# Some physics about covering the Earth with Internet

Before I read that Elon Musk’s insanely-huge satellite fleet was to orbit at around 1100 kilometers above the Earth, I did some back-of-the-envelope math to figure out what satellite altitude would be necessary to carpet the planet with coverage. From this came an interesting little exercise in hypothetical logistics.

First, figuring the minimum altitude of a 4,000 station constellation: We can use a formula for the fraction of the Earth visible from a point:

$$A = \frac{1}{2} \left( 1 – \frac{R_e}{R_e + h} \right)$$

…where $$R_e$$ is the radius of Earth, $$h$$ is the altitude of the observer, and $$A$$ is a fraction of the Earth’s surface area. We can divide up the Earth into 4000 pieces and solve for h to know how high a satellite must be to see its portion of the Earth. When we do this, we get…

1.98 miles high.

What.

If you want to blanket the entire planet with internet coverage, and you’ve got 4000 things to do it with, those things only need to be floating about 2 miles off the ground!

Obviously nobody’s putting satellites in two-mile orbits. And we’d like some redundant coverage, so let’s demand that every point on Earth be covered by at least three stations and try again. What do we get? Continue reading

# On Elon Musk’s Ridiculously Huge Satellite Fleet

On Friday, Elon Musk announced SpaceX’s intent to launch a fleet of over 4000 communications satellites, whose primary purpose is to provide a low-latency broadband internet to us Earthlings:

He’s drumming up interest among potential hires for SpaceX’s new satellite branch in Seattle for, fittingly, building satellites.

This is an interesting move. 4000 is a huge fleet— Musk says in the video that this is more than double the number of currently active satellites, but from this database, it looks to be actually more than triple that number.

# Why?

## It’s faster

Currently, to travel a long distance, a packet of data must be routed through thousands of miles of fiber optic cable. Because the fiber is made of glass and is refractive, the speed of light in the cable is about 60% of the speed of light in a vacuum— so this limits how quickly data can go from one place to another on Earth.

Musk plans to route data directly from satellite to satellite through the vacuum of space, bypassing the speed limit of fiber.

Another factor is the number of hops: as a packet makes its way around the internet, it makes pit stops at routing stations, which might hold the data in memory as it waits in line to be sent to the next stop. This takes time, and may happen several hundred times for a long distance communication. This can add up to a significant fraction of a second.

Musk plans to route long distance packets in just a few hops, reducing the delay between endpoints even further. This is possible because each satellite will be in view of many others— it’s possible they’ll transmit data by pointing lasers at each other. For data, it’s the difference between taking the backroads and taking the freeway.

Notably, the proposed fleet would be orbiting at around 1100 km; far lower than the altitude of current high speed internet satellites, which hang out in geostationary orbit. Current satellites 22,000 miles away, which demands a minimum round-trip signal time of about 1/4 second. In contrast, Musk’s satellites will orbit just 3 milliseconds from the surface.

## Money

It seems there is opportunity here to enter a relatively sparse niche. Long distance communication is dominated by fiber, which is expensive to lay. Satellites are expensive too, but in a matter of months that cost will drop enormously— perhaps by an order of magnitude— when SpaceX succeeds in landing its reusable first stage rocket. Elon is in the unique position of knowing exactly how cheap space launches are about to become, and also has the ability to fully reap the benefits of cheap launches without any of the markup that he’ll be charging his competitors.

Musk says he intends to capture about 10% of consumer traffic, and 50% of the “long distance” traffic. This means the network will be focused on backhaul, routing large batches of data between endpoints for telcos and ISPs. It seems he hopes to attract backhaul customers by offering significantly (“order of magnitude”) improved latency over current capability.

10% of consumer traffic is a lot— There are about 2.5 billion subscribers on the internet (that will surely grow), and Musk is therefore hoping for about 250 million SpaceX internet users. That’s incredibly ambitious, although a comparatively small amount of infrastructure (as opposed to laying fiber for the same consumer base) will provide him easy access to the entire globe. If he succeeds, and we suppose a consumer cost of, say, a conservative \$10 a month, that’s a total of 2.5 billion dollars of revenue per month. This would pay off a 15 billion dollar satellite network (Musk’s cost estimate) in just half a year. 10% of all internet subscriber traffic is pretty ludicrous, but if he’s overestimating by a factor of 10, it’s still looks to be easy money. This isn’t counting the backhaul traffic he intends to capture (whose revenue I don’t know how to estimate).

And money is the goal. Musk primarily wants something that will generate cash for his a future Mars colony venture; he seems to believe this will be a cash cow.

## Will I get satellite internet on my phone anywhere on Earth?

Probably not; the radio frequencies SpaceX is likely to get their hands on don’t penetrate buildings. It’s more likely that homes and businesses will need to pay a few hundred dollars for a dish installation on the exterior of the building.

This is maybe something for Musk to worry about; it’s quite possible that in developed countries individual subscribers will soon stop paying for high speed internet at home, and start using their phones, almost in the same way that landlines were outmoded some years ago.

Overall, it appears that Musk is vying to become a major player in the telecom industry. New competitors are rare here, though he did manage to bust into the immensely expensive and entrenched space industry. We’ll soon see how he performs.