the present invention will be described in detail with reference to the accompanying drawings . it should be noted that although a ground vehicle is mainly used in the following description , the present invention can be advantageously implemented to other types of vehicles such as humans , motorcycles , commercial aircraft , etc . fig1 a - 1c show examples of possible arrangement for three - axis accelerometer sensor units that the present invention is applicable . an example of fig1 a is a three - axis accelerometer on the sensor board . an example of fig1 b is a combination of a double - axis accelerometer and a single - axis accelerometer formed o the sensor board . an example of fig1 c is a combination of three single - axis accelerometers formed o the sensor board . either of one chipset of three - axis accelerometer , one double - axis accelerometer and one single - axis accelerometer , or three single - axis accelerometers on the sensor board is applicable as long as the three axes conform a cartesian coordinate system ( i . e ., side - by - side angles between two axes are right angles ). x , y , and z represent the three axes of the sensor coordinate system in which each axis is the sensing direction of an accelerometer . fig2 a is a schematic diagram showing an input - output relationship with respect to a stationary status detection unit 36 . the inputs are three - axis accelerometer outputs : ax , ay , and az . the output is a flag to indicate the platform stationary status : “ 0 ” for “ stationary ”, “ 1 ” for “ in motion ”, for example . the stationary status detection unit 36 is a processor for conducting the computational operation as will be described in detail later based on the prescribed program . fig2 b is a schematic block diagram showing an example of basic configuration of the integrated ins / gps navigation system 20 implementing stationary status detection method of the present invention . the integrated ins / gps navigation system 20 is typically mounted on a ground vehicle but can be applied to other devices such as airplanes , motorcycles , and human bodies , etc . in this example , the ins / gps navigation system 20 includes an inertial navigation system ( ins ) 30 , a global positioning system ( gps ) receiver 40 , and a kalman filter 50 ( kf - 2 ). unlike ordinary gps navigation systems widely used in automobiles today , the integrated ins / gps navigation system 20 does not use any speed pulse sensor to detect the moving distance of the automobile . therefore , it is no longer necessary to establish complicated wiring in the vehicle for connecting a speed pulse sensor to a navigation system &# 39 ; s processor . further , it is unnecessary to adjust the outputs of the speed sensor which was necessary in the conventional technology because the definition of speed pulse is different from manufacturer to manufacturer of speed pulse sensors . in fig2 b , the ins 30 is configured by an angular sensor unit 31 , a three - axis accelerometer sensor unit ( acc unit ) 32 , a low - pass filtering unit 34 , a stationary status detection unit 36 , and a navigation equation unit 38 . the gps receiver 40 includes a kalman filter 42 ( kf - 1 ) in addition to gps signal receiver circuits and a microprocessor ( not shown ). the kalman filter 50 ( kf - 2 ) includes a kalman gain unit 52 which provides a gain ( coefficient ) to each parameter associated with position tracking operations . the angular sensor unit 31 could be either one of one through three - axis gyroscopes , cameras , three - axis magnetic compasses , multiple accelerometers distantly attached with known geometry , or combinations of those . the acc unit 32 is configured as a manner of either one of fig1 a - 1c and is created through mems ( microelectro mechanical system ) technologies . the angular sensor unit 31 detects angular motions and the acc unit 32 detects accelerations of a platform such as a ground vehicle . the outputs from the angular sensor unit 31 and the acc unit 32 are supplied to the low - pass filtering unit 34 in which high frequency components in the outputs thereof are removed . the output data from the low - pass filtering unit 34 is supplied to the navigation equation unit 38 where the current position , velocity and orientation of the vehicle are estimated through the inertial navigation technology . the output data from the acc unit 32 is also supplied to the stationary status detection unit 36 where the stationary or in - motion status of the vehicle is detected . the output data from the stationary status detection unit 36 is then supplied to the navigation equation unit 38 to halt the accumulation computation of sensor outputs and make corrective action in case the stationary status is detected . the gps receiver 40 receives signals from a plurality of gps satellites and calculates the estimated location and velocity of the vehicle by comparing clock signals and position and velocity data from the satellites . typically , the gps receiver 40 optimizes the obtained position and velocity data by the kalman filter ( kf - 1 ) 42 to minimize the noises on the satellite signals . the estimated position data from the ins 30 and the estimated position data from the gps receiver 40 are combined by the kalman filter ( kf - 2 ) 50 which optimally estimates , in real time , the states of the navigation system based on such noisy measurement data . the kalman gain unit 52 provides weight or gain to each parameter in the measurement data . the output of the kalman filter 50 is provided to the navigation equation unit 38 to calibrate the estimated position , velocity , and orientation of the vehicle which will be displayed on a navigation monitor screen ( not shown ). the physical concept and advantages in using the three - axis accelerometer outputs are described in fig3 and 4 . it must be noted that , although a side view of an automobile is used in fig4 , there could occur a motion in the lateral direction as well , e . g ., centripetal acceleration in cornering . first , a few items regarding basic vector algebra are introduced for explanation purpose in fig3 : { circumflex over ( x )}, ŷ , and { circumflex over ( z )}: unit vectors along with x , y , and z axes with magnitude of 1 ax , ay , and az : accelerometer raw outputs along with x , y , and z axes using these terminology introduced , we can represent the measured total acceleration vector , or { right arrow over ( a )}, as { right arrow over ( a )}= ax { circumflex over ( x )}+ ay ŷ + az { circumflex over ( z )} ( m / s 2 ) ( 1 ) fig4 shows that , by virtue of using the three axis accelerometer unit , the measured total acceleration vector , { right arrow over ( a )}, is equivalent to the sum of the following vectors regardless of the sensor unit orientation or vehicle motion direction : { right arrow over ( a )}={ right arrow over ( g )}+{ right arrow over ( e )}+{ right arrow over ( m )} ( m / s 2 ) ( 2 ) here , the gravity vector { right arrow over ( g )} is always vertically upward with respect to the local horizontal surface with magnitude of 9 . 8 ( m / s 2 ) ( although the magnitude of gravity is dependent on a specific locale , the change is too small to be detected by mems sensors ). the bias component in { right arrow over ( e )} may slowly change depending upon temperature but almost constant with respect to the sensor unit coordinate system for a short time period . comparing equations ( 1 ) and ( 2 ), we have { right arrow over ( g )}+{ right arrow over ( e )}+{ right arrow over ( m )}= ax { circumflex over ( x )}+ ay ŷ + az { circumflex over ( z )} ( m / s 2 ) ( 3 ) equation ( 3 ) simply tells that it is always possible to evaluate the total acceleration vector of { right arrow over ( g )}+{ right arrow over ( e )}+{ right arrow over ( m )} using the three - axis accelerometer outputs regardless of the sensor unit orientation or vehicle motion direction . the other way around , if only the acceleration vertical to the vehicle is analyzed as suggested in the japanese patent no . 3404905 noted above , motion acceleration vector { right arrow over ( m )} in the forward or backward direction will not be detected resulting in undetected motion and misjudge of a stationary status . now , in case of stationary status , { right arrow over ( m )} disappears ({ right arrow over ( m )}={ right arrow over ( 0 )}) to have { right arrow over ( a )}={ right arrow over ( g )}+{ right arrow over ( e )} ( m / s 2 ) ( 4 ) also , by virtue of the almost constant nature of { right arrow over ( g )} and { right arrow over ( e )}, precise estimation of { right arrow over ( g )}+{ right arrow over ( e )} can be obtained by an average the latest samples , or mean , of the measured total acceleration with a specific time window during a stationary period : { right arrow over ( a )} mean ={ right arrow over ( g )}+{ right arrow over ( e )} ( m / s 2 ) ( 5 ) { right arrow over ( a )}−{ right arrow over ( a )} mean ≈{ right arrow over ( 0 )} ( m / s 2 ) ( 6 ) is guaranteed during a stationary period . at the same time , it is also true that { right arrow over ( a )}−{ right arrow over ( a )} mean 6 ≠{ right arrow over ( 0 )} happens more likely during an in - motion period . therefore , it is possible to take the amplitude of { right arrow over ( a )}−{ right arrow over ( a )} mean , or |{ right arrow over ( a )}−{ right arrow over ( a )} mean | as the measure of detecting stationary status . the procedure of the present invention for detecting the stationary status of the platform is described with reference to the graphs of fig5 - 10 and the flow chart of fig1 . it should be noted that the underlined parameter values shown in the flow chart of fig1 may be altered according to the platform ( vehicle , motor cycle , etc . ), sensor performance , and other conditions . it should also be noted that , although not specifically shown in the flow chart of fig1 , there is a step of mounting the three - axis accelerometer on the platform such as a vehicle where orientation of the accelerometer can be freely made without suffering accuracy in the detection of stationary status . the process starts at step 101 of fig1 by setting the stationary status counter reading i = 0 . in step 102 , the process measures the raw outputs of the three - axis accelerometer . fig5 shows the actual raw outputs from the three - axis accelerometer unit attached to an automobile in which , x , y , and z - axes are directed approximately to the vehicle forward , right hand , and downward directions , respectively ( although not necessary ). these are the outputs from an actual 1 - hour drive including intensive cornering made inside a spiral parking tower ( from 900 to 1150 seconds ). according to the vector analysis , these are the components of a total acceleration vector , { right arrow over ( a )}, namely , { right arrow over ( a )}= ax { circumflex over ( x )}+ ay ŷ + az { circumflex over ( z )} ( m / s 2 ) then in step 103 of fig1 , the process takes the latest mean of each sensor output with a specific time window . a one - second time window is used here as an example , i . e ., the mean of the latest 25 samples measured at 25 hz . fig6 shows an example of the latest one - second mean of the actual accelerometer outputs . according to the vector analysis , these are the components of a mean acceleration vector { right arrow over ( a )} mean , namely , { right arrow over ( a )} mean = ax mean { circumflex over ( x )}+ ay mean ŷ + az mean { circumflex over ( z )} ( m / s 2 ) in step 104 of fig1 , the process then simply subtracts the mean value of fig6 from each raw component of fig5 . an example of the results is shown in the graph of fig7 . according to the vector analysis , these are the components of a differential acceleration vector { right arrow over ( a )}−{ right arrow over ( a )} mean , namely , { right arrow over ( a )}−{ right arrow over ( a )} mean =( ax − ax mean ) { circumflex over ( x )} +( ay − ay mean ) ŷ +( az − az mean ) { circumflex over ( z )} since { right arrow over ( a )}−{ right arrow over ( a )} mean ={ right arrow over ( 0 )} holds during a stationary period , the process seeks for the period meeting ≡{ right arrow over ( a )}−{ right arrow over ( a )} mean |= 0 ( or very small ) |{ right arrow over ( a )}−{ right arrow over ( a )} mean =√{ square root over (( ax − ax mean ) 2 +( ay − ay mean ) 2 +( az − az mean ) 2 )}{ square root over (( ax − ax mean ) 2 +( ay − ay mean ) 2 +( az − az mean ) 2 )}{ square root over (( ax − ax mean ) 2 +( ay − ay mean ) 2 +( az − az mean ) 2 )} which is shown in fig8 . note that |{ right arrow over ( a )}−{ right arrow over ( a )} mean |= 0 ( or very small ) appears as the vehicle stops . in a practical application , as in step 105 , it is preferable to set a threshold value , e . g ., 0 . 05 ( m / s 2 ), to detect possible stationary status by |{ right arrow over ( a )}−{ right arrow over ( a )} mean |& lt ; 0 . 05 ( m / s 2 ): for possible stationary status in step 107 of fig1 , to make sure the stationary status firmly , it is preferable to count the endurance of the successful small amplitude increasing a counter , “ i ”, when |{ right arrow over ( a )}−{ right arrow over ( a )} mean |& lt ; 0 . 05 ( m / s 2 ) is met . namely , every time when the difference between the raw outputs and the latest mean is smaller than the predetermined threshold value , the reading of the counter is incremented by one . otherwise , the counter is reset to 0 in step 106 . fig9 shows the consequent stationary counter “ i ” in which it is possible to determine that the platform is stationary when “ i ” exceeds a certain time period , e . g ., 1 second corresponding to i = 25 at 25 hz as indicated by step 108 of fig1 . finally , it is set stationary flag = 0 when i ≧ 25 indicating the “ stationary ” status in step 110 otherwise let stationary flag = 1 indicating the “ in motion ” status in step 109 . it should be noted that “ 0 ” and “ 1 ” can be defined in the other way around . fig1 shows the stationary flag by “•” or thick lines overlapped by accurate speed - pulse - indicating speed with “+” as reference . note that when the reference speed of “+” is 0 , the method of the present invention also accurately indicates that it is stationary by “ 108 ” at 0 . fig1 and 13 suggest an application of the present invention . fig1 shows navigation trajectory using mems six degrees of freedom integrated ins / gps without using speed pulses , and without using the present invention . because of gps dropouts during certain stationary time periods , erroneous accelerations are accumulated as indicated by the arrows : vehicle trajectories are unnecessarily extended straightly at a and b . fig1 shows the results of applying the present invention to the same navigation system to execute corrective actions during the detected stationary status : extra measurement of “ velocity = 0 ” condition applied . note that unnecessary extensions have now disappeared in fig1 .