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.
Description of VISNAV
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  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  for subsequent navigation solution by the use of colinearity equations. Among other competitive systems  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 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.