Ultrasonic sensor SRF02 - fake ranging mode

Hi everyone,
I have been trying to measure flight time of a 40khz burst. I have two SRF02 sensors each connected to a serial line of my arduino mega, I have one that sends the burst (I see the led lighting up) and the second one which should be waiting for the burst.
The problem is that I always read 0 from the receiving sensor instead of the time passed from when I started the fake ranging and when he got the burst.
Anyone ever used these sensors in fake ranging mode?

Here is also my code:

void setup()

void loop()

Serial1.print(0x0, BYTE);//address of sonar
Serial1.print(0x58, BYTE);//enter fake ranging which just waits for a 40khz burst
Serial3.print(0x0, BYTE);
Serial3.print(0x5C, BYTE);//send the burst
Serial1.print(0x0, BYTE);
Serial1.print(0x5E, BYTE);//get the result of the fake ranging


Currently I am doing a project involving ultrasonic sensor.
It will trigger a video player to play playback when it detects an object within a specified range,250 cm.
When i put it on the wall and move closer to it,the ultrasonic sensor has no problem in detecting.
However,when I put it on the ceiling,the sensor does not seem to detect the object(human) when I move closer to it.

Do anyone know the reason why?