Uplink

In satellite telecommunication, a downlink is the link from a satellite down to one or more ground stations or receivers, and an uplink is the link from a ground station up to a satellite. Basically any signal from the ground to satellite can be known as Uplink. A space link is a communications link between a spacecraft and its associated ground system or between two spacecrafts. A space link protocol is a communications protocol designed to be used over a space link, or in a network that contains one or multiple space links. The basic data flow over a space link is made of Telemetry(TM) and Telecommand (TC) data. Telemetry generally translates to downlink and telecommand to uplink.
Telecommand can further be divided into two types:

1. Direct commands for reconfiguration of spacecraft: These are commands that state the satellite to operate some pre-coded commands. A few of such commands are:

• Kill command used to shutdown all systems permanently, done in case of the satellite transmitting out of frequency band
• Reset command used to reset a particular subsystem or all subsystems back to initial state
• Sleep command used to temporarily shutdown all electronics on the device, typically done in a few belts in orbit where radiation or other factors could harm satellite functioning

1. Application specific commands: It involves commanding the satellite to start, end or enhance a particular functionality. Also these commands come with some data that can be input to the satellite to enhance a functionality. A few examples:

• Control data can be uploaded to satellite to improve the stability of the satellite and make the control law of the satellite more effective
• If high accuracy in time is required, even time is uploaded, since in space, clocks may behave differently
• Imaging satellites may use tele commands to focus on imaging particular areas or change resolution of imaging
• Commands to activate or deactivate a particular device
• Command to change the polarity of the magnetorquers

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