studie:
My goal is to output x,y coordinates of the transmitter. The problem is how do I accurately track the position of the user? Could I use a Wifi module that sent time stamps and then calculated an accurate distance from that information? Is there any type of device that gives a tunable accurate range strength or angle? For example, the transmitter is 1 ft from the receiver and outputs a steady rss of 10.0. the transmitter is 20 ft from the receiver and outputs a steady rss of 1.0. Maybe there are other methods I’m not thinking of? If GPS can work from 40,000 at less than 5m accuracy, why isn’t it thousands of times easier to implement this method in a single room/building?
Thanks again,
Studie
In time difference of arrival methods (TDOA), aka multilateration, the relative range and bearing can be determined (not absolute). Problem is, TDOA requires a few nSec of precision and that's very costly, and to synch all receivers to that precision. So companies like Ekahau, Time Domain, Wherenet and others sell TDOA systems. Atomic clock references like GPS satellites use isn't done for these vendors' products. Instead, they share a synch signal, usually via cat5 to each fixed unit. And some jazzy nanosec precision pulse leading edge DSP chips. The challenge is in non-line-of-sight, where the direct path (attenuated) is not the strongest signal.
Simple RSSI (signal strength) in low power RF cannot provide, with ideal line of sight at all times, better than about 50 ft, due to the laws of physics: Inverse Square Law in RF propagation. And due to that math, the precision degrades related to the log of the distance. Happy it works that way so we can talk to satellites millions of miles away. If attenuation vs. distance was linear, we’d couldn’t.
Ultrawideband is useless at more than 50 ft or so due to the extreme low power rules from the FCC and the very high frequencies.
A way to do what you want: strong IR diode on the moving unit. Two or more IR-sensitive TV cameras. Digitize the TV rasters. Do the photogrammetry - focal length of lens, distance between cameras, location of diode’s signal within the 2D TV raster, etc. It’s been done. Best to have the IR diode emit an ID code in pulses, and the analysis software does an FFT on the raster to find the coded pulses amidst noise.
Another technique is ultrasonic transducers, but these are limited to about 20 ft.
Another (costly) method in use is magnetic flux fields: fixed units emit lines of flux. moving unit detects the flux lines and does the trig. Pulse coding tells the receiver which sender is which and there’s a priori knowledge of where the senders are.
One more: line-scanning lasers, immobile. Detect reflected pulse coded light from the target. Do the trig. Companies sell these for creating 3D line drawings of inside buildings, and the like.
It a’int easy, indoors. GPS does penetrate some one story buildings without too much multipath. But it’s marginal. Remember that GPS is like 20dB more sensitive after it finds the satellites (code correlates) and then goes into tracking mode. This is called post-detection correlation gain. If your GPS receiver can be “told” which satellites are overhead (and what codes they are using is implied by the satellite’s ID), it can warm-start easier. But there’s that catch-22 before code lock where much stronger signals are needed. Some good GPS receivers have clever assisted-GPS methods for this.
