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INTRODUCTION
Now days there are several navigation systems for positioning
the objects. Several research efforts have been carried out in the field of
Six Degrees Of Freedom estimation for rendezvous and proximity
operations. One such navigation system used in the field of Six Degrees
Of Freedom position and attitude estimation is the VISion based
NAVigation system. It is aimed at achieving better accuracies in Six
Degrees Of Freedom estimation using a more simpler and robust
approach.
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
Sensor geometry
We have discussed that Position Sensitive Diodes are used for
sensing purpose. The Position Sensitive Diode (PSD) is a single substrate
photodiode capable of finding or locating a light beam within defined
sensing area. When photons meet the PSD sensor active area electrical
currents are generated that flow through its four terminals. The closer
the incident light centroid is to a particular terminal, the larger the
position of current that flows through that load comparison of these four
currents determines the centroid location of the incident light
With regards to the above figure the normalized voltage are as
follows
Vy=k (Iright-Ileft) / (Iright+Ileft) 8(1)
Vz=k (Iup-Idown) / (Iup+Idown) 8(2)
Visnav
Where K is a constant value 1 ohm.
This equation (1) is an indication of the angle the incident light
beam makes about the object space X axis. Similarly equation (2) is
determined by the angle that the incident light beam makes about the
object space Y axis