Zoologists track animals using global-positioning-system(GPS)tags(标签) which then return their data via satellite. Marine(海洋的)biologists have a harder time of it, though, because radio signals can’t pass through seawater. This makes it impossible either to receive GPS signals or to send any data collected back to base.
That does not stop people tagging sea creatures. Data collected and stored in a tag can be sent to a satellite in bursts if the species in question is one that comes to the surface from time to time. A tag may also be recovered if the animal carrying it is caught by a fishing boat. Fisherfolk are typically paid a few hundred dollars per tag returned to its home laboratory.
None of these methods, though, keeps accurate track of where the animal carrying the tag has been. For these and other reasons, it would therefore be useful to have a marine equivalent of GPS. And one is now being employed. The Woods Hole Oceanographic Institution, in Massachusetts, hopes to fill the seas with sonic beacons(声波信标) that will play the role of GPS satellites.
The sea is divided into distinct layers that have different temperatures. During the Second World War, American scientists showed that some of these layers act as sonic waveguides. They called them “sound fixing and ranging” (sofar) channels. Sound sent out in one of these channels echoes(回响) between the layers above and below, thus staying in the channel. Thus constrained(被约束)，a sound wave can travel hundreds of kilometers before it becomes too weak to detect.
The sofar transmitters from Woods Hole are usually at an appropriate depth for the channel concerned. Every 12 hours they broadcast a 32-second-long location signal known as a pong. Pongs are so called because they are similar to sonar “pings”, but of lower frequency. In typical conditions a pong can be picked up 1,000 km away. By listening to the pongs from several beacons a receiver can calculate its location. Existing receivers for the two sofar transmitters are currently carried on free-floating instrument packs. But the plan is to have two more transmitters this year, and more in future years.
【小题1】Which sea creatures can GPS tags be applied to?| A．Those feeding on other sea animals. |
| B．Those following fishing boats constantly. |
| C．Those coming out of the sea sometimes. |
| D．Those swimming deep under the water. |
| A．The echoes among them are weak. |
| B．The water has a high temperature over there. |
| C．The layers among them are quite similar. |
| D．The sound remains there and stays strong for a while. |
| A．Pings are of higher frequency. |
| B．Pings can travel faster than pongs. |
| C．Pings can be received 1,000 km away. |
| D．Pings are broadcasted every 32 seconds. |
| A．Tags for sea creatures |
| B．“GPS” for the oceans |
| C．Strange deep sea creatures |
| D．Data from distant satellites |
