Investigating the integration of a foot-mounted IMU and GNSS antenna

Low cost MEMS sensors have been shown to provide high accuracy positioning when mounted on a user´s foot through the use of zero velocity updates (ZUPT) every time the user takes a step. Although position drift is greatly reduced using ZUPTs, position errors will still accumulate over time, particularly due to heading errors which are weakly observable through zero velocity updates. Measurements from GNSS can be used to restrict position drift; however, for good GNSS reception, the GNSS antenna is usually placed on the user´s back which results in a non-constant separation between the GNSS antenna and IMU. This reduces the effectiveness of GNSS measurements since the lever arm is unknown. Instead, it is much better for IMU integration if the IMU and GNSS antenna are collocated. This is because the GNSS receiver and IMU experience the same dynamics, and the GNSS measurements can be used directly to correct the IMU. This should improve INS error observability, however, the antenna position results in a significantly compromised view of the sky. This paper examines the use of GPS and GLONASS measurements from a foot mounted antenna and explores two different methods for integration with the foot mounted IMU. The first method uses GNSS derived course over ground measurements to reduce INS drift, and the second tightly integrates carrier phase measurements and attempts to resolve ambiguities. It is shown that the combination of a MEMS IMU with GNSS carrier phase measurements can be used to maintain a centimeter level accuracy trajectory even though the sky view is compromised.