In a general sense, telemetry is one half of a communication system for the transmission of information. For example, multiple remote seismic measuring devices might send information about a tremor or earthquake to a central monitoring station for analysis. Space telemetry is a system developed specifically for use by space vehicles to send information back to Earth.
Data From Space
Spacecraft send data to NASA using telemetry. The reports can be information about the spacecraft’s condition, or perhaps a photograph or other measurement. The data can be sent back to Earth on a previously programmed schedule, when certain events happen, or on command from an Earth based control station.
Some spacecraft travel deep into space and continue to provide information about their journey long after their intended mission has been completed. Voyager I is a good example: Although now more than 10 billion miles from Earth and nearly 31 years after completing its primary mission, Voyager I’s telemetry (and telecommand—command and control information for the probe) systems continue to operate and send data back to Earth even though it was first launched in 1977. The radio signals from Voyager I’s telemetry system travel at the speed of light and take more than 16 hours to reach Earth.
Space telemetry is not limited to deep space probes and both manned and unmanned spacecraft use telemetry to send data back to Earth. Often during a space mission, events happen too quickly for humans to read the information and provide it to mission control. Critical information collected by sensors is compiled by computers and sent back to Earth in real time, where it is assessed by more computers and disseminated to the people controlling the flight as it is needed.
With data streaming into the computers at a rate no human or group of humans could comprehend, the computer can pick out an anomaly and bring it to the attention of the appropriate controller or group of controllers for further assessment. The controller might issue a command to correct the problem or have an astronaut make a correction or take some other action.
How It All Works
On Earth there are many possible methods of telemetry but in space, telemetry is (currently) limited to radio signals. Similar to a cell phone receiving an email or visiting a web page, Earth based control systems receive radio signals from a spacecraft. Various methods of encoding the information exist and have changed over time. Originally the signals were very simple but have become increasing complex as the need and capacity for information has increased.
The data capacity of a telemetry system is called bandwidth and is the amount of information the system is capable of sending in real time back to Earth. Some of the very first space telemetry systems were only capable of sending two or three pieces of information. Modern telemetry systems can send tremendous amounts of data at once. Improvements in data compression, error correction and transmission speeds have also contributed to increased bandwidth.
Space telemetry transmits information in the form of a radio signal. Sensors on the spacecraft collect analog data–a photograph of Mars for example–and send it back to Earth. Early space telemetry systems used Pulse Position Modulation (PPM) and Pulse Width Modulation (PWM) to transmit data. Modern telemetry uses Pulse Coded Modulation (PCM) to transmit data. There are various ways of implementing these data communication methods, but the general principle for each of them form the basis of what is space telemetry and how space telemetry works.
Pulse Position and Pulse Width Modulation
PPM encodes analog information based on a pulse within a specific time frame.
As a very simple example, you could set the time frame to one second and send a pulse in the first half of the second to represent a binary 1, and in the second half to represent a binary 0. The pulse could begin—within the time frame of 1 second—at 2/10 of a second and end at 4/10 of a second, or it could start at 7/10 of a second and end at 9/10 of a second.
Obviously this is an impractical example as it would take a full eight seconds to send just one byte of data. In practice, transmission of data happens much faster. The advantage of PPM data transmission is that you only need to measure the time delay between pulses to read the data.
PWM varies the length of a data pulse to represent a specific piece of data within a specific time frame. A pulse that lasts 1/10 of a second might equal a zero, while a pulse that lasts twice that length might equal a 2. An advantage of PWM over PPM is the ability to encode more information into a given time frame.
Pulse Code Modulation
PWM and PPM both served their purpose as data transmission systems in space telemetry in the 1940s, early 1950s and even the 1960s to some degree. Newer technology uses Pulse Code Modulation (PCM). Data transmission by this method later found its way into the music and video entertainment industry and is how compact discs (CDs), digital video discs (DVDs) and similar media are encoded. There are many methods of implementing PCM, but the basic principle is the same.
PCM is an encoded binary signal of ones and zeros and is used to send both analog and digital information. The accuracy of analog information sent by PCM relies on the bit depth and sampling rate of the analog to digital encoder. Bit depth is the number of bits used to represent a value, while sampling rate determines how often a sample of the information is taken.
For an analog example, consider a photograph taken with a digital camera. At high resolution the file size is large since there are many pixels (sample rate) and colors (bit depth). At low resolution with fewer colors , the bit depth and sample rate are low with lower image quality and a lower file size.
Sending a high resolution photograph of Saturn’s rings back to Earth using PCM requires a lot of data. On the other hand, reporting the temperature of the spacecraft’s battery might require just a few bytes of data, along with the overhead required to send it. Besides the capacity for sending a lot data quickly, a major advantage of PCM is the ability to encrypt information.
Very early rockets developed by Germany during World War II used multiplexed radio signals to report on the rocket’s flight and control system functions. As few as four pieces of data were reported by the system and it was considered to be highly unreliable.
Later, PPM and PWM methods of space telemetry were developed separately for use on missile systems by the United States and the USSR. These methods of telemetry were first used in the 1940s, and various methods of implementing the technologies found their way into use.
As the need for more information and two-way communication between the spacecraft and Earth evolved, so did the methods for transmitting data to and from the spacecraft. By using more than one radio signal to transmit data, more data could be sent at one time.
Methods of encoding data into radio signals has improved greatly since the first steps into space and PCM has become a generic term for how data is transmitted. NASA implements PCM into telemetry using multiple microwave and radio signals along with various methods of encoding data within those signals
As space exploration and development continues, “what is space telemetry?” and its answers will continue to evolve.
Credits and References
Earth-Satellite image by NASA
Voyager spacecraft image by NASA