Page 441 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

434 CHAPTER 16 Acoustic Positioning
submersible, it would show three different distances—none of which would be correct, since none of these is line of sight. Sounds “1” and “2” have multiple reflections to get to the receiver, while sound “3” has just one reflection. If the sound source is a measured output level (units measured in dB or decibels), the receiver can be set to reject all reception below a certain dB level, thereby rejecting all multipath signals.
Acoustic positioning systems update at regular intervals. The update rate is limited by the source and reception offset. Once an acoustic sound signal has been generated, it must go to the end of its reception range and come back to the transmitter/receiver (thereby generating a range through timing difference) before the next pulse can be sent. This is the reason acoustic positioning cannot maintain real-time positioning feedback.
16.6 Types of positioning technologies
Any discussion of positioning must first begin with an explanation of the term “frame of
reference.”
16.6.1 Frame of reference
Whenever a position is given for anything anywhere in the universe, that position must be made within a coordinate reference frame. If a geo-referenced position is given, there are many unan- swered questions simply to specify which mathematical model is used to generate that position. Some frame of reference examples are:
• “Two feet from my front door” (house reference frame)
• “The left seat of my car” (car reference frame)
• “Twenty feet off the port beam” (ship reference frame)
• “Control Yoke is located at station 45,” that is, 45 inches aft of the originating datum on the
engineering drawings of an example aircraft (aircraft reference frame)
• “Latitude 30 North/Longitude 90 West (WGS 84)” (earth reference frame)
In acoustic positioning, the raw positioning data is resolved from the acoustic beacon/tracking device to the transducer array (arranged in a known pattern). Your raw position will be given in an x/y/z offset from that referenced transducer array. The imaginary line forming the sides of the refer- ence triangle(s) is known as a “baseline.” The frames of reference can be fixed to the transducer array itself, a physical object (such as a dam, a ship, or a pier), or the earth. All offset coordinates may then be given with reference to that frame of reference.
The arrangement of the various acoustic sensors as well as their relative placement will deter- mine both the accuracy of the positional resolution as well as the ease of placement and system cost. All arrangements use some form of active send/receive in order to positively measure the time of transmission and reception over the array of transducers. The known spacing and angular offset of the transducer array forms what is known as a “baseline” from which to measure angular dis- tance (Figure 16.7). A comparison of different types of acoustic positioning arrangements is depicted in Figure 16.8.