CubeSat VLF Lightning Wave Receiver — Rev A

The intended payload would measure the magnetic component of VLF signals, initially across 3 to 30 kHz, and identify candidate lightning-generated events. For each it would record the waveform and its spectrum, attach UTC time and orbital position, and store it for downlink. Detections would then be compared against independent lightning observations.

This is a receiver mission: the only signal it is meant to generate is a calibration tone injected into its own front end. That is intended behaviour — Rev A is a schematic and demonstrates none of it.

The receiver core is drawn. The review was about the distance between that and an instrument whose measurements would mean something.

The sensor is the first gap. The coil is external and not shown, so its geometry, its sensitivity in volts per tesla and its calibrated response are undefined — and each sits in the relationship between a measured voltage and the field that produced it. A coil mounted near spacecraft wiring measures the spacecraft.

Then the analog path. Gain is unverified against the requirement that 10 microvolts RMS give at least 10 dB SNR, and one fixed setting may not serve weak and strong events alike. The filter must be designed and measured rather than assumed: 3 to 30 kHz flat within 3 dB, 20 dB attenuation above roughly 40 to 45 kHz. The 1.65 V bias and stable 80 to 100 kSPS sampling need checking too.

The flight subsystems are requirements, not parts of the drawing: UTC within a millisecond, storage with 99 per cent integrity, a spacecraft-computer interface. USB-C, the OLED and the MCU's 2.4 GHz path are development conveniences — none is the downlink.

A VLF receiver is sensitive enough to detect the spacecraft it is bolted to. That is not a nuisance to tidy up at integration; it decides whether the instrument has a noise floor low enough to see anything.

Switching and digital subsystems can radiate or couple energy into the receiver band. The review names them: Wi-Fi, the OLED, the LEDs, switching circuits, unnecessary radios, high-current wiring near the coil. Any can put a line exactly where a natural signal would sit, and a clock harmonic does not look different from lightning once it has been through the same amplifier.

So noisy subsystems are disabled during sensitive measurements rather than trusted, repeated noise frequencies are identified so they can be recognised later, and the coil is mounted away from high-current wiring.

The verification approach is the part worth writing down, because it is the only way to know which subsystem is responsible. Record a baseline with everything off. Enable one subsystem at a time and record what changes. Build an interference database. Then repeat it in the assembled satellite, because a subsystem quiet on a bench is not necessarily quiet beside everything else.

None of it has been carried out.

A receiver that cannot check itself produces numbers nobody can defend. The intended calibration system injects a known signal so coil response, gain, filter response, converter accuracy and drift can each be checked against a traceable reference — stable within 5 per cent, and never mistaken for a natural event.

The drawing contains two devices that do not obviously belong to a receiver: CID45N65MD8, annotated as a gallium-nitride transmitter, and CI02S120C3, annotated as silicon carbide. The review's reading is that they might be intended for that calibration role.

I am not going to settle it here. The source says their connections and purpose need clarification, and that is where it stands. Whether they are the injection path or something that should not be on the board is Rev B work.

A VLF receiver is not an amplifier connected to a converter. I drew it as though it were, and the review is largely a list of what that assumption left out.

The instrument does not end at the connector. The coil's geometry and sensitivity are part of the measurement chain; the filter decides whether energy from outside the band arrives disguised as signal inside it; the sampling has to be steady enough that a frequency means what it says; the timing has to be good enough to match an event against somebody else's record of the same lightning. Filtering, aliasing, calibration and timing are not support details around the science. They are the science.

The spacecraft also becomes part of the environment being measured — most of the noise the instrument must reject comes from the vehicle carrying it. And two devices labelled as transmitters sat on a mission that receives, which forced a separation I had not made clearly enough: receiving, calibrating and talking to the spacecraft are three different jobs.