The Reflector (Retro-reflector Ensemble For Laser Experiments, Calibration, Testing
& Optical Research) microsatellite is a passive reflective spatial reference test
object consisting of separate prism retroreflectors placed in reference points (nodes). It
will be used for retroreflection of laser radiation coming from a ground-based laser radar
to analyze and investigate the return signal structure for determination of spatial
(angular) resolution, testing, and identification of spacecraft which may be defined as
space debris. The REFLECTOR microsatellite was launched as a piggyback load on board of
the Meteor-3M 1 satellite.
REFLECTOR is a passive satellite who's only instrumentation onboard is a retroreflector
array. The REFLECTOR microsatellite is a system of 32 retroreflectors in holders placed in
defined node points of a spatial test object (see picture). Depending on the
microsatellite's long vertical axis orientation (the extendable beam may be pointed
towards the zenith or nadir), only one of the 16-retroreflector groups is actually working
at any moment. Because of spatial diversity of several groups of retroreflectors, there
will be several simultaneous tracks with range differences up to 1.365 meters.
The specifications of the retro-reflectors carried on REFLECTOR are:
- Fused silica prism, 1.4607 index of refraction at 532 nm
- Aluminum reflective coating on side facets
- Reflection coefficient at 532 nm is 0.62
- Input aperture = 28.2 mm × 16 mm (rectangular shape)
- Diffraction-limited angular extent of return pattern = 4 × 6.1 arcsec (FWHM)
- Acceptance angle approximately + 36 degrees (half-max reflected power)
The basic retro-reflector has a hexagonal shaped aperture of 28.2 mm diameter so the
diffraction-limited beam returned from one of these retros is a roughly symmetric pattern
of about 4.0-arc sec (FWHM) in angular extent. However, to address the effects of the
relativistic velocity aberration that dislocates the return beam along the orbital ground
path, the retros on REFLECTOR were modified to improve the efficiency of light returning
to the illuminating site. This was done by placing a 16 mm wide rectangular aperture
across the face of each retro. When the spacecraft is in orbit, the slits will be oriented
orthogonal to the orbit direction. The slits cause two effects - diffraction from the slit
elongates the pattern along the orbital direction (to about 6.1 arc sec, FWHM) but the
slit also limits the input acceptance angle to the retro. The net effect is an improvement
in energy returned to the illumination ground site but a limitation on the acceptable
illumination angles.
To make REFLECTOR more useful to the general laser ranging and remote sensing
community, the group of retros at the mid-section of the vehicle were modified to cause a
polarization change to the reflected light. Polarization sensitive measurements using
laser systems is a growing approach for sensing applications. In most cases,
retro-reflectors have little effect on the polarization of the incident light. No other retro-reflectors are currently in orbit that provide a specific polarization
signature to the returning light. For REFLECTOR, 1/4 wave plates were placed over the
entrance apertures of the 4 center retros. The fast axis of the 1/4 wave plate was aligned
with the vehicle's central body tube. Light returned from these retros will pass
through the 1/4 wave plate twice, once on the way in to the retro and once on the way out.
The polarization change imparted to the light will depend on the incident angle, which
will be a relatively complicated but predictable function of the vehicle orbit parameters
and the ground site position. Bench measurements with and without the wave plates in front
of a test retro showed no significant effects on the divergence of the return beam.
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