Terrain Relative Navigation (TRN) System for Landing Applications [SPLICE subsystem]

With the focus of future human spaceflight missions shifting back to landing on the surface of the moon, Draper recently achieved a milestone in demonstrating that its terrain relative navigation (TRN) system is a viable technology and candidate system that can help return humans to the moon. On September 11, 2019, Draper demonstrated its TRN system onboard the Xodiac lander vehicle, developed and provided by Masten Space Systems, Inc., Mojave, Calif. The flight demonstration was carried out as part of the Safe and Precise Landing Integrated Capabilities Evolution (SPLICE) project led by NASA’s Johnson Space Center and funded by NASA’s Flight Opportunities (FO) Program. Using an independent package comprised of two cameras, an inertial measurement unit (IMU), barometer and onboard computer, the TRN system successfully identified features located around the launch site and continuously at 300 feet in altitude, and maintained an accurate navigation solution to the landing. 

What is terrain relative navigation? 

Returning humans to the moon requires vision-aided navigation systems, such as the Draper TRN system, because such systems enable complete autonomy from deorbit insertion to landing, according to Matthew Fritz, space systems engineer at Draper and the principal investigator for the TRN flight demonstration. “Terrain relative navigation is a method of determining the state of a vehicle relative to the local terrain as viewed through an onboard camera. By overlaying images captured during flight onto preloaded satellite maps with known features identified, the TRN system can map features from the captured image to those in the satellite map and determine its location relative to these features,” Fritz said. Draper has developed and field tested its TRN technology during the past decade with the primary application focused on higher altitude trajectories. For lunar applications, the TRN system will support vehicle navigation to just before landing and over more aggressive trajectories. In order to achieve this mission, Flight Opportunities secured a terrestrial flight demonstration onboard the Xodiac lander vehicle to provide a trajectory similar to what would be expected during a lunar landing mission. Demonstrating the TRN system on a platform similar to what is expected for future missions allows for valuable data collection that will be used to further mature the TRN system with the ultimate goal of advancing its technical readiness level. 

Flight Performance 

During the September 11 flight demonstration, the Draper test team set out to determine the performance of the TRN system in terms of 1) lower altitude limitation and 2) vehicle state estimation accuracy via a test flight on Masten’s Xodiac launch vehicle. The TRN system successfully matched known Masten Xodiac launch vehicle with Draper TRN system ground features above an altitude of approximately 300 feet. Draper expected this outcome based on the performance of the TRN system on previous higher altitude test flights for other programs. Once below the 300 feet altitude, the TRN used visual odometry to track unknown features in the camera image from frame to frame. 

In considering vehicle state estimation accuracy, the TRN’s largest error during the flight demonstration was approximately 30 feet in magnitude off the “truth” state (onboard GPS measurement), which occurred near the apogee of the trajectory. Position error at landing was less than 10 feet, which was better than expected. It was determined that the error continued to decrease based on a strong TRN measurement just prior to the TRN system switchover to visual odometry, which led to the more accurate state estimate at landing. 

Technology Maturation 

The flight demonstration served as the next step in developing a TRN for an extraterrestrial landing, Fritz said. “The Draper TRN system was originally developed to target more benign, Earth-based applications and has achieved a higher TRL for this application. However, for space applications, the TRL is not as advanced. While the algorithms generally remain the same for space applications, there are unknowns ranging from hardware interactions and restrictions, to software updates, that are expected for space flight. By using flight opportunities on similar vehicles, Draper is able to identify and account for the unknowns and advance the TRN and its technical readiness level as we target future space flight opportunities.” 

Funding for the flight demonstration of the TRN system on the Masten Xodiac vehicle was provided by NASA’s Flight Opportunities (FO) Program. FO provides funding that allows for technology advancement by pairing system developers with flight providers, such as Masten. “Opportunities to test on established flight platforms are rare,” Fritz said. “Testing the TRN system on a flight-like vehicle and demonstrating a real-world application has provided Draper an opportunity to validate assumptions used during algorithm development.” 

Going forward, the Draper test team will use the results from the flight test to make updates to both software and hardware before targeting additional terrestrial test flight platforms to further evaluate their TRN system. With these upcoming flight test opportunities as well as performing algorithm testing using third party surface rendering technologies, Draper plans to have the TRN system ready for deployment by 2021.