One challenge driving Mars entry, descent and landing technology development is to land at higher elevation sites without decreasing the horizontal landing accuracy relative to that achieved in the 2012 Mars Science Laboratory (MSL) Mission. Higher elevation landing capability would allow missions to the southern highlands. A combined multi-mode guidance and navigation strategy for high elevation landing is developed and tested, building on previous work on reference trajectory planning and tracking and on final position alignment. A new enhanced inertial navigation strategy is investigated to reduce the state knowledge error especially for shallower, longer duration entries.

The new contributions to the high elevation entry guidance scheme are computationally efficient reference trajectory updating requiring the optimization of a single parameter at a time and improved downrange performance in the final position guidance. To reduce the error in the estimated vehicle state on which the guidance law operates, unscented Kalman filter (UKF)-enhanced inertial navigation is proposed and tested. The UKF-enhanced inertial navigation scheme accounts for the nonlinearity of entry dynamics and factors in the modeling errors.

The performance of the multi-mode guidance algorithm, combined with the inertial navigation system, is assessed for an MSL-type entry vehicle. Using the simulation testing, the performance with realistic levels of vehicle modeling, atmospheric density, inertial sensor, and entry delivery errors is characterized. A range of nominal entry flight path angles is considered, encompassing the MSL value as well as shallower entries that might be considered for future missions to reduce the peak acceleration and heat rate to which the vehicle is subjected. The results show that, using the multi-mode guidance and conventional inertial navigation, landing elevations as high as 1.5 km relative to the Mars Orbiter Laser Altimeter (MOLA) reference are achievable for the steep entry with -14 deg initial flight path angle. For shallow entry with -12 deg initial flight path angle, the multi-mode guidance with UKF-enhanced navigation can achieve the landing elevations above 0 km MOLA for 99\% of the cases. For all the tested entry flight path angles, the requirement of the horizontal accuracy within 10 km has been achieved by the guidance and navigation system. For 1-2 km elevation landing, as required to access more of the southern highlands, the IMU-based navigation error is too large and modifications, possibly further improvement in the guidance and navigation algorithms or the addition of navigation sensors, would be required.