What to buy, what it buys you, and what to skip.

The tier 1 reference host. Two gigabytes is genuinely sufficient because the node agent holds bounded ring buffers and never queues decoded frames; see firmware/README.md on the memory budget. Choosing 2 GB over 8 GB saves USD 110 at July 2026 prices, which is more than a quarter of the entire tier 1 budget. The Pi 5's Ethernet MAC supports hardware timestamping, which is what makes sub-microsecond cross-node timing achievable at all.

The single most important part in the entire build and the one most builders skip. Without a pulse-per-second signal wired to a GPIO and disciplined through chrony, a node's timestamps are good to milliseconds and it can never contribute to multi-node geometry. With it, the node holds a few hundred nanoseconds and becomes a real element of an interferometric array. Budget for this before budgeting for a second camera.

Rolling shutter is a real limitation for fast targets: a fast-crossing object is geometrically skewed, and that skew must not be read as anomalous kinematics. The firmware records the readout time per frame so the discriminator can correct for it. Where budget allows, the global-shutter IMX296 removes the problem entirely at the cost of resolution.

The cheapest genuinely additional band in the platform. A second HQ camera body with the IR-cut filter removed and a hard 850 nm bandpass in front of it produces a channel that is physically independent of the visible camera rather than a filtered copy of it. Without the bandpass filter this is not a NIR channel, it is a visible channel with extra noise, and the registry will mark it as such.

The V3 is the current reference, not the V4. RTL-SDR Blog announced the V4 as end-of-line in 2026 after Rafael Micro stopped producing the R828D tuner and the remaining stock proved faulty; the V3 remains in stable production. A V4L using the R828S is expected but was not shippable at the time of writing. Any BOM still specifying a V4 is out of date. The 1 ppm TCXO matters here: an uncompensated dongle drifts enough to smear a narrowband detection across the analysis window.

Context only, never a detection channel. Its job is to make a refraction argument settleable after the fact instead of arguable forever. Pressure trend and dewpoint spread are what let the discriminator recognise a temperature inversion, which is the single most common cause of a genuinely strange-looking optical track near the horizon.

Records where each sensor was actually pointing rather than where the operator believes it was bolted. A mast that shifts two degrees in wind invalidates every bearing taken during the gust unless the shift is recorded. The onboard sensor-fusion output is used directly; running fusion on the host wastes CPU that the trigger pipeline needs.

The honest budget thermal option and a fully supported variant, not a downgrade to be apologised for. At 768 pixels it cannot image a shape, but it can absolutely register that something warm crossed the field, and that is enough to make a coincidence trigger with the optical channel. The discriminator knows the difference: detections from this part are scored for thermal presence and never for thermal morphology. This is the variant registry working as intended.

Tier 3 host. Needed once SWIR imaging and wideband SDR are both running, since each wants a multi-hundred-megabyte buffer of its own.

The single most important part in the entire build and the one most builders skip. Without a pulse-per-second signal wired to a GPIO and disciplined through chrony, a node's timestamps are good to milliseconds and it can never contribute to multi-node geometry. With it, the node holds a few hundred nanoseconds and becomes a real element of an interferometric array. Budget for this before budgeting for a second camera.

Rolling shutter is a real limitation for fast targets: a fast-crossing object is geometrically skewed, and that skew must not be read as anomalous kinematics. The firmware records the readout time per frame so the discriminator can correct for it. Where budget allows, the global-shutter IMX296 removes the problem entirely at the cost of resolution.

The cheapest genuinely additional band in the platform. A second HQ camera body with the IR-cut filter removed and a hard 850 nm bandpass in front of it produces a channel that is physically independent of the visible camera rather than a filtered copy of it. Without the bandpass filter this is not a NIR channel, it is a visible channel with extra noise, and the registry will mark it as such.

Context only, never a detection channel. Its job is to make a refraction argument settleable after the fact instead of arguable forever. Pressure trend and dewpoint spread are what let the discriminator recognise a temperature inversion, which is the single most common cause of a genuinely strange-looking optical track near the horizon.

Records where each sensor was actually pointing rather than where the operator believes it was bolted. A mast that shifts two degrees in wind invalidates every bearing taken during the gust unless the shift is recorded. The onboard sensor-fusion output is used directly; running fusion on the host wastes CPU that the trigger pipeline needs.

Radiometric output is the reason this is the reference rather than a cheaper array: every pixel carries an absolute temperature, which turns a thermal track into an energy measurement the discriminator can reason about. The flat-field shutter fires every few minutes and blanks the stream for roughly half a second; the firmware marks those windows invalid rather than letting them register as a detection. Export-controlled in some jurisdictions, and the 8.7 Hz frame rate is a deliberate export-compliance limit, not a technical one.

Three separate UV channels rather than a single index, which is what makes the part useful for discrimination instead of merely for sun exposure. A corona discharge, a lightning leader, and direct sunlight have distinguishable ratios across the three. Needs a fused-silica or PTFE diffuser window; ordinary acrylic blocks UVB and UVC entirely and will silently turn this into a one-channel sensor.

The first part in the stack that produces a genuine three-dimensional point cloud with per-point Doppler. Range to a few hundred metres for a drone-sized cross-section under good conditions, far less in rain. Every site develops a fixed clutter map from buildings and terrain within the first hour; that map is learned, stored per node, and subtracted before anything is called a detection.

Magneto-inductive rather than Hall-effect, which is why it reaches tens of nanotesla instead of hundreds. Mount it at least two metres from the node's own electronics and from any of the active-emission hardware, on a non-ferrous mast section. In practice the dominant signal at most sites is the operator walking past with a phone, and learning that local signature is part of commissioning rather than a nuisance.

Digital output straight to the Pi's I2S peripheral, which keeps the analogue path short and avoids the ground-loop noise that plagues USB audio interfaces on a solar-powered mast. Rolls off below 50 Hz, so this covers audible acoustics only. Infrasound needs a dedicated differential pressure sensor and a mechanical wind filter, which is a tier 3 addition.

Open hardware, fully documented, and the only practical route to a real energy spectrum rather than a bare count rate at this price. A spectrum is what separates a cosmic-ray shower from an isotope line. This is also the instrument that reads back the active-emission module's own 59.5 keV americium line, which is how the node proves the lure is operating and correctly characterises its own contribution to the background before subtracting it.

The single largest line item in any tier and the reason tier 3 exists as a category. It buys haze penetration and material discrimination that no other band provides, and it exploits night-sky airglow to illuminate targets passively at 1.5 to 1.7 micrometres with no emitter at all. Export-controlled in most jurisdictions; check ITAR and EAR status before shipping across a border. Most builders should skip this and put the money into a second complete node instead, which buys geometry, and geometry beats spectral coverage.

A 10 Hz element does not reach the infrasound band and is not a substitute for a broadband seismometer. Its role at tier 3 is ground-coupled acoustic detection of low overflights and, more importantly, characterising the site's vibration background, which is the prerequisite for ever siting a gravimeter there.

Part of the optional active-emission module. Sources are the encapsulated foils already present in ionisation smoke detectors, aggregated behind aluminium and acrylic so that alpha and beta are fully absorbed while the 59.5 keV gamma line passes. Total activity stays under the US NRC 10 CFR 30.15 general-license threshold that already covers these devices in ordinary domestic use. Read docs/safety before building this. It is genuinely optional, every passive band works without it, and jurisdictions outside the US have their own rules that the operator is responsible for checking.

LED rather than laser, keeping the emitter below any Class 3R ocular hazard threshold while still producing a distinctive pulse-coded signature. The randomised duty cycle is what makes it identifiable in the archive: the node knows its own emission schedule exactly, so any NIR return correlating with the code is self-illumination and is subtracted rather than reported. Point it above the horizon, never at a road or a flight path.

Sealed enclosures trap moisture rather than excluding it: the air inside is humid when you close the lid, and the first cold night condenses it onto the coldest surface, which is always the optics. A Gore breather vent plus reusable desiccant solves this. Every optical port needs its own gasketed window rather than a hole, and germanium for the LWIR port because glass is opaque at 10 micrometres.

Sized for a tier 2 node drawing roughly 11 W continuous, which is 264 Wh per day. Three days of autonomy through overcast is the design point, and LiFePO4 rather than lead-acid because the duty cycle is a shallow daily discharge for years. The current monitor is not an accessory: node power draw is itself telemetry, and an unexplained current step is one of the more reliable early indicators of a sensor failing.