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  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
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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VISNAV is Vision Based Sensor and Navigation System for Autonomous Aerial Refueling
Now a 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.
VisNav Sensor Systems
The Texas A&M team has engaged in significant sensor and sensor fusion research over the past few years One focus of our sensors and sensor fusion work has been to address the demands of real-time, high resolution, and high reliability systems; we seek optimal integrated designs of advanced sensors, fusion algorithms, and computing architectures to solve particular set of problems. The integration of sensor design, sensor fusion algorithms and advanced computing results in a class of embedded systems we refer to as smart sensors. VisNav, our vision based navigation sensor suite has a special mention among our sensor results. StarNav, one of Texas A&M's other navigation solution is described here
VisNav came in to being, with a primary objective of enabling (anticipating a worst case scenario) a single sensor to provide full 6 degree of freedom relative navigation of two air vehicles â€œ with sufficient precision and bandwidth to enable the challenging goal of fully autonomous air refueling. A second objective was to achieve an extremely robust navigation solution. We have fully achieved these goals in ground demonstrations, and the flight experiment program is presently mid-stream. We mention that the small mass (<1 kg) and low power (< 20 watts) of this modular sensor design allows obvious paths to dual and higher parallel redundancy architectures..
Flight Test Demonstration Program :
To validate and demonstrate the Autonomous Air Refueling System (AARS), composed of the VisNav relative navigation sensor, and the Reference Observer Tracking Controller (ROTC) control laws, both developed by our team, we conducted the following flight test program. The program is being conducted from 3 April 2006 â€œ 31 October 2007 at the Flight Mechanics Laboratory, under sponsorship from FRL/MN and Boeing, under sub-contract to StarVision Technologies. Fight testing of the first vehicle began in June 2006, and is still proceeding. To date, more than 10 validation and verification flights of the tanker and receiver have been conducted. The first air-to-air docking of the tanker aircraft and the receiver aircraft, without human supervision or intervention, is scheduled for 2nd Quarter 2007.
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