Decode and deliver radio data

CPUT’s prelaunch description proposed an improved AIS receiver payload, raw-data capture, software updates from the ground and a more efficient data interface. These were design capabilities to test, not proof that every function later worked in orbit. Raw samples preserve information about a received signal before its full interpretation. A decoded message is the result of applying a particular processing chain to that signal.

Keeping raw samples can help engineers investigate interference or compare decoder behavior. It also uses storage and downlink capacity. If a decoder reports no valid message, the absence may reflect weak reception, interference or a processing problem; it does not establish that no transmitter existed. Raw samples may help distinguish explanations, provided their timing and receiver configuration are known.

In a deliberately simplified example, a link has 600 kilobits of usable capacity during a contact. One decoded record occupies 2 kilobits; one diagnostic capture occupies 120 kilobits. Sending two captures consumes 240 kilobits, leaving 360 kilobits for 180 records. These are invented sizes and a fixed capacity, not MDASat specifications.

A team might send routine decoded records while reserving some contacts for diagnostic samples. The tradeoff depends on the service and the fault under investigation. Compression, packet headers, retransmissions and error protection can change the real calculation. If the 600-kilobit figure is already usable payload capacity, do not subtract the same overhead a second time. Define the budget before doing arithmetic.

Allocate useful downlink capacity

The ITU describes the VHF Data Exchange System, or VDES, as combining AIS, application-specific messages and VHF data exchange through terrestrial and satellite components. AIS is therefore one component of a larger architecture. Sharing that architecture does not make every AIS receiver a fully capable two-way VDES terminal. Reception, transmission, channel support and protocol behavior must each match the intended role.

SANSA’s 2023/24 report describes M2MSat as part of a technology roadmap toward operational maritime capability, with an upgraded transceiver on VDES channels. A receiver detects incoming information; a transceiver also transmits. A future design’s added transmit function should not be projected backward onto an earlier flight’s demonstrated results. Compare the actual equipment, tested functions and dates.