The Odin AOS instrument was built by a consortium of three French laboratories: the LAS space astronomy laboratory (Marseille), which is responsible for the qualification tests and the technical management of the whole instrument and is in charge of the AOS processor; the CESR space radiation research centre (Toulouse), which built the digital interface and power supplies sub-systems; and the Paris-Meudon observatory (ARPÈGES laboratory), which built the I.F. sub-system and is in charge of scientific management of the instrument.
AOS IF processor
This provides the following functions:
- selection of one from five available channels (four sub-millimetre and one millimetre bands). The channel bandwidth is 1 GHz centred at 3.9 GHz with an input level of - 60 dBm/MHz
- amplification and down conversion of the 3.9-GHz frequency band to 2.1 GHz, by frequency mixing with a phase-loop locked DRO at 6 GHz
- adjustment of the input level in 1 dB steps over a 15-dB range
- internal calibration of the AOS frequency response using a 100-MHz Comb generator (stability of 10-3 between - 30° and 70°C)
The technology used involves reliable, microwave components, including thin films on alumina substrates and underground lines.
AOS Acousto-Optical processor
The AOS processor is directly derived from an acousto-optical design from Meudon Observatory for ground-based instrumentation (Lecacheux et al., 1993). The AO processor uses a 780-nm laser diode (Hitachi HL 7851 G) driven by a constant current. The lithium niobate Bragg cell (from Thomson), after anisotropic diffraction of the light, provides about 1000 resolution elements over a bandwidth of 1 GHz. The photodetector is a 1,728-pixels, linear CCD (Thomson TH 7803). The whole AO processor is temperature stabilized over several, distinct functioning steps ranging between 10° and 30°C, in order to avoid variations due to short term (satellite platform) and long-term (ageing) temperature changes.
AOS data handling and interfaces
The CCD video output is readout every 5 ms and digitized over 12 bits, in order to keep the noise statistics of the signal unmodified over a dynamic range greater than 10 dB. A pre-adder delivers the sums of four consecutive read-outs to transputer (Thomson T805), and performs the final double-buffer integration in synchronism with the radiometer and antenna-pointing information. The transputer also performs instrument monitoring and manages communications with ODIN's on-board computer.
AOS instrument characteristics
All components are reliable and space-rated wit the exception of the laser diode and the Bragg cell. These two parts have been extensively tested and space-qualified for two specific programmes undertaken by CNES.
The main electrical and environmental characteristics of the instrument are:
|input frequency (5 channels)||3.9 GHz|
|spectral resolution||1 MHz|
|active thermal stabilization||±0.02°C|
|gain control||15 dB in 1 dB steps|
|internal frequency calibration||100 MHz Comb|
|stability (Allan variance)||100 sec.|
|power consumption (including thermal stabilization)||19.5 W|
|operating temperature||0° to 30°C|
The instrument has been extensively tested in the laboratory: all the measured spectrometric parameters (bandwidth and flatness, spectral resolution, amplitude stability, frequency accuracy and stability, etc.) greatly exceed the required specifications.