The SonarCE application is a real sonar based on Pocket PC device. It uses the internal Pocket PC sound system to detect echo from different obstructions. It is quite precise with error of 5-10 cm at distance of 10m in air. It can be use in air or in water but in case of water you need external microphone and speaker closed in water proof case. The main window toolbar contains the most important functions: send one chirp, sonar loop and analyze window. More advanced functions are hidden under settings menu. Additionally this program is equipped with testing functions which helps to test the device parameters crucial for the Sonar CE application. 

Physical background

The short sound (chirp) is sent by the speaker in interesting direction. When the sound “finds” on its way any obstruction, then is reflected and comes back to the device’s microphone. The device is in recording mode (after sending a chirp) and records the incoming sound wave. The recorded wave is correlated with the output signal (played sound data). If the recorded data have the same or similar course as played signal then the autocorrelation function returns maximum. Unfortunately the recorded signal is deformatted by the sound line (amplifiers, speakers, microphone and digitalization) and thus is not really the same. Additionally the correlation function itself generates sinusoid like course (due to its mathematical properties) thus the correlation data needs a post processing. The correlation function can be improved by played the sound with different frequencies within the played time (1). This make the autocorrelation function more sharp. The post processing function takes absolute value from the correlation function and reduces the output data to less points (230 in this case). Every point represents the maximum value from the window operating on correlation function. For example, if the correlation function has 2300 points and the data window is 10 points wide then the first point from post processing data represents the maximum value from the first 10 point (of the absolute values) of the correlation data. The absolute distance measured by any sonar depends on the time the chirp is played (longer time more sound energy is cumulated in the chirp), from sound amplitude and in small degree from chirp frequency (different absorption wave in different medium eg. low freq. waves can travel longer).