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Instrument

SIR-2 consists of three main subsystems, Instrument Sensor Unit (ISU), Power Supply Unit (PSU) and the Instrument Control Unit (ICU) which is designed by UiB.

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Instrument Sensor Unit (ISU)

The InGaAs PhotoDiode Array (PDA) is located in the ISU subsystem, the rest of the electroncis which is the PDA readout electronics, a 16-bit analog-to-digital converter (ADC), a microprocessor for data and command handling is located in the ICU.

Reflected sunlight enters the Optical Box (O-box) , see figure below. The O-box is locate outide the spacecraft with the lunar surface in its field of view. The reflected light form the surface is consentrated and sent via a fiber cable to the instrument Sensor Head (SH) where it is split by the dispesive grating to its diffeernt wavelengts and and detected by the linear PDA IR-sensor consisting of 256 pixels.The dispersed light passes through a quartz body and a second order filter glued to the detector window. After passing this window the light is absorbed by the photosensitive pixelated detectors, and the elctrons collected in each pixels are sequentially read out as voltages to the ICU electroncs after at defined and programable integration time (exposure time)



Instrument Control Unit(ICU)

The voltages for each of the ISU pixels are digitized by a 16-bit ADC in the ICU. The data obtained are finally embedded in the data packages that are sent to the spacecraft's mass memory for downloading to the earth.

To improve sensor stability the PDA temperature is actively controlled by the ICU built in controll algorithm and a Peltier cooler and heater system, and the exessive heat is radiated out to free space by the Radiator Unit (RU). The ICU also controls the power on/off of the SH so it only can be switched on when temeprature is not lower than -55 deg C. SH temperatures and other instrument essential temperatures are monitored and telemeterd as Housekeeping data embedded in the instrument dowlink data stream.

The ICU electronics in addition to instrument control also handles instrument commands through the 1553B MIL-BUS and the transfer of datapackages to the spacecraft LVDS signal interface. The core of the ICU electronics design is based on the ACTEL RTAX FPGA technology (anti-fuse) where all the system logics and the processing capacities are implemented. An FPGA embedded microcontroller core with a watchdog circuit controls the electronics and ensures the instrument autonomy. The instrument default system program resides in the EEPROM/flash memory which enables the possibility of having stored the latest and optimized program and front setup parameters for the instrument. The RAM allows general purpose storage of programs and data. The watchdog works as an automatic power-on reset, starting the system in standby mode. An overview of the ICU interfaces to the ISU (J3) and to the spacraft (J1) is shown in the figure below.

The ICU is located inside the spacecraft, and therefore more protected from cosmic radiation and temperature effects



Power supply unit(PSU)

PSU contains the DC/DC converters, ON/OFF relays, the slowstart, and the EMC filtering which is connected to the spacecraft power bus. The power supply unit also contains the HK parameter signal conditioning circuits with MUXes and an ADC (16 bit) for the HK parameter monitoring.

The PSU is a custom designed DC/DC converter which powers the various parts of the electronics and contains the interfaces to the instrument health and temp monitoring sensors.