Telemetries Overview

What are telemetry packets?

The space segment (also known as "the satellite") of a space mission generates a lot of data, which must be transferred to the ground segment (aka the ground station) via the radio-communication link. These data which go from space to ground (downlink) are called telemetries, and can be of several different sorts, from science data to house-keeping data, or even error reports.

The Picsat mission uses a specific packet communication protocol, based in large part on the CCSDS (Consultative Comittee for Space Data Systems) standards, to transfer telemetries generated in space to the Earth. In this standard, a packet, which represents the elementary piece of information which can be transferred, may contain multiple datapoints, but all of the same type (house-keeping data are never mixed with science data, error reports are not mixed with house-keeping or science data, etc.). During radio communications, a packet is considered to be absolutely indivisble. If only one byte is missing at the receiving end, the packet cannot be decoded, and all the data contained within are lost.

Telemetry packet categories

• ADCS actuator: these packets contain data about the different actuators of the Attitude Determination and Control System (ADCS). They can be seen as part of the "monitoring" data, used to check the global health of the satellite. They are not automatically sent to ground if not explicitly requested.
• ADCS attitude: also coming from the ADCS system, these packets contain data about the attitude pointed by the system (how it is oriented in space).
• ADCS sensor: these packets contain data about the health of the ADCS sensors (magnetometers, star tracker, etc.).
• ADCS standard: these are the standard telemetry packets emitted by the Attitude Determination and Control System. They contain important information used to check the global health of the system, and its current operating mode.
• beacon: the beacon is a very important packet, containing a whole lot of information about the satellite, and the status of its subsystems. This packet is automatically emitted every 10 s, and can be received by anyone when the satellite is above their head.
• boot error report: this category of packets is only emitted when the On-Board Computer initializes.
• counters: these packets contain a list of identifiers for the data stored on board of the satellite. They are very useful to prepare data download operations.
• datalist: this category is similar to the previous one, except that more information about the data stored on the satellite are available.
• dump answer data: a category used to retrieve large amount of data from the satellite, when they cannot fit into a telecommand answer, and do not have a dedicated category (example: a dump of the raw bytes contained in the memory of the satellite will generate such packets)
• exe acknowledgement(Execution acknowledgement): automatically emitted to acknowledge the proper execution of a telecommand sent by the ground, or to inform the user of a problem encountered when executing it.
• event report: this category is used for packets containing general information about specific event happening at software level in the On-Board Computer.
• flux mean (Reduced science data - Flux Mean): these packets are part of the science data which are sent to Earth on request. This category is used to send the mean value of the photometry measured by the payload, after an initial step of processing performed on-board. Interpeting these packets also requires to have access to the "flux variance" packets.
• flux variance (Reduced science data - Flux Variance): also part of the processed science data, these packets contain some information related to the pointing precision of the payload, and thus to the way the "flux mean" packets must be further processed on-ground to retrieve a high precision measurement of the photometry of the target star.
• fmt acknowledgement (Format acknowledgement): these packets are emitted by the satellite and/or the payload when a command has been received. They contain an error code indicating if the format of the packet received was valid or not.
• HF science (High-frequency raw science data): these are the raw science data, acquired at high-frequency (1000 Hz) by the payload of PicSat. These data are too big to be routinely retrieved by the ground segment. They are stored on-board, and only a minimal subset of them will be retrieved during the mission, to callibrate the instrument.
• house keeping: these data are used to check the health of the satellite. They contain much more information that the beacon, but are not automatically sent by the satellite. They must be explicitly requested by the ground station.
• L1 housekeeper (House-Keeping level 1): some more health-check data, concerning the higher-level part of the On-Board Computer Software.
• patch list segment: uh-oh! If some of these packets are sent to Earth, this means that something is going seriously wrong up-there! These packets contain the list of code segments sent by the ground station and received by the satellite. They are only emitted when software updating activities are ongoing.
• pld beacon (Payload beacon): this is the payload beacon, emitted at a given frequency (usually 10 s, like the satellite beacon). It contains information about the status of the payload (current operation mode, temperature, etc.). It is not always active, but when it is, it can be received by anyone listening to the satellite.
• payload HK (Payload House-Keeping): these are high-frequency data acquired by the sensors embedded on the payload electronic board (mostly temperatures and voltages). These are important packets, as some of these data (temperatures) are required to correct the photometry for instrumental bias.
• pld report (Payload report): Implemented but not used.
• rec acknowledgment (Reception acknowledgement): these packets are automatically emitted by the satellite upon reception of a telecommand from the ground segment.
• SD filelist (SD card list of files): No kidding, data are stored onboard PicSat as files on a standard SD card, like in your camera or mobile phone! These packets contain the list of files stored on the card (to Linux user: yes, this is the response to your usual 'ls' command. Except it is coming from space! How cool is that?)
• scheduler Fifo: contain the list of all the operations currently scheduled by the On-Board-Computer.
• tc answer (Telecommand answer): as suggested by its name, this category was designed as a generic way to store and retrieve the answer to the telecommand sent from Earth. As a consequence, they can contain very different types of data.

What is the life cycle of a telemetry packet?

Multiple different data are constantly being genereated withing the satellite subsystems (sensors are acquiring measurements, the processor is converting raw science to reduced science, event reports are generated, crucial information about the states of the different subsystems are stored in memory, etc.). These data stored in the memory of the satellite cannot be analyzed by our team of Earthling scientists until they are physically somewhere down here. The only way to get these data on the ground is to sent them from space to Earth using the PicSat onboard radio transmitter. But for them to be properly received by a ground station located at least 600 km below the satellite, they need to be properly sorted and organized. These data are thus aggregated by the processor to form homogenous data packets (or frames). These CCSDS packets typically have a size of about 200 bytes, and can be directly sent to the ground, or stored in the SD card to be retrieved later on.

When a packet is transmitted by the satellite, and retrieved by a station on Earth, it is unpacked, and the data contained within are automatically read by a computer, and stored in a dedicated database located at LESIA, in Meudon, France. This database can then be accessed by our automatic data reduction software to be further processed, and then by our team to be analysed. The database is also directly accessed by this website to generate the graphs and plots you can see here and there.

If you have set up your own station and have already started to receive data from our satellite, you also have access to the unpacked data you received and uploaded on this site. You can use our download page (availble after the launch) to retrieve them on your computer and do whatever you want with them. These data are yours!