Satellite subsystem for performing telecommunication and ranging measurements
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In spacecraft design, the tracking, telemetry, and command (TT&C), known also as telemetry, tracking, and control, refers to three specific functions of the spacecraft, usually implemented as single subsystems[1].
As indicated by the name, these three functions implemented by the TT&C subsystem are:
command (also known as telecommand, TC) reception, for controlling and having the spacecraft executing the actions required by the operator
telemetry (TM) transmission, for receiving the data about the status of the spacecraft, and that generated by the scientific instruments
tracking, adopted for determining the radial distance and velocity, with respect a reference ground station.
The satellite TT&C subsystem design can be tailored to the specific mission needs, that pose requirements on performance (e.g., minimum bit rates), physical sizing (e.g., subsystem's mass, power consumption, and volume), reliability, and security.
In all cases, the TT&C subsystem comprises the following units[2]:
TransponderBepiColombo X/Ka-Band Deep Space Transponder, that comprises a transmitter for sending telemetry, a receiver for receiving commands, and a ranging channel (i.e., receives and re-transmit the ranging signal as received from the ground station).
Low noise and power amplifiers, for having the received command and transmitted telemetry signal reach the design operating points. These are tipically TWTA or SSPA
Antennas, for radiating and receiving the RF signals for command and telemetry
Radio frequency distribution network (known as RFDN), that comprised the coaxial cables, waveguides, switches.
The TT&C subsystem closely operates in interface with the satellite on-board data handling (OBDH) subsystem. In particular the TT&C is charge only of the RF modulation and demodulation of the TC/TM signals, while the coding, processing, generation, and execution or the related bit streams are usually done by the OBDH. The interface with the OBDH is usually by means of specific electrical lines, that can be based on RS-422, MIL-STD-1553, Wizard Link (TLK211), SpaceWire, SpaceFibre[3][4], or other electrical data interface standard.
The other main interface of the TT&C subsystem is the ground segment, represented by the ground stations, the mission control center, and the associated infrastructure. In particular, the satellite TT&C subsystem shall be RF compatible with the ground segment, meaning, that the same modulation formats, coding layer, data link, and upper level protocols shall be supported by the two entities.
Spacecraft missions that generate a high data volume from the scientific instruments (e.g., Earth Observation missions), require a high-rate downlink that becomes a driver for the sizing of the telemetry bit rate[5].
Block diagram of the payload data transmission subsystem of Sentinel 1,2,and 3 satellites[6]
For this reason, in these kind of space missions, it is preferable for the spacecraft to have two separated downlinks: one with low bit rate, for just retrieving the spacecraft status, and one with high bit rate, for downloading the scientific instruments data.
The latter is identified as payload data telemetry (PDT), and often implemented in the spacecraft as a separated subsystem[7].
^Davalle, Daniele (2023). "SpaceWire/SpaceFibre Analyser Real-Time (SpaceART) system extension to the Wizardlink Protocol". Proc. of European Data Handling & Data Processing Conference (EDHPC).
^Nannipieri, Pietro; Fanucci, Luca (2022). "Satellite High-Speed On-Board Data Handling: From a Wizardlink Equivalent Transceiver To a Full SpaceFibre Interface". Proc. IEEE Aerospace Conference (AERO).
^Wertz, P.; Hespeler, B.; Kiessling, M.; Hagmanns, F.J. (2016). "Next generation high data rate downlink subsystems based on a flexible APSK modulator applying SCCC encoding". proc. International Workshop on Tracking, Telemetry and Command Systems for Space Applications (TTC).
^L'Abbate, Michelangelo; Venditti, Paolo; Svava, Carlo; Bagaglini, Flavio; Roscigno, Rita (2014). "From Mbps to Gbps: Evolution of Payload Data Handling and Transmission system for future earth observation missions". IEEE Metrology for Aerospace (MetroAeroSpace): 576–581.
