The Neutral Meter comprises two sensors that together measure the neutral wind velocity vector The component of the neutral wind along the sensor look direction are measured by a Ram Wind Sensor (RWS). The two mutually perpendicular components of the neutral wind perpendicular to the sensor look direction are derived from neutral gas arrival angle measurements made by a Cross-Track Wind Sensor (CTS)
The ram wind sensor (RWS) measures the kinetic energy of the neutral gas entering the sensor. It functions by ionizing a fraction of the incoming neutral beam and determing the energy of the beam by using a retarding potential analyzer. The instrument must be designed so that the neutral gas does not collide with any of the sensor walls prior to being ion ized. Only a small number of ions can can be produced in the flow-through source and thus a channel electron multiplier is used as the detector.
Ambient charged particles are prevented from entering the instrument by a pair of parallel deflector plates inside the entrance aperture. Neutral particles proceed uninhibited to the ion source. The ion source consists of redundent hot filaments enclosed in a magnetic field that produces a narrowly confined electron beam.
Passing through the electron beam, a fraction of the neutral gas is ionized to produce ions which have the same flow energy as the parent neutral gas
The ions pass through a series of grids that act as a planar retarding potential analyzer. The retarding grids have positive potentials applied to them which determine the energy of ions that have access to the detector.
For a satellite in orbit around the Earth in the ionosphere, the vehicle velocity is about 8 km/s. This velocity gives the ions an energy of about 1/3 eV per mass unit. Thus if the ions being detected are O+ (16 amu) these ions have about 5 volts of energy with respect to the sensor. Thus for a retarding potential of say 7 volts, no O+ ions will pass through the retarding grids , while for a retarding potential of say 3 volts, all the ions will pass through the retarding grids . On exiting the retarding grids the ions enter a large aperture before being deflected into a channel electron multiplier for detection. The channel electron multiplier is extremely sensitive to the impact of solar photons and thus the electrostatic deflection is necessary to ensure that the detector does not directly view the sun through the entrance aperture.
The precise flux of ions reaching the collector will depend, not only on the retarding potential, but also on the temperature and the ambient drift of the ions ( and thus the neutral gas) along the sensor look direction.
The ion flux is measured as a current at the output of the elecron multiplier. The precise functional form for the ion flux as a function of retarding potential is quite complex but sophisticated computer analysis can be used to locate the position at which the current is reduced by a factor of 2. Changes in the location of this point are used to determine the drift of the ambient neutral species.
The Cross-Track Wind Sensor presents four small entrance apertures to quadraspherical chambers in which the neutral gas pressure is allowed to build up due to the ram motion of the vehicle. Inside each chamber the neutral gas undergoes many collisions with the chamber walls and becomes thermally accomodated with the chamber. Then the enhanced pressure in the chamber is proportional to the ambient neutral density and the angle of approaching neutral gas with respect to the aperture normal.
The pressure inside each chamber is measured witha miniature ruiggedized Bayert-Alpert gauge similar in function to the devices conventionally used in laboratory vacuum systems.
A pressure equalization valve is activated periodically to equalize the pressure in all four chambers. Then the outputs of each gauge for equal pressures can be determined.
The arrival angle of the neutral gas entering the sensor is determined by noting that the pressure in each chamber is proportional to the ambient neutral density and the cosine of the angle of the incoming neutral density with respect to the aperture normal.