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GPS-Independent Navigation at the Core of Space Development Agency’s New LEO Constellation

GPS-Independent Navigation at the Core of Space Development Agency’s New LEO Constellation

By Dee Ann Divis

The Space Development Agency (SDA) is building a satellite constellation with a new, self-generated navigation capability that, if successful, will enable it to support the military with early warning and tracking information, even beyond-line-of-sight targeting, without input from GPS.

The still-unnamed web of Department of Defense satellites will use a proliferated architecture comprising hundreds of small spacecraft instead of dozens of larger, perhaps more-tempting-to-target satellites like those in the GPS constellation. Set in low Earth orbit (LEO), the relatively petite spacecraft will be far less expensive to build, launch and update and the signal latency from space to the ground will be less.

The SDA satellites will incorporate optical crosslinks creating a resilient, mesh network of connections that should enable the system to move information through the constellation from one side of the globe to another without interim downlinks to the Earth’s surface. The cross-links will also enable the satellites to share clock data and determine their position relative to each other. With this capability, and a link to the ground to add an Earth-centered reference frame, the constellation has the potential to self-generate its own positioning, navigation and timing (PNT) information and be truly independent of GPS.

The primary purpose of the new SDA network is to enable beyond-line-of-sight pinpointing of ground and maritime targets as well as advanced warning and tracking of hypersonic and missile launches. The system’s sensor layers will detect and track threats, feeding that information through data transport satellites to warfighters and defense systems via downlinks to the military’s Link 16 tactical data link network.

“Basically, we detect a missile; we calculate a track on that missile; send that to the Transport Layer; and then the Transport Layer can get that to an interceptor or get that to a shooter,” SDA Director Derek Tournear told the FY2022 Defense Programs Conference produced by McAleese and Associates in May.

Both the capability to move data and generate PNT information will reside in the constellation’s Transport Layer. The sensing satellites that support missile warning, tracking and targeting will reside in the Tracking and Custody Layers at other levels of an overall architecture comprising multiple layers.

The initial Transport Layer will comprise roughly 150 satellites. The first 20 of those spacecraft are already being built by Lockheed Martin and York Space Systems as a part of Tranche 0 (zero) — also called T0. (T0 will also incorporate eight tracking satellites). The plan is to launch those first 20 Transport Layer satellites into 1,000 km orbits in two different planes in September 2022 and March 2023.

With Tranche 0, SDA plans to demonstrate some of the capabilities it intends to provide beginning with Tranche 1 (T1), SDA Technical Director Frank Turner told Navigation Outlook.

“Tranche 0 will not have what I would call a navigation solution that we would provide to the warfighter,” Turner said, “but we're going to start working on those capabilities and make sure that we can do ranging with our optical links — that we can understand where the constellation's space vehicles are located — those kinds of things. So really, it's sort of getting our toes into the water on the whole navigation idea in T0 — looking to really, then, proliferate out some capabilities, beginning with T1.”

Tranche 1 of the Transport Layer (TLT1 for short) is already in the works. SDA is now seeking up to three contractors to build as many as 144 more satellites to be launched into six orbital planes in late fiscal year 2024.


Though SDA will lean on GPS during the development of its new constellation, the crosslinks connecting the satellites are the foundation for what they hope will be a fullyindependent navigation capability.

“The fundamental way the architecture selfnavigates is through the exchange of information over the optical two-way links, said an SDA technical adviser who spoke on background.

The two-way links enable independent ranging, said the adviser, “so each node [satellite] will be able to range to its nearest neighbor.” Each satellite will also be able to do time transfer of its clock data with its nearest neighbor’s clock.

“The fact that you've got this crosslink capability — that's the core requirement,” said satellite navigation expert Logan Scott of LS Consulting. It enables the satellites to establish where they are on their own, he told Navigation Outlook. “There’s some good thought put into this.”

“Each node in that inertial system will know where the others are — and we now have a self-determined inertial satellite system. But that doesn't mean anything because we have to tie it to a reference frame called the Earth,” the adviser old Navigation Outlook.

That will space-to-ground links, the adviser said. “Once we tie in the space-to-ground links, we now have absolute position and time because we'll be bringing in the UTC-USNO [Coordinated Universal Time - U.S. Naval Observatory] and we'll be bringing in the information to tie into an Earth-centered, Earth-fixed coordinate system.

The goal with the Tranche 0 satellites, said the SDA adviser, is to determine if they can successfully do independent ranging and time transfer between first-level satellites. With Tranche 1, “we will then start to execute the navigation filter on a ground basis.”

GPS, however, will still be the primary navigation system for Tranche 1, the technical expert said. “GPS will be a high-quality input to the navigation filter. So even though we will have the intended GPS-independent navigation filter starting to be prototyped, GPS will truly provide the time and the positioning of each satellite, and then it will also be brought into the filter to give us a sense of truth,” said the adviser. “By the time we get to T2 (Tranche 2), then we expect to take GPS out as a primary navigator, but it will always exist as one of the inputs into the sensor fusion engine.”

Will this approach will be able to provide the same accuracy and reliability as GPS? It’s too early to know, said Turner.

“If you look at what SDA is doing right this minute, we're still trying to understand and trying to work out, really, what the concepts of reliability and things like that mean for a proliferated LEO architecture,” Turner told Navigation Outlook. When you drop down orders of magnitude in cost and spacecraft lifespan and go up an order of magnitude in the number of space vehicles, those concepts begin to be a little bit different, he said. “We still don't have that specific understanding of exactly what's going to happen just yet. So, using the terms of reliability and things like that, I think, is a little bit early for a proliferated LEO architecture.”

Future Steps

SDA’s plans for the constellation stretch into fiscal year (FY) 2030 with the goal of adding more PNT capabilities already penciled in for Tranche 3 — which would launch around FY28.

Exactly what those additions would be remains to be determined though SDA has a wish list of the improvements it wants. That list includes candidate navigation signals (waveforms) and frequency bands with included concepts of operations designed to maximize performance, availability and integrity. The agency is seeking cryptographic systems to protect PNT integrity, confidentiality and/or availability. They are also looking for timekeeping systems and atomic clocks that offer better SWaP (size, weight and power) characteristics as well as better orbit and timing prediction applications, hardware or software to support alternate PNT and satellite-to-satellite/ground ranging via optical crosslinks.

“At this point in time,” Turner told Navigation Outlook, “I think really it's making sure that the door is open, making sure that our partners — be they commercial, be they other government agencies, mission partners, the war fighters themselves — they understand that we have not settled on anything; that we're still in discussions about everything.”

There is also an openness, said Turner, once the constellation’s self-navigation capability has been developed, to expanding the program’s mission to providing PNT more directly to warfighters, if that is what the users of the system want.

“Step one is we're going to make sure that our constellation is GPS independent; that's going to take us a little bit to get done,” said Turner, “…then we can start looking at how — and this is in the future, probably T2 and beyond — how we would provide that to the warfighter and in what manner they could ingest it and use it.”

Images provided courtesy of the Space Development Agency