Abstract:
An air cargo power drive unit has a motor, at least one driver roller element coupled to said motor, a light source, a light detector, and a processor having memory associated therewith, said memory storing instructions. The device is configured to emit light from the light source, receive reflected light from the light detector when an air cargo is overhead, and convert the detected light into a time series of a digital samples representing a time-varying intensity of the received light. The processor then performs calculations on the digital samples to determine whether the unit load device is moving. This determination may be based, for instance, on spikes among the digital samples, and/or on first, second, or even higher-order, statistics of the detected samples.

Description:
FIELD OF THE INVENTION 
       [0001]    One embodiment of the present invention is directed to a power drive unit for transporting cargo on an aircraft. More particularly, one embodiment of the present invention is directed to a power drive unit having the capability to detect the presence and motion of a cargo-carrying unit load device above the power drive unit. 
       BACKGROUND INFORMATION 
       [0002]    A large variety of motorized systems for moving cargo are known. Motor driven rollers are typically employed in these systems. Cargo and passenger airplanes in particular often employ a series of motor driven power drive units (“PDU”s) to quickly and efficiently propel cargo containers and pallets, otherwise known as unit load devices (“ULD”s), within the aircraft cargo compartment. This configuration can allow for the transportation of cargo from the external loader to the interior of the airplane by one or more operators controlling the PDUs. 
         [0003]    Cargo within an airplane cargo deck is typically supported by a system of freely rotating floor-mounted conveyance rollers. Sets or banks of PDUs can be simultaneously elevated from beneath the cargo deck to a level just above the conveyance rollers. Each PDU may be a separate electromechanical actuator which includes one or more rubber coated wheels or drive rollers. The drive rollers of the elevated PDUs contact and move cargo above the conveyance rollers in the commanded direction upon energization. The movement of cargo depends on the coefficient of friction between the PDU drive rollers and the bottom surface of the ULD, as well as the lifting force generated by the PDU lift mechanism. When the PDUs are deenergized, roller rotation ceases and the ULD stops moving. 
         [0004]    Several sets of PDUs can be arranged along a common path of conveyance, and each set can be operated separately, thereby allowing for the transfer of multiple pieces of cargo. An operator supervising the transportation of cargo into the cargo deck area can guide cargo by means of a joystick and an on/off switch or similar controls. 
         [0005]    PDUs can be damaged when they continue to operate beneath immobilized cargo, a condition known as scrubbing. Scrubbing occurs when cargo is too heavy or has come upon an obstruction such as a wall guide within the cargo compartment. Scrubbing can quickly wear away the rubber coating on the rollers (or the roller itself) necessitating their replacement and can result in damage to the PDU motor. 
         [0006]    Cargo container stall sensors integrated within a PDU are used to sense a stalled container and to remove power to the PDU motor after a predetermined delay to avoid PDU damage. Some PDU control systems have a manual de-select switch for removing power to the PDUs when a stall condition is determined. Unfortunately, this de-select switch is often not used properly by operators, who are focused on loading cargo rather than protecting PDUs. Thus, damage to PDUs when scrubbing conditions occur is a common problem. 
         [0007]    Known stall sensors include mechanisms for monitoring the temperature of the PDU motor, which is subject to measurement error, or require additional electromechanical mechanisms on the PDU, which are susceptible to wear and other maintenance issues. 
         [0008]    Further, in the aircraft cargo area, it is important to keep track of the location of the ULDs. The most common method of keeping track of these ULDs, while they are in the cargo area, is by detecting them as they pass over a ULD sensor which is located on the floor of the cargo compartment. One known sensing method is the use of infrared (IR) light to determine the presence of the ULD. For instance, U.S. Pat. No. 5,661,384 discloses a PDU having an IR sensor to detect the presence of cargo directly above a corresponding PDU. U.S. Pat. No. 7,014,038 also discloses employing an IR or other sensor data to detect cargo. Such systems typically employ a digital sensor which only allows two states (i.e., “ULD present” or “ULD not present”). While prior art PDUs are configured to use IR information to detect an ULD, they are not configured to detect whether the ULD is moving. 
       SUMMARY OF THE INVENTION 
       [0009]    In one aspect, the present invention is directed to a method of detecting motion of an unit load device in a cargo hold of an aircraft. The method includes providing a floor of the cargo hold of an aircraft with a power drive unit having a motor, at least one driver roller element coupled to said motor, a light source, a light detector, and a processor having a memory associated therewith for storing instructions. The method also includes emitting, with the light source, a plurality of pulses of light in a direction of an underside of an unit load device when an unit load device is over the power drive unit; detecting, with the light detector, light reflected from the underside of the unit load device; sampling the detected light to form a time series of digital samples, each digital sample representative of an intensity of reflected light; and performing calculations on a plurality of said digital samples to determine whether the unit load device is moving, wherein the calculations include at least one from the group consisting of: (a) determining the number of spikes among the digital samples; and (b) taking at least one statistic of the digital samples and comparing said at least one statistic with a corresponding at least one threshold. 
         [0010]    In another aspect, the present invention is directed to an air cargo power drive unit. The unit comprises a motor, at least one driver roller element coupled to said motor, a light source, a light detector, and a processor having a memory associated therewith for storing instructions. When executed by said processor, the instructions cause the processor to: cause the light source to emit a plurality of pulses of light in a direction of an underside of an unit load device when an unit load device is over the power drive unit; obtain a time series of digital samples, each digital sample representative of an intensity of light reflected from an underside of an unit load device and detected by the light detector when an unit load device is over the power drive unit; and perform calculations on a plurality of said digital samples to determine whether the unit load device is moving. The calculations include at least one from the group consisting of: (a) determining the number of spikes among the digital samples; and (b) taking at least one statistic of the digital samples and comparing said at least one statistic with a corresponding at least one threshold. 
         [0011]    yet another aspect, the present invention is directed to a cargo aircraft having an air cargo loading system including at least one power drive unit and unit load device overlying the power drive unit. The at least one power drive unit comprises: a motor, at least one driver roller element coupled to said motor, a light source, a light detector, and a processor having a memory associated therewith for storing instructions. When executed by the processor, the instructions cause the processor to cause the light source to emit a plurality of pulses of light in a direction of an underside of the unit load device; obtain a time series of digital samples, each digital sample representative of an intensity of light reflected from an underside of an unit load device and detected by the light detector; and perform calculations on a plurality of said digital samples to determine whether the unit load device is moving. The calculations include at least one from the group consisting of: (a) determining the number of spikes among the digital samples; and (b) taking at least one statistic of the digital samples and comparing said at least one statistic with a corresponding at least one threshold. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  illustrates the underside of an aircraft and  FIG. 2  an aircraft cargo deck that can be used to implement an embodiment of the present invention. 
           [0013]      FIG. 3  is a top view of a PDU in accordance with one embodiment of the present invention. 
           [0014]      FIG. 4  is an end view of the PDU in accordance with one embodiment of the present invention. 
           [0015]      FIG. 5  is a block diagram of the electronics of the PDU in accordance with one embodiment of the present invention. 
           [0016]      FIG. 6A  shows an ideal waveform comprising a train of light pulses and  FIG. 6B  shows an ideal output for when no ULD covers the PDU. 
           [0017]      FIG. 7A  shows an ideal waveform comprising a train of light pulses and  FIG. 7B  shows an ideal output for when a stationary ULD covers the PDU. 
           [0018]      FIG. 8A  shows an ideal waveform comprising a train of light pulses and  FIG. 8B  shows two received pulses representative of a stationary ULD and two other received pulses representative of a moving ULD.  FIG. 8C  shows a detailed view of a pulse from  FIG. 8B  that is representative of a stationary ULD and  FIG. 8D  shows a detailed view of a pulse from  FIG. 8B  that is representative of a moving ULD. 
           [0019]      FIG. 9A  shows a process flow diagram for one embodiment of how the processor can determine whether there is ULD motion using spikes in the received data. 
           [0020]      FIG. 9B  shows a process flow diagram for a second embodiment of how the processor can determine whether there is ULD motion using statistics of the received samples. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The contents of aforementioned U.S. Pat. Nos. 5,661,384, and 7,014,038 are incorporated by reference to the extent necessary to understand the present invention. In view of these references, one skilled in the art would know how to use an IR light source and an IR light detector to detect whether an ULD is present over a PDU. 
         [0022]      FIG. 1  illustrates the underside of an aircraft  25  and  FIG. 2  an aircraft cargo deck  26  that can be used to implement an embodiment of the present invention. A generally H-shaped conveyance surface  26  forms a deck of an aircraft, adjacent a cargo bay loading door  23 . However, there are many other aircraft cargo deck configurations to which the embodiments of the invention can be implemented. For example, some aircraft, particularly those configured primarily for the transportation of cargo without passengers, have the upper passenger deck removed and an additional larger cargo deck installed. Other aircraft may have three or more parallel longitudinal tracks rather than the H-shape shown in  FIG. 2 . 
         [0023]    The cargo compartment includes a cargo loading system comprising a plurality of freely rotating conveyance rollers  27  mounted in the cargo deck to define the conveyance plane. Cargo loaded onto the aircraft cargo deck can be moved manually throughout the cargo bay upon the freely rotating conveyance rollers. However, it is desirable to electro-mechanically propel the cargo with minimal or no manual assistance. To this end, the H-shaped cargo surface includes a number of PDUs  28 , that provide a mechanism upon which cargo is propelled over the conveyance rollers  27 . Each PDU  28  typically includes a drive roller element which can be raised from a lowered position beneath the cargo deck to an elevated position. These PDUs are referred to as “self-lift” PDUs. In the elevated position, the drive roller element contacts and drives the overlying cargo that rides on the conveyance rollers. Other types of PDUs, which can also be used as embodiments of the present invention, are above the conveyor plane all of the time and held up by a spring. These PDUs are referred to as “spring-lift” PDUs. 
         [0024]    In the longitudinal direction, the H-shaped conveyance surface  26  includes a left track and a right track along which cargo is to be stowed in parallel columns during flight. In the transverse direction, the cargo deck is also separated into a tail (or “aft”) section  11  and a forward section  12 . Thus, the left and right tracks are divided into four sections, two forward sections  13  and  15  and two aft sections  17  and  19 . In addition to the four sections, there is an additional path  21  between both tracks at the cargo door  23 . This additional path  21  divides the cargo bay between the forward and aft sections  11  and  12 . This path is used to move cargo into and out of the aircraft, and also to transfer cargo between the left and right storage tracks. 
         [0025]    In one embodiment, a human operator manipulates control elements to selectively and electrically energize PDUs  28  in each of the five aforementioned sections  13 ,  15 ,  17 ,  19  and  21 . Typically, these controls are mounted in an operator interface unit. The control elements may be mounted on a wall or other structure within the cargo bay or may be portable, e.g., the controls may be in a hand held pendant. These controls will typically have an on/off switch and a joystick which, depending on the direction pushed, will energize a set of PDUs  28 , causing groups of drive roller elements to be elevated (if not already elevated) and rotated in one of two possible directions (i.e., forward or reverse). A section of PDUs will remain energized as long as the joystick is held in a corresponding position. When the joystick is released, the selected set of PDUs is de-energized. In the case of self-lifting PDUs, the drive roller elements are returned to their retracted position below the plane of the conveyance rollers  27 ; in the case of spring-lift PDUs, the PDUs remain biased in the upward position and brakes are applied to hold the cargo containers in place. Control systems of this type are known in the art. 
         [0026]      FIG. 3  is a top view of a PDU  28  in accordance with one embodiment of the present invention. PDU  28  includes a housing  30  which incorporates a pair of wheels  51  and  52  that function as drive roller elements. Wheels  51  and  52  are coupled to a drive shaft (not shown). PDU  28  further includes necessary motor and gear assemblies and other necessary components (not shown) for turning and/or raising wheels  51  and  52  so that wheels  51  and  52  are positioned above the cargo deck and are able to contact the bottom of a ULD. PDU  28  further includes an electronics cavity that is separated from the rest of the PDU by a wall  53  for housing the necessary electronics (disclosed in more detail below), and includes an electrical connector  56  for coupling the electronics to a power and a control source. PDU  28  further includes a light source  57 , such as an infrared light (“IR”) transmitter having a light emitting diode (“LED”), for emitting infrared light. PDU  28  further includes a light detector  57 , such as an IR receiver having a photo diode or photo transistor and perhaps other circuitry such as signal amplifiers, automatic gain control, bandpass filters and the like, for detecting the presence of infrared light. In other embodiments, other types of light besides IR can be used. It is understood by those having ordinary skill in the art that when the light source  57  emits light of a particular center wavelength (e.g., infrared), the light detector  58  will be selected based on its response characteristics in the relevant wavelength, and may be accompanied by appropriate optical filters, lenses and the like. 
         [0027]      FIG. 4  is an end view of PDU  28  in accordance with one embodiment of the present invention, and illustrates the relationship of PDU  28  with the bottom surface  60  of a ULD that is passing over and being propelled by PDU  28 . The light source  57  emits light that bounces off the bottom surface  60  (assuming a ULD is present) and is reflected back to light detector  58  where it is processed by the electronics of PDU  28 . 
         [0028]      FIG. 5  is a block diagram of the ULD sensor and scrub sensor electronics of PDU  28  in accordance with one embodiment of the present invention. Coupled to light detector  58  is an analog to digital (“A/D”) converter  70  that takes an analog input from the light detector  58  and converts it to a digital value representative of an instantaneous intensity of light. Coupled to A/D converter  70  is a processor  72  and memory  74 . Processor  72  may be any type of general purpose processor, and memory  74  may be any type of storage device that stores instructions to be executed by processor  72 . In one embodiment, processor  72  may include A/D converter  70  and/or memory  74 . Light source  57  is coupled to an output pin of processor  72 . In one embodiment, a power driver is included between processor  72  and the light source  57 . 
         [0029]    A processor-adjustable variable resistor may be coupled to processor  72  and light detector  58 . The variable resistor is used to set the sensitivity of A/D converter  70 , which selects the window of light that the sensor will measure (i.e., the minimum strength of light that will be detected and the greatest strength of light that can be measured before the A/D output reaches its maximum value). 
         [0030]    In one embodiment, the A/D converter  70  is a 10-bit A/D converter, although A/D converters of other bit resolutions may be used instead. In one embodiment, the A/D converter  70  samples the time-varying light intensity at a rate of 200 samples/second, or at 5 msec intervals. Thus, for a one-quarter second pulse, a time series of 50 digital samples are taken, and these are provided to the processor  72  for further calculations. It is understood that not all 50 samples may be used due to start-up transients in the first few digital samples. It is further understood that other sampling rates may be used, depending on the A/D converter  70  and processor  72  speed. 
         [0031]      FIG. 6A  shows an example of an ideal output waveform  202  emitted by the light source  57 . The output waveform  202  comprises a train of light pulses  204  with a nominal pulse height represented by some voltage V 0 . In the embodiment shown, these pulses comprise square waves with an ON period  206  of W 1 , an OFF period  208  of W 2 , and a total period of W 3 =W 1 +W 2 . In a particularly preferred embodiment W 1  =W 2  for a 50% ON-time duty cycle, though it is possible to have other duty cycles, as well. 
         [0032]      FIG. 6B  shows an ideal waveform  222  output by the light detector  58  (i.e., the received light) when no ULD is covering the PDU. In the absence of an object, e.g., an ULD, covering the PDU  28 , the emitted light pulses  240  are not reflected off of the bottom surface of that object, and so no light energy (i.e., 0 volts) should be received at the light detector  58 , whose output is therefore flat. In reality, however, there may be some small amount of ambient light of the appropriate wavelength, such as ‘bleed’ from the light source  57 , that impinges on the light detector  58 , thus resulting in minimal received light energy. However, this minimal received light energy is generally below some threshold value and therefore is ignored by the processor  72 . 
         [0033]      FIG. 7A  shows the same ideal output waveform  202  seen in  FIG. 6  while  FIG. 7B  shows, for comparison, an ideal waveform  242  output by the light detector  58  when a stationary object is covering the PDU  28 . When a stationary object such as an ULD covers the PDU  28 , the emitted light pulses  240  are reflected off of the bottom surface of that object and the reflected light energy is received at the light detector  58 . The output of the detector  58  is representative of the time-varying intensity of the reflected light. However, since the object is stationary, ideally, the detector output will perfectly track the emitted waveform, with the intensity of detected light on the output side of the detector  58  being represented by some voltage value Vc. 
         [0034]      FIGS. 8A and 8B  also correspond to the situation in which an ULD covers the PDU  28 , but shows more realistic, non-ideal output.  FIG. 8A  shows the same ideal output waveform  202  seen in  FIGS. 6 and 7 .  FIG. 8B  shows a waveform  260  comprising four pulses of detected light. In this instance, the ULD is initially stationary during the first two pulses  262 ,  264 , and then is in motion during the last two pulses  266 ,  268 . 
         [0035]    When the ULD is stationary, the detected pulses  262 ,  264  have intensity values that are all confined in a narrow band defined  270  between V LO  and V HI . This is because the emitted waveform  202  impinges on the same location on the underside of the ULD, and so the reflected light is substantially unaffected by variations in the surface of the underside of the ULD.  FIG. 8C  shows a magnified view of the detected pulse  264  and shows that all intensity values within the pulse  264  are between the lower limit  270 L and the upper limit  270 H of the band. 
         [0036]    In contrast, when the ULD is in motion, the detected pulses  266 ,  268  have intensity values that go outside this band  270  from time to time. This happens because as the ULD moves, different portions of its underside pass over the PDU  28 , and variations in the surface of the underside cause corresponding variations the instantaneous intensity of the reflected pulses. Generally speaking, at least some of these instantaneous sample values go outside the band  270 .  FIG. 8D  shows a magnified view of the detected pulse  267  and shows that some of received intensity values, designated  280 A-H are outside the band  270 . Digital sample values which fall outside the band  270  are referred to as “spikes”. 
         [0037]    In one embodiment, the processor, which dictates when the light source  57  emits pulses, only processes samples received from the A/D converter  70  when the light source  57  is emitting a pulse. For instance, the processor  72  may begin to accept samples from the A/D converter  70  when the light source  57  is energized and discontinue accepting samples when the light source  57  finishes emitting a pulse  204 , or perhaps some very short predetermined time thereafter. This cycle is then repeated for the next pulse. In another embodiment, the processor  72  continuously accepts and processes samples from the A/D converter  70 , and employs an algorithm to detect pulses, such as by looking for a rising pulse edge, in a known manner. For an A/D sample rate of 200 samples/sec, and a pulse width of 0.25 second, in the case where the processor  72  only processes samples when the light emitter is outputting light pulse  204 , roughly 100 samples are taken per ON/OFF cycle. It is within these 50 or so digital samples during which the light source is on that, in one embodiment of the present invention, the processor  72  looks for spikes. 
         [0038]      FIGS. 9A and 9B  depict two general flow diagrams  910 ,  930 , respectively, illustrating the functionality performed by PDU  28  in order to determine whether an ULD is in motion, in accordance with two general embodiments of the present invention. In both embodiments, the functionality is preferably implemented by software stored in memory  74  and executed by processor  72 . In other embodiments, the functionality can be performed by hardware, or any combination of hardware and software. 
         [0039]    In the general embodiment represented by  FIG. 9A , the flow diagram  910  depicts the principal steps carried out by the processor  72  in looking for spikes in a received pulse. 
         [0040]    In step  912 , the processor  72  accepts digital samples from the A/D converter  70 . 
         [0041]    In step  914 , the processor looks for pulse edges so that it can focus on the pulse data, rather than on data corresponding to where no pulse is present. 
         [0042]    In step  916 , the processor  72  calculates a mean for the digital samples within the received pulse. In some embodiments, the mean may constitute a running mean which is calculated based on digital samples from earlier returned pulses and also digital samples from a current pulse. A new running mean may be calculated from a weighted average of a current running mean and newly acquired digital samples. In one embodiment, the digital samples are de-meaned prior to further processing, though this is not an absolute requirement. It is noted that for some embodiments, however, the mean may not need to be calculated to identify spikes. 
         [0043]    In step  918 , the processor  72  establishes the criteria of determining spikes. In one embodiment, this can entail establishing the band  270 . Digital samples falling outside this band are deemed to be spikes. The band  270  can be established in a number of ways. For instance, the band  270  may be determined by using a threshold value based on predetermined fraction or percentage of the mean. Thus, if a predetermined fraction of 1/16 (i.e., for a predetermined percentage of 6.25%) is used, then the threshold value T 1  would be 1/16 of the mean and the band  270  would straddle either side of the mean by 1/16 of that mean value. Other predetermined fractions or percentages may be used instead. Alternatively, the threshold value T 1  may be determined through other heuristics. Regardless of how the band is established, in this embodiment, spikes are defined as those values that vary from the mean by at least T 1 , i.e. V HI =Mean+T 1  and V LO =Mean−T 1 . 
         [0044]    In another embodiment discussed below, a spike is found to be present if the values of two digital samples within a window differ by some minimum threshold T 2 . 
         [0045]    In step  920 , the processor  72  performs calculations on the digital samples, looking for spikes. 
         [0046]    Finally, in step  920 , the processor  72  determines whether the spikes collectively meet the requisite criteria for the PDU  28  to determine that the ULD is in motion. 
         [0047]    A number of different criteria may be employed to determine whether or not the ULD is in motion. 
         [0048]    In one embodiment, the ULD is deemed to be moving only if at least one received pulse contains an integer number K spikes, where K is at least 1. In another embodiment, the ULD is deemed to be moving only if two or more successive pulses each contain at least K spikes. 
         [0049]    In still another embodiment, a ‘voting’ system may be used in which the ULD is deemed to be in motion only if a first integer number X out of a second integer number Y successive received pulses each contain at least K spikes. 
         [0050]    In still another embodiment, the ULD is deemed to be moving only if at least K digital samples within at least one pulse differ from the mean value for the digital samples by at least a first threshold. In one variation, a “local” mean value based on a first number of digital samples falling within a window of, e.g., 20 samples, may be used for this purpose, and a number windows within a pulse may be studied. The windows may be overlapping, such as having a 50% overlap. Whether or not a local mean value is used, in a further refinement, the ULD may only be deemed to be moving if at least one digital sample is greater than the mean value and another digital sample is below the mean value. In other words, at least one digital sample must fall on either side of the band, before it is determined that the ULD is in motion. In some embodiments of this approach when windows are used, once K such “outliers” are identified, the remainder of the pulse is not processed, since the criterion has been met for deciding that the ULD is in motion. 
         [0051]    In yet another embodiment, the criterion for finding a spike is that at least one digital sample differs from another digital sample within a received pulse by at least some threshold value. Again, both digital samples may be required to fall within the same window. For example, the processor may use a window of, say, 15 samples and determine whether two samples within that window differ from each other by at least some threshold value T 2 . If so, it is determined that a spike exists in the window, and thus, in the received pulse. This calculation is then performed for a number of such (possibly overlapping) windows along the received pulse. If at least K such spikes are identified in the pulse, then it is decided that the ULD is in motion. Again, in some embodiments of this approach using windows, once K such spikes are identified, the remainder of the pulse is not processed, since the criterion has been met for deciding that the ULD is in motion. Also, in this embodiment, since one is simply trying to determine whether two digital samples within a window have values that differ by the threshold T 2 , it is not necessary to first calculate the mean or de-mean the digital samples. 
         [0052]    In any of the above embodiments, the precise number for K generally will be determined through trial runs and testing where the ULD is known to be in motion. Thus, it is possible that K can be any integer number, such as 1, 2, 3, or even more. 
         [0053]    In the general embodiment represented by  FIG. 9B , the flow diagram  930  depicts the principal steps carried out by the processor  72  to employ second order statistics to determine whether a received pulse indicates motion of an ULD. 
         [0054]    In step  932 , the processor  72  accepts digital samples from the A/D converter  70 . 
         [0055]    In step  934 , the processor  72  looks for pulse edges so that it can focus on the pulse data, rather than on data corresponding to where no pulse is present. 
         [0056]    In step  936 , the processor  72  calculates a mean for the digital samples within the received pulse. Again, in some embodiments, the mean may constitute a running mean which is calculated based on digital samples from earlier returned pulses and also digital samples from a current pulse. A new running mean may be calculated from a weighted average of a current running mean and newly acquired digital samples 
         [0057]    In step  93   8 , the processor  72  calculates one or more statistics for a pulse using the mean that was calculated in step  936 . In one embodiment, the statistics may be of the general form: 
         [0000]    
       
         
           
             
               P 
               n 
             
             = 
             
               
                 1 
                 M 
               
                
               
                 
                   ∑ 
                   
                     i 
                     = 
                     1 
                   
                   M 
                 
                  
                 
                     
                 
                  
                 
                   
                      
                     
                       
                         X 
                         i 
                       
                       - 
                       
                         X 
                         0 
                       
                     
                      
                   
                   n 
                 
               
             
           
         
       
     
         [0058]    where: P n  is the calculated n th  order statistic that is compared against some corresponding threshold value T 3   n ; i is an index, M is the number of samples from the pulse (e.g., M=50) that are used to calculate a given statistic; X i  is the value of the i th  digital sample in the pulse (or window within a pulse); X 0  is the mean (whether it is the mean for that pulse or a moving average); and n is the order of the statistics and so corresponds to the power to which the absolute value of the difference (X i −X 0 ) is taken. 
         [0059]    It is understood that first, second, third, fourth, or even higher order statistics may be taken. It is also understood that a decision may be made based on a single statistic of a single order, or on a vector comprising a plurality of statistics of different orders, in which instance a corresponding plurality of thresholds {T 3   n } may be established. Finally, it is also understood that in other embodiments, statistics other than those represented by the general formula above may be taken. Regardless of which statistic(s) is/(are) used, they may be calculated either on a received-pulse-by-received pulse basis, or for each of a plurality of (possibly overlapping) windows within a single pulse. For each such window (or for the entire pulse, if so calculated), the processor determines whether each corresponding statistic exceeds some predetermined threshold T 3   n . If so, it is then determined that the ULD is moving. 
         [0060]    Several embodiments of the present invention are specifically illustrated and/or described herein. However, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.