Retrieving the data
The interval between the readings of the data varies according to their nature. For example, gyro sensors have to be sampled more frequently while temperature sensors will not vary so fast and, therefore, can be sampled at longer intervals. Another issue is data that is sporadically sampled, where there is no fixed or known time interval between the readings, such as the number of commands received and general payload data. However, it is interesting to relate the data from one sensor to another, for example if there was a variation in the temperature of the satellite it is interesting to identify if it was due to solar radiation or a short circuit. This is possible by searching for variations in radiation sensors or current sensing circuits readings. In this way it is imperative that the readings are stored together with others that occurred at the same instant.
In view of this, as a project decision, a frequency to write new data in the memory of 1 Hz was chosen, that is, 1 new package is written every second. This package contains the last reading of all sensors and submodules. The adopted approach proposes the use of bit-flags to indicate which data groups were updated in the last reading. The respective data fields to these groups are updated, while the others are replicated from the previous packet.
When responding to a Telemetry Downlink request, when it requests several subsequent packets, the first packet is sent in full, while the subsequent packets will only send the data that was updated, saving bandwidth by not sending redundant data. The group bit-flags of each packet are always sent, so that the terrestrial station knows what data is contained in the referred packet. This also gives the ground station the flexibility to choose which data groups it wants to request, sending in the request only the bit-flags referring to the interested groups. The OBDH then compares the requested bit-flags with the ones present in each packet and only sends the data belonging to the groups that have been requested AND have been updated in the referring packets. This technique generates bandwidth savings and make the telecommand requests more flexible, without need to create specific telecommands to request certain types of data.
Another way to retrieve the data from OBDH is over periodic transmissions. When in "Normal" operation mode, the last read packet is send through radio, at intervals according the satellite Energy Level. The interval can be 60 or 120 seconds when in Energy levels L1, L2 or L3. Then, the data is packed over NGHam protocol and sent.
Assumptions:
1 - Sending all the data that is saved is impractical, so it is necessary to select only the desired data;
2 - There is also a bandwidth when sending redundant data (repeated, which was not updated in the last reading).
The flowchart below was made to meet the previous assumptions:
