Why Now?

One of the most frequently asked questions we receive is "How are your satellites different?" While it is important to distinguish between Kepler and the work of incumbents such as Iridium, Globalstar, or Inmarsat, I believe the better question is "Why now?". Here's why.

All communications systems are subject to a theoretical maximum throughput called the Shannon Limit. The Shannon limit dictates that the fastest data rate you can theoretically achieve for any communications system depends only on the signal and noise power received, and the bandwidth it is received upon. This applies equally to TV remotes, two tin cans on a string, the International Space Station, and of course Kepler.

It turns out that most modern communications satellites are pretty efficient. Firstly, they need to be to transfer the most data for the lowest power. Secondly, unlike cellular network or WiFi, each satellite has (both by design and regulation) their own dedicated spectrum. That implies that there are only a few fundamental ways to increase data rates: up the power, drop the noise, or increase the bandwidth.




We really only have two levers to play with on the satellite if it's transmitting. These are the size of the antenna relative to the wavelength, and the amount of power we can pump into that antenna. Of course, if it's receiving, antenna size is really the only lever we have to pull. Power is more or less limited by the efficiency of commercially available amplifiers, and the number of solar panels you can cram onto a satellite (more = better). While power amplifiers and solar panels are always improving, their gains are measured in percent per year. That really only leaves us with the antenna-lever to create big improvements in data rates. We can up the power one-for-one by either increasing the antenna size, or by increasing the frequency. Both are challenging. As the frequency increases, radios and antennas become much more challenging to build and less efficient, while larger antennas are both mechanically and electrically complicated. We will return to this momentarily.



Bandwidth is the second major determinant of throughput and the only practical way to get more is to go higher in frequency where there is more available. For example, the 433 MHz ISM (Industrial, Scientific, and Medical) band used for some IoT devices has about 1.7 MHz of bandwidth, whereas the 2.4 GHz band for WiFi has 100 MHz. Non-practical ways of increasing the bandwidth involve running a space-earth fiber line, or petitioning the ITU. The reader is left to determine which course of action is more feasible for their application.

These power and bandwidth constraints are fundamentally why communication technologies are trending towards higher frequencies (e.g. 5G, high-throughput satellites), and why the Kepler satellites are designed for Ku-band between 10-15 GHz. Compare this to the customer-facing antennas on incumbent satellites systems operating in C-band (4-8 GHz), L-band (1.6 GHz), or even down to VHF (below 300 MHz). Through the use of high frequencies, Kepler can provide equal or better throughput using a much smaller antenna, which translates to much smaller satellites, which translates into substantially lower cost.




Back to my original point. Until a few years ago (ca. 2010), most if not all nanosatellite communications systems were limited to under 2.4 GHz. This, however, recently began to change. There are two key reasons for this change. The first is that industry trends towards 5.8 GHz WiFi and 5G have pushed chip makers to design high-frequency components that are more efficient, cheaper, and easier to integrate into radios and antennas. Secondly, as nanosatellites have proven more capable every day, their communications needs have steadily grown, meaning more work is put into improving nanosatellite communications. Both of these factors have combined to enable Kepler to develop high-throughput nanosatellites for a fraction of the cost of traditional satellites.

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