Background

Picosats represent an emerging category of low-cost and lowcomplexity LEO satellites. As of 2022, around 2000 picosats have been launched [8]. The defining characteristic of this burgeoning industry is its ability to rapidly expand network coverage at a reduced capital outlay, enabling end-users to access services at lower prices. This feat is accomplished by miniaturizing picosats (Fig. 1) and simplifying their hardware design. This stands in contrast to the satellite constellations for broadband Internet, bent-pipe transponders, and directto-cell connectivity (e.g., 4G/5G NTN), which entail substantial capital expenditure. These constellations employ larger satellites equipped with high-end hardware, often exceeding 10x the cost and size of picosats.

皮卫星（Picosats）是新兴的一类低成本、低复杂度的低地球轨道（LEO）卫星。截至2022年，已发射的皮卫星数量约有2000颗[8]。这个蓬勃发展的行业的决定性特征是，它能够以较低的资本支出快速扩展网络覆盖范围，从而使终端用户能以更低的价格获取服务。这一成就是通过将皮卫星小型化（图1）并简化其硬件设计来实现的。这与用于 宽带互联网、弯管转发器和直连蜂窝网络连接（例如4G/5G NTN） 的卫星星座形成了鲜明对比，后者需要大量的资本支出。这些星座采用的卫星更大，配备了高端硬件，其成本和尺寸通常是皮卫星的10倍以上

DtS IoT: In an IoT-picosat network, IoT devices on Earth directly communicate with picosats using the direct-to-satellite (DtS) model (Fig. 2a). These IoT devices share nearly identical characteristics with traditional terrestrial IoT devices in terms of hardware design, power profile, and cost. On the other end, 10s - 100s of picosats in a constellation serve as gateways, aggregating data from these IoT devices and downloading it to ground stations. Due to their simplistic design, picosats employ very basic hardware configurations, including omnidirectional antennas without any beamforming capability, off-the-shelf radio and computation units, and low-capacity batteries (Fig. 1). The main advantage of the DtS model is its simplicity in usage and deployment. IoT device users can simply turn a device on and connect to the Internet from anywhere on Earth without requiring a terrestrial gateway. This connectivity proves particularly advantageous in remote areas lacking terrestrial infrastructure, such as farms, forests, and oceans. Moreover, in urban environments, DtS models enable setup-free deployment.

直连卫星物联网（DtS IoT）：

在一个物联网-皮卫星网络中，地球上的物联网设备采用直连卫星（DtS）模型直接与皮卫星通信（图2a）。这些物联网设备在硬件设计、功率特性和成本方面，与传统的地面物联网设备几乎完全相同。在另一端，一个星座中数十到数百颗皮卫星充当网关，汇聚来自这些物联网设备的数据，并将其下载到地面站。由于其简约的设计，皮卫星采用非常基础的硬件配置，包括无波束成形（beamforming）能力的全向天线、现成的无线电和计算单元，以及低容量电池（图1）。DtS模型的主要优势在于其使用和部署的简便性。 物联网设备用户只需在地球上任何地方打开设备即可连接到互联网，无需地面网关。 这种连接性在缺乏地面基础设施的偏远地区（如农场、森林和海洋）尤其具有优势。此外，在城市环境中，DtS模型也实现了免安装部署。

Data Communication: Despite being power-constrained, IoT devices achieve the necessary link budget for direct transmission to the satellite by employing low-power modulation techniques, such as LoRa [58, 61, 62], which is a popular choice among commercial IoT-picosat service providers like Wyld Networks [13], EchoStar [4], Lacuna [48], SWARM [10], and FOSSA [5]. Furthermore, IoT-picosat networks operate within the VHF to S bands (100 MHz - 2.4 GHz), where low-frequency data communication ensures that the wireless links between the IoT devices and picosats are not significantly affected by weather and atmospheric conditions, unlike high-frequency broadband satellites such as Starlink [9]. Low frequencies also experience lower path loss, reducing the power requirement on IoT devices and picosats.

数据通信：

尽管功率受限，物联网设备通过采用低功耗调制技术（如LoRa [58, 61, 62]）来获得与卫星直接传输所需的链路预算。LoRa是Wyld Networks [13]、EchoStar [4]、Lacuna [48]、SWARM [10] 和 FOSSA [5] 等商业物联网-皮卫星服务提供商的热门选择。此外，物联网-皮卫星网络工作在VHF至S波段（100 MHz - 2.4 GHz）。与星链（Starlink）[9]等高频宽带卫星不同，低频数据通信确保了物联网设备和皮卫星之间的无线链路不会受到天气和大气条件的显著影响。低频信号的路径损耗也更低，从而降低了对物联网设备和皮卫星的功率要求。

An IoT-picosat network operates exclusively within a licensed spectrum or spectrum allocated for weather watch and meteorology in some countries. However, the allocated operational bandwidth is very limited, typically around one MHz, posing a significant challenge with spectrum availability [58, 59, 62]. The network operator divides the allocated spectrum into multiple channels, each with bandwidths ranging from 10s to 100s of kHz. The combination of low channel bandwidth and low-power modulation techniques like LoRa results in a low data rate satellite-ground communication link of around one kbps. Here, uplink (ground to satellite) and downlink (satellite to ground) operations occur in different channels. However, due to the scarcity of spectrum, network operators face limitations in allocating multiple channels in any direction. For instance, SWARM can hardly avail more than two uplink channels of 125 kHz (standard LoRa channel bandwidth), while FOSSA can only utilize one [66].

物联网-皮卫星 网络专门在授权频谱内运行，或在某些国家使用分配给气象观测和气象学的频谱。然而，分配的可用带宽非常有限，通常在一兆赫（MHz）左右，这给频谱可用性带来了巨大挑战 [58, 59, 62]。网络运营商将分配的频谱划分为多个信道，每个信道的带宽从几十到几百千赫（kHz）不等。低信道带宽和LoRa等低功耗调制技术的结合，导致卫星与地面通信链路的数据速率很低，约为1 kbps。在这里，上行链路（地面到卫星）和下行链路（卫星到地面）在不同的信道中进行。然而，由于频谱稀缺，网络运营商在任何方向上分配多个信道的能力都受到限制。例如，SWARM几乎无法获得超过两个125 kHz（标准LoRa信道带宽）的上行信道，而FOSSA只能使用一个[66]。

Ground Stations: Picosats maintain contact with two types of ground stations – Telemetry, Tracking, and Control (TT&C) and data downloads. Unlike traditional multi-million dollar ground stations, these are low-cost facilities equipped with off-the-shelf radios and rotation-capable Yagi antennas (Fig. 6) [62]. Due to the simplistic and low-power design choices, picosats communicate with the ground stations using low data rate modulation like LoRa. On average, a picosat passes over a ground station 2-3 times a day, with contact times of 6-8 minutes each. However, with such a low data rate and short contact time, a small number (6-8) of ground stations are insufficient to download data from a large constellation comprising 100s of satellites, which aggregate data from 10s of thousands of IoT devices [62, 69]. As a solution, a new concept of distributed ground stations has emerged in both academia and industry [16, 52, 63, 69]. For picosats, such distributed architectures are increasingly being adopted. For example, TinyGS [12] has deployed 1000+ ground stations for picosats, with support for multiple constellations. These ground stations are highly cost-effective (∼ $100), utilizing LoRa radios with omnidirectional antennas and WiFi as the backhaul. They are receive-only (no uplink) and utilized for downloading sensor data and health status from picosats.

地面站：

皮卫星与两类地面站保持联系 —— 遥测、跟踪和控制（TT&C）站以及数据下载站。 与传统的耗资数百万美元的地面站不同，这些是配备了现成无线电和可旋转八木天线（图6）的低成本设施[62]。 由于简约和低功耗的设计选择，皮卫星使用像LoRa这样的低数据速率调制方式与地面站通信。平均而言，一颗皮卫星每天飞越一个地面站2-3次，每次联系时间为6-8分钟。然而，在如此低的数据速率和短的联系时间下，少量（6-8个）地面站不足以从一个由数百颗卫星组成的大型星座中下载数据，而这些卫星汇聚了来自成千上万个物联网设备的数据[62, 69]。作为一种解决方案，分布式地面站的新概念在学术界和工业界都已出现[16, 52, 63, 69]。对于皮卫星而言，这类分布式架构正被越来越多地采用。例如，TinyGS [12]已经为皮卫星部署了超过1000个地面站，并支持多个星座。 这些地面站成本极低（约100美元），使用带全向天线的LoRa无线电和WiFi作为回程链路。它们只能接收（无上行链路），用于从皮卫星下载传感器数据和健康状态信息。