Satellite Communications

Geostationary orbits, used by some communications satellites.  Image credit: Lookang (CC-BY-SA).
Geostationary orbits, used by some communications satellites. Image credit: Lookang (CC-BY-SA).

One important aspect of field work in remote places is keeping lines of communication open. At a minimum, the ability to call for help is needed. Sending status updates, checking email, talking with loved ones, and a number of other uses are good to have. Even in this day and age, though, not every remote place has good cell phone coverage. These places are where satellite phone systems are extremely useful.

There are two main types of satellite systems: geostationary satellite systems, and low-Earth-orbit satellite systems.

Geostationary satellite systems have satellites over fixed locations above Earth’s equator, at an altitude of roughly 36,000 km (22,000 mi). Geostationary satellites are nice in that they are always in the same spot relative to a location on Earth, so there are no signal hand-offs where calls may drop, nor do the stations on the ground need to have any kind of tracking mechanism to keep the antenna pointed at the satellite. Unfortunately, because the geostationary satellites are located over the equator, they do not work well pole-ward of 70° latitude, because they are too close to the horizon for reliable, interference-free signals. Geostationary satellites also have a noticeable delay, because the round-trip light time is a minimum of ~0.25 seconds, and the time to receive a response back doubles that.

Low-Earth-orbit satellite systems require many more satellites, but the satellites are much closer to Earth, generally only 650–1100 km above the surface. Many of these satellites are in a polar or near-polar orbit, which gives them good coverage near the poles. Each satellite is only over any given area for 4–15 min, so hand-offs are necessary (and are not always reliable). One advantage of low-Earth-orbit systems is that the transmitter and antenna on the ground do not need to be especially powerful or carefully aimed. Low-Earth orbit systems have substantially less data throughput than the geostationary systems (9600 kbps for LEO vs. 60–512 kbps for geostationary). For reference, the LEO throughput is much less than dial-up modems, and geostationary throughput is up to 10x higher than dial-up, though still far short of broadband internet access (4 Mbps down, 1 Mbps up).

I mentioned that the antennas (and power) for a geostationary satellite setup need to be better than ones for low-Earth orbit satellites. This is because of the inverse-square law, where the as the distance is increased, the power which reaches the receiver drops by the square of that increase. Think of standing outside at night with a friend (representing the ground station and satellite), and each of you has a flashlight (representing the radio transmitters) and eyes (the radio receivers). When you are close, the light is very bright, and you probably have to look away. As you move away from each other, the lights appear dimmer and dimmer. Each time you double the distance between you, the brightness of the light dims by a factor of four. If you need a certain level of brightness at the receiver (your eye, or the satellite antenna), then there has to be either a sufficiently bright light shining (power level), or it needs to be focused enough—and harvested enough by a sufficiently large receiver—to achieve that level of signal.

Inverse-square law in action; as the distance increases (e.g. from r to 2r), the area the energy is directed over increases as the square of the distance (e.g. from 1 to 4 units).  Image credit: Borb (CC-BY-SA).
Inverse-square law in action; as the distance increases (e.g. from r to 2r), the area the energy is directed over increases as the square of the distance (e.g. from 1 to 4 units). Image credit: Borb (CC-BY-SA).

With a difference in altitude of ~40x between low-Earth orbit and geostationary orbit, there is a difference of 1600x in the signal level, all else being equal. For that reason, satellite phones for low-Earth-orbit satellites can get away with less powerful radios and smaller antennas that are less sensitive to proper positioning. It’s handy to not need exact positioning for the low-Earth-orbit satellites, because their quick movement across the sky can be difficult to track without a motorized, computer-driven antenna. Mobile or ship-based satellite communication systems tend to rely more on the low-Earth-orbit satellites precisely because the aim of the antenna is much less important. Nobody wants to try to hold an antenna pointing in a certain direction while pitching about on a ship in 4 m seas in the wind and the cold.

As an amateur radio operator, one thing I enjoy doing is going outside when the International Space Station is flying over, and listening to the radio signals it sends down. During the morning or evening passes on clear days where the space station is visible, it is quite easy to point in the right direction. Spot the station, then point your hand-held antenna toward it. During the day, in the depths of night, or when it’s cloudy, tracking the station can be more difficult (at least without computer assistance). Still, it’s pretty neat to hear astronauts answering questions from the local middle school students, all the while knowing that the signal coming from the space station is coming directly to your radio, no internet or commercial broadcast station required.

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