Part 3 — The fleet engine: turning signals into meaning

Part 2 delivered a clean, quality-graded stream of tracker positions into the database. This part is where those positions stop being dots on a map and become an operating diary: this truck is driving; it stopped at the border for six hours; it lost signal in a dead zone north of Garoua and came back forty kilometres later; forty litres disappeared while it was parked overnight.

The fleet engine is a set of small, cooperating state machines. Each one owns a single question and remembers its answer between batches of telemetry:

• Vehicle state — what is this truck doing right now, and what does the engine need to remember to keep answering that as new signals arrive.
• Trips — the driving windows: what opens one, what keeps it open, and every way it closes. This is where the engine works hardest to tell a real departure from GPS noise.
• Stops — when a truck goes still, how long it has to stay still before that counts, and how the engine labels why it stopped: border, customs, weighbridge, rest, breakdown.
• Signal gaps — the silence story. When a truck goes dark, what the engine gathers while it waits, and how it decides what happened once the truck comes back.
• Fuel — from a capacitive probe reading a liquid height, through a calibration curve, to a fuel-loss alert with a location and a confidence score.

A single rule underlies all of them: the engine processes one vehicle's signals strictly in order, one batch at a time. Everything in this part depends on that guarantee. It is the first thing the next chapter explains.