Finding SEIS North on Mars: Comparisons between SEIS sundial, Inertial
and Imaging measurements and consequences for seismic analysis
Abstract
In this paper, we present the results obtained in the determination of
the true North direction on Mars by using a gnomon on the InSight
mission and compare the measurements with either the North determination
from the Inertial Measurement Unit and Imaging analysis. The obtained
measurement has been use to populate the SEIS orientation information in
the archived SEIS data. Images taken during December 2018 and January
2019 allows to determine the gnomon shadow position and length over a
target. By calculating the Sun local coordinates using planetary
ephemeris VSOP87, the images are used to estimate the true North
direction on the landing site. By using eight different images, we
obtain the true North direction with an accuracy up to
$2.5^{\circ}$, which is confirmed by the IMU and
Imaging analysis. The true North direction is also confirmed by an image
taken near local noon, when the sun crosses the meridian. The North
determination precision is then discussed in view of the seismic
determination of the back-azimuth.