RAMS: A Miniature Ram Angle and Magnetic Field Sensor Pre-Flight Calibration Results

The popular and well-established concepts for satellite attitude sensing, including Earth horizon sensing, Sun sensing, geomagnetic field sensing, and star sensing have had almost no new revolutionary additions in decades. In this paper we introduce a new attitude sensing concept and prototype miniature sensor called RAMS (Ram Angle and Magnetic field Sensor). This novel instrument directly measures the in-situ 2-axis ram direction of a LEO satellite by collecting the incoming thermospheric flow field through a wide field of view entrance aperture, ionizing the neutral molecules in a thermionic cathode chamber, adjusting the kinetic energy of the charged molecules in an electric field, and measuring their impingement location in two coordinate axes on a quad detector. Coupled with its own built-in magnetometer, RAMS provides an estimate of the satellite's 3-axis attitude relative to the local orbit frame regardless of roll angle or sunlight conditions as long as the ram direction is within the field of regard of the entrance aperture. If available, an external Sun sensor can be utilized in place of the magnetometer during daylight conditions. As a by-product of interest to the space weather community, the sensor also provides a measure of the in-situ cross track winds and density of the thermospheric neutrals.

The RAMS sensor head has a mass of 23 g, occupies a volume of 22.5 cubic cm, and an expected power draw of ~0.5 W. The measurements consist of the two angles (α, β) defining the projection of the ram vector R relative to the x-axis of the sensor frame, and the components (Bx, By, Bz) of the local geomagnetic field vector B from the built-in magnetometer. The ram velocity is the vector sum of the satellite velocity vector V and the total wind vector W, our measurement accounts for the vector sum of V and the component of W perpendicular to V. We will describe in detail the sensor components and software modules necessary for estimating the horizontal wind angle and the satellite attitude from the instrument measurements. Additionally, we will provide both high-fidelity simulations and ground calibration from a flight RAMS unit highlighting its potential for sub-degree attitude estimation in a light-weight, low-volume, low-power, and low-cost package compatible with the extreme limitations of CubeSat-class missions.