Data Processing Unit
The MENA DPU consists of a single 16-bit Harris RTX2010 microcontroller operating at 4.91 MHz, look-up tables used to process the raw data, a low-voltage power suppy, a high-voltage controller, and, for each of the three sensors, the front end time-of- flight and pulse height electronics and high-voltage power supplies. The DPU communicates with the sensor heads and the Central Instrument Data Processor (CIDP). It monitors instrument health and safety and receives and processes the raw sensor data, producing one image every 2 minutes (i. e., each spacecraft spin period). It transmits this image, together with a selection of the raw sensor data, event rate data, and housekeeping data, to the CIDP for downlink to Earth.
How MENA Works
The time of flight for each ENA detected is determined by the front end electronics from the start and stop pulses triggered in the MCP. This value, together with the positions of the start and stop pulses on the MCP, is processed by the look-up tables to compute the incidence angle of an incoming ENA, the length of its path through the detector, and its velocity. The amplitude of the start and stop pulses -- their "pulse height" -- is also measured. This information can in principle be used by the look-up tables to estimate the mass of the incident ENAs and, together with the computed velocity, their energy. Because the pulse height distributions for hydrogen and oxygen are broad and overlap one another, however, species (mass) discrimination on the basis of the pulse height will be challenging. In addition to the raw sensor data, the DPU receives information on the spacecraft spin phase from the CIDP. Knowledge of both the spin phase and the incidence angle is needed to determine the position in the sky from which the detected ENAs are emitted and to produce the image of the ENA emission region.