The generation of spacecraft data has a direct correlation with the value of any mission. More data typically means more mission value. That is why we at VisionSpace have been looking into compression of satellite telemetry data. To understand the unique requirements, we give you in the first part of our series an introduction to the process of satellite housekeeping data generation and systems involved.

The Telemetry, Tracking, and Control (TT&C) subsystem is an integral part of every spacecraft, providing an interface to monitor and control all onboard subsystems and functions. Based on the evaluation of collected and processed data on the ground, called Telemetry (TM), a spacecraft's health is determined. The execution and processing of received telecommands is another essential task of this subsystem.

The first step in the generation of TM is the measurement of physical parameters like temperatures, voltages, and currents. Monitoring includes available resources like fuel, memory, and battery charge. TM data further consists of the current operational mode of all subsystems. Analog values, representing actual physical parameters, are converted using Analog-to-Digital-Converters. All digital values are formatted and pre-processed.

A large spacecraft can have hundreds of just temperature sensors. Measuring the spacecraft's status is essential to evaluate its overall health and the payload condition, which is the central element of every mission. Each mission must determine before launch and throughout the mission lifetime which TM is processed only onboard and forwarded to the ground. Both options can lead to a more cost-effective solution depending on the mission requirements and the overall situation.

Satellites orbiting earth might not have a constant link possible to the ground systems. Besides global coverage limitations, also the costs for a connection with the satellite must be considered. It is impossible to downlink TM all the time in most orbits because worldwide coverage with a ground station network is impossible due to international borders and oceans. A satellite in these orbits usually has multiple, relatively short passes over ground stations per day or can be equipped with a radio frequency or optical uplink terminal to a higher orbit satellite offering better ground coverage.

Ground Station at Very Large Array, Socorro, United States

For spacecraft exploring the solar system, specialized large antennas are required, and communication is often only possible at certain times a day. All interplanetary missions share these resources, which are very limited due to their high operational costs. The TT&C subsystem usually has a lower bandwidth requirement than payload subsystems but must also be more reliable. The data bandwidth requirement typically lies between 500 and 2000 kilobits per second but can also be lower. Combining tailored coding techniques, modulations, and protocol layers ensures reliable data transmission for spacecraft operations.

This is the first post in a two part series on data compression. In the next post we will present the new and yet-to-be-released CCSDS standard for housekeeping data compression, so stay tuned.

If you are interested in spacecraft data compression or want to know more about our offered products and services, please contact us!

Satellite Housekeeping Data