OPALS, Optical Payload for Lasercomm Science, is a demonstration mission that will test optical communications technologies from a platform on the ISS. Using off-the-shelf hardware, the flight system will transfer video data from orbit to a ground station at JPL's Optical Communications Telescope Laboratory (OCTL) in Wrightwood, California. It will show the feasibility of laser communications with low cost commercial hardware and prove, like Curiosity, that lasers can be used for good instead of evil.
Traditional communications in space use radio waves. On the longer end of the electromagnetic communication spectrum, S-band radio is typically around 10 centimeters (about 4 inches) long. They can't transmit a lot of data in one go, but they are easy to receive; the relatively long wavelength of radio causes them to quickly spread out in all directions from their point of origin. They also aren't interrupted by things like cloud cover or rain, making them a popular communications wavelength from the shuttle program to Sirius XM radio.
Laser communications on the other hand use much shorter wavelengths in the high end of the visible light spectrum. OPALS will operate with wavelengths around 1,500 nanometers or 0.00015 cm. The physical properties of operating at a higher frequency means communications at this wavelength can transmit a lot more data in one go.
But there's a catch. Laser communications, just like laser pointers, send data in a very focused and narrow beam of light. If you point a laser pointer at a wall and walk slowly backwards, the dot will get a little bigger and fuzzier, but it's still just a dot. The same is true from space. Receiving laser communications is tricky.
To take advantage of the high data transfer of laser communications, a spacecraft will have to aim its laser at a receiver on Earth with extreme accuracy. It's hard but not impossible, and OPALS is going to prove just that.
The OPALS team is on track to launch their flight hardware to the ISS on one of SpaceX's Falcon 9 rockets next July. Once installed, it will demonstrate one way communications (from space to Earth) every time the station passes over the telescope at Wrightwood, about once every 2 or 3 days.
The team will keep a close eye on the ISS, tracking its passes over Wrightwood by calculations instead of a tracking system. Just before an overhead pass, the telescope will look to where the team predicts the ISS will be. Once the station comes into view (the telescope is surrounded by trees) the flight system will detect a beacon on the ground then use its gimbals to get into a position where it's looking down to the telescope. Then the laser will fire, transferring a preselected loop of video data for the duration of a the 100 second pass.
As soon as the ISS passes out of range, the test ends. The team will debrief, go through their post-demonstration activities, then start planning for the ISS' next pass. This will go on for at least three months, the length of the OPALS primary mission. But if all goes well (and funding comes through) they may get a chance to run demonstrations and gather data for a full year.
What the OPALS team finds may go beyond the one mission to support another NASA laser communications program, the Laser Communications Relay Demonstration run by the Goddard Spaceflight Center. The long-duration LCRD mission will push the state of the art of laser communications further, perhaps with data and results from the OPALS missions.
Communicating with space is about to get way more awesome.