Abstract:
Systems and methods for measuring aircraft altitude less than 10 meters. Two signals having different frequencies are simultaneously transmitted. Returns or echos of the two signals are received. The phase of the return of the first and second signals are determined. A distance value based on the determined phase of the first and second return signals is determined.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application Ser. No. 60/621,588, filed Oct. 22, 2004, which is hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Commercial aircraft exhibit their highest take-off fuel efficiency when departing the ground most rapidly. Rapid ground departure escapes ground effect drag quickly and improves fuel economy. However, the possibility of striking the aircraft tail on the ground if the angle of attack is too steep exists. If the distance between the tail and the ground could be accurately measured, the flight controls could limit the angle of attack automatically to prevent tail strikes as well as provide the peripheral benefit of improved take-off fuel efficiency.  
         [0003]     Presently, Frequency Modulation, Continuous Wave (FMCW) radar altimeters require frequency sweep rates of &gt;1 GHz to measure distance to within an accuracy of ˜2″. This is an impractical implementation for a radar altimeter as well as illegal, as the bandwidth of such a transmitter would exceed the allowable frequency allocation for radar altimeters. However, there exists a need for accuracy of ˜2″ without the complexity or the high cost. Therefore, there exists a need for more accurate close range radar altimeters for use in applications, such as tail strike warning system.  
       BRIEF SUMMARY OF THE INVENTION  
       [0004]     Embodiments of the present invention include systems and methods for measuring aircraft altitude when the aircraft is less than 10 meters above ground level. Two modulating signals having different frequencies are simultaneously transmitted. Returns or echos of the composite signal are received. The phase of the return of the first modulating and second modulating signals are determined. A distance value based on the determined phase of the first and second return modulations is determined. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0005]     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings.  
         [0006]      FIG. 1  illustrates an exemplary system formed in accordance with an embodiment of the present invention;  
         [0007]      FIG. 2  illustrates a flow diagram illustrating an exemplary process performed by the system shown in  FIG. 1 ; and  
         [0008]      FIGS. 3 and 4  illustrate two signals outputted by the system of  FIG. 1  for use by that system in determining distance. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0009]      FIG. 1  illustrates an example system  20  that generates highly accurate low range distance calculations in an aircraft  18 . In one embodiment, the system  20  includes a radar altimeter  22 , an altitude processor  40 , and a tail strike warning system  42 . The radar altimeter  22  includes a transmitter  24 , a receiver  26 , and a circulator  28 , nulling circuitry  30 , and an antenna  32 . The altitude processor  40  is in signal communication with the transmitter  24  and the receiver  26 . The tail strike warning system  42  is in signal communication with the altitude processor  40 .  
         [0010]     The circulator  28  is configured to direct signals generated by the transmitter  24  to the antenna  32  and to direct signals received by the antenna  32  to the receiver  26 . The nulling circuit  30  provides additional isolation of the receiver  26  from the transmitter  24  by reducing the level of the transmitter signal present in the receiver beyond the isolation provided by the circulator  28 .  
         [0011]     The altitude processor  40  receives radar return signals from the receiver  26  and determines a distance value based on the signals received from the receiver  26 . The distance value is sent to the tail strike warning system  42 . The tail strike warning system  42  activates an alert if the distance value is below a threshold amount. The distance value may be sent to other aircraft systems  48 , such as a navigation or flight management system. The navigation or flight management system may command flight controls in order to maximize climb angle and protect against over-rotation and attendant tail strikes.  
         [0012]      FIG. 2  illustrates a flow diagram of an example process  60  performed by the system  20  shown in  FIG. 1 . First, at a block  62 , a signal having two different modulating frequencies is transmitted via the antenna  32 . The radar return of the transmitted signal is sent to the altitude processor  40 . The altitude processor  40  determines the phase of the returned first modulation, see block  64 . At a block  66 , the altitude processor  40  uses the determined phase of the first modulation (modulating signal) to identify a range value using the returned second modulating signal. In this embodiment, the first modulation has a lower frequency than the second modulation (modulating signal). Because the first modulation has a lower frequency, the return of the first modulating signal is used to roughly determine a distance value or determine a phase location that is used when analyzing the return second modulating signal. Because the second signal is a higher frequency than the first signal, it is more accurate for determining the distance value.  
         [0013]     The present invention measures coarse distance using a low frequency amplitude modulated signal and resolves this to highly accurate value using a second higher frequency amplitude modulated signal. Range accuracy is dependent on the accuracy of the phase measurement.  
         [0014]     Range accuracy improves by the ratio of the frequency of the primary signal to the frequency of the secondary signal, provided the secondary frequency is not so much higher than the primary frequency that range error of the primary frequency exceeds the wavelength of the secondary frequency minus the range accuracy of the secondary frequency, see  FIGS. 3 and 4 .  
         [0015]     The phase angle can be measured only up to 360° without ambiguity and the accuracy of the phase measurement is the same for all modulation frequencies. Converting from wavelength, λ 1 , to frequency using c=f*λ 1 , where c=speed of light, f 1(max) &lt;8*c/λ 1 , or f 1(max) &lt;8*c/R max .  
         [0016]     Because the phase angle can be accurately measured to 360° without ambiguity, the maximum useful range is 2*R max &lt;λ 1 , which makes f max &lt;2*c/R max . However, if the phase of the first modulating frequency is allowed to approach 360°, it is possible that a return of greater than 360° may be received and this would be undetectable without additional measures, such as a third modulating frequency. To prevent this from occurring, it may be advantageous to design the receiver to have insufficient sensitivity to detect a signal returned from a distance corresponding to greater than 90°. For this example, twice the maximum useful range, 2*R max , is limited to &lt;λ 1 /4, or a 90° phase difference. Therefore, R max &lt;λ 1 /8, or λ 1 &gt;8*R max .  
         [0017]     With regard to range accuracy, if the accuracy of the phase angle measurement is limited to ±x 1 °, range accuracy uncertainty is dx 1 =λ 1 *x 1 /360. Thus, we know the coarse range only to within a distance of ±dx 1 . In order to use amplitude modulation signal # 2  to resolve fine range, the wavelength of signal # 2  must be longer than the range uncertainty from modulating frequency f 1  of the first signal. 
 
Therefore, λ 2 &gt;2*λ 1   *x   1 /360, or 
 
 f   2   &lt;f   1 *360/(2*i x 1 ) 
 
         [0018]     However, the phase angle of modulating frequency f 2  can be measured only to some limited accuracy, ±x 2 °. Therefore, the frequency f 2  of signal # 2  must be decreased by a factor of (360−x 2 )/360.  
         [0019]     Thus, 
 
 f 2 ′=f 2*((320− x   2 )/360)=((360− x   2 )/360)* f   1 *360/(2 *x   1 )=((360 −x   2 )* f   1 /(2 *x   1 ) 
 
         [0020]     Where f 2 ′ is the corrected secondary range measurement frequency.  
         [0021]     The ultimate range accuracy is dx 2 =λ 2 *x 2 /360.  
         [0022]     Depending on the required accuracy of the range measurements, one is free to choose frequencies lower than the ones calculated.  
         [0023]     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. For example, other radar altimeter configurations may be used, such as a dual antenna radar altimeter. Another example is a radar altimeter configuration using three or more modulating frequencies. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow here.