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How can you approximate distance using RF?

I want to get an approximate distance between an RF transmitter and RF receiver . Which one of the following methods would work : 1. Count how many successful RF transmissions are received per second - that means there is a linear relationship between successful transmission and distance. 2. Adjust the auto-gain of the RF receiver until you get correct RF transmissions - means there is a linear relationship between gain value and distance 3. Somehow get a signal strength output pin on the RF module - any one know which RF module can do that? Is there a better way to do it? And also can someone verify those methods above

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1. You can get a reasonably accurate measurement using microwave or VHF to transmit short pulses, and receive the passive or active reflections of these. This is called primary or secondary radar, but there are specific systems based on this. Basically this measurement needs some reference in time in the signal, so you can measure the forward and return time using a transponder that returns the signal to the sender. The sender knows when a particular part of the signal was sent, so it can measure the time it took in microseconds, calculate the distance. The time through the transponder is subtracted. This method is used with aircraft and is known as DME (first link below). It is accurate to about 200m. A variation of this is to sweep the frequency and measure the phase of the returned modulation. That helps eliminate reflections that can confuse the pulse method. This is a tellurometer, accuracy maybe 0.1m, now obsolete and replaced by laser equivalents, also being made obsolete by satellite methods. GPS uses another variation. The time is sent with the signal, so that the receiver has a precision time reference. A code in the signal is used to tell the receiver when it was sent. Sophisticated processing involving reference stations and corrections for bending of waves, relativistic effects gives it a higher accuracy. GPS also compares the signals sent by several satellites, coordinated by the precision clock. The common method to locate a transmitter is to transmit its position from a GPS receiver attached to it. GPS also gives an accurate time reference that could be used to time the signal from a remote transmitter, but why bother. Another satellite method is to measure the doppler shift of the satellite signal. That determines when the spacecraft is passing over, so if the orbit is accurately known the position on earth can be determined. About the methods: Method 1. Can only work if there is some sort of precision time or position or angle reference. With time either the signal is returned by transponder or passive reflection. If transmitters in different locations send a pulse at known times exactly the same time, the receiver can measure the time difference and triangulate. Not easy to synchronise those transmitters though they could send signals between each other somehow, or use a GPS. The receiver can also triangulate off several signals using some kind of directional antenna to triangulate. Method 2 and 3 are basically the same methods. The auto gain control signal is an analogue of signal strength, with reservations. There are methods to accurately measure received signal level, and this may give some vague idea of how far the transmitter is. In free space it is more meaningful, because the signal level can be predicted with some accuracy. However, if both points are near the ground this is never so. The signal is subject to interference patterns (multiple reflections cancel or add to the signal), so the level is unpredictable. The variation is typically up to 40 dB.