Technique Background

A. Space Edge: A New Intelligent Computing Paradigm

On-board hardware evolution in aerospace industry. Onboard network and computation capabilities have increased significantly over the past decade. The high-speed inter-satellite and ground-satellite links (i.e., ISLs and GSLs) have been successfully deployed on satellite platforms, which can offer Gbps-level data transmission for satellite communication [13], [14] and facilitate the inter-networking of a large number of satellites. Besides, on-board processing capability has also been promoted via low-power commercial off-the-shelf (COTS) hardware accelerators, e.g., graphics processing unit (GPU) [8], vision processing unit (VPU) [15], and field programmable gate array (FPGA) [16]. In 2020, European Space Agency (ESA) launched the Φ-sat-1 sensing satellite equipped with Intel Movidius Myriad 2 to verify the on-board performance of deep neural network (DNN) and achieved great success [17].

Space edge computing (SEC) enables in-orbit intelligence. With such space hardware evolution above, recently we have witnessed a new intelligent computation paradigm: space edge computing (SEC). SEC exploits edge-like computing capabilities on satellites and within satellite networks to process data in orbits. SEC can be used in many innovative space applications such as autonomous remote decisions, inorbit detection for disaster discovery, climate monitoring and maritime rescue [18]–[20]. SEC enables faster data processing, reduced latency, and improved efficiency by handling data in orbit rather than sending all data back to the ground.

航空航天工业的星上硬件演进

在过去十年中，星载网络和计算能力已取得显著增长。高速的星间链路（ISL）和星地链路（GSL）已成功部署于卫星平台，能够为卫星通信提供 Gbps 级的数据传输速率 [13], [14]，并促进了大量卫星的互联互通。此外，通过采用低功耗的商用现货（COTS）硬件加速器，例如图形处理单元（GPU）[8]、视觉处理单元（VPU）[15] 和现场可编程门阵列（FPGA）[16]，星上处理能力也得到了提升。2020年，欧洲航天局（ESA）发射了搭载英特尔 Movidius Myriad 2 芯片的 Φ-sat-1 遥感卫星，旨在验证深度神经网络（DNN）的在轨性能，并取得了巨大成功 [17]。

空间边缘计算（SEC）赋能在轨智能

基于上述空间硬件的演进，我们最近见证了一种新的智能计算范式：空间边缘计算（SEC）。SEC 利用卫星上以及卫星网络内部的类边缘计算能力，直接在轨道上处理数据。SEC 可用于许多创新的空间应用，如自主远程决策、用于灾害发现的在轨探测、气候监测和海上救援等 [18] – [20]。通过在轨处理数据而非将所有数据传回地面，SEC 能够实现更快的数据处理、更低的延迟和更高的效率。

B. Battery Energy Consumption: The Achilles’ Heel to SEC

However, while the advanced on-board hardware provides intelligence, it also involves additional energy consumption, which not only affects the execution of SEC tasks, but also affects the lifetime of the SEC system itself.

然而，先进的星上硬件在提供智能能力的同时，也带来了额外的能源消耗。这不仅影响了SEC任务的执行，还影响了SEC系统本身的寿命。

Satellite power supplement. Satellites orbit around the earth, and during a portion of their orbit, they enter the earth’s shadow, causing an eclipse. Fig. 1 plots a typical architecture of existing SEC power systems (SPS). During the sunlight phase, solar panels generate energy from the received photons, and SPS distributes power to other subsystems of the satellite. On-board rechargeable batteries store excess power generated by solar panels when the satellite is in sunlight, and the stored energy is used to power the satellite during eclipse periods. Typically, among all subsystems the communication and computation modules can contribute copious energy consumption [7].

卫星的电力补充

卫星环绕地球运行，在其轨道的一部分会进入地影区，发生“食”的现象。图1 展示了现有SEC电源系统（SPS）的典型架构。在光照阶段，太阳能电池板通过接收到的光子产生能量，SPS 将电力分配给卫星的其他子系统。当卫星处于光照区时，星上可充电电池会存储由太阳能电池板产生的多余电能，而这些存储的能量则用于在 “食”期间 为卫星供电。通常，在所有子系统中，通信和计算模块是能耗大户 [7]。

Depth of discharge (DoD) and battery lifetime. The lifetime of a battery (as well as the satellite itself) is tightly affected by its power usage during the charging-discharging cycles of the battery. In particular, DoD characterizes the percentage of discharged energy relative to the maximum capacity. DoD is equal to 0 if the battery is full and 100% if the battery is empty. As the battery is used, the maximum capacity of the battery will gradually decay. Typically, when the maximum battery capacity falls below 80% of its initial volume, the battery should be retired, which is defined as its lifetime [21], [22]. Previous works [23], [24] have uncovered that if we regularly discharge a battery at a higher DoD, its lifetime will be shorter (e.g., about 20% DoD increase can reduce the lifetime by half). Fig. 2 shows how DoD affects the total number of available chargingdischarging cycles and the lifetime of two kinds of Lithium batteries. Considering that battery usage can jointly affect task performance and satellite lifespan, optimizing the battery energy consumption and accomplishing energy-efficient in-orbit task processing is important for futuristic SEC networks.

放电深度（DoD）与电池寿命

电池（以及卫星本身）的寿命与其在充放电循环期间的电力使用情况密切相关。其中，放电深度（DoD） 是一个关键指标，它表征了电池已释放电量相对于其最大容量的百分比。当电池满电时，DoD 为 0；当电池完全放空时，DoD 为 100%。随着电池的使用，其最大容量会逐渐衰减。通常，当电池的最大容量低于其初始容量的 80% 时，电池就应被淘汰，这个时间点被定义为其寿命终点 [21], [22]。先前的研究 [23], [24] 已经揭示，如果一个电池经常在较高的 DoD 水平下放电，其寿命将会缩短（例如，DoD 增加约 20% 可能导致寿命减半）。图2 展示了 DoD 如何影响两种锂电池的总可用充放电循环次数和寿命。考虑到电池使用会共同影响任务性能和卫星寿命，优化电池能耗并实现高能效的在轨任务处理对未来的SEC网络至关重要。