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Definition
The VISNAV system uses a Position Sensitive Diode (PSD) sensor for 6 DOF estimation.
Output current from the PSD sensor determines the azimuth and elevation of the
light source with respect to the sensor. By having four or more light source called
beacons in the target frame at known positions the six degree of freedom data
associated with the sensor is calculated. The
beacon channel separation and demodulation are done on a fixed point digital signal
processor (DSP) Texas Instruments TMS320C55x [2] using digital down conversion,
synchronous detection and multirate signal processing techniques. The demodulated
sensor currents due to each beacon are communicated to a floating point DSP Texas
Instruments TMS320VC33 [2] for subsequent navigation solution by the use of colinearity
equations. Among other competitive systems
[3] a differential global positioning system (GPS) is limited to midrange accuracies,
lower bandwidth, and requires complex infrastructures. The sensor systems based
on differential GPS are also limited by geometric dilution of precision, multipath
errors, receiver errors, etc.These limitations can be overcome by using the DSP
embedded VISNAV system
FACTORS AFECTING
MEASUREMENT There is likely to be a large
amount of ambient light at short wavelength and low carrier frequencies due to
perhaps the sun, its reflections, incandescent or discharge tube lights, LCD and
cathode ray tube displays etc. In many cases this ambient energy would swap a
relatively small beacon signal and the PSD centroid data would mostly correspond
to this unwanted background light.
In order to avoid this problem by
modulating the beacon controller current by a sinusoidal carrier of high frequency.
The resulting PSD signal currents then vary sinsuoidally at approximately the
same frequency and have to be demodulated to recover the actual current proportional
to the beacon light centroid. This modulation or demodulation scheme leads high
degree of insensitivity to variations in ambient light and it is a key to make
the PSD sensing approach practical.
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