AlfaCrux: understand when a radio message can arrive

Alfa Crux, usually styled AlfaCrux by its team, launched on April 1, 2022. The University of Brasília developed the educational mission through its aerospace systems laboratory with institutional partners including AEB, FAP-DF and Finatec. The contemporary university report describes student involvement across the mission life cycle and objectives involving amateur radio, packet relaying and storage followed by retransmission. Those objectives connect spacecraft engineering with the experience of communicating through a moving satellite.

A radio link requires more than a transmitter somewhere above Earth. The spacecraft and receiving station need compatible equipment, suitable geometry and enough signal quality for the selected communication method. A satellite in low Earth orbit is not continuously above one location. Operators therefore plan opportunities for contact, and a mission may store information while waiting for an appropriate receiving station to become available.

An immediate relay forwards received information during a suitable contact opportunity. Store-and-forward — open definition retains information for later transmission, allowing the receiving opportunity to occur at another time or place. Neither method removes limits on radio access, available power or data capacity. The choice changes what the system must remember and how users should interpret the delay between sending a message and receiving it.

Imagine a fictional satellite that accepts a sensor packet at 10:00 and forwards it at 10:12. The twelve-minute interval includes waiting for the later contact; it is not twelve minutes of radio-wave travel. If the packet contains a measurement from 09:55, its observation time is earlier still. Preserve the measurement timestamp, reception time and forwarding time separately so a user can judge whether the information is fresh enough for its intended task.

Follow the information through time

A successful file or packet transfer also needs a way to detect damaged or incomplete information. Sequence numbers and integrity checks can help a receiving system recognize missing pieces or corruption, depending on the chosen protocol. Repeated reception can improve confidence only when the messages are correctly identified and interpreted. These are general communications examples, not claims about a particular AlfaCrux packet format or guaranteed delivery rate.

UnB’s April 2022 report describes early monitoring and the mission’s educational and technology goals. It does not establish uninterrupted global service or the lifetime ultimately achieved. AEB’s later inventory records decommissioning in April 2024. Reading the two records together keeps the initial ambition distinct from later history. AlfaCrux is useful for learning how opportunities, stored information and verification determine what a small communications spacecraft can actually demonstrate.