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Name: eb
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993


Question:
I am working on a autonomous robot and I need to figure out a way to triangulate its position remotely. The robot has an on-board data acquisition and signal processing capabilities (d/a a/d, and other assorted goodies). The environment is a 100 ft by 100 ft lunar surface (simulated) and we need an accuracy of about 3 cm. Internal mapping is a possibility, but we are looking for a way to establish a external reference. One idea was to set up three radio beacons at different frequencies and measure the power received. We could take the FFT of this and measures its strength, hoping the power will drop off as a function of 1/d^2. We experimented with this idea using a RF generator and a spectrum analyzer, but there was not a constant radiation pattern from our antenna (no 1/d^2 relationship was observed). The room was full of other electronics that may have disturbed our experiment. Are we going in the wrong direction? I have looked into many triangulation schemes, but ours seemed the simplest. Before we invest the money to perform the experiment outside of the room that was full of electronics I wanted a professional opinion. I am well aware of the many variables in this situation (antenna directivity, atmospheric losses, VSWR problems, etc.).



Replies:
You are right, there are many ways to do this. The way you are doing this may not be that far off the track. Common Radio telemetry nowadays, though, is done by triangulation, not by relative field strength. With a single or multiple receivers, you can detect the angular change of the strongest portion of the NDB (Non-Directional Beacon) and by simple calculations determine its current position and the incremental vector of movement. My suggestion, is to use a directional receiving antenna connected to an angular shaft encoder. Rotating the antenna and noting peak signal strength at the various frequencies can give you position very well. This is the basic principal of LORAN devices used today. If you close the position loop, you can control the motion on the device. To be very, very accurate, however, I suggest using an inertial guidance device internal to your unit, and use the LORAN type radio-telemetry for reference setting and periodic checking of position. I hope this helps. Without knowing all of the other parameters, I may be way off base.

dipper



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