Probe hardware

The probe is a program on a computer carried on the aircraft. The aircraft supplies a radio front end, a position, and a path to the controller. The radio, the antenna, a GPS receiver, and the link are part of the aircraft. Installing the program is described in probe-installation.md. Running it is described in probe-administration.md.

Probe computer

The probe runs on a small Linux computer on the airframe. An NVIDIA Jetson is one example. A later Jetson module, another ARM board, or a small x86 computer can host it when meshint-probe is built for that computer's libraries.

The archive in the current release is built for the Jetson TX1 userland: aarch64, glibc 2.23, and OpenSSL 1.0.2. Install that file on a TX1. A different computer needs its own build.

The computer stores the program, the probe state, any recording, and a software update the controller has offered. Samples stay on that computer. The link to the controller carries detection reports, not the recording.

More than one probe can share a computer. Each has its own state. How those identities are set is in probe-administration.md.

Radio

A software-defined radio on the probe computer is the front end that collects energy in the band of interest and produces complex baseband: little-endian interleaved float I and Q, with a sample rate and a centre frequency. The analyser measures one dwell of that form. The antenna is aircraft equipment, chosen for the same band, and may be omnidirectional or directional.

MeshInt does not tune the radio and does not include a scanner. In the current probe, a dwell is the start of a cf32 recording on the computer when the recording matches the stop, and silence otherwise. A live dwell from the radio is the same kind of input, supplied by software outside this product.

The integrator chooses the radio. The names below are examples of front ends that can sit with a small probe computer. MeshInt does not include a driver for them:

A suitable radio covers the band the aircraft is there to watch and can deliver a dwell at the sample rate and centre frequency of that watch. The controller later names the stops. The radio still has to be able to visit them.

Position from the airframe

The probe reports where the aircraft was when it measured an emission. That fix comes from the airframe. The probe has no GPS receiver of its own.

The airframe supplies latitude and longitude in degrees and altitude in metres. A position is reported only when all three are present. Heading, in degrees from north, and ground speed are sent when the airframe has them. Heading matters when the antenna is directional, because the reported sector is about the airframe heading.

The probe is given the fix as coordinates at start, or by a navigation file the airframe keeps up to date. The airframe writes that file. The probe reads it.

The clock label is gpsdo when the radio reference is disciplined from the airframe GPS, and free otherwise. The probe records which one it was given. It does not lock the reference.

Link to the controller

The probe opens the connection to the controller. The airframe needs an IP path from the probe computer to the controller's probe address. The controller waits for that connection.

Starlink on the aircraft is one such path. A line-of-sight IP radio, a cellular modem, or a ground network while the aircraft is connected are others. The probe uses whichever path can open TLS to the controller it was given. It trusts only that controller's certificate, and it does not follow a redirect.

The path carries the session: a hello, detection reports, status, and the configuration coming back. It does not carry I/Q. When the path is down, the probe keeps its configuration and holds reports on the aircraft until the next connection succeeds.