Monday, November 26, 2018

A cool simulation of SpaceX's revised satellite broadband plan

On November 15, the FCC approved a revision to the plan for Starlink, SpaceX's forthcoming broadband satellite service. The new plan reduces the number of satellites from 4,525 to 4,409 and lowers the altitude of the phase-1 satellites from 1,100 to 550 km.

Revised broadband satellite plan (Sources 1 and 2)

Mark Handley, a professor at University College London, has created videos based on simulations he ran of both the original and revised phase 1 plans. You should watch the videos, but it will help to first look over a few images from his simulation of the new plan.

Starlink phase-1 is shown below. Handley assumed a phase offset between adjacent planes that would minimize the chance of a collision. The minimum crossing distance between satellites is a little over 90 km -- an improvement over the previous plan.

Starlink phase 1

Looking up, there are several satellites in view at any one time (but fewer than there would have been with the previous 1,100 km plan):

View from the ground

Radio transmission will be used for links between the satellite constellation and the ground and laser links will be used between satellites. Handley assumed that each satellite links to the one before and after it in its plane and to the second closest in the adjacent planes in order to improve east-west routes. In his simulation of the first plan, he assumed a fifth laser linked to a satellite in the crossing plane, but that would be more difficult at this fast-moving lower altitude. It will be interesting to see how SpaceX uses the inter-satellite links.

Each satellite is linked to four others, forming a mesh network.

A simulated route between San Francisco and London is shown below. Note that latency is about half that of current, terrestrial routes. (The latency advantage of satellite routes over terrestrial is generally greater for long distances).

San Francisco-London -- twelve hops in space

Nine parallel routes between New York and London are shown below -- eight have lower latency than the current, terrestrial route.

Eight New York to London routes are faster than a terrestrial route

The following image shows all three phases -- Handley is not sure if the other two will interact with the phase 1 satellites.

Red: phase 1, white: phase 2, Blue, phase 3

Only Elon Musk knows what motivated this revision in SpaceX's Starlink plan, but we can speculate. They have had two test satellites in orbit at around 550 km altitude for some time, and that experience must have informed the decision. Musk reported that the links between the test satellites and the ground were performing well, but did not mention the laser links between satellites. Perhaps they feared difficulty with the longer laser links at 1,100 km or some other engineering problem. (Musk recently fired several managers in an effort to keep the project on schedule).

Because SpaceX and others are planning to launch thousands of new low-Earth orbit satellites, the FCC is focusing their attention on debris mitigation. The chance of a collision with the new plan is reduced since the minimum distance between satellites increases from over 40 to slightly over 90 km and.

The lower-altitude satellites may require more fuel to overcome a slight increase in atmospheric drag, but they will be easier to de-orbit when their life (around 5 years) is up, further reducing the likelihood of a collision.

Starlink's primary goal is affordable global broadband service to individuals and organizations, but Musk has also predicted that they would carry half of the global long-distance traffic. Handley's simulations predict roughly 2X latency improvement over current terrestrial cables on long routes and he points out that low-latency long-distance service will appeal to financial companies with offices in Europe, North America, and Asia. If capacity turns out to be sufficient, I imagine Starlink will also attract tier 1 ISPs and large companies like Google.

Finally, SpaceX also has approval for a constellation of 7,518 very low-Earth orbit satellites operating at altitudes from 335km to 346km. These will not be launched until SpaceX has satellite engineering, regulatory and market experience and time to assess potential cooperation/competition from 5G terrestrial networks.

I recommend watching the videos of both of Handley's simulations because the narratives are different and both are inciteful. You should also read his paper Delay is Not an Option: Low Latency Routing in Space.

Friday, November 16, 2018

An in-depth interview of OneWeb Executive Chairman Greg Wyler

OneWeb is building a large constellation of low-Earth orbit (LEO) Internet-service satellites and Via Satellite has published the "definitive 2018" interview of OneWeb founder and Executive Chairman Greg Wyler. The following are some of the quotes that caught my eye:
  • The system has been designed. The satellites have been tested. They are going through final stages of testing now before the launches begin. The satellites have actually performed better than expected in many ways, especially with their Radio Frequency (RF) performance which is really positive.
  • I think we will have customers up and running in 2020.
  • Whether [our satellites] are $500 thousand (the estimate in 2015) or $1 million is virtually irrelevant because what they are not is $50 million, and that is where it started.
  • The initial customers will be in the mobility and emergency services markets (paraphrase).
  • Aviation is a big [market] for us.
  • Why not let Sprint, DT, roam onto the plane? You can give the customers 4G/5G on the same devices they are used to using in their car, at the gate, or in other places.
  • The plane itself can become a Local-Area Network (LAN) party! I have been in aviation my whole life so this is always something I have been interested in.
  • OneWeb with its first constellation will be able to make a big impact on health centers and schools.
  • I would like to keep [the number of satellites up in five years time] below 1,500.
The tone of the interview was positive, but the early emphasis on emergency and mobile services (where they will have competion from other, relatively focused LEO satellite companies like Telesat and Leosat) makes me wonder whether their goal of eliminating the digital divide by 2027 might be slipping.

If I could have asked one question, it would have been about the objection to OneWeb that has been raised by the Russian Federal Security Service (FSB). If the FSB succeeds in stopping OneWeb in Russia, they will lose access to a potential market. Furthermore, it would jeopardize their contract for 21 launches with the Soviet space agency Roscosmos and perhaps cost and delay the project.

This has been a quick summary of a long interview -- you should check out the full interview.