Simulation for MSS-2 low-perigee elliptical orbit satellites: an example of lithospheric magnetic field modelling

A future constellation of at least four geomagnetic satellites (designated Macau Scientific Satellite-1 (MSS-1) and Macau Scientific Satellite-2 (MSS-2)) was recently proposed, to continue high-quality geomagnetic observations in the post-Swarm period, focusing especially on collecting data that will provide a global, three-dimensional survey of the geomagnetic field. In this paper, we present a simulation of two years of orbits (2020.01.01-2022.01.01) of two satellites (tentatively denoted as MSS-2) that are constellated in elliptical (200 x 5,300 km) low-perigee orbits. By comparing error variances of Gauss coefficients, we investigate the sensitivity of lithospheric magnetic field modelling to data collected from various satellite orbits, including a near circular reference orbit of 300 x 350 km, and elliptical orbit of 180 x 5,300 km, 220 x 5,300 km, 200 x 3,000 km and 200 x 1,500 km. We find that in two years the two MSS-2 satellites can collect 35,000 observations at altitude below 250 km, data that will be useful in advancing the quality of lithospheric magnetic field modelling; this number of observations reflects the fact that only 4.5% of the flight time of these satellites will be below 250 km (just 6.4% of their flight time below 300 km). By combining observations from the MSS-2 satellites' elliptical orbits of 200 x 5,300 km with observations from a circular reference orbit, the variance of the geomagnetic model can be reduced by a factor of 285 at spherical harmonic degree n = 200 and by a factor of 1,300 at n = 250. The planned lower perigee of their orbits allows the new satellites to collect data at unprecedentedly lower altitudes, thus dramatically improving the spatial resolution of satellite-derived lithospheric field models, (up to 80% at n=150). In addition, lowering the apogee increases the time interval during which the satellites fly at near-Earth altitudes, thus improving the model predictions at all spherical harmonic degrees (around 52%-62% at n = 150). The upper limit of the expected improvement to the field model at the orbital apogee is not as good as at the perigee. However, data from the MSS-1 orbit can help fill the gap between data from the MSS-2 orbits and from the circular reference orbit for the low-degree part of the model. The feasibility of even lower-altitude flight requires further discussion with satellite engineers.