the preferred embodiment of this invention is illustrated in fig2 . the invention employs vacuum micro - electronic device pulse sonar transceivers 34 and 44 ( transmitter / receiver ), which are radiation hardened and temperature tolerant in order to perform rod position indication with significantly better accuracy than the current system . vacuum micro - electronic devices such as commercially available ssvds supplied by innosys inc ., emeryville , calif ., are suitable for this purpose . a description of a vacuum micro - electronic device can be found in u . s . pat . no . 7 , 005 , 783 . the existing magnetic coils 12 are replaced by a primary and redundant ceramic - based miniature antenna 36 with a very narrow beam width of approximately two degrees beam spread , which are located and supported in the interior of each control rod drive rod housing 26 . each control rod drive rod housing 26 is equipped with a primary and redundant set of hardware , which wirelessly transmit the control rod position data with a unique frequency in order to avoid interference . the primary hardware is shown to the right of fig2 and includes the primary pulse vacuum micro - electronic device transceiver 34 . the primary vacuum micro - electronic device data transmitter 38 and a thermoelectric generator 40 whose hot junction is coupled to the control rod drive rod housing 26 by a heat pipe 42 . the redundant hardware shown to the left of fig2 includes the redundant pulse vacuum micro - electronic device transceiver 44 , the redundant vacuum micro - electronic device data transmitter 46 and the thermoelectric generator 48 whose hot junction is coupled to the drive rod housing 26 through a heat pipe 50 . prior to the installation of the hardware , an electromagnetic interference site survey is performed in order to identify potential sources of noise or interference . the transmission frequency for the wireless data transfer is chosen such that the transmission frequency occupies an unused frequency band at the rod housing location . the received measured signal is also filtered as needed in order to minimize electrical interference and other related issues impacting the accuracy of the transmitted signal . the dc power required by the pulse sonar transceivers 34 and 44 and the wireless data transmitters 38 and 46 is generated by way of one or more of the thermoelectric generators 40 and 48 that have the hot junction connected to the drive rod travel housing 26 through heat pipes 42 and 50 and a cold junction of the thermoelectric device located on the opposite side of the thermoelectric generators . it should be noted that the transceivers 34 and 44 and the wireless data transmitters 38 and 46 can be powered by conventional existing cables if necessary . the data can also be transmitted through conventional field cables if necessary . fig3 depicts a schematic of the system of this invention . this system employs a time of flight , phase difference and amplitude change methodology in order to determine the precise control rod position . time of flight compilations are accomplished by measuring the time it takes for the signal to travel from the pulse transmitters 34 and 44 back to its source . the system , as illustrated in fig3 , comprises a pulse repetition frequency oscillator 54 which together with the pulse transmitter 56 makes up the pulse generator that transmits the pulses at predetermined and known times , which allow the echoes 60 reflected from the top of the control rod 32 to return to the antenna 36 and be transmitted to the base station 52 for signal processing . as the pulse transmitter 56 sends the signal 58 into the drive rod travel housing 26 , the same signal is coupled and converted to a dc signal through a rectifying diode 68 and compared in the comparator 64 to a referenced voltage 62 . the comparator output goes high when the transmitted signal is present , which energizes the relay coil k 1 , which opens the normally closed contact k 1 a and closes the normally open contact k 1 b . as the echo 60 is received , it cannot go into the pulse transmitter , because k 1 a is open ; therefore , its only path is through k 1 b into the amplifier 66 and the wireless data transmitter 38 . the phase and amplitude of the original signal are also known to the system by continuously splitting the pulse transmitter output and providing the frequency spectrum information to the wireless data transmitter . the wireless data transmitter sends the original raw signal ( containing amplitude and phase ) as well as the return signal once it bounces off the control rod , to a signal processing base station 52 for further computation . it should be noted that the time of flight measurements do not assume a constant speed of sound , because the speed in the coolant will be affected by the temperature , therefore , a series of thermocouples 70 are spaced along the control rod drive rod travel housing 26 as part of the system in order to adjust the speed of sound based upon the temperature . in order to achieve the accuracy required for this invention , the system will include multiple thermocouples submerged in the coolant within the control rod drive rod travel housing at different elevations along the axial dimension of the housing 26 . as the rod moves , abrupt coolant temperature changes will be known by the system through these thermocouples and transmitted to the base station 52 to correct the signal for the temperature changes . typical pressurized water reactors operate at 2 , 220 psig ( 15 . 3 megapascal ) and 626 degrees fahrenheit ( 330 ° celsius ). according to the n . bilaniuk and g . s . k . wong model 1 , the speed of sound in water is c = 5 , 062 feet / second at 212 degrees fahrenheit ( 100 ° celsius ). using equation one , the round trip travel of the signal can be calculated : where d is the distance of the top of the control rod and c is the speed of sound in water at a specific temperature . based on the above equation and assuming that the transreceiver antenna is located approximately one foot above the control rod drive rod when the rod is fully withdrawn , and that the total distance the rod can travel is sixteen feet , we can obtain the following round trip travel times : 1 n . bilaniuk and g . s . k . wong ( 1993 ), speed of sound in pure water as a function of temperature , j . acoust . soc . am . 93 ( 3 ) pp 1609 - 1612 , as amended by n . bilaniuk and g . s . k . wong ( 1996 ), erratum : speed of sound in pure water as a function of temperature [ j . acoust . soc . am . 93 , 1609 - 1612 ( 1993 )], j . acoust . soc . am . 99 ( 5 ), p 3257 if the control rod drive rod moves one step ( ⅝ inches ( 1 . 59 cm .)) from the fully withdrawn position , the round trip time becomes the delta in the received signal for a ⅝ inch ( 1 . 59 cm .) movement would be 19 . 75 μseconds . this case would be the most demanding in the time of flight calculations and can be easily resolved with a 1 mhz clock in the processing base station . as stated above , the phase of the received signal and the amplitude information will also be used to determine the rod position . phase and amplitude also change proportional to rod movement and are used as error correction terms . the estimated accuracy of this system is plus or minus 0 . 4 inch , which is over seven times better than the current system . accordingly , this system provides improved accuracy , which would be mainly limited by the control rod drive mechanism step accuracy ; with the conservatism built into the current safety margin essentially eliminated by the better accuracy . the accuracy of the current prior art system is +/− 3 . 125 ″. the accuracy of the system of this invention is only limited by the smallest rod movement which is 0 . 625 ″. the improved safety margins provided by this invention ultimately means that the reactors employing this concept can generate more electricity than their counterparts employing conventional technology . redundant sensors provide the same accuracy instead of a half accuracy as with the current design . only initial installation calibration is needed and sensor interaction issues within the rod travel housing and with adjacent rod housings are eliminated . furthermore , the system enables elimination of the signal and powering cables which facilitates a more efficient fuel reload outage . furthermore , the control rod position monitoring system of this invention provides substantially improved accuracy over current systems . additionally , the drive rods no longer have to be ferromagnetic . while specific embodiments of the invention have been described in detail , it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure . accordingly , the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof .