The emergence of new hypersonic weapon systems poses a major challenge for the existing missile defense architecture, due to the low flight altitude and maneuverability of these systems, tracking them with regular ground-based radars is more challenging. Hypersonic weapons are still detectable by ground-based radars, but only during the last stages of their flight, but at this point, the defender’s reaction and decision window are severely restricted by the hypersonic missile’s speed. This in turn can result in scenarios where only one engagement is possible, with the outcome of the strike being dependent on that single engagement.

HGV Versus RV Terrestrial-Based Detection (Source: Hypersonic Missile Nonproliferation RAND RR2137-2.2)

In contrast, satellites and other elevated terrestrial sensors such as Joint Land Attack Cruise Missile Defense Elevated Netted Sensor System (JLENS), aren’t as limited by the horizon to the same extent as ground-based radars. As such, satellites in Low-earth orbit (LEO) can have a large view of the earth which extends for thousands of kilometers in all directions. Furthermore, due to the speed at which hypersonic missiles travel, which is beyond Mach 5, or five times faster than the speed of sound, the weapons’ outer layers can reach blisteringly high temperatures which are detectable by infrared satellites, even against the warm Earth background.

Therefore, a sensor architecture centered around terrestrial elevated sensors and a constellation of satellites can extend the time window available to missile defense systems. Aside from allowing engagements at longer ranges, the extended time window gives defensive systems the chance to carry out follow-on engagement attempts if the first doesn’t succeed.

Aside from the recent emergence of hypersonic weapon systems, LEO satellite constellations can also aid in challenging scenarios associated with traditional ballistic missile defense, such as mid-course tracking, which the United States has long sought after. But this task is considered quite challenging for the current missile warning architecture, which is made up of satellites in geosynchronous orbits that are part of the Space-Based Infrared System (SBIRS) and the aging Defense Support Program (DSP).

Although these existing systems are capable of detecting very faint infrared signals on earth and in space, they each cost billions of dollars and lack the necessary optics and sensitivity to track dim objects in mid-course flight. Therefore, any attempts involving midcourse tracking will require multiple Wide-Field of View (WFOV) satellites in LEO. The SBIRS program initially had a planned Low-earth constellation dubbed the SBIRS-LOW, but after multiple delays and ballooning costs, the effort was discontinued after the launch of  two satellites. The two satellites, now called the Space Tracking and Surveillance System (STSS), remained in experimental service under the Missile Defense Agency (MDA) before being quietly deactivated in 2022. 

Building the constellation

A notional depiction of the NDSA components (Source: Space Development Agency)

The National Defense Space Architecture is a new attempt to make a relatively cheap and robust satellite constellation a reality. The new space architecture has been moving at a fast pace due to the SDA making use of existing commercial-off-the-shelf satellite buses, communication systems, and various other technologies that have already been derisked through years of private industry investments. Some of these new satellites will also be much smaller and spend a shorter time in orbit, this leads to a major reduction in costs and complexity compared to the existing dedicated multi-billion dollar military satellites.

Overall, the NDSA calls for a layered constellation that features

  • A Custody Layer that will keep track of ground launchers and pass on the data for targeting. This layer will be made up of Electro-optical (EO) and Synthetic Aperture Radar (SAR) satellites. The SDA hopes to leverage existing commercial and military ISR space vehicles and fuse the data from these sensors, this is done to not duplicate existing capabilities. 
  • A Tracking Layer that consists of Wide Field of View (WFOV) satellites that will detect launch signatures over a wide area thanks to their field of view.
  • A Tracking Layer that consists of Medium Field of View (MFOV) satellites that will be cued by the WFOV satellites. The MFOV layer will trade a slightly decreased field of view for better sensitivity and accuracy, thus allowing it to pass precise track information to interceptor systems. 
  • A Transport Layer with satellites that will disseminate information from the Tracking layers to end-users through the use of lasers. This layer will also have onboard processing capabilities and will act as a backup Navigation constellation.

The timeline so far

In October 2020, the SDA awarded $193.5 million to L3Harris and $149 million to SpaceX for the Tranche 0 Tracking Layer satellites. According to SpaceNews, each vendor is to deliver four space vehicles each with a WFOV Overhead Persistent Infrared Payload by September of 2022. The satellites will also be equipped with optical inter-satellite link (OISL) to pass data to  Transport satellites.

Contracts for the Tranche 1 Transport Layer were awarded to Lockheed Martin, York Space Systems, and Northrop Grumman in April of 2021. The agreements which were collectively worth $1.8 billion covered 126 satellites, 42 satellites from each vendor. Each space vehicle in the constellation is to be equipped with:

  •     Optical Communications Terminals (OCTs) enabling a minimum of four simultaneous optical communications links,
  •     Link 16 mission communications payload,
  •     Ka-band RF mission communications payload, and
  •     Battle Management Command, Control, And Communication (BMC3) module enabling on-orbit data processing, storage and fusion.

In July of 2022, the SDA awarded agreements for Tranche 1 Tracking Layer. L3Harris was awarded a prototype agreement worth $700 million, while Northrop Grumman Strategic Space Systems’ award was worth $617 million. Both vendors are to produce 14 space vehicles each, all equipped with the WFOV Overhead Persistent Infrared Payload and optical inter-satellite link (OISL).

Present and future roadmap

The SDA’s near-term launch plan (Source: Space Development Agency)

Although contracts for Tranche 2 haven’t been awarded yet, it will likely include hundreds of new satellites and will mark the full operational capability of the constellation. All the layers will provide persistent global coverage. Follow-on tranches will keep improving the constellation every two to four years.

Cooperation and challenges

Areas of cooperation with the NRO include the possible integration of optical inter-satellite links on NRO imaging satellites, allowing the imaging satellites to use the SDA’s Transport Layer. This would enable the SDA to utilize the data from NRO’s satellites as a part of the Custody Layer. Even if the NRO satellites do not integrate with the Transport Layer directly, they can still support the Custody Layer by sending satellite imagery through ground stations. These stations would beam the information back up to be fused with other satellite data from the Custody Layer and subsequently disseminated through the Transport Layer.

Initial plans for cooperation between the SDA and MDA were: the MDA would build the HBTSS and the SDA would integrate the sensor in its satellite and provide launch services for the HBTSS satellites. But according to a report released by the Government Accountability Office (GAO) in June, the division of labor and cooperation on the HBTSS has fallen apart. GAO states that the MDA’s latest plans for the HBTSS do not include the inter-satellite links as originally agreed on, and the agency has declined to procure launch services through the SDA. 

Furthermore, the report states that the MDA plans to expand HBTSS beyond its current phase to include an operational constellation of its own, leading to a possible capability overlap with the SDA’s constellations. MDA officials have explained that their MFOV constellation will provide more accurate intercept-quality data for systems like the GPI, but according to the same report, MDA officials acknowledge the SDA constellation can also provide intercept-quality data to the GPI. 

Although it’s still in its early stages, the National Defense Space Architecture has already overcome many challenges that have plagued past satellite constellations, such as the Strategic Defense Initiative Organization’s Brilliant Eyes and Brilliant Pebbles. When fully implemented, the NDSA provides the United States with a constellation for monitoring high-value adversarial systems, a high-bandwidth global communication constellation, a backup navigation constellation, and an extremely accurate hypersonic and missile tracking constellation.