Distance between 2 arduino's using rf links - communication

Question

I currently have a setup where I send a char using a Tx of 434MHz and an Uno to a Mega with a Rx. The Mega counts how many times it receives the char and then if it falls below a certain number it triggers an alarm. Is this a viable way to measure the distance between two microcontrollers while indoors or is there a better way.
Transmitter (Mega)
#include <SoftwareSerial.h>
int rxPin=2; //Goes to the Receiver Pin
int txPin=5; //Make sure it is set to pin 5 going to input of receiver
SoftwareSerial txSerial = SoftwareSerial(rxPin, txPin);
SoftwareSerial rxSerial = SoftwareSerial(txPin, rxPin);
char sendChar ='H';
void setup() {
pinMode(rxPin, INPUT);
pinMode(txPin,OUTPUT);
txSerial.begin(2400);
rxSerial.begin(2400);
}
void loop() {
txSerial.println(sendChar);
Serial.print(sendChar);
}
Receiver
#include <SoftwareSerial.h>
//Make sure it is set to pin 5 going to the data input of the transmitter
int rxPin=5;
int txPin=3; //Don't need to make connections
int LED=13;
int BUZZ=9;
int t=0;
char incomingChar = 0;
int counter = 0;
SoftwareSerial rxSerial = SoftwareSerial(rxPin, txPin);
void setup() {
pinMode(rxPin, INPUT); //initilize rxpin as input
pinMode(BUZZ, OUTPUT); //initilize buzz for output
pinMode(LED, OUTPUT); //initilize led for output
rxSerial.begin(2400); //set baud rate for transmission
Serial.begin(2400); //see above
}
void loop() {
for(int i=0; i<200; i++) {
incomingChar = rxSerial.read(); //read incoming msg from tx
if (incomingChar =='H') {
counter++; //if we get bit "h" count it
}
delay(5); //delay of 10 secs
}
Serial.println(incomingChar);
Serial.println(counter); //prints the the bits we recieved
if(counter<55) {
//if we receive less than 100 bits than print out of range triggers alarm
Serial.println("out of range");
tone(BUZZ,5000,500);digitalWrite(LED,HIGH);
}
else {
noTone(BUZZ);digitalWrite(LED, LOW);
//if we get more than 100 bits than we are within range turn off alarm
Serial.println("in range");
}
counter = 0;
incomingChar=0;
}

Answer 1

In theory you could achieve distance measuring by making the uno send a message which the mega would echo back. That would give the uno a round-trip time for message propagation between the arduinos. You would have to approximate the processing delays. After that it is basic physics. That is basically the same as how radar works. The actual delay would be something like
troundtrip = tuno send + 2*tpropagation + tmega receive + tmega send + tuno receive
I am guessing the distance you are trying to achieve is in the order of meters. Required resolution is going to be an issue, because s = vt => t = s/v, where s is the distance between your arduinos and v = c in case of radio waves. As the transmission delays should stay constant, you have to be able to measure differences in the order of 1/c second intervals, basically. I am not very familiar with arduinos, so I do not know if they are capable of this kind of measurements.

Answer 2

I would suggest you use an ultrasonic range finder like the Maxbotix HRLV-EZ4 sold by Sparkfun.
It is within your price range and it should be able to measure distances up to 5m/195 inches with 1mm resolution.

Answer 3

It is actually possible to do it, I have seen it be done with other microcontrollers. Therefore using arduino you would have to solve the equations,fit in arduino language and make a lot of measurements to value discrepancies over communication itself. Do not forget about atmospheric attenuation wich need to be known and fit in the equations. Humidity may deviate electromagnetic waves.