Proximity space links are defined to be short-range, bi-directional, fixed or mobile radio links, generally used to communicate among probes, landers, rovers, orbiting constellations, and orbiting relays. These links are characterized by short time delays, moderate (not weak) signals, and short, independent sessions.

ISO 21460:2006 defines the Proximity-1 space link protocol physical layer. The specification for the channel connection process, provision for frequency bands and assignments, hailing channel, polarization, modulation, data rates, and performance requirements are defined in ISO 21460:2006. Currently, the physical layer only defines operations at UHF frequencies for the Mars environment.

ISO 21460:2006 does not specify

• individual implementations or products,
• implementation of service interfaces within real systems,
• methods or technologies required to perform the procedures, or
• management activities required to configure and control the protocol.