Acoustic signaling and sound sources
for RAFOS navigation Signaling


fig1.jpg - 114978 Bytes In this companion webpage we give a brief overview of the acoustic signaling system used to provide navigation for SOFAR and RAFOS float applications. In the early SOFAR float days the floats transmitted short CW (continuous wave) pulses at 250 or 260 Hz. These signals, each about 1.2 seconds long, were sent exactly on the minute mark for 90 minutes every six hours. The digital recorder at the listening stations, using a hydrophone at the depth of the SOFAR channel (1200 meters deep), would record the acoustic intensity in this narrow listening band. In the subsequent processing on a computer, we basically searched for a pattern in which the signal intensity increases regularly every minute. By noting when this occurs, we know the arrival time to the second. By knowing the previous location of the float, we can determine how many minutes to add to obtain the total travel time. Figure 1 shows an example of this early signaling system. The bright band in the center against the irregular gray background shows the incoming signal. The scale here is one minute across days and 24 hours from top to bottom. The float was deployed at about 930am (Rossby and Webb, 1970). This method worked well, but it didn't take advantage of the fact the a signal transmitted in the ocean preserves its phase structure (or pattern) for several minutes. In other words, it looked for the presence of acoustic energy at a given frequency, but it didn't take advantage of the shape of the signal.

fig2.jpg - 65310 Bytes In the mid-1970s Doug Webb proposed that we instead transmit a single long signal, 80 seconds long, in which the frequency increases linearly 1.523 Hz from start to end. At the receiver, we now listen not only in a certain frequency band for the signal, but for one that has a specific phase structure. The key to doing so is to have a replica of the 80 second signal for comparison. By storing and comparing the most recent 80 seconds of listening against the reference signal, we determine at what time the two agree best within a given period of listening. This is illustrated in Figure 2 which shows the ideal signal and several recorded signals. To demonstrate for yourself how the signal detection method works, print the figure and make a transparent copy on a copier machine. Then put the transparency over the print and slide it slowly along the signal. You will notice that there is only one position when the entire signal lines up everywhere. At all other positions only short parts of the signal will line up whereas other sections of the signal are out of phase. It is this best line-up of the signal everywhere we look for. The detection scheme can be further simplified if we keep only information on whether the signal is positive or negative (all amplitude information in figure 2 is removed so all that is left would be a square wave switching positive or negative according to the zero crossings). The advantage of doing this is we only need to work with a single bit of new information at each 0.1 second time step, 0 for negative or 1 for positive. The detection technique becomes what is known as an exclusive-or plus summation. The time step at which this correlation differs most from random is considered to be the time of arrival. The method works very well, and is easy to implement in small, energy-efficient microprocessors. The latter development was a major factor leading to the RAFOS float concept in which each float does its own listening. Just a few moored SOFAR sound sources provide the navigation for as many floats as desired.

Sound sources

fig3.jpg - 231811 Bytes The sound source used for all early RAFOS work consisted of the standard SOFAR float, fabricated by Webb Research Corporation, Figure 3, at left. These are moored at sound channel depth, typically around 1200 m depth. They are typically programmed to send a signal once, twice or thrice per day depending upon the requirements of program. Starting in 1993 we also have used a source fabricated by Sparton of Canada, Figure 4, at right. This source is more efficient and can insonify larger areas, and thanks to its dipole radiation pattern it puts most of its energy in the horizontal plane at sound channel depth such that less energy is lost towards the surface. This source has been used in two major studies of the circulation in the northern North Atlantic (see the NAC and ACCE webpages). It, too, has been very successful. The electronics for the Sparton source come from Webb Research corp.

fig5.jpg - 27014 Bytes fig6.jpg - 208039 Bytes Recently, we have developed a new source that has some similarity to the Sparton source, but instead of using a piezoelectric ring around the outside of the pipe to excite it, we suspend a monopole in the center of the pipe. In both cases the active element excites pressure fluctuations in the pipe, but it is only those at the right frequency, set by the length of the pipe (which thus must be carefully trimmed to achieve resonance at the desired frequency) that will excite standing waves in the pipe and thus generate the desired acoustic output. This source was developed to meet the needs for regional applications for RAFOS float studies at a lower cost. Figure 5, at left, shows schematically the transducer and Figure 6, at right, shows the complete system on deck awaiting deployment. This source is undergoing its first tests and we plan to deploy it in its first prototype application in Spring 2001. When operational, this source will join the others in providing acoustic navigation for various underwater applications.