Voices in the Sky - Satellite Communication Methods

by Monican

There are more than one thousand active satellites orbiting the Earth, several thousand more that are abandoned, and around 20 deep space satellites and spacecraft.  There are plans in the next three years to launch nearly a thousand more, many of which belong to new small-sat fleet operators like OneWeb, Planet Labs, and BlackSky Global.

In this article, I will cover the current methods that satellite operators use to communicate with satellites and spacecraft, security issues, and the future of laser communications and what it means for us Earthlings.

One thing that every single satellite has in common is a need to communicate with the Earth, and this is done with Radio Frequency (RF) communications in several frequency bands.  As many readers probably know, the Federal Communications Commission (FCC) in the United States, and equivalent regulators in other parts of the world, have to carefully police usage of the electromagnetic spectrum so that people don't interfere with each other and with critical services like ambulance radios, air traffic control, and radio stations.

This creates a problem for satellite operators who have to deal with multiple regulatory agencies since satellites broadcast across continent-sized swaths of the Earth.  This wide field of transmission also creates security issues around eavesdropping which I'll cover more below.  The other problem that arises from RF communications is how slow they are.

For example, when NASA tries to download data from the Mars Science Lab rover, speeds can be as low as several kilobits per second.  One of the causes of this low data rate is the wide beam angle over such a great distance which wastes energy, and the low-frequency nature of UHF, X-band, and S-band radios.

UHF, the lowest frequency transmission spectrum of those three examples stands for "Ultra High Frequency," so why do I say they use low-frequency?  When compared to the frequency of visible light, their limitations become apparent.  Visible light is three orders of magnitude higher frequency than traditional radios, operating in the terahertz band.  This allows not only faster communication because more information can be encoded in a given unit of time, but also much tighter beamwidth, which cuts down on wasted energy sending photons elsewhere other than your target.

Thus there are two huge benefits to using lasers to communicate over long distances rather than RF: higher data throughput and lower energy needed to transmit.

One more benefit is very important to the militaries of the world: the tighter beam-width means eavesdropping on the signal is much more difficult.  Here's an example: the LADEE spacecraft which went to the Moon and spent a year orbiting it tested out laser comms between the Moon and Nevada.  The laser beam hitting the Earth was only six miles in diameter, centered approximately on the trailer-sized receiving telescope.  This means that anyone trying to listen in on this conversation would have to be within six miles of the receiver in the desert, which would be very easy to spot and prevent.  Laser communications are also immune to jamming unless the adversary is directly in view of the telescope, which is much harder due to the vastly narrower field of view compared with traditional RF.

The main barrier to implementing Earth-space laser communications is interference from the Earth's atmosphere.

Water vapor and other gases in our air diffract visible light (and infrared and ultravoilet) and only recently have scientists developed reliable single-photon detection, ensuring that even if the light scatters upon entering the atmosphere, reception is still possible.  Receiving telescopes on spacecraft are restricted to very small sizes, which means that an Earth-based transmission must blast quite a bit of power at the spacecraft.  The way operators get around the low power limits of the space-to-Earth transmissions and the small size of the receiver on the spacecraft is by having a very large cryogenically cooled receiver on the Earth with a really powerful transmitter.  In some sense, energy for a terrestrial transmitter is "unlimited' compared to the tight power budgets of a spacecraft.  So the powerful ground stations allow the other end to be quite small and low power.

Another drawback of optical laser communications is weather dependence.

Clouds block visible and infrared light, so the ground stations have to be in very dry, clear areas, such as the high deserts of Chile or the arid regions of New Mexico, Spain, and Australia.  Due to this limitation and the still experimental nature of laser communications, future satellites and spacecraft will still need to have an "old fashioned" X-band, S-band, or UHF antenna, but these will increasingly be seen as just emergency backups rather than primary systems.

The higher level of security that comes from the tighter beamwidth of lasers still has classic weak points: the terrestrial communications network used to send these signals between the desert transceiver and end users will still rely on classic encryption and suffer from any problems experienced by the network on Earth.

These developments will have a noticeable impact on our lives in the coming decades.

Vastly higher data rates between the Earth and space, or between satellites in orbit, will improve our global communication network as well as allowing far more scientific data to be downloaded from future scientific missions.

The militaries of the world are busy developing laser communications to make eavesdropping more difficult and get around jamming.  Laser communications will also be used more for point-to-point communications on the Earth in locations where it is not feasible to wire fiber optic cable.