  This work addresses the challenge of developing a localization system for an
uncrewed ground vehicle (UGV) operating autonomously in unstructured outdoor
Global Navigation Satellite System (GNSS)-denied environments. The goal is to
enable accurate mapping and long-range navigation with practical applications
in domains such as autonomous construction, military engineering missions, and
exploration of non-Earth planets. The proposed system - Terrain-Referenced
Assured Engineer Localization System (TRAELS) - integrates pose estimates
produced by two complementary terrain referenced navigation (TRN) methods with
wheel odometry and inertial measurement unit (IMU) measurements using an
Extended Kalman Filter (EKF). Unlike simultaneous localization and mapping
(SLAM) systems that require loop closures, the described approach maintains
accuracy over long distances and one-way missions without the need to revisit
previous positions. Evaluation of TRAELS is performed across a range of
environments. In regions where a combination of distinctive geometric and
ground surface features are present, the developed TRN methods are leveraged by
TRAELS to consistently achieve an absolute trajectory error of less than 3.0 m.
The approach is also shown to be capable of recovering from large accumulated
drift when traversing feature-sparse areas, which is essential in ensuring
robust performance of the system across a wide variety of challenging
GNSS-denied environments. Overall, the effectiveness of the system in providing
precise localization and mapping capabilities in challenging GNSS-denied
environments is demonstrated and an analysis is performed leading to insights
for improving TRN approaches for UGVs.
