In order to receive data from the satellite it is helpful to design a packet scheme that can guarantee reliable and redundant communication. The Consultative Committee for Space Data Systems (CCSDS) provides its recommendation for space data system standard, which we have chosen to follow and apply to our project.

Packet Telemetry

Is a concept which facilitates the transmission of satellite-acquired data from the source to a user in a standardized highly automated manner. This is done so the ground system can recover the individual data units with high reliability by using Source Packets and Transfer Frames, which are data structures.

Source Packet

The Source Packet contains a Primary Header that is used to route the packet to its destination, and it contains information about the length, sequence, and other characteristics of the packet. There can be a secondary header which carries a standardized time-tagging, the satellite position, attitude and data that can support the primary functions of the packet.

Source Packet Illustration

 

Transfer Frames

This data structure provides an envelope for transmitting data packets over a noisy satellite-to-ground channel. It also contains a header that provides information that helps in the data routing and classification. This frame has a fixed length and it is used to encode various variable length Source Packets together for a reliable transmission to the ground. By using this method there is no need to have a predetermined size of each packet and guarantees that all the information is transmitted and if not, the users would be able to recognize what data was missing.

Transfer Frame Illustration

Idle Packets

What happens when a Source Packet is too small to fit in a Transfer Frame? Idle Packets are used, which are data that carries no information and have no specified bit pattern. They are used to meet synchronization requirements and error protection techniques for the transmission of data.

 

Source: public.ccsds.org/publications/archive/102x0b5s.pdf

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