Antenna Beam-Sharing: Progress Toward Multiple Uplinks Per Antenna

Within a decade, the number of spacecraft requiring support from NASA’s Deep Space Network (DSN) is expected to increase by a factor of 2 to 3. Key drivers behind this expectation include NASA’s increasing reliance on more affordable, targeted smallsat missions, a more robust science program in general, and the emergence of multi-element human lunar exploration missions. Given that a proportional increase in the number of antennas needed to communicate with all of these missions might prove prohibitively expensive, NASA has been exploring ways to gain increased capacity from its existing antennas. One way to do this is through antenna beam-sharing. While successfully employed for simultaneous downlink, beam-sharing for simultaneous uplink has not been an option, within the same frequency band, due to the severity of the intermodulation products. An alternative approach to achieving near-simultaneous uplink involves multiplexing the Forward Communications Link Transmission Units (FCLTUs) intended for all of the participating in-beam spacecraft onto a single uplink frequency, which they all receive. Each spacecraft then identifies which of the transfer frames within the received FCLTUs are intended for it on the basis of spacecraft ID. Known as Multiple Uplinks Per Antenna (MUPA), this technique has been under study for several years and is now at the point where key components are being prototyped and implementation pathfinding is occurring. This paper describes the progress in these efforts to date. These efforts fall into three broad categories: the FCLTU multiplexer, ground system interfaces, and the spacecraft radio. The FCLTU multiplexer efforts include developing and testing the software needed to: receive FCLTUs from disparate Mission Operations Centers (MOCs), assess whether their destination spacecraft are in view or not, queue them accordingly, multiplex the in-view FCLTUs, send them on to the DSN under a virtual spacecraft ID for radiation, and feedback the appropriate radiated FCLTU statistics to each of the originating MOCs. The ground system interface efforts include identifying the modifications to DSN systems needed to successfully schedule MUPA sessions, distribute appropriate support data products to the uplink and tracking data systems, and ensure that the coherent turnaround of the shared uplink carrier frequency to each of the individual spacecraft downlink carrier frequencies is executed and communicated in ways that fully support simultaneous 2-way Doppler and ranging. The spacecraft radio efforts include designing, simulating, and/or testing radio features on the JPL Iris radio that will enable it to support variable turnaround ratios, selectable uplink frequencies, onboard Doppler compensation when trying to acquire and lock onto the shared uplink frequency, and filtering of the received transfer frames on the basis of spacecraft ID. These efforts have also involved examining MUPA’s compatibility with different types of uplink encryption, coding, and modulation and how these factors would enter into the scheduling of a MUPA session. Together, all of these efforts and the underlying system engineering for them, are “setting the stage” for the next key step on the path to implementing MUPA: flight demonstrations. Preparations with the SunRISE and EscaPADE projects for a multiplexer demonstration are already underway, and the team is working to identify other flight projects amenable to demonstrating the Iris radio’s MUPA-related capabilities.

未来十年内，需要美国国家航空航天局（National Aeronautics and Space Administration，NASA）深空网络（Deep Space Network，DSN）提供支持的航天器数量预计将增长2至3倍。推动这一增长的核心因素包括：NASA对成本更低、针对性更强的小卫星任务的依赖度不断提升，整体科学项目体系愈发健全，以及多单元载人月球探测任务的兴起。鉴于为满足所有此类任务的通信需求而按比例增加天线数量的成本可能高得令人望而却步，NASA一直在探索通过现有天线提升通信容量的方法。其中一种途径便是天线波束共享技术。该技术已成功应用于同时下行链路传输，但受互调产物的严重影响，同一频段内的同时上行链路共享此前一直无法实现。一种实现近乎同时上行链路传输的替代方案，是将所有参与波束共享的航天器对应的前向通信链路传输单元（Forward Communications Link Transmission Units，FCLTUs）复用至单个上行链路频段，供所有航天器接收。随后每艘航天器可通过航天器标识码，从接收到的FCLTUs中识别出专为其设计的传输帧。这项被称为单天线多上行链路（Multiple Uplinks Per Antenna，MUPA）的技术已被研究多年，目前已进入关键组件原型开发与实施路径探索阶段。本文将阐述迄今为止相关工作的进展。此类工作主要分为三大方向：FCLTU复用器、地面系统接口，以及航天器无线电系统。FCLTU复用器相关工作包括开发并测试以下所需软件：接收来自不同任务运行中心（Mission Operations Centers，MOCs）的FCLTUs，评估其目标航天器是否处于天线可视范围内，并据此进行队列调度；将可视范围内的FCLTUs进行复用；以虚拟航天器标识码的形式将复用后的信号发送至DSN进行辐射传输；并将对应的辐射FCLTU统计数据反馈至各发起任务的MOC。地面系统接口相关工作包括：确定需对DSN系统进行的改造，以顺利调度MUPA任务会话；为上行链路与跟踪数据系统分发适配的支持数据产品；确保共享上行链路载波频率向各航天器下行链路载波频率的相干转换能够顺利执行，并以完全支持双向多普勒与测距同步的方式完成相关信息传递。航天器无线电系统相关工作包括：为喷气推进实验室（Jet Propulsion Laboratory，JPL）的Iris无线电设计、仿真并/或测试相关功能，以支持可变转换比、可选上行链路频段，以及在捕获并锁定共享上行链路频段时的星上多普勒补偿，同时实现基于航天器标识码对接收传输帧的筛选。此类工作还涵盖了研究MUPA与不同类型上行链路加密、编码及调制方式的兼容性，以及这些因素如何影响MUPA任务会话的调度。上述所有工作及其背后的系统工程，正为实现MUPA的下一关键步骤——飞行验证——奠定基础。目前，团队已借助SunRISE与EscaPADE项目开展复用器演示的准备工作，并正寻找其他适合验证Iris无线电MUPA相关功能的飞行任务项目。