VISNAV

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.

Aerial Refueling

        The aim this application is to extend the operational envelop of unmanned aerial vehicles by designing an autonomous in flight refueling system. One of the most difficult technical problems in autonomous flight refueling is the accuracy. That is it needs high accurate sensor to measure the location of the tanker and the aircraft. Currently Global Positioning System (GPS) is limited by an accuracy of one foot approximately.

Dsp Implimentation

         The beacons are multiplexed in FDM mode. A low power fixed point DSP, TMS320C55x [2] is utilized for the algorithm of beacon separation and demodulation. Asynchronous analog to digital converter samples the sensor’s four currents to feed estimates to the TMS320C55x [2]. Each current has frequency components corresponding to the frequencies of different beacons. For the case of eight beacons the carrier frequencies are starting from 48.5 kHz with an interchannel separation of 0.5 kHz, in order to distinguish from low frequency background noise.

Demodulation

        Considering that it is needed to determine the amplitude of the sinusoidal signal and the associated signal due to the relative movement of the sensor, an approach similar to AM demodulation is used here. The main difference, however, is that we are also interested in the carrier amplitude. Although analog circuits can be used to perform the channel separation and demodulation, the DSP based approach provides a more cost effective solution with a higher degree of reliability, programmability and scalability.

Abstract

        Spacecraft missions such a spacecraft docking and formation flying requires high-precision relative position and attitude data. Although a global positioning system (GPS) can provide this capability near the earth, deep space missions require the use of alternative technologies. One such technology is the vision-based navigation (VISNAV) sensor system developed at Texas A&M university .J comprises an electro optical sensor combined with light sources or beacons. This patented sensor has an analog detector in the focal plane with a rise time of a few microseconds.

Conclusion

        A new method for operating beacons and demodulating the beacon currents for the VISNAV sensor system is introduced here. It is shown that target differentiation based on FDM yields higher signal to noise ratios for the sensor measurements and the demodulation in the digital domain using multirate signal processing techniques brings reliability and flexibility to the sensor system. The algorithm that is implemented on DSP is robust when there are four or more of line of sight measurements except near certain geometric conditions that are rarely encountered.