Jack Schuss, SpaceEM; Christopher Cox, Keystone Mission Solutions Corporation
Keywords: LEO, radar, network
Abstract:
The population increase of LEO satellites over the last decade has clarified the need for rapid discovery and precision tracking of space objects and events. Meeting this need requires a worldwide network of accurate, high sensitivity tracking radars that have the capability to execute search operations to rapidly detect new space objects or maneuvers. Since many radars are required globally to enable frequent updates of space object state vectors, each radar’s architecture must be optimized in order to make the network affordable. Both track and search/discovery will drive radar and network cost and complexity, with wide angle search requirements being the more stressful. These network cost drivers can be mitigated by leveraging two architectural features into the system: (1) use of a Limited Field of View (LFoV) architecture for the radars, and (2) sharing the required global PA among the network radars (here P is the radar average transmitted power and A is the aperture area of the radar), while maintaining the timeliness of the global network to discover new space events. This latter feature is made possible by recognizing that radar network search timeliness is a function of global radar power-aperture, not individual radar power-apertures, and that this global parameter can be leveraged to profoundly reduce radar network costs, as discussed below. The required global PA is a function of the type of event being discovered, and varies depending on the need to detect small maneuvers vs. new space events.
The consequence of these mitigating factors is an alternative architectural approach for achieving LEO surveillance, namely fielding a global network of many, small radars, instead of deploying a network of a few, large radars. Both networks have comparable capabilities to discover new space events, but the large network of small radars provides for better custody of known space objects. Furthermore, the smaller radars are easier to instantiate and maintain in remote locations, and the resulting network is easier to grow.
Regarding the first feature, namely the LFoV architecture, it should be noted that radar track sensitivity is a function of PA^2, and search operation is a function of PA (“power – aperture”). To achieve a given sensitivity, the required average transmit power and the transmit complexity decrease as the aperture area increases; since the transmit function is a radar cost driver, increasing aperture area dramatically decreases radar cost. Cost effective increases in aperture area can be obtained by employing a LFoV radar architecture, in which the number of electronic phase-controlled elements is much smaller than the number of square half-wavelengths in the aperture area. While this LFoV architecture reduces radar power and cost, it also results in reduced scan coverage; nevertheless, this reduced scan volume can be tailored to yield accurate state vectors, as will be shown in the paper.
Network search requirements are a stronger driver of radar cost and complexity than track. Search operation sensitivity scales as PA and scales inversely to the angular width of the search fence. Since the LFoV radar architecture increases aperture, it also reduces search transmit power, but not by as much as it does for track. The cost and complexity of a radar and of the network can be fundamentally reduced by noting that global power-aperture is the dominant parameter that governs LEO surveillance radar systems, whether this power-aperture is located in a small number of large radars, or in a large number of small radars. Choosing the latter architecture of a large network of small radars permits high track revisit rates for space objects due to the large number of radars, while cost-effectively maintaining the same network response time for discovery of unplanned space events. Each radar then supports both global track and search requirements by providing a large scan angle Field of View (FoV) for track, but a much smaller search sector. The required size of this search sector varies whether small maneuvers, large maneuvers, or totally new events need to be detected quickly.
This paper will discuss how radar size and complexity are impacted by track and search requirements, using both simple general models and detailed simulations. The timeliness of a global network of radars will be presented, showing that the mean time to detect a LEO space event is the same for both a large and a small network of radars, if the networks have approximately the same total power-aperture. This scaling will be elucidated for discovering small maneuvers, large maneuvers, and new space events. Finally, the implications of this scaling to radar and network architecture will be discussed.
Date of Conference: September 17-20, 2024
Track: Space Domain Awareness