in one embodiment , as shown in fig1 , the buoy - to - sail boat distance indicator system ( system ) 100 has a buoy transmitter 200 and a boat receiver 300 . the buoy transmitter 200 has a transmitter power source 210 , a transmitter central processor unit ( cpu ) 220 , a radio transmitter 240 , an acoustic transmitter 260 , and a speed of sound calibrator 280 . the boat receiver 300 has a receiver power source 310 , a receiver cpu 320 , a radio receiver 340 , an acoustic receiver 360 , and an information display 380 . the transmitter cpu 220 controls the speed of sound calibrator 280 . the speed of sound calibrator 280 provides the transmitter cpu 220 with the speed of sound through water for the operating environment in which system 100 is being used . the speed of sound calibrator 280 , which can be implemented in many ways , measures the speed of sound for the current conditions . one way to implement a speed of sound calibrator would be to measure the temperature , water salinity , and water pressure , and then calculate the speed of sound for those conditions using the appropriate known equations . another way , somewhat more empirically , would be to measure the time for a signal to transit a known distance . the ways for measuring the speed of sound through water are well known to one of average knowledge in the art . the transmitter cpu 220 controls both the radio transmitter 240 and acoustic transmitter 260 . the transmitter cpu 220 causes the radio transmitter 240 to transmit a radio signal 140 and causes acoustic transmitter 260 to transmit an acoustic signal 160 . radio signal 140 and acoustic signal 160 should be omnidirectional signals . in the simplest embodiment transmitter cpu 220 is programmed with the range distance at which the radio signal 140 and acoustic signal 160 should simultaneously arrive . using the calibrated speed of sound , the transmitter cpu 220 computes the transmit time delay . the transmit time delay is the amount of time after sending the acoustic signal 160 at which to transmit the radio signal 140 so that both signals simultaneously arrive at a specified range distance . the transmitter cpu 220 causes the acoustic transmitter 260 to send immediately the acoustic signal 160 , and then at transmit time delay later causes the radio transmitter 240 to send immediately the radio signal 140 . just as the transmitter cpu 220 drives both radio transmitter 240 and acoustic transmitter 260 , the receiver cpu 320 listens to both the radio receiver 340 and acoustic receiver 360 . in the simplest embodiment , receiver cpu 320 observes which signal , radio signal 140 or acoustic signal 160 , arrives first . if acoustic signal 160 arrives first , receiver cpu 320 then knows the range result that boat receiver 300 is closer to buoy transmitter 200 than the specified range distance . if the radio signal 140 arrives first , receiver cpu 320 then knows the range result that boat receiver 300 is farther from buoy transmitter 200 then the specified range distance . the receiver cpu 320 displays range result on the information display 380 . in the simplest embodiment the information display could be just a light which turns on or off when the range result is that boat receiver 300 is closer to buoy transmitter 200 than the specified range distance . or the light could turn on or off when the range result is that the boat receiver 300 is farther from the buoy transmitter 200 than the specified range distance . or the information display could be two lights , a first light for when the range result is that boat receiver 300 is closer to buoy transmitter 200 than the specified range distance , and a second light for when the boat receiver 300 is farther from buoy transmitter 200 than the specified range distance . or , the information display 380 could be a text message saying “ less than ” or “ greater than ,” as appropriate . or the information display could use audio signals vice visual signals . in an alternative embodiment , the transmitter cpu 220 can use the radio transmitter 240 to send the speed of sound information to the receiver cpu 320 via radio signal 140 and radio receiver 340 . in this embodiment , receiver cpu 320 measures the received time delay between arrival of the radio signal 140 and the acoustic signal 160 . using the speed of sound and received time delay , the receiver cpu 320 can then compute the measured range distance and sign ( i . e ., plus or minus ) which the boat receiver 300 is from the specified range distance . the measured range distance can be shown on the information display 380 as appropriate . as a special case of this alternative embodiment , when the specified range distance is set to zero , the measured range distance will be the distance from the buoy transmitter 200 to the boat receiver 300 . in yet another alternative embodiment , boat receiver 300 will have a plurality of acoustic receivers 360 . by knowing the relative positions of each acoustic receiver 360 , the receiver cpu 320 can use the different arrival times for acoustic signal 160 at each acoustic receiver 360 to determine the relative direction from boat receiver 300 to buoy transmitter 200 . the details of how to do such calculations are well known to one of average knowledge in the art . the unambiguous range is the range under which , in the simplest embodiment , the two correlated signals ( radio signal 140 and acoustic signal 160 ), will have been received at the boat receiver 300 before the next radio signal 140 is received . this range is determined by the time between radio transmissions and the speed of sound for the operating environment . the rate at which the system 100 can measure the distance between transmitter 200 and receiver 300 is also determined by the time between radio transmissions . in an alternative embodiment , the concept of staggered pulse repetition intervals , well known to one of average knowledge in the art in the area of traditional radar , can be applied to this system 100 so as to provide an alternative method for either increasing the unambiguous range , or increasing rate at which the distance measurements are made , or some combination there of . transmitter power source 210 powers all other parts of buoy transmitter 200 . receiver power supply 310 powers all other parts of boat receiver 300 . transmitter power 210 and receiver power 310 can be any sort of electrical storage or generation source . in this system 100 the buoy transmitter 200 is not interrogated in any way by boat receiver 300 . one buoy transmitter 200 can service an unlimited number of boat receivers 300 . depending on the method of implementation , speed of sound calibrator 280 may or may not have parts in common with acoustic transmitter 260 . system 100 components , especially the housings for buoy transmitter 200 and boat receiver 300 , should be brightly colored such that they can be easily seen . for example , buoys normally used during sailboat races are brightly colored so that they can be easily seen . thus , for the same reasons , buoy transmitter 200 should be brightly colored . having boat receiver 300 brightly colored can facilitate people being able to see that a sailboat has a boat receiver 300 . also , for both buoy transmitter 200 and boat receiver 300 , should either fall into the water , having them brightly colored will make them easier to see by people trying to find them . because the radio signal 140 travels over the water &# 39 ; s surface and the acoustic signal 160 travels below the water &# 39 ; s surface , the operations of system 100 can be identified as radio over audio below , and abbreviated to create the word roab . a buoy - to - sailboat distance indicator system 100 for determining the distance between a buoy and a boat both floating in a common body of water , has a buoy transmitter 200 and at least one boat receiver 300 . the buoy transmitter 200 , deployed on the floating buoy , has a speed of sound calibrator 280 for producing a measurement of the speed of sound through the body of water , a radio transmitter 240 for producing a radio signal 140 transmitted through the air above the body of water , an acoustic transmitter 260 for producing an acoustic signal 160 transmitted through the body of water , and a transmitter cpu 220 . the transmitter cpu 220 receives the measurement of the speed of sound , calculates the trigger times , and triggers the radio transmitter 240 and the acoustic transmitter 260 to transmit respectively the radio signal 140 above , and acoustic signal 160 through , the body of water such that both simultaneously arrive at a predetermined distance across the body of water . a boat receiver 300 , deployed on a boat , has a radio receiver 340 for receiving the radio signal 140 transmitted through the air above the body of water , an acoustic receiver 360 for receiving the acoustic 160 signal transmitted through the water , a receiver cpu 320 for determining the relative arrival time between the radio signal 140 and the acoustic signal 160 , and determining thereby the distance of the boat from the buoy relative to the predetermined distance . an information display 380 displays the distance information about the distance of the boat receiver 300 from the buoy transmitter 200 relative to the predetermined distance . in an alternative embodiment a buoy - to - sailboat distance indicator system 100 for determining the distance between a buoy and a boat , both floating in a common body of water , has a buoy transmitter 200 and a boat receiver 300 . the buoy transmitter 200 deployed on the buoy has a speed of sound calibrator 280 for producing a measurement of the speed of sound through the body of water , a radio transmitter 240 for producing both a radio signal 140 transmitted through the air above the body of water and for transmitting the measurement of the speed of sound , an acoustic transmitter 260 for producing an acoustic signal 160 transmitted through the body of water , and a transmitter cpu 220 . the transmitter cpu 220 receives the measurement of the speed of sound , calculates the trigger times and triggers the radio transmitter 240 and the acoustic transmitter 260 to transmit respectively their radio signal 140 above , and acoustic signal 160 through , the body of water such that both simultaneously arrive at a predetermined distance across the body of water . the boat receiver 300 has a radio receiver 340 for receiving both the radio signal 140 transmitted through the air above the body of water and the transmitted measurement of the speed of sound , an acoustic receiver 360 for receiving the acoustic signal 160 transmitted through the body of water , a receiver cpu 320 for determining the relative arrival time between the radio signal 140 and acoustic signal 160 , and determining thereby the distance and sign of distance of the boat receiver 300 from the buoy transmitter 200 and from the predetermined distance . an information display 380 displays the distance and the sign of distance of the boat receiver 300 from the buoy transmitter 200 and from the predetermined distance . although various preferred embodiments of the present invention have been described herein in detail to provide for complete and clear disclosure , it will be appreciated by those skilled in the art , that variations may be made thereto without departing from the spirit of the invention or the scope of the appended claims .
this system is intended for use during a regatta to indicate the distance between a sailboat and a race course marker buoy being rounded as part of the race . the system involves a transmitter on the buoy and a receiver on the boat . the buoy transmitter sends both a radio signal and an acoustic signal . the boat receiver detects both the radio signal and acoustic signal . the boat receiver measures the time delay between the radio and acoustic signals . this time duration is converted to a distance and the boat receiver displays the distance information .