Velocity

2013

NovAtel's Annual Journal of GNSS Technology Solutions and Innovation

Issue link: http://velocitymagazine.epubxp.com/i/164724

Contents of this Issue

Navigation

Page 15 of 51

Little Bird " With a GNSS/INS system, the INS can 'coast' through periods of GNSS signal blockage or degraded GNSS solution quality. " RTK algorithms solve for the position-offset vector from the base to the rover receiver. The base receiver does not have to be stationary, and it does not need a highly accurate known coordinate if the only quantity of interest is the relative displacement of the rover with respect to the base. An algorithm used with two GNSS receivers that do not move with respect to each other—a fixed-baseline RTK implementation—can solve for the heading and pitch of the fixed baseline. This algorithm can also be used with two receivers that are moving with respect to each other— a moving baseline implementation. In this case, the base receiver obtains a single-point (autonomous) GNSS position solution and transmits code and carrier phase corrections to the rover based on that position. The rover then uses those corrections to compute the vector from the base to itself, resulting in an RTK-quality solution between the two receivers, even though the absolute position solutions for the two receivers are only of single-point quality. The moving baseline RTK solution has the same benefits and drawbacks as a fixed baseline RTK solution. The main benefit is a very precise relative solution because the distance between the base and rover is quite short. The drawbacks are the usual challenges of requiring constant communication between the rover and the base, as well as maintaining enough common satellites in view during the landing maneuvers as the helicopter approaches the ship deck. An inertial navigation system (INS) is typically added to a GNSS solution to address issues such as these. With a GNSS/INS system, the INS can "coast" through periods of GNSS signal blockage or degraded GNSS solution quality. An INS provides good relative accuracy over time, allowing it to "hang onto" a high-accuracy solution. VTOL UAV & Relative Navigation Methodology The UAV VTOL application requires continuously precise and accurate relative positioning of 16 velocity 2013 the helicopter and the ship. The solution implemented on the Little Bird uses GNSS positioning and inertial navigation and is a modernization of a system previously demonstrated in 2005. Both the ship and the helicopter were outfitted with SPAN-SE-dual antenna GNSS/INS receivers. The relative RTK solutions was provided by NovAtel's ALIGN® algorithm. The accompanying sidebar describes the evolution of the integrated navigation solution to achieve the relative navigation used in the Little Bird tests. In landing a helicopter aboard a moving ship, the quality of the attitude solution on the ship's system plays the most significant role in determining the overall relative accuracy. The ship's GNSS/INS system is mounted in a convenient location away from the landing pad, but the landing pad is the true point of interest. Similarly, the landing gear is the point of interest on the helicopter, not the location of the inertial measurement unit (IMU). Both GNSS/INS systems must project their solutions from the IMU to the point of interest. To implement this coordinate projection, the offset vector from the IMU must be measured in the IMU frame, and the rotation matrix between the IMU reference frame and the GNSS's Earth-centered Earth-fixed (ECEF) frame must be known. The accuracy of the solution at the point of interest therefore depends on the quality of the measured offset as well as the quality of the rotation matrix from the IMU frame to the ECEF frame. This rotation matrix is maintained as part of the INS solution. The quality of the rotation matrix is very dependent on the quality of the initial INS alignment (i.e., finding the IMU's orientation with respect to gravity and north), and the overall convergence of the GNSS/INS solution. The longer the offset vector is to the landing pad, the larger the effect of the rotation matrix errors (i.e., a classic pointing error in survey terminology). Attitude errors in GNSS/INS are best observed with vehicle dynamics. In particular, horizontal accelerations allow the azimuth error to be observed and controlled. Depending For more Solutions visit http:/ /www.novatel.com

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