Why use interval time as the identifier for transmitters?

Sonotronics has historically used interval time along with code and frequency as the identifier for particular transmitters. The reason for this is that when transmitting data, the range that can be achieved is proportionate to the complexity of the data signal being transmitted. When using the pulse interval (time between pings), a receiver needs only to detect one ping, then another. Once this has occurred, the particular transmitter can be identified. There can be small variances in interval time due to effects in the water during the transmission, but the tags are always spaced apart further apart than these effects can cause problems. For example, a transmitter with an interval of 900ms might show up as 902ms, but the next transmitter will be placed at 910ms. This may sound confusing, but when doing automated tracking (SUR's, USR-90-DH's) this is all abstracted from view by our data processing software. What this all means is that we want researchers to get the best range possible!

Why do you not quote source levels for transmitters?

As a part of Sonotronics philosophy, going back 30 years, we have always elected to quote achievable ranges rather than source level. The reason for this has to do with the physics involved in transmitting a sonar signal through water. Ultimately what our customers desire is to know what the range they can expect in their environment is. There are more factors than just source level involved. Fundamentally, it is a combination of source level of the transmitter, losses in between the transmitter and the hydrophone, and sensitivity of the receiver/hydrophone combination. These three components together result in the range that a signal can actually be detected. Sonotronics historically has had very sensitive receivers, with the ability to pick transmitters from the background noise over good distances. This has allowed us to use lower source level transmitters that can achieve the same or greater ranges than other transmitters on the market.

What kind of range can I expect to get from my SUR?

As mentioned in the FAQ above, this depends on the sensitivity of the SUR, the losses in between transmitter and receiver, and the transmitter source level. For this one, lets give some real life examples.

1. In the St. Claire River Near Detroit, MI. 6/23/2005 . SUR is in 5 feet of water near the shore. Depth goes up to 50m. Tag was a few in water - 1m. Achievable range was 400m. The tag was a CT-04-1 - a medium powered "up to 1km" transmitter.
2. In Lake Pleasant at Cave Creek, AZ high powered transmitters were detected regularly between 400m and 500m with an array of 6 SUR's.

My Fish seem to have disappeared - where did they go?

This question comes up from time to time, and of course we do not know exactly where anyone's fish are, we can offer a few interesting comments.

1. In one particular case, a researcher told us that he would hear his fish intermittently in a particular area, but the signal would mysteriously appear and disappear. It turns out that the fish had died wearing the ultrasonic transmitter. The fish was on the shore at the edge of the water. As the water level rose and fell, the fish would go in and out of being underwater. The important point in this example is to remember that ultrasonic transmitters can be heard well in water, but only from a few feet in air.
2. In another interesting case, a researcher was tracking a fish that was caught by an eagle. When the bird took the fish out of the water (and flew off with it in its talons) of course the signal disappeared.
3. In another case, a researcher had several sturgeon disappear in a narrow river system. These particular sturgeon also had radio transmitters attached, which can be detected on land. The radio signal was tracked to what turned out to be an alligator lair on land near the river. Poor sturgeon!
4. In another case, a researcher had many fish disappear over a period of a month. It was later found that the fish were in a space between lily pads and the shore. This was a problem because the stalks of lily pads contain air pockets, and they were dense enough in this case to severely attenuate the acoustic signal. Note vegetation by itself is not necessarily a problem, but trapped air is.

Certainly these examples do not explain anywhere near the number of situations that can exist, but they only serve to show that there are a lot of factors involved.

What kind of range can I expect when manual tracking?

When manual tracking, this is when you can expect to get the advertised range of our transmitters. (Up to 1km, Up to 3km, etc.) It is important to note that this is in good conditions. Of course, good conditions is the caviat, which we will explain. There are a few main factors that reduce range

1. Shallow environments - Environments in less than 10m of water can reduce the range you can achieve from acoustic transmitters. For example, in a recent case, a transmitter that can normally be detected up to 3km in good conditions was being detected at around 500m range in 3m depth water.
2. Temperature or Salinity stratified environments - When there is a temperature or salinity gradient in the water column where a transmitter is located, there is an effect called ray bending that occurs. Because the speed of sound in water is a function of temperature and salinity, you can end up with "layers" in which the speed of sound is different. Due to snell's law, the sound signal is bent at the interface between each layer. The end result of this is that sound can be warped downward, reducing range. As you can then imagine, a thermocline (distinct division between two temperature layers) can be very bad. In some cases, it can cause sound to not reach the surface at all. One common trick to avoid this is to use an omnidirectional hydrophone with a long cable to penetrate the thermocline.

What is the difference between ultrasonic (acoustic) transmitters and radio transmitters, and when do you use each?

Ultrasonic (acoustic) transmitters actually emit high frequency sound, while radio transmitters emit electromagnetic energy. Below is a table which details some of the differences between ultrasonic and radio transmitters, followed by some additional information.

Ultrasonic transmitters contain a piezoelectric element which emits the sound, much like a tweeter, but at higher frequencies. Ultrasonic transmitters are typically found anywhere from 30 to 300kHz, while radio transmitters typically start in the 20's of MHz area.

Radio transmitters contain an antennae, like any other radio device. Because receiving a radio transmission is not dependent upon having a hydrophone physically in the water, they can be detected while on land or even in an airplane.