Making Things Talk

HOW TO LOCATE (ALMOST) ANYTHING 275

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/*

Once you've got a packet, you need to extract the useful data.

This method gets the address of the sender and RSSI.

*/

void parseData(int[] thisPacket) {

// read the address -- a two-byte value, so you

// add the two bytes as follows:

address = thisPacket[5] + thisPacket[4] * 256;

// get RSSI:

rssi = thisPacket[6];

}

Radio signal strength is measured in decibel-

milliwatts (dBm). You might wonder why the

signal reads –65dBm. How can the signal strength be

negative? The relationship between milliwatts of power

and dBm is logarithmic. To get the dBm, take the log of

the milliwatts. So, for example, if you receive 1 milliwatt of

signal strength, you’ve got log 1 dBm. Log 1 = 0, so 1 mW =

0 dBm. When the power drops below 1 mW, the dBm drops

below 0, like so: 0.5 mW = (log 0.5) dBm or –3.01 dBm.

0.25mW = (log 0.25) dBm, or –6.02 dBm. If logarithms

confuse you, just remember that 0 dBm is the maximum

transmission power, which means that signal strength

is going to start at 0 dBm and go down from there. The

minimum signal that the XBee radios can receive is –92

dBm. Bluetooth radios and Wi-Fi radios typically have

a similar range of sensitivity. In a perfect world with no

obstructions to create errors, relationship between signal

strength and distance would be a logarithmic curve.

Figure 8-5

Output of the XBee RSSI ...