-end package for Earth-skimming neutrino simulations and states that it is being used to support the development of balloon experiments including PBR (Reno et al., 18 Sep 2025). Its ULDB example instrument is based on EUSO-SPB2 and PBR Cherenkov telescopes, with optical Cherenkov triggering and a co-located radio instrument in the $-90^\circ$1 band, and is used to study energy-dependent neutrino aperture, ToO sensitivity, and the effect of neutrino cross sections, $-90^\circ$2-energy-loss models, and EAS parameterizations (Reno et al., 18 Sep 2025). This establishes a unified design-to-analysis chain spanning optical Cherenkov and radio channels.

The PBR literature also contains an instructive ambiguity: not all quoted numbers are fixed mission constants. Flight duration, radio band edges, and some performance estimates differ between papers, especially between 2024 overview papers and 2025–2026 subsystem papers (Battisti et al., 2024, Eser et al., 4 Sep 2025, Adams et al., 27 Jan 2026). These differences are best read as evidence of an evolving pre-flight baseline. Another recurrent misconception is to treat PBR as primarily a radio experiment because of its name. The published record does not support that. PBR is consistently described as a hybrid Schmidt observatory with fluorescence, Cherenkov, radio, X/γ, and IR capabilities; the radio system is essential, but it is one element of a deliberately multi-detector architecture (Battisti et al., 2024, Mayotte et al., 7 Sep 2025).

A final interpretive point concerns scope. Some papers use PBR to explore beyond-Standard-Model implications of extreme neutrino events such as KM3-230213A (Olinto et al., 3 Jul 2025). Those studies are horizon-use cases for the instrument, not definitions of the baseline mission. The baseline mission remains the three-part program stated in the overview papers: UHECR fluorescence from above, simultaneous optical-radio measurements of HAHAs, and ToO searches for very-high-energy neutrinos (Eser et al., 4 Sep 2025, Adams et al., 27 Jan 2026).