Since Nasa first ventured into space, it has relied on radio frequency (RF) communication to transmit data to and from space. ‘But why isn’t that good enough now?’ asked Donald Cornwell, head of Nasa’s Space Communications and Navigation Programme, at the Lase plenary session at Photonics West. The laser, photonics, and biomedical optics conference and exhibition took place in San Francisco from 10-12 February.
Radio frequency is reaching its limit as demand for more data capacity continues to increase, Cornwell explained. The expanding number of missions, combined with the increase in sensor resolution, demands orders of magnitude enhancement in communications capacity. Moreover, optical communications will be necessary for high-definition TV images of future human missions to Mars.
During the presentation, Cornwell gave an example of the current download speeds that can be achieved from the surface of Mars. Using the best RF system available, it would take up to nine years to transmit a 30cm resolution Google map of the Martian surface at one bit per pixel.
‘The point of this is that we’re leaving 90 per cent or more of the data on the surface of Mars,’ he said. ‘We’re orbiting mars and have to pick and choose, and I think we’re missing moments of serendipity because we’re not bringing enough data back.’
By using free-space optical communication, it would be possible to transmit an entire Google map of Mars in just nine weeks.
The idea of free-space optical communication has been around for more than 50 years, Cornwell said, with many projects launched over the years only to be delayed or cancelled due to funding issues or other problems.
It wasn’t until October 2013 when Nasa demonstrated the first two-way, high-rate laser communication from the Moon, 4,800 times faster than current RF systems, that Nasa started to get excited about optical communication, according to Cornwell.
The Lunar Laser Communication Demonstration (LLCD) was mounted on the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft. It transmitted data more than 400,000km at a download rate of 622Mb/s, and an uplink rate of 20Mb/s, in a system that was half the mass and used 25 per cent less power than state-of-the-art lunar RF systems. ‘This achievement inspired Nasa to follow up this laser optical communication,’ noted Cornwell.
Following the LLCD mission, through a government-funded programme, Nasa is currently developing a similar optical communications terminal that will be sent to Mars. According to Cornwell, the deep space optical communications (DSOC) system will be space qualified by 2017, which Nasa will then provide as government-funded equipment (GFE) to the Mars 2020 mission. Nasa anticipates that the DSOC will be able to deliver approximately 250Mb/s from Mars to Earth, a large increase from just 6Mb/s which is possible today.
The reason why using light waves instead of radio waves to transmit data improves performance is that because RF wavelengths are longer and the size of their transmission beam covers a wider area. Therefore, the receiving antennas for RF data transmissions must be very large. Laser wavelengths are 100,000 times shorter, allowing data to be transmitted across narrower, tighter beams.
‘This means that instead of a radio system where our beam diffracts out over large distances, we can narrow our beams so diffraction is less of a problem − we can deliver more concentrated energy at a distance,’ Cornwell explained at the conference. ‘This is very attractive to Nasa if you’re going deep into the solar system − you want to be able to efficiently deliver signal power at a distance.’
Laser-based communication therefore allows for higher data rates with lower mass, volume and power requirements, extremely desirable for future missions.
Promising as the technology is, however, the distance to Mars is around 1,000 times greater than the distance to the Moon. Therefore a number of technical factors will need to be considered for the DSOC system that will be sent to Mars, Cornwell stated.
One of the enabling technologies for the LLCD mission from the Moon was created by the Massachusetts Institute of Technology’s (MIT) Lincoln Laboratory. The optical module contained an inertial reference system which was used to measure micro-vibrations caused by motions on the spacecraft, such as doors closing. The system compensated for these vibrations to ensure that the 1,550nm four-inch aperture collimated laser beam was kept stable.
This is particularly important because although having a narrow beam allows energy to be delivered more efficiently, it also makes it extremely challenging to reach the receiver on Earth. ‘Our beam from the Moon getting to the Earth is 6km in diameter, but hitting a 6km target from 400,000km away is a real challenge,’ Cornwell told the audience. ‘The [vibration measurement system] was one of the breakthroughs required to make this work.’
But for the DSOC terminal which will be used to transmit data from Mars, targeting the Earth from 1,000 times further away will be much more of a challenge. For the LLCD mission, Nasa used a receiver a little less than a metre in size, but a much larger receiver will be needed for the DSOC. Initially, Nasa is looking to rent time on the Mount Palomar Hale telescope located in California, Cornwell remarked, but in the future the plan is to develop a 12-metre ground station to support data transmission for deep space missions.
The disturbance isolation system will need to have even more rejection, Cornwell added, because the narrow beam has to travel a much larger distance and accurately point back to Earth.
The increased distance will also mean that the laser on the ground will have to be much more powerful to create a strong enough signal. For the LLCD mission, a 40W laser was used, but for the DSOC mission, a kilowatt laser will need to be used, as well as a photon counter receiver in space so that the uplink beam can be seen.
In terms of the RF systems, Cornwell noted that he didn’t see them ever being made redundant, but instead complemented by laser systems. And, by making use of technology that is commercially available, it allows Nasa to remain competitive with radio frequency systems. ‘We heavily leveraged the commercial telecom industry. We used their components so that we could build something we could get components for without costing us heavily. We’re using that to drive the cost of these systems down, and we are fairly competitive with some of the high-end RF systems out there,’ he said.
About the author
Jessica Rowbury is a technical writer for Electro Optics, Imaging & Machine Vision Europe, and Laser Systems Europe.
You can contact her on jess.rowbury@europascience.com or on +44 (0) 1223 275 476.
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