Design and Flight Performance of the Orion Pre-Launch Navigation System

The Orion capsule, the successor of the Space Shuttle as NASA’s flagship human transportation vehicle, is designed to take men back to the Moon and beyond. The first Exploration Mission (EM1) is scheduled for 2018, while its first flight test, EFT-1 (Exploration Flight Test-1), was successfully completed on December 5th, 2014. The main objective of the test was to demonstrate the capability to re-enter the Earth’s atmosphere and achieve safe splash-down into the Pacific Ocean. This un-crewed mission completes two orbits around Earth, the second of which was highly elliptical with an apogee of approximately 5908 km, higher than any vehicle designed for humans since the Apollo program. The trajectory was designed in order to test a high-energy re-entry similar to those crews will undergo during lunar missions. In order to have a good navigation solution during entry, the navigation system operated during pre-flight operations, and during the entire flight, even when Orion was not controlling itself but under the control of the launch vehicle or the upper stage. Reference ? describes the navigation design of Orion’s EFT-1 mission and the flight performance for the post-lift phase. The objectives of this paper are to: i. introduce the pad align algorithm design of the Orion vehicle, both the Exploration Flight Test 1 design and the changes made in preparation for Exploration Missions 1 and 2, and ii. document the performance of the pre-launch navigation system during EFT-1, which relies on the classic extended Kalman filter (EKF). Reference 3 introduced the preliminary EFT-1 navigation design, while pre-mission simulation performance was shown in reference. 4. The UDU factorization as introduced by Bierman is employed in the filter design, and measurements are included as scalars employing the Carlson and Agee-Turner Rank-One updates. The possibility of considering only some of the filter’s states (rather than estimating all of them) is included in the design. Prior to launch the extended Kalman filter is initialized with the estimated vehicle’s attitude from gyro compassing (coarse align algorithm) and an inertial position derived from the current time and the coordinates of the pad. This pre-launch navigation phase is called fine align and the only measurement active in this mode during EFT-1 was integrated velocity, which is a pseudo-measurement consisting of a zero change of Earth-referenced position over a 1 second interval. The GPS receiver measurement are not available during fine align because the vehicle, including the GPS antennas, are covered by the launch abort fairing. The main purpose of fine align is to better estimate the attitude and the IMU error states.