Method for determining a distance between two nodes

1. A method performed by a first node for determining a distance between the first node and a second node wherein said method comprises:

time stamping a data packet to be transmitted from the first node to the second node with a first time stamp,

transmitting the data packet to the second node,

receiving the transmitted data packet back from the second node,

estimating the distance between the first node and second node based on a round trip time between the first node and second node for the data packet,

determining a number of repetitions for a repeating step based on an estimated distance,

repeating the transmitting step and the receiving step according to the number of repetitions determined in the determining step,

time stamping a last received data packet from the second node with a second time stamp, and

calculating the distance between the first node and the second node based on the first time stamp and the second time stamp, the number of repetitions of the repeating step, and internal delays in the first node and the second node,

wherein the number of repetitions of the repeating step decreases when the estimated distance increases, i.e., the shorter the estimated distance is the more repetitions are performed.

2. The method according to claim 1 , wherein the number of repetitions is in a range of 2 to 10000 repetitions, preferably between 10 to 1000 repetitions.

3. The method according to claim 1 , wherein the step of calculating the distance between the first node and the second node comprises:

determining an overall time between the first time stamp and the second time stamp,

subtracting accumulated internal delays in each of the first node and the second node from the overall time to obtain a total travelling time, and

dividing the total travelling time with two times the number of repetitions to obtain an average time for the data packet to travel from the first node to the second node.

4. A first node for determining a distance between the first node and a second node, wherein the first node comprises a processing unit, a radio transceiver, an antenna, a battery, and a memory, the memory comprising instructions which when executed by the processing unit causes the first node to:

time stamp a data packet to be transmitted from the first node to the second node with a first time stamp,

transmit the data packet to the second node,

receive the transmitted data packet back from the second node,

estimate the distance between the first node and the second node based on a round trip time between the first node and the second node for the data packet,

determine a number of repetitions for a repeat step based on the estimated distance,

repeat the transmit and the receive steps according to the determined number of repetitions,

time stamp a last received data packet from the second node with a second time stamp, and

calculate the distance between the first node and the second node based on the first time stamp and the second time stamp, the number of repetitions of the repeating step, and internal delays in the first node and the second node,

wherein the first node is further caused to decrease the number of repetitions of the repeating step when the estimated distance increases, i.e., the shorter the estimated distance is the more repetitions are performed.

5. The first node according to claim 4 , which is further caused to repeat the repeating step up to 10000 repetitions, preferably between 10 to 1000 repetitions.

6. The first node according to claim 4 , which is further caused to:

determine an overall time between the first time stamp and the second time stamp,

subtract accumulated internal delays in each of the first node and the second node from the overall time to obtain a total travelling time, and

divide the total travelling time with two times the number of repetitions to obtain an average time for the data packet to travel from the first node to the second node, in order to calculate the distance between the first node and the second node.

7. The first node according to claim 4 , wherein the processing unit further comprises a system clock for generating a time base, said system clock being unsynchronized with a system clock in a processing unit of the second node.

8. A computer program comprising computer program code, the computer program code being adapted, if executed by the processing unit of the first node, to implement the method according to claim 1 .

9. A non-transitory computer readable storage medium storing the computer program according to claim 8 .