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Link (or datalink): the means of transmitting data from one place to another. In the case of the radio module a RF transmitter/receiver pair is a link (a simplex one).
Simplex: a link where data can only be transmitted sent in one direction.
Duplex: a link where : data can be sent in both directions.
Half-duplex: data can be sent in both directions but not at the same time.
Full-duplex: data can be transmitted sent in both directions at the same time.
Downlink: a simplex link, where the data is sent from the rocket to ground (hence "down" physically).
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APRS: automatic packet reporting system, a packet protocol for routing packets across multiple repeaters. Generally sits over stacked on top of 1200 baud AFSK and AX.25. For rockets it is usually used to transmit GPS location.
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