Communications

Lasers in space? More space comms than Star Wars

by Yokogawa Test&Measurement January 30, 2023

Mention space lasers and many people might think of Star Wars – but for the European Space Agency (ESA), it’s all about comms rather than science fiction.

Today’s satellites produce immense amounts of data and none more so than ESA’s Sentinels. This constellation of satellites is tasked with monitoring the Earth, capturing imagery of vegetation, soil and water cover, inland waterways and coastal areas. Data also includes sea-surface topography and sea- and land-surface temperature, as well as polluting gasses.

All this data is of huge value – but only if it can be sent back to Earth for analysis. ESA does this by sending the captured data to a number of geostationary satellites. Fixed in position above the Earth, they send this flood of raw data down to the surface where it becomes useful knowledge.

But there’s a problem. This torrent of data threatens to overwhelm the bandwidth available from radio communication links – there will simply not be enough capacity to meet ESA’s data transfer needs.

This is where lasers come in. As the backbone of the Internet, optical communications routinely carry thousands of terabytes a day around the globe. And ESA already has experience of lasers, using them to communicate between the low Earth orbit satellites and those in geo synchronous orbit.

But getting a reliable communication by laser between Earth and space is a very different matter. For one thing, there’s the atmosphere itself to contend with. Clouds, smoke, dust, fog and rain can all interfere with the signal. Differences in pressure and temperature also play a role.

Avoiding this interference comes down to getting a good signal-to-noise ratio. This is achieved by transmitting signals at a very at a very precisely specified infrared wavelength of 1064.625 nm. This allows the receiver to lock on to the transmitted narrowband signal and to eliminate interfering signals.

ESA is implementing optical Earth-to-satellite communications technology in its optical ground stations (OGS) in Spain and Greece.

Ensuring that the transmitter wavelength remains constant is critical if the satellites are to send and receive data successfully. To achieve this, the transmitter laser is pumped by an 808nm laser diode to generate an accurate 1064.625 nm.

Of course, highly accurate wavelengths need a highly accurate way of measuring them. To ensure this accuracy, the ESA chose a specialist optical wavelength meter, the AQ6151B from Yokogawa.

The AQ6151B can achieve an accuracy to ±0.2 ppm and measures wavelengths from 900 nm to 1700 nm, exactly the range that ESA needs.

It also offers the speed that ESA’s researchers were looking for. The AQ6150 series takes only 0.2 seconds to acquire, analyze and transfer a measurement to a PC. The AQ6150 Series can measure up to 1024 wavelengths simultaneous and handles input signal power as low as -40 dBm.

Ease of use was also high on ESA’s list – the AQ6151B scores again here, with built-in analysis functions and no need for programming.

ESA expects that lasers could take on the burden of handling high bandwidth traffic, replacing radio as the main means of sending and receiving data from satellites.

It’s not Star Wars, but there is certainly a new hope for space communications.