York clears Nemesis GEO spacecraft design review

York Space Systems has cleared Delta Critical Design Review for the US Space Force’s Tetra 3 and Tetra 4 spacecraft, keeping both vehicles on schedule for fourth-quarter delivery as the company prepares its first prime spacecraft work for geosynchronous space domain awareness missions.

The two spacecraft form the programme York calls Nemesis and are being developed for demonstrations involving resident space objects, rendezvous and proximity operations and experimental payloads. York is responsible for the spacecraft design, payload integration and overall mission execution rather than supplying only a standard satellite bus to another prime contractor.

Delta Critical Design Review was required after a propulsion system change was introduced to achieve the full mission capability. A late propulsion modification can affect spacecraft structure, mass distribution, electrical demand, thermal behaviour, software and launch integration simultaneously, making the company’s reported ability to absorb the change without wider redesign a central part of the milestone.

York says the revised propulsion system introduced no structural, thermal or power impacts to the spacecraft baseline and did not require changes to launch vehicle integration. Only minor modifications were required to harnessing and flight software. The company has not identified the propulsion system or disclosed the additional manoeuvre capability it provides, so the extent of the underlying hardware change cannot be assessed from public information.

Propulsion is central to a rendezvous and proximity operations mission because the spacecraft has to control its relative position around another object rather than simply maintain a conventional operational orbit. Manoeuvres alter velocity and geometry continuously, while the guidance system has to ensure that planned approaches remain inside defined safety limits.

Nemesis will use several sensors to support that task. The disclosed suite includes a long wave infrared camera, LiDAR capable of ranging resident space objects at up to 20 kilometres and a GPS side lobe receiver qualified for operations in GEO. Each provides a different measurement that can contribute to navigation or characterisation of the object being observed.

Infrared imaging provides passive observation based on emitted thermal radiation, allowing the spacecraft to build imagery without transmitting towards the target. LiDAR operates differently by sending light towards the object and measuring the returned signal to establish range. Used together, the sensors can provide complementary information as the distance and viewing geometry change during an approach.

The GPS receiver addresses the spacecraft’s own navigation. GEO lies far above the region for which conventional GPS user geometry is optimised, so spacecraft in that orbit can make use of weaker signals transmitted beyond the main beams of GPS satellites. Side lobe reception provides another navigation input that can be combined with onboard propagation and other sensor information.

York has not disclosed the infrared camera resolution, LiDAR measurement accuracy or navigation algorithms, which prevents meaningful assessment of the final proximity performance before flight. The stated 20 kilometre LiDAR range does establish an operating point at which active ranging is expected to contribute before the spacecraft reaches very close observation distances.

The missions also carry government furnished equipment intended to support realistic on-orbit threat presentation, independent collection of range data, small satellite technology risk reduction and experimental payload hosting. Those additions make Tetra 3 and 4 test platforms as well as space domain awareness spacecraft, increasing the number of interfaces York has to manage within the available mass, power, thermal and data budgets.

Operating in geosynchronous orbit brings different constraints from the proliferated low Earth orbit constellations that dominate many newer satellite programmes. GEO spacecraft operate roughly 36,000 kilometres above Earth and are widely used for persistent communications, missile warning and other missions requiring continuous coverage of large areas. Observing activity in that region therefore has direct relevance to the resilience and protection of high value national security systems.

Rendezvous operations around those assets require accurate relative navigation because the observing spacecraft and resident object are both moving while appearing comparatively close within the local orbital frame. Errors in state estimation or propulsion execution can accumulate into larger miss distances than expected, which makes the interaction between sensors, flight software and propulsion particularly important.

The propulsion redesign tested that integration before hardware reached the final build stage. Avoiding structural and thermal changes suggests the existing spacecraft carried enough margin to accept the replacement system, while limiting software and harness modifications reduced the amount of requalification required elsewhere. York has not disclosed the exact margin consumed by the change or whether any reserves were reduced.

Clearing Delta CDR now allows the revised configuration to proceed through manufacturing, assembly and test ahead of the planned fourth-quarter deliveries. Delivery will still leave launch, commissioning and the on-orbit demonstration ahead, where the sensor suite and manoeuvre system will have to operate together around actual resident space objects rather than design-review models.

The programme’s more revealing milestone will come after deployment, when infrared imaging, LiDAR ranging, navigation and propulsion can be assessed as one proximity operations chain in GEO. For now, the review establishes that York has absorbed a consequential propulsion change without reopening the spacecraft architecture or losing its stated 2026 delivery schedule.