Satellites in low Earth orbit (LEO) constellations benefit greatly by being able to communicate with one another. Since they’re so close to Earth, they can only “see” relatively small portions of the ground at one time, making inter-satellite links essential in creating a robust global LEO network.
While most satellite communications from the spacecraft to the ground travel within the radio frequency (RF) portion of the electromagnetic spectrum, oftentimes transmissions between satellites are in the optical bandwidth. Sometimes called laser links, these optical inter-satellite links — or OISLs — have some special characteristics that make them ideal for satellites to communicate with each other. These include:
- Higher data rates: Because of the higher frequency and shorter wavelengths of OISLs, they can carry more information than RF links with less power consumption.
- Less interference: OISLs use a highly focused beam of light directed precisely at the other satellite. This makes them less susceptible to interference from RF signals from Earth or from other satellites.
- Greater security: Because OISLs use those narrow beams and high frequency, they’re hard to detect and decode.
Because of all this, it’s tempting to wonder why OISLs aren’t used for other types of satellite communication. One of the great advantages of RF is that the longer wavelengths can more easily travel through the atmosphere without interference from clouds or other weather. But visible light at higher frequencies is more vulnerable to this so-called atmospheric attenuation, so laser links are much less common for Earth-space communications due to the potential for interference.
In the vacuum of space, attenuation is not a factor, which is why these optical links are ideal for inter-satellite communications as well as future communications between Earth and the moon and even Mars.
One other limiting factor is cost. OISLs are simply more expensive due to the need for more precise and powerful equipment to transmit and receive these signals. There has been some downward movement on that cost, so expect to see optical links becoming more prevalent in the coming years.
At E-Space, we’re using OISLs to optimize our upcoming LEO satellite system, which will create a new class of ubiquitous, real-time Internet of Things (IoT) solutions and services. Our OISL will be integrated with our payload processor, with the optical transceiver designed around our specific mechanical dimensions, target performance, volume production and environmental requirements.