LEO Networks 101

Low Earth orbit satellites operate at an altitude of less than 2,000 km above the Earth’s surface. Given the altitude, a satellite’s velocity and orbital period are determined by orbital mechanics. For instance, for an altitude of 550 km, used by the Starlink satellites SpaceX has thus far placed in orbit, the satellites travel at 27,306 km/h, completing each orbit in 95 min 39 sec.

LEO satellites offer an extremely different type of service compared to geostationary satellites. GEO satellites are stationary with respect to the Earth, and achieving this stationarity requires operating at 35,786 km altitude. By flying at a lower altitude than GEO satellites, LEO satellites offer much lower propagation latency 65× for the 550 km example — but at the cost of losing stationarity. Another distinction is that the higher altitude of GEO satellites provides them a larger cone of coverage, with a handful of satellites sufficient to cover the entirety of the globe. In contrast, LEO satellites necessarily cover much smaller areas, and achieving global coverage requires the use of many more satellites. The coverage and velocity characteristics of LEO satellites imply that a ground station sees a particular LEO satellite only for a few minutes. After this time, if continuous connectivity is desired, the ground station must execute a connection hand-off to another LEO satellite that becomes reachable.

To provide such continuous, low-latency connectivity, several companies have proposed large LEO constellations comprising hundreds to tens of thousands of satellites. In particular, SpaceX Starlink, Amazon Kuiper, and Telesat, are all planning constellations with more than a thousand satellites. SpaceX’s plans are the most ambitious, with 42,000 planned satellites. SpaceX has already launched more than 400 satellites, making Starlink the largest-ever satellite constellation.

Besides a large number of satellites connecting to ground stations, most of the proposed networks also feature inter-satellite links (ISLs), such that a connection between two distant ground stations traverses an uplink, a series of ISLs, and a downlink. The upand down-links are planned to be radio, and more limited in bandwidth, on the order of 10 Gbps, while for ISLs higher bandwidths may be achievable [2, 36].

This design approach enables LEO mega-constellations to offer low-latency broadband Internet connectivity. With suitably designed satellite orbits, an LEO constellation can provide truly global coverage: from any location on Earth, at all times, one or more satellites are reachable.

TL; DR

• 高度: 2000km 以内
• 速度: 27306 km/h
• 周期: 95 min (完成一次轨道运行)

低地球轨道（LEO）卫星在距离地表低于 2,000 km的高度运行。根据轨道力学原理，卫星的速度和轨道周期由其高度决定。例如，在 550 km的高度上（SpaceX迄今部署的星链卫星所采用的高度），卫星以 27,306 km/h的速度飞行，每 95 分 39 秒完成一次轨道运行。

与地球静止（GEO）卫星相比，LEO卫星提供一种截然不同的服务类型。GEO卫星相对于地球是静止的，而实现这种静止性要求其在 35,786 km的高度运行。通过在比GEO卫星更低的高度飞行，LEO卫星能提供低得多的传播延迟——在 550 km的例子中，延迟降低了65倍——但代价是失去了静止性。另一个区别是，GEO卫星较高的轨道使其拥有更大的覆盖锥域，仅需少量几颗卫星便足以覆盖全球。相比之下，LEO卫星的覆盖区域必然小得多，实现全球覆盖需要使用数量远超于此的卫星。LEO卫星的覆盖和速度特性意味着一个地面站只能在几分钟内观测到一颗特定的LEO卫星。此后，如果需要持续连接，地面站必须执行一次连接切换，转接到另一颗进入可达范围的LEO卫星。

为了提供这种持续的低延迟连接，数家公司已提出建设由数百到数万颗卫星组成的大型LEO星座。特别是，SpaceX的星链（Starlink）、亚马逊的柯伊伯（Kuiper）和Telesat公司，都在规划超过一千颗卫星的星座。SpaceX的计划最为宏大，计划部署 42,000 颗卫星。SpaceX现已发射超过400颗卫星，使星链成为有史以来规模最大的卫星星座。

除了大量卫星连接到地面站之外，大多数规划中的网络还具备 星间链路（Inter-Satellite Links, ISLs） 功能，使得两个遥远地面站之间的连接可以经由一条上行链路、一系列星间链路和一条下行链路进行传输。上行和下行链路计划采用无线电技术，其带宽较为有限，约为 10 Gbps量级，而星间链路则可能实现更高的带宽[2, 36]。

这种设计方法使LEO巨型星座能够提供低延迟的宽带互联网连接。通过合理设计的卫星轨道，LEO星座可以提供真正的全球覆盖：在地球上任何地点、任何时间，都有一颗或多颗卫星处于可达范围之内。