Background

The recent interest in satellites has been precipitated by the low cost of manufacturing small satellites like cubesats and launching them into low earth orbits (300 to 500 miles above Earth surface) using rideshare agreements. Together, these factors have ensured that cheap hardware can be deployed in space for tens of thousands of dollars. This is in contrast to costs of tens of millions for traditional satellites [17]. In this paper, we focus on Earth observation satellites. The primary goal of these satellites is to get very frequent, near real-time data about Earth.

近期对卫星兴趣的激增，源于立方体卫星（cubesats）等小型卫星的制造成本以及利用拼车发射协议将其送入低地球轨道（距离地表300至500英里）的成本降低。综合这些因素，使得廉价硬件能够以数万美元的成本部署到太空中。这与传统卫星动辄数千万美元的成本形成了鲜明对比[17]。在本文中，我们专注于地球观测卫星。这些卫星的主要目标是获取关于地球的非常频繁、近乎实时的数据。

Imaging Equipment: Earth observation satellites serve multiple high-value applications like agriculture, forestry, smart traffic, natural disaster response, and geopolitical analysis. As such, they carry a wide array of sensing mechanisms in different parts of the spectrum. The most prominent ones are visible imagery and SAR-imagery [7] (synthetic aperture radar), but sensing in various parts of the spectrum (infrared, microwave, ultraviolet) is also prevalent [59]. As orbits get lower and equipment becomes precise, the resolution of this imagery has been steadily improving. Of late, several constellations offer meter-level pixel resolution [7, 20, 30].

Orbital Dynamics: The LEO satellites typically operate in polar orbits (see Fig. 1). One orbit period is around 90 minutes for satellites in LEO orbits. Since the orbit period is different from the period of Earth’s rotation, the satellite observes a different part of Earth during each orbit. A single satellite can image the Earth across several days. Therefore, Earth observation satellites operate in constellations to achieve frequent image capture. The planned constellation sizes consist of hundreds of satellites [7, 20, 30] to get a new image every few minutes to few hours.

成像设备 (Imaging Equipment): 地球观测卫星服务于农业、林业、智能交通、自然灾害响应和地缘政治分析等多种高价值应用。因此，它们携带了在不同光谱范围内工作的多种传感设备。最主要的是可见光影像和合成孔径雷达（SAR）影像[7]，但在其他各种光谱（红外、微波、紫外）的传感也十分普遍[59]。随着轨道越来越低、设备越来越精密，这些影像的分辨率在稳步提高。近来，已有多个星座能够提供米级像素分辨率的影像[7, 20, 30]。

轨道动力学 (Orbital Dynamics): LEO卫星通常在极地轨道上运行（见图1）。 对于LEO轨道上的卫星来说，一个轨道周期约为90分钟。由于轨道周期与地球自转周期不同，卫星在每个轨道周期都会观测到地球的不同部分。单颗卫星可以在几天内对整个地球进行成像。 因此，地球观测卫星以星座的形式运行，以实现频繁的图像捕获。 规划中的星座规模包含数百颗卫星[7, 20, 30]，以实现每隔几分钟到几小时获取一幅新图像。

The location of the satellite with respect to an observer on Earth is defined by two angles: azimuth and elevation (Fig. 3a). The azimuth defines the angle with respect to the north-south direction in the Earth-surface plane. The elevation is the angle measured perpendicular to the Earth’s surface. An elevation of 90 degrees corresponds to the satellite directly overhead. Due to their low orbits, an observer on Earth would see a LEO satellite rise from the horizon, travel through the sky, and fade below the horizon on the other end within few minutes. An observer would get four to five such contact periods in one day (see Fig. 3), with each contact achieving different peak elevation. When the satellite gets to a higher elevation, it is closer to the Earth, can deliver a stronger signal to a ground station on the Earth, and has a longer contact period.

Data Communication: For a traditional ground station on Earth, the uplink comprises primarily of low rate TT&C (tracking, telemetry, and control) data. Consequently, the typical design of ground stations uses a narrowband uplink (tens to hundreds of kbps) and a high bandwidth downlink (hundreds of Mbps to tens of Gbps) [18, 19, 34]. This design choice also accounts for the choice of spectrum. Public documents [22, 23, 52] show that the uplink uses the lower frequency and lower-bandwidth S-band (2025-2110 MHz) 1 while the downlink uses the higher frequency X-band (8025-8400 MHz). Some designs are also exploring higher frequencies (Ku band – 12 to 18 GHz and Ka band – 26.5 to 40 GHz) for downlink [19].

卫星相对于地球上观察者的位置由两个角度定义：方位角（azimuth）和仰角（elevation）（图3a）。方位角定义了在地球表面平面上相对于南北方向的角度。仰角是垂直于地球表面测量的角度。90度的仰角表示卫星位于正上方。由于轨道较低，地球上的观察者会看到一颗LEO卫星从地平线升起，划过天空，然后在几分钟内从另一端的地平线消失。一个观察者在一天内会经历四到五个这样的通信窗口期（contact periods）（见图3），且每次通信的峰值仰角各不相同。当卫星达到更高仰角时，它离地球更近，能向地面站传输更强的信号，并且通信窗口期也更长。

数据通信 (Data Communication): 对于地球上的传统地面站而言，上行链路主要包含低速率的 跟踪、遥测和控制（TT&C） 数据。因此，地面站的典型设计采用窄带上行链路（数十至数百kbps）和高带宽下行链路（数百Mbps至数十Gbps）[18, 19, 34]。这一设计选择也影响了频谱的选择。公开文件[22, 23, 52]显示，上行链路使用频率较低、带宽较窄的S波段（2025-2110 MHz），而下行链路则使用频率较高的X波段（8025-8400 MHz）。一些设计也在探索更高频率的波段（Ku波段 – 12至18 GHz和Ka波段 – 26.5至40 GHz）用于下行链路[19]。

Today, most satellite operators deploy their own ground stations [18, 19]. These ground stations cost millions of dollars to deploy and maintain due to three factors: (a) high-end specialized equipment, (b) expensive licensing process for transmission, (c) large antenna sizes require acquisition of dedicated space. In contrast, L2D2’s ground stations operate with commodity hardware, use small antennas deployable on rooftops, and majority of them are receive-only.

State-of-the-Art: The best known ground station design today can achieve a data rate around 1.6 Gbps by combining six frequencypolarization channels at the optimal satellite-ground station link [19]. The 1.6 Gbps link in [19] can download data up to 80 GB in a single pass. Note that, the max rate can only be sustained when the satellite is at the shortest path. As the satellite reaches closer to the horizon, the link quality degrades and the satellite has to downgrade its rate. Each satellite can do four to six passes per ground station per day, but the passes have varying quality. The typical amount of data that needs to be downlinked to image the Earth every day can go up to few Terabytes per day [13]. Multiple satellites need to collaborate to make this happen.

如今，大多数卫星运营商都部署自己的地面站[18, 19]。这些地面站的部署和维护成本高达数百万美元，原因有三：(a) 高端的专业设备；(b) 昂贵的传输许可申请流程；(c) 大型天线需要征用专用场地。相比之下，L2D2的地面站使用商用硬件，采用可部署在屋顶的小型天线，并且其中大多数是仅接收的。

当前最先进技术 (State-of-the-Art): 目前已知的最先进地面站设计，通过在最佳星地链路条件下组合六个频率-极化信道，可以实现约1.6 Gbps的数据速率[19]。 在[19]中描述的1.6 Gbps链路可以在单次过境中下载高达80 GB的数据。需要注意的是，最大速率只能在卫星处于最短路径时维持。当卫星接近地平线时，链路质量会下降，卫星必须降低其速率。 每颗卫星每天可以与每个地面站进行四到六次过境通信，但每次过境的质量各不相同。为实现每日地球成像，需要下行的数据总量可高达每天几TB[13]。这需要多颗卫星协同工作才能完成。