Aliasing and clutter

In the rapid pulse mode, the maximum detection range for the sonar system is limited by the interpulse interval. If an object has an echo time that is greater than one pulse period, it is detected by the system in the next pulse cycle. It is then perceived as having an echo time that is one pulse period less than it actually is. Since this distortion is caused by sampling related to each pulse, we call it aliasing. This is demonstrated in Figures ​Figures3,3, ​,4.4. While the perceived direction of this “phantom” object is unchanged, the range is wildly incorrect and may even overlap the echo from the tracked target. The techniques presented in this paper aim to keep the range and angle measurements of the target clean. This can be done by keeping other echoes far enough away (in time) to not overlap the target echo (~ 0.5 ms). If that is not possible, the goal is to reduce the amplitude of the obstructing echo as much as possible.

Aliasing visualized. In this cartoon example, each timeline has pulses (represented by tall lines) and received echoes (represented by shorter lines). Each pulse and its echoes are given a unique color. From top to bottom, the interpulse interval decreases until a new pulse occurs before all echoes from the previous pulse are received, shown in the bottom timeline. The echo is misinterpreted as a closer object associated with the latest pulse. This is the aliased echo, and is labeled with an asterisk.

Transducer envelope of pulses and echoes at different repetition rates demonstrating aliasing in the bottom graph. The outgoing pulse peaks at 0.4 V, overlapping echoes from two closely-spaced PVC pipes are seen peaking at 0.2 V, and a single loud echo made by a square poster board is seen peaking at 0.25 V. The interpulse interval is decreased in each graph until a new pulse occurs before all echoes from the previous pulse are received, causing an aliasing condition where the poster board incorrectly appears at short range.

Two strategies specific to problem of aliased echoes overlapping the target echo are presented: First, by using an adaptive delay, the interpulse interval can be manipulated to change the relative time of the aliased echo. This changes the perceived range of the alias to prevent it from overlapping with the target. Second, the sonar system can use movement to prevent objects in the background from falling in the main path of the sonar beam. This reduces the magnitude of clutter echoes.

These strategies may not always work, particularly if the aliased object is close in range to the target and the sonar beam is too wide for the movement strategy to avoid illuminating the aliasing object. In this case, beam forming of the transmitted pulse by firing both transducers in a phased manner can be used to increase the amplitude of the target echo and decrease the amplitude of the aliased object echo. This can also be effective in non-aliasing situations where a distractor object at the same range (but different angle) is causing interference.