Ping Length
The signal sent by the ROVL transmitter is called a "ping." It is a burst of sound which is kept to the minimum practical length in order to mitigate multipath effects. ROVL Mk II transmitters, ROVL Mk III transponders, and ROVL Mark III transceivers all transmit pings.
There are two types of ping:
A basic ROVL ping (one that does not carry an ID nybble).
A basic ROVL ping plus an ID (one that carries an ID nybble).
The ping is sent at 25kHz. In water, the speed of sound is about 1500 meters per second but varies depending on the water temperature, pressure, and salinity. The ROVL defaults to assuming 1484 meters per second (but can be easily configured to assume something different). This makes a single cycle of the ping measure about 60mm in length.
A basic ROVL ping is twelve cycles long. Thus, the length of a basic ping in water is about 710mm (0.71m).
An ROVL ping with ID nybble normally 108 cycles long. Thus, the length of a ping with ID in water is about 3560mm (3.56m).
All ROVL pings are generated by piezoelectric transducers which are resonant near the ping frequency. The transducers reverberate for a few cycles after excitation stops, so if you put a hydrophone in the water you will see a few extra cycles beyond what is stated above. However, the extra cycles are ignored by the receivers, so they do not come into play when thinking about interference.
Considering this section and the previous section, it should be clear that multipath will be a bigger issue when using pings with IDs than with baseline pings, because the receiver and transmitter need to be farther away from reflectors before there is no chance of interference.
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