referring to fig1 an air traffic control system according to an embodiment of this invention comprises , as a position sensor , a magnetic three dimensional position sensor 1 which serves to detect a position of an air traffic controller ( not shown ). the magnetic three dimensional position sensor may be , for example . fastrak ( product name ) sold and manufactured by polhemus in usa . as known in the art , the magnetic three dimensional position sensor 1 has a transmitter for generating , a magnetic field and a receiver for detecting the magnetic field . in the illustrated example , the receiver is attached to a body of the air traffic controller . for example , the receiver may be worn at a part adjacent to eyes of the air traffic controller . at any rate , the magnetic three dimensional position sensor 1 detects the position of the air traffic controller at a predetermined period to transmit to a display renewal judgement portion 2 , a position detection signal which is representative of the detected position of the air traffic controller and which will be called position information . the display renewal judgement portion 2 calculates an amount of movement of the air traffic controller on the basis of the position detection signal . in addition , the display renewal judgement portion 2 further analyzes a movement of the air traffic controller in accordance with the amount of movement ( namely , movement amount ) calculated and detects a moving state or a stationary state of the air traffic controller . furthermore , the display renewal judgement portion 2 further judges whether or not a display image should be changed or switched to another direction or angle in consideration of the position of the air traffic controller ( namely , the position information ). thus , the display renewal judgement portion 2 transmits a result of judgement and the detection position to a display data generator 4 . the result of judgement is representative of whether or not the display renewal is needed and may be called judgement information . in the illustrated example , the air traffic control system further comprises an air traffic control information management portion 3 which receives , from an outside system , air traffic control information related to a flight plan and an airport surface state . such air traffic control information is managed by the air traffic control information management portion 3 and is transmitted to the display data generator 4 as newest or latest air traffic control information . thus , the display data generator 4 is supplied with the newest air traffic control information from the air traffic control information management portion 3 and with the result of judgement and the detection position from the display renewal judgement portion 2 . the display data generator 4 generates display data and supplies the same to a display portion 5 . the display data is displayed on the display portion 5 . referring to fig2 in addition to fig1 the above - operation will be described more in detail . in fig2 the position of the air traffic controller is detected as position data at a step a 1 by the magnetic three dimensional position sensor 1 ( step a 1 ). the detected position data or information is transmitted as a current position detection signal to the display renewal judgement portion 2 and is stored therein ( step a 2 ). the display renewal judgement portion 2 stores a previous position detection signal concerned with a previous position of the air traffic controller . under the circumstances , the display renewal judgement portion 2 calculates an amount of movement of the air traffic controller by comparing both the previous position detection signal with the current position detection signal and produces a movement amount signal d representative of a difference or deviation between the previous and the current positions ( step a 3 ). subsequently , the display renewal judgement portion 2 compares the movement amount signal d with a reference signal d representative of a predetermined reference value ( step a 4 ) to judge whether or not the air traffic controller is now being moving . a result of comparison is stored as a comparison result signal in the display renewal judgement portion 2 ( step a 5 ). if the movement amount signal d is greater than the reference signal d , the display renewal judgement portion 2 judges that the air traffic controller is now moving and returns the processing back to the step a 3 . on the other hand , if the movement amount signal d is not greater than the reference signal d , the processing is moved from the step a 4 to a step a 6 . at the step a 6 , the display renewal judgement portion 2 compares the previous and stored comparison result signal with the current comparison result signal to judge whether or not the current comparison result signal is identical with the previous comparison result signal . when the current comparison result signal is identical with the previous comparison result signal , the air traffic controller does not move from a previous position , namely , is in the stationary state by the display renewal judgement portion 2 , processing is returned back to the step a 3 , as shown in fig2 . in consequence , the next following movement amount is calculated by the display renewal judgement portion 2 . on the other hand if the result of judgement ( current comparison result signal ) at the step a 6 is not equal to the previous comparison result signal , the display renewal judgement portion 2 judges that the air traffic controller stops moving . thus , the processing is moved from the step a 6 to a step a 7 . at the step a 7 , the display renewal judgement portion 2 informs the display data generator 2 that a display position is to be renewed in accordance with the detected position of air traffic controller . now , the air traffic control information is given from the outside system and is received by the air traffic control information management portion 3 ( step a 8 ). the air traffic control information management portion 3 renews stored air traffic control information into the received air traffic control information ( step a 9 ) and notifies the display data generator 4 of display renewal of the air traffic control information ( step a 10 ). at a step a 11 , the display data generator 4 generates the display data on the basis of the indications issued at the steps a 7 and a 10 by the display renewal judgement portion 2 and the air traffic control information management portion 3 . the display portion 5 visually displays the display data received from the display data generator 4 ( step a 12 ). thus , display renewal processing is finished . referring to fig3 and 4 , description will be made about another embodiment according to this invention . a hardware structure according to an air traffic control system of this embodiment is similar to that illustrated in fig1 and 2 and will not be described later . the air traffic control system according to this embodiment is specified by a front display region a hatched , as shown in fig3 . the hatched front display region a of the display portion 5 represents a front display region watched from a predetermined front position of the display portion 5 . in addition , it is assumed in fig4 that the display portion 5 further internally stores an additional display region b in addition to the front display region a . the illustrated additional display region b is divided into sub - areas a , b , c , and d in addition to the hatched display region a , as shown in fig4 . the sub - area a is representative of a display area selected when the display portion 5 is seen from a lower side of the screen of the display portion 5 while the sub - area b is representative of a display area selected when the display portion 5 is seen from an upper side of the screen . likewise , the sub - areas b and d are representative of display areas selected when the display portion 5 is seen from righthand and lefthand sides of the screen , respectively . with this structure , when the screen of the display portion 5 is watched from the lower side of the screen , the sub - area a is read out of the air traffic control information management portion 3 and is displayed on the display portion 5 . this applies to any other sub - areas b , c , and d . as a result , the display data generator 4 ( fig1 ) selects the sub - areas a , b , c , and d on the basis of the position of the air traffic controller and sends the display data to the selected sub - area to display the display data on the screen ( a ) of the display portion 5 . thus , the sub - areas a , b , c and d are switched from one to another in response to a movement of the position of the air traffic controller and is selectively displayed on the screen hatched in fig4 . herein , there is an air traffic control system such that a display image is manually switched each time when the position of the air traffic controller is moved or changed . if the embodiment according to this invention is applied to such a system , this invention effectively dispenses with any manual operation for switching the display portion and is effective to reduce a load imposed on the air traffic controller . herein , it is often preferable that the image on the screen seen from a plurality of angle positions is identical with the image seen from the predetermined front position . on the other hand , when the image is seen from an inclined position , a visible image area on the screen is reduced in comparison with the image area seen from the front position of the screen . alternatively , it is preferable that the air traffic information displayed as the image would be changed to another information with reference to the position of the air traffic controller . referring to fig5 description will be made about the display data generator 4 ( fig1 ) which can fulfill the above - mentioned requirements and which is operable to modify the image seen from the front position . therefore , the display data generator 4 shown in fig5 may be called a modifying portion . the display data generator 4 illustrated in fig5 has first , second , and third memories 41 , 42 , and 43 and a processing unit 44 supplied as the position information with the detected position of the air traffic controller and a mode signal indicative of the above - mentioned displays . in the illustrated example , the first memory 41 stores the air traffic control information seen from the front position as a reference image . when the display data generator 4 is given the mode signal representative of displaying the reference image , the reference image is displayed on the screen of the display portion 4 under control of the processing unit 44 even when the position information is representative of the movement of the air traffic controller . this means that , when the screen is watched by the air traffic controller at a position inclined from the front position , the first memory 41 and the processing unit 44 serves to display the reference image on the screen and will be referred to as a first circuit . the second memory 42 stores a shaped image of the newest air traffic control information in consideration of the case where the image on the screen is reduce when the image is watched from an oblique position in comparison with that seen from the front position . the shaped image may be reduced in an amount of information and may be prepared at every angle . as a result , the shaped image is displayed as a reduced viewing area image . at any rate , the second memory 42 is operable to compensate for a reduction of a display areas of the display portion 5 in cooperation with the processing unit 44 . in this case , the processing unit 44 is helpful to select the reduced viewing area image and to send the same to the display portion 5 and a combination of the second memory 42 and the processing unit 44 may be called a second circuit . furthermore , the third memory 43 successively stores a plurality of images each of which is representative of air traffic control information different from one another . the processing unit 44 reads either one of the images out of the third memory 43 with reference to the position information and sends the same to the display portion 5 . thus , the third memory 43 and the processing unit 44 serves to detect the position of the air traffic controller to select one of the images and may be named a third circuit . from this fact , it is readily understood that the air traffic controller can automatically select and display necessary air traffic control information . as mentioned before , it is readily understood that the embodiments according to this invention detects the position of the air traffic controller by using the magnetic three dimensional position sensor and produces the necessary air traffic control information in consideration of the position of the air traffic controller . in addition , the display of the air traffic control information is renewed by judging the movement of the air traffic controller and by detecting completion of the movement of the air traffic controller . this means that consideration is made in this invention about movement hysteresis of the air traffic controller . thus , this invention is very effective to improve visibility of the display portion because an image can be displayed which is seen from a front with reference to the position of the air traffic controller or which is selected in consideration of the order predetermined with reference to necessity and importance of the image . alternatively , this invention can automatically switch the image on the basis of the position of the air traffic controller . therefore , no manual switching is needed in this invention . at any rate , the displayed image is automatically switched from one to another on the basis of the position of the air traffic controller . this means that the air traffic controller devotes to navigation of the aircraft without manual , operation of switching the images . in other words , this invention is advantageous in that it is possible to alleviate a load imposed on the air traffic controller and to realize a safe flight . while this invention has thus far been described in conjunction with a few embodiments thereof , it will readily be possible for those skilled in the art to put this invention into practice in various other manners . for example , this invention may be used to control a mobile body , such as an vehicle or a ship and can obtain similar effects . in this event , an image on a screen can be varied in accordance with a position of a viewer .