Laser communication systems have high potential for use in spacecrafts. The same module could be used for both data transmission and ranging, or establishing the direction and distance of the target. Along with combining two systems, lasers have a lower electrical power requirement and are smaller, lighter, and more compact compared to traditional radio frequency (RF) systems. These benefits are due to the small size of optical wavelengths, which are measured in microns (10^-6 m), while RF wavelengths are on the order of 10^-2 m. The drawback is that establishing a link and remaining in contact is more difficult due to the smaller size of transmissions.
CubeSat: Application of Laser Communications
The CubeSat Project was developed by California Polytechnic State University, San Luis Obispo and Stanford University's Space Systems Development Lab. The goal of the program is to give launch opportunities for universities previously unable to access space. Over 60 high schools and universities currently participate in the CubeSat Program.
A CubeSat is a 10cm cube miniature satellite. The small size and weight allow for CubeSats to easily join rockets and other spacecraft into space. Once jettisoned, CubeSats roughly orient and stabilize themselves in their irregular path in Low Earth Orbit, anywhere from 100 to 1,240 miles. The satellite we are designing for has an orbit of about 90 minutes, five of which put it within range to communicate with Goddard. Connection and data transfer between the CubeSat and Goddard must happen within those five minutes.
As the CubeSat nears Goddard, it will begin scanning the ground with its laser, trying to hit every possible position of the receiving station at Goddard. It will continue to scan until the receiver responds with its own laser beam, confirming the connection. The CubeSat may then have to backtrack to where it found the receiver. Another challenge is keeping the link while the satellite is moving.
Our ground based receiver will be located at NASA's Goddard Space Flight Center. It will have the capability, given an incoming signal, to locate the position of the source (in our case, the CubeSat). The receiver will then fire a laser back at the CubeSat, establishing the two-way communication link.
Assuming the CubeSat's laser connects with the lens of the receiver, the receiver will be able to determine the direction of the CubeSat in relation to the receiver. The laser is redirected through the lens to be projected onto a surface. A webcam viewing the surface, once properly calibrated, will be able to extract the radius and angle of the laser's position from the optical axis. This information is processed into the direction of the incoming laser.
With the direction of the CubeSat from the ground, stepper motors with gear trains for high-resolution movement position a laser signal back at the CubeSat. This signal will confirm that the communication link is established, allowing for the data transfer of the satellite to Goddard.
Our Test Setup
To recreate the CubeSat scenario, we will place our mock-up CubeSat laser system on a cart at some distance away from a stationary receiver. The distance between them, size of the laser beam, and size of the target will all be adjusted to replicate the difficulty of accomplishing this task from 2000km above the Earth.