Ionospheric Plasma Irregularity Payload — Rev A

Measure small-scale variations in ionospheric electron density, and map their location, size, strength and relationship with geomagnetic conditions.

The intended investigation covers plasma-density enhancements and depletions, equatorial plasma bubbles, auroral plasma structures, steep electron-density gradients, irregularities from metre to kilometre scale, changes associated with geomagnetic storms, and the regions capable of disturbing navigation and radio signals.

Irregularities matter because they change the amplitude and phase of radio signals crossing them. Severe ionospheric scintillation can reduce GNSS accuracy or prevent a receiver from maintaining lock, so mapping where these structures occur can help identify regions where navigation and radio links may become less reliable.

That is the argument for measuring density directly, in situ, rather than inferring it. A probe in the plasma reports what is there along the orbit track, at the spatial resolution the sampling rate and the spacecraft velocity allow.

A conductive surface held at a known potential in a plasma collects current from the electrons and ions around it. That current is the measurement.

It varies with electron density, electron temperature, probe voltage, spacecraft potential, spacecraft velocity, probe geometry and probe surface condition — which is why several of those have to be either controlled, measured, or explicitly carried as uncertainty. Holding the bias fixed and known removes one of them, and is why fixed-bias survey is the primary science mode rather than a continuous sweep.

The main science sensor is an exposed conductive Langmuir-type probe connected to JP1. A practical assembly includes a conductive probe tip or surface, an insulating probe support, a coaxial or guarded signal connection, a deployment or external mounting structure, a contamination-resistant surface, a low-leakage connector, and a plasma-facing mechanical enclosure.

JP1 is only the electrical connector. The current schematic shows it and nothing of the probe itself. That matters more than it sounds: the probe is not packaging around the instrument, it is the first component in the measurement chain, and it has not yet been defined.

Everything about it enters the result. The material and the surface condition set how the collected current relates to density, and a contaminated or oxidised surface changes that relationship. The exposed area scales the current directly. The shape and orientation determine the collection geometry and how the spacecraft's motion through the plasma is sampled. The cable sets the stray capacitance and leakage at the most sensitive node in the design. And the mounting decides whether the probe sits in the ionosphere or in the spacecraft's own disturbed boundary layer and wake — a probe that measures the wake measures the spacecraft, not the ionosphere.

The probe input is exposed to a spacecraft charging environment, so it needs transient protection: D1 and D2 are low-leakage clamp arrays placed to keep electrostatic events away from U3 and the converter.

This is the sharpest trade-off in the design. A protection device sits directly on the most sensitive node in the instrument, and every protection device leaks. At the currents this front end is meant to resolve, clamp leakage can be comparable to or larger than the plasma current being measured — the protection can be noisier than the signal it protects.

Nothing about that has been resolved experimentally, because there is no hardware. What the review established is the shape of the problem and the acceptance criterion: the front end must still meet its original noise and leakage limits *after* transient testing, and the clamps must be verified rather than assumed quiet. The design targets that bound the trade are an input leakage requirement below 10 pA under controlled laboratory conditions and input-referred current noise below 100 pA RMS across the primary science bandwidth.

U4 and U6 buffer the front-end output and limit broadband, switching and aliasing noise before digitisation. The requirement is not simply "filter": out-of-band signals must not produce false in-band density structures, because a false structure is indistinguishable from a real irregularity once it reaches the detection logic.

The amplifier also has to stay inside its valid output range across nominal plasma conditions, and recover properly after saturation rather than latching or taking an unbounded time to return.

One fixed transimpedance gain may not be enough. Quiet and dense plasma can demand different measurement ranges, and a single setting risks either losing resolution at the quiet end or saturating at the dense end. Selectable gain is a Rev B design direction with its own acceptance criterion — each gain within plus or minus 2 per cent of its calibrated value. The current schematic does not have it.

U7 digitises the conditioned probe signal against an external reference, with differential inputs. Its resolution has to support the required measurement accuracy, and offset, gain and nonlinearity have to be characterised for every operating range so that corrected converter error stays inside the current-measurement uncertainty budget.

The sampling requirements are targets: continuous plasma-current sampling at no less than 2 kSPS, and a high-rate burst mode at up to 10 kSPS or the maximum validated converter rate. Saturated readings are flagged rather than silently clipped, and a one-hour acquisition should contain no unexplained missing or duplicate samples.

Reference connections and decoupling are on the Rev B list to be verified, which is the kind of item that costs nothing to fix on a drawing and a great deal to discover on a board.

U1, an RP2040, with U2 external SPI flash for firmware and configuration. It controls probe bias, reads the converter, timestamps measurements, detects density irregularities, stores science data, sends data to the spacecraft computer, and manages calibration and safe mode.

The detection work happens here rather than on the ground: the payload has to recognise a steep gradient quickly enough to enter burst mode while the structure is still being crossed. Ground processing then does the calibrated analysis on the stored record.

U8 with the JP2 antenna connector and a TIMEPULSE output. It supplies UTC measurement time, latitude, longitude, altitude, velocity, and — combined with measurement time — the along-track location of a detected irregularity. TIMEPULSE disciplines RP2040 sampling and timestamps.

The targets: absolute timing error below 1 ms, position error below 20 m under valid reception. Firmware identifies missing or invalid GNSS time, and data collected without valid synchronisation receives a quality flag rather than being stored as though it were located.

This is not a scientific scintillation receiver. It supports timing, position, velocity and geolocation. A normal navigation GNSS module can provide position and time but usually cannot produce scientific S4 or phase-scintillation measurements, because those need raw carrier observables the module may not expose. The review lists confirming that the receiver supplies raw carrier measurements as a precondition for any scintillation claim — a claim this project does not currently make.

U9, an LIS3MDL, measures the local three-axis magnetic-field direction. Its value is in checking whether measured plasma structures align with Earth's field, recording the magnetic environment alongside each measurement, and characterising interference produced by the spacecraft itself.

It is a contextual sensor unless it receives full magnetic calibration and is mounted away from spacecraft-generated fields. Hard-iron and soft-iron coefficients are stored, out-of-range readings are flagged, and the acceptance criterion is that calibrated field direction agrees with a reference within a selected angular error — including with major spacecraft subsystems operating, not only on a quiet bench.

Placement is a Rev B item: the magnetometer currently sits near current-carrying traces.

U11, a high-accuracy digital temperature sensor, sits beside the front end and measures its temperature to a target of plus or minus 0.5 degrees Celsius.

Amplifier offset, resistor value, converter gain and bias output all move with temperature, so calibration coefficients correct for all four. The acceptance criterion is that corrected current readings vary by less than 5 per cent across the supported temperature range, verified by repeating the current calibration at several chamber temperatures rather than extrapolating from one.

A microSD card holds the bulk science record: raw current, bias, time, position, temperature and quality flags. U2 SPI flash holds firmware, configuration and temporary data.

The requirements are about failure rather than capacity. At least 99 per cent of stored science records should pass integrity checks. High-gradient and burst-mode records are protected from automatic deletion, so that filling the card cannot quietly discard the most valuable events. Interruption during a write must not corrupt unrelated files, and a full card must not stop the payload.

The main irregularity-detection mode, and the one that gives the highest spatial resolution.

The probe is held at a constant positive bias, current is measured continuously at a target of 2 kSPS or higher, rapid relative changes are detected in flight, and time, position, temperature and magnetic context are recorded alongside every sample. Holding the bias fixed removes one of the variables that otherwise moves the collected current, which is why this is the primary mode rather than a continuous sweep.

Activated when a steep gradient or strong disturbance is detected. Sampling increases toward a 10 kSPS target, full-rate raw measurements are saved, and data from before and after the trigger is preserved so that the entry into the structure is not lost to the detection latency. The event is marked high priority, which also protects it from automatic deletion when storage fills.

Optional. Sweeping the probe bias through multiple voltages produces a current-voltage curve, and fitting that curve estimates electron density, electron temperature and spacecraft potential — quantities the fixed-bias mode cannot separate.

A complete bipolar sweep requires a negative supply or a level-shifting circuit that is not present in the current schematic. The 3.3 V path as drawn cannot take the probe negative relative to the spacecraft, so the full Langmuir analysis this mode exists for is not achievable from Rev A. Specifying the bias range, and deciding whether bipolar capability is required at all, is a Rev B item. The mode is a design intention, not a Rev A capability.

Entered on excessive temperature, excessive current, bias error or repeated faults. Probe bias is set to zero or a protected value, high-rate recording stops, stored data is preserved, temperature and supply voltage continue to be monitored, and the payload waits for a valid command or an automatic recovery condition.

The requirement that shapes the design is that the payload stays commandable with the sensitive input protected — a safe mode that cannot be commanded out of is a failure, not a safe state. A hardware watchdog and a hardware bias shutdown are both Rev B additions.

Software identifies rapid density increases, depletions and gradients. The acceptance criterion is that simulated changes of at least 5 per cent above the noise floor are detected with at least 90 per cent probability, verified by replaying simulated density profiles mixed with measured noise.

Rejection matters as much as detection. Processing has to reject converter spikes, bias changes, resets and known spacecraft interference, and the false-event rate has to stay below the mission's selected limit — verified with radios, processors, converters and other spacecraft loads operating during acquisition, because that is when false events actually appear.

Calibration software converts measured probe current into electron density, with a target uncertainty below 20 per cent in supported chamber conditions, checked against an independent plasma diagnostic.

Two further quantities depend on the sweep mode. If voltage-sweep mode is added, fitting the current-voltage curve estimates electron temperature against an uncertainty established during chamber calibration. If the bias range supports it, sweep analysis also estimates spacecraft potential against a chamber reference. Both are conditional on circuitry Rev A does not have.

Ground software combines measurement time with spacecraft velocity to give each sample a valid along-track coordinate, then maps density, gradients and irregularity events along the orbit.

Validation is deliberately blind: a simulated or chamber-generated dataset with known input structures is processed without the answers, and the pipeline is judged on whether it reproduces those structures at the right locations. Event records are then compared with geomagnetic indices and other ionospheric observations on common UTC timing.

H1 is the main spacecraft connector. The payload sends science products and status data to the spacecraft computer and receives commands from it.

The acceptance criterion is a 24-hour interface test with no uncorrected communication errors, run against a spacecraft-computer emulator. Clearly labelling the H1 signals is a Rev B item — the kind of omission that is free to fix on a drawing and expensive to discover during integration.

The requirement is blunt: the RP2040, microSD, GNSS, USB and spacecraft electronics must not dominate the probe signal, and self-generated current-equivalent noise must stay below the science requirement.

Verification is by comparison — noise measured with each subsystem activated individually and then together, so that a contribution cannot hide inside the total. The design responses are separated analog and digital return paths, the ferrite between rails, physical guarding at the high-impedance input, and disabling USB and unnecessary microSD activity during the most sensitive measurements. All four are Rev B items.

The payload has to tolerate launch vibration, vacuum, thermal cycling, radiation effects and atomic-oxygen exposure, and pass calibration and functional tests afterwards rather than only before.

Atomic oxygen is the one specific to this instrument. An exposed conductive probe surface sitting in low Earth orbit is subject to erosion and contamination, and the probe's surface condition is one of the things that sets the relationship between collected current and density. A probe that survives launch mechanically but changes its surface in orbit has changed its calibration.

Planned qualification is vibration and shock, thermal-vacuum cycling, a materials review, radiation analysis, and a repeat of calibration afterwards to measure what the testing changed.

The review compared the schematic against the sixteen top-level requirements and the thirty-five mission functions, and asked of each whether Rev A could contribute to it.

Most of the electronic measurement chain is present, which is the thing that distinguishes this design from one that has only its support electronics: probe interface, clamps, electrometer, feedback network, bias source and buffers, conditioning, converter, processor, GNSS, storage, context sensors, power and protection, and the spacecraft interface are all drawn.

What is missing is the sensor. JP1 is a connector, and the physical probe — material, area, shape, orientation, cable, mounting and cleaning method — is undefined. Without it, TR-01 has no hardware behind it, and the relationship between collected current and electron density has no basis.

What is unresolved is precision. Three devices (U3, U5, U7) are not yet specified, and U5's pinout suggests it is not the part its label claims. The bias range is undefined, and the 3.3 V path cannot produce a bipolar sweep. One fixed gain may not cover the required current range. The high-impedance input has no guarding. Clamp leakage has not been verified against the signal it protects. Analog and digital returns are not deliberately separated, and the magnetometer sits near current-carrying traces.

None of that makes the drawing wrong. It is the distance between a circuit that is logically correct and an instrument that can carry a picoampere.