Spacecraft Communication Basics

Introduction

The first satellite, Sputnik, was primarily a communications platform. It beeped on a frequency channel of 20 and 40 MHz that anybody with the right kind of radio could listen to. So, from day one, just about everything that’s ever flown in space has had communications as a primary component. 

A communications engineer is responsible for many things. Two things customers always want more of, though, is how much time they can talk with their system and faster data transfers. In fact, many satellites end up using a fraction of their potential simply because operators cannot get all the data down that the sensors could collect. Operators spend significant time and money trying to optimize data collection that fits within the limits.

In small companies, the communication engineer who designs the link between Earth and space is the same engineer who figures out the ground station plan. They’ll work with ground stations on scheduling time, setting up ways to get data into their servers, and keeping the operations center working. In big companies, this may be several different teams, each doing one part.

Photons Everywhere

Something that sometimes confuses people is all the different names for photons and groups of photons. You’ll hear people talk about radio frequencies, radio wavelengths, electromagnetic waves, electromagnetic radiation, and lasers. However, these are all describing the same phenomenon: photons are flying around. Our eyes can only see photons of a certain energy level. If we could see the energy level of the photon coming from a WiFi hotspot, the WiFi would look like a lightbulb. Same with our cellphones making calls… we would see people holding flashlights to their ears.

This is a little bit of a simplification, but I think it’s better than the overcomplicated way people describe things today. At the end of the day, all we’re talking about is sending photons with the right energy level at the right time to the right place.

Link Budgets

Communication engineers do a lot of things and one of them is understanding if and when you can talk with your satellite. And, when you are talking, how great the connection is. All of these things are often wrapped up in a “Link Budget”, where the word “link” means you’re linking two things. We’ll have a more in-depth article on this later. For now, let’s cover the major concepts.

If you were to sit down and think about what you’d need to know about your link, you’d probably come up with the following questions:

1. How far away are the things from each other?
   
   
2. How strong is the signal from the source?
   
   
3. How low of a signal can the receiver hear?
   
   
4. Is there a lot of noise and interference between the points?
   
   

Those four questions lead to multiple answers. For instance, there are different types of noise from things like the atmosphere, the cables you’re using, and other photons flying around. When you answer all those questions, you usually end up with a spreadsheet of numbers that you add and subtract from like you were making your monthly financial budget. The bottom line number at the end of it tells you if your system is going to work or not.

Radios vs. Antennas

On Earth, a lot of people treat these as the same thing. On spacecraft, a radio is not the same thing as an antenna. Communication engineers will tell you that a radio is a box that creates and reads data while an antenna sends and collects data. Said with a little more complexity: radios work with electricity and antennas work with photons. On small programs, the same engineer might design both the radio and the antenna. On big programs, you usually have different people designing and testing those things.

Antennas can be different shapes, depending on what you're trying to do. In the picture of Cassini above, the big dish is the antenna that talks with Earth. The thin, white rods sticking out of it are different antennas that can detect the photon emitted by lightning on Saturn.

Radios, like the one below from General Dynamics, most often look like boring boxes with a place to plug in cables. They're packed with electronics and are usually protected pretty well from interference and radiation.

Lasers in the Sky

One of the problems with radios is they send their photons over a pretty big area. This takes a lot of power and it can make it harder for other satellites when their photons overlap. Lasers can solve both of these problems by sending a focused beam of photons. You can also pack more data into them more easily, so you can get much higher data rates.

On the downside, if you’ve got a relatively thin beam of photons, you’ve got to point it pretty accurately. It’s kind of like using a hand-held laser pointer from space to point at a small area on the ground. This means you need better control systems and that’s not cheap. Small satellites may not have the room or power to point that accurately, even if they want to. Like everything else in space, you’ve got to decide what you care about more.

Getting Permission

The International Telecommunication Union (ITU) is in charge of saying who gets to use which photons, when they can use them, and where they can use them. In the United States, the organization that controls things (and works with the ITU) is the Federal Communications Commission (FCC). Each country has its own version of the FCC which you’ll talk with.

Unfortunately, working through all the paperwork can take years and a pile of money. Not all types of photons can make it through the atmosphere and everyone wants permission to use the best types of photons. Your country’s organization will want to make sure you’ll make the best use of what they give you and that you’ll be responsible.

If you’re an amateur group not trying to make a profit, like a high school or university, it’s easier because there are special channels you can use. They aren’t good for streaming lots of data, but you can still do good science with what you get. Timewise, you can expect it to take between six months and a year to get permission.

If you’re using lasers, it’s easier to get permission as nobody is regulating it just yet from a communications standpoint. You do, though, need to talk with the Federal Aviation Administration (FAA) since you can’t shine lasers from the ground just anywhere due to eye hazards and interference with things like airplanes. Technically, there is a laser clearinghouse (LCH) that manages government satellites and you can talk with them to get their buy-in. As of today, you do that on a volunteer basis and it takes months. Companies should probably do that since the government is likely to be a present or future customer and you want to get on their good side.

Curated Videos

1. https://www.youtube.com/watch?v=xNEDNOQnwD8
   The video spends most of its time talking about how talking with space is so hard and covers a little bit about how communications work.
   
   
2. https://www.youtube.com/playlist?list=PL3rE2jS8zxAxamj-MY7FvzOZkHUALNndQ
   This is a series of videos related to satellite communications used in an academic course. You can look for your topic of interest and skip around as you need.
   
   
3. https://www.youtube.com/watch?v=DEAVirFERU4
   They use computer graphics to show how spacecraft deep in space send data back to Earth. It's a great overview and they've got other space videos, too.
   
   
4. https://www.youtube.com/watch?v=e8U6S08uP0o
   Good overview video talking about radio frequency systems and the transition to laser systems. It's an interesting time for space communications.

Curated Links

1. https://www.nasa.gov/smallsat-institute/sst-soa/communications#9.5.5
   NASA maintains an excellent special section on small spacecraft technology. This link is for communications and it has lots of great information.
   
   
2. https://www.spaceacademy.net.au/spacelink/radiospace.htm
   It’s an older website, but physics don’t change and there’s good information here.