Optical Data Transmission
Small satellite designers rely on the current state-of-the-art in microelectronics to perform the mission-critical function of relaying information back to Earth. The majority of kilogram-class CubeSats have downlink data rates below 10 thousand bits per second, although a few have attained rates in the 10 million bit per second range. We need higher downlink rates to support the incredible amounts of data generated each second by the current generation of small, relatively inexpensive sensors such as high-definition color video cameras that generate 22 million bits of data each second. These sensors can operate continuously and collect data over a whole day; however, the problem is that the spacecraft has only about 30 minutes each day to send the data back to Earth while it is in view of a single mid-latitude ground station. One day’s worth of high-definition video data can take almost 4 months to download at 10 million bits per second, and almost 300 years using the slower, 10-thousand-bits-per-second data rate more typical of CubeSats. Fortunately, we can boost our downlink data rates by a factor of 200 or more by using light instead of radio waves to transmit the information. The laser transmitter in our Pathfinder OCSD spacecraft will be able to transmit at rates up to 50 million bits per second. The next two OCSD Mission spacecraft will boost that rate to over 600 million bits per second. This means that the same high-definition video data that takes 4 months to download with the current-state-of-the-art CubeSats would take about two days with the OCSD technology.
|data transfer rate
(bits per second)
|time to download one day of high-definition data|
|10 thousand||300 years|
|10 million||4 months|
|600 million||2 days|
The success of the OCSD mission may open possibilities of using small satellites for synthetic aperture radar and hyperspectral Earth imaging missions, which would generate vast amounts of data to send back to Earth. Additionally, the success of the OCSD mission may enable a low-latency, high-rate communications service for orbiting satellites.
After the launch of the pair of OCSD Mission CubeSats, we will use GPS receivers and optical proximity sensors made up of cameras, LED beacons, and laser range finders to track the relative position of each spacecraft with respect to the other. These low-cost sensors may enable future missions with several small spacecraft to operate cooperatively in close proximity. Gross relative positioning will be controlled by varying the attitude, and hence drag, of one spacecraft with respect to the other. Fine control will use a propulsion system fueled by water. We expect these technologies to enable science and exploration missions, in-space servicing of spacecraft, or the formation of larger space systems with multiple small satellites that would otherwise be more expensive to build and launch.