Prototyping Operational Autonomy for Space Traffic Management

Current state of the art in Space Traffic Management (STM) relies on a handful of providers for surveillance and collision prediction, and manual coordination between operators. Neither is scalable to support the expected 10x increase in spacecraft population in less than 10 years, nor does it support automated manuever planning. We present a software prototype of an STM architecture based on open Application Programming Interfaces (APIs), drawing on previous work by NASA to develop an architecture for low-altitude Unmanned Aerial System Traffic Management. The STM architecture is designed to provide structure to the interactions between spacecraft operators, various regulatory bodies, and service suppliers, while maintaining flexibility of these interactions and the ability for new market participants to enter easily. Autonomy is an indispensable part of the proposed architecture in enabling efficient data sharing, coordination between STM participants and safe flight operations. Examples of autonomy within STM include syncing multiple non-authoritative catalogs of resident space objects, or determining which spacecraft maneuvers when preventing impending conjunctions between multiple spacecraft. The STM prototype is based on modern micro-service architecture adhering to OpenAPI standards and deployed in industry standard Docker containers, facilitating easy communication between different participants or services. The system architecture is designed to facilitate adding and replacing services with minimal disruption. We have implemented some example participant services (e.g. a space situational awareness provider/SSA, a conjunction assessment supplier/CAS, an automated maneuver advisor/AMA) within the prototype. Different services, with creative algorithms folded into then, can fulfil similar functional roles within the STM architecture by flexibly connecting to it using pre-defined APIs and data models, thereby lowering the barrier to entry of new players in the STM marketplace. We demonstrate the STM prototype on a multiple conjunction scenario with multiple maneuverable spacecraft, where an example CAS and AMA can recommend optimal maneuvers to the spacecraft operators, based on a predefined reward function. Such tools can intelligently search the space of potential collision avoidance maneuvers with varying parameters like lead time and propellant usage, optimize a customized reward function, and be implemented as a scheduling service within the STM architecture. The case study shows an example of autonomous maneuver planning is possible using the API-based framework. As satellite populations and predicted conjunctions increase, an STM architecture can facilitate seamless information exchange related to collision prediction and mitigation among various service applications on different platforms and servers. The availability of such an STM network also opens up new research topics on satellite maneuver planning, scheduling and negotiation across disjoint entities

A Concept for Civil Space Traffic Management

As technology has improved, operators have sought to use cubesats, as well as smallsats more generally, to perform increasingly more ambitious and sophisticated functions. Despite this, practical concerns associated with cubesat infant mortality, conjunctions, limited maneuverability, and debris generation have been relatively muted because most cubesats have been launched to lower orbits that limit both their orbital lifetime and consequences should a collision occur. NASA ARC has developed a concept for a highly-automated and distributed space traffic management (STM) architecture, drawing on similar work done to provide traffic management for small unmanned aerial systems (UAS) operating at low altitudes. The system proposes a strategy to accommodate growing space traffic volume safely, as well as pave the way for a transition of civil STM authority to a civilian governmental entity. The architecture envisions an open-access software platform architecture of data and service suppliers, consumers, and regulators, connected via a set of application programming interfaces (APIs). The platform would build on, rather than replicate existing integration and coordination efforts within the space situational awareness ecosystem, using existing standards for data message formats from organizations like the Consultative Committee for Space Data Systems and wrapping, rather than replacing existing integrations. We will present an initial STM architecture in this presentation, with a few examples showing how stakeholders can interact structurally, but flexibly, within this architecture

Genetically-barcoded SIV facilitates enumeration of rebound variants and estimation of reactivation rates in nonhuman primates following interruption of suppressive antiretroviral therapy

<div><p>HIV and SIV infection dynamics are commonly investigated by measuring plasma viral loads. However, this total viral load value represents the sum of many individual infection events, which are difficult to independently track using conventional sequencing approaches. To overcome this challenge, we generated a genetically tagged virus stock (SIVmac239M) with a 34-base genetic barcode inserted between the <i>vpx</i> and <i>vpr</i> accessory genes of the infectious molecular clone SIVmac239. Next-generation sequencing of the virus stock identified at least 9,336 individual barcodes, or clonotypes, with an average genetic distance of 7 bases between any two barcodes. <i>In vitro</i> infection of rhesus CD4+ T cells and <i>in vivo</i> infection of rhesus macaques revealed levels of viral replication of SIVmac239M comparable to parental SIVmac239. After intravenous inoculation of 2.2x10⁵ infectious units of SIVmac239M, an average of 1,247 barcodes were identified during acute infection in 26 infected rhesus macaques. Of the barcodes identified in the stock, at least 85.6% actively replicated in at least one animal, and on average each barcode was found in 5 monkeys. Four infected animals were treated with combination antiretroviral therapy (cART) for 82 days starting on day 6 post-infection (study 1). Plasma viremia was reduced from >10⁶ to <15 vRNA copies/mL by the time treatment was interrupted. Virus rapidly rebounded following treatment interruption and between 87 and 136 distinct clonotypes were detected in plasma at peak rebound viremia. This study confirmed that SIVmac239M viremia could be successfully curtailed with cART, and that upon cART discontinuation, rebounding viral variants could be identified and quantified. An additional 6 animals infected with SIVmac239M were treated with cART beginning on day 4 post-infection for 305, 374, or 482 days (study 2). Upon treatment interruption, between 4 and 8 distinct viral clonotypes were detected in each animal at peak rebound viremia. The relative proportions of the rebounding viral clonotypes, spanning a range of 5 logs, were largely preserved over time for each animal. The viral growth rate during recrudescence and the relative abundance of each rebounding clonotype were used to estimate the average frequency of reactivation per animal. Using these parameters, reactivation frequencies were calculated and ranged from 0.33–0.70 events per day, likely representing reactivation from long-lived latently infected cells. The use of SIVmac239M therefore provides a powerful tool to investigate SIV latency and the frequency of viral reactivation after treatment interruption.</p></div

Plasma viral loads, PBMC CA-vRNA and CA-vDNA in animals in study 2.

<p>(A) 6 rhesus macaques, DEJX (red), DFGV (yellow), DEJW (blue), H090 (purple), DEPI (light green), and H105 (dark green) were infected with SIVmac239M and cART was initiated on day 4 post-infection. Pairs of macaques were removed from therapy on days 305, 374, and 482 post-infection. Viral RNA copies were measured in plasma collected up to 550 days. Bars over the viral load data indicate duration of therapy for each animal. (B-C) Cell-associated viral DNA (B) and RNA (C) was measured in PBMCs over the duration of cART therapy in animals infected with SIVmac239M. Measurements obtained are shown overlaying the detected plasma viral loads. Open symbols represent measurements that were below the limit of detection.</p

Evaluation of clonotypes found in stock and monkeys.

<p>(A) Each individual clonotype was plotted by its rank order in the stock versus the number of animals in which it was found. A linear correlation was generated with an R² value of 0.77. Density of clonotypes at any single point are colored using a log scale heat map where red points depict 2 logs of clonotypes and dark blue points represent single clonotypes. (B) The number of barcodes was plotted against the number of monkeys in which the barcodes were found. Of the 9,336 total stock barcodes, 7,991 were found systemically in at least one animal, and 1,345 were not found in any of the 26 animals. (C) The mean relative frequency of each individual barcode (grey circles) was plotted against the number of monkeys in which the barcode was found. The relative frequencies of the barcodes demonstrate the comparative homogeneity of all clonotypes across all animals.</p

Pairing of relative viral loads and time of reactivation.

<p>Approximate day of reactivation of cells harboring clonotypes was estimated in study 2 animals following interruption of long-duration therapy for animals DEJX (A), DFGV (B), DEJW (C), H090 (D), DEPI (E), and H105 (F). Total viral load at the time point selected is comprised of the relative proportions of clonotypes determined by sequencing analysis. Each individual clonotype’s growth rate was estimated during the maximum exponential phase of the total viral load curve, and the slope of the growth of each clonotype was extended below the limit of detection to estimate the approximate time of reactivation. The grey line represents the plasma concentration of one viral copy in the total plasma, and the tick marks represent the theoretical reactivations based on calculated reactivation rate for each animal.</p

Insertion of genetic barcode into SIVmac239.

<p>(A) A 34 base cassette (yellow) bearing a stretch of 10 random bases was inserted between <i>vpx</i> and <i>vpr</i> of wild-type SIVmac239 to generate the genetically barcoded virus SIVmac239M. The MluI restriction site used is outlined in black, and the sequences of the barcode flanking regions are colored in blue and green to depict the two possible insertion orientations. (B) Representative sequences in single stock aliquots depicting the bimodal distribution of authentic barcodes (green) versus barcodes containing PCR error (red).</p

SIVmac239M <i>in vitro</i> and <i>in vivo</i> replication.

<p>(A) CD8+ T-cell-depleted PBMCs infected with either SIVmac239 (teal) or SIVmac239M (maroon) were monitored over 13 days and samples collected were assayed by ELISA specific for reverse transcriptase (RT) over time. (B) Infectivity was tested <i>in vivo</i> in two rhesus macaques, ZK37 (light blue) and ZK56 (purple) following intravenous infection. Viral RNA copies were measured in plasma over 100 days post infection with a lower limit of detection of 15 copies/mL.</p