The probe is part of the instrument, not just a connector

I drew the electronics for a plasma instrument and left the part that touches the plasma for later, as though it were packaging.

Ionospheric Plasma Irregularity Payload — Rev AInstrumentationDesign Review

Problem

JP1 sits on the schematic as a small coax connector, and for most of the design that is all it needed to be. Reviewing against the requirements, the first one — measure plasma current across a density range of roughly 1e8 to 1e12 per cubic metre — has nothing behind it, because the thing that collects the current does not exist yet in any defined form.

Decision

I stopped treating the probe as a mechanical item to specify during integration and wrote out what actually enters the measurement: material and surface condition, which set how collected current relates to density; exposed area, which scales the current directly; shape and orientation, which set the collection geometry; the cable, which adds stray capacitance and leakage at the most sensitive node in the design; and the mounting, which decides whether the probe samples the ionosphere or the spacecraft's own wake.

Test / evidence

The source is explicit that the schematic shows the probe connector but not the physical probe, and the first item on the correction list is to define it. The verification plan says the same thing from the other side — probe placement has to be validated by spacecraft-plasma simulation and chamber measurement before any current reading means anything.

Result

A probe mounted in the wake measures the spacecraft, not the ionosphere, and no amount of front-end precision recovers from that. The probe is the first component in the measurement chain.

Next step

Define material, geometry, area, mounting, orientation, cable and cleaning method as Rev B work, before refining anything downstream of it.