Abstract:
Systems and methods for radar altimeter antenna performance monitoring via reflected power measurements are provided. In one embodiment, a single antenna radar altimeter comprises: an antenna; a circulator coupled to the antenna; a transmitter coupled to the circulator; a receiver coupled to the circulator; wherein the circulator provides coupling of the transmitter and the receiver to the antenna while providing isolation between the transmitter and the receiver; a reflected power monitor positioned between the circulator and receiver; and a processor coupled to the reflected power monitor via a first analog-to-digital converter, the processor configured to compute and track reflected power measurement statistics from data generated by the reflected power monitor and provide a performance output indicating when one or more of the reflected power measurement statistics exceed a predetermined deviation threshold.

Description:
BACKGROUND 
     Radar Altimeters are used by aircraft for determining the aircraft&#39;s distance to the ground. The formation of ice or fluids on radar altimeter antennas results in degradations of the radar pattern shape which can cause the radar altimeter to provide hazardous misleading information to the aircraft&#39;s flight crew and/or flight computer. 
     For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the specification, there is a need in the art for systems and methods that provide a dynamic means to monitor performance of radar altimeter antennas. 
     SUMMARY 
     The Embodiments of the present invention provide methods and systems for to monitor performance of radar altimeter antennas and will be understood by reading and studying the following specification. 
     Systems and methods for radar altimeter antenna performance monitoring via reflected power measurements are provided. In one embodiment, a single antenna radar altimeter comprises: an antenna; a circulator coupled to the antenna; a transmitter coupled to the circulator; a receiver coupled to the circulator; wherein the circulator provides coupling of the transmitter and the receiver to the antenna while providing isolation between the transmitter and the receiver; a reflected power monitor positioned between the circulator and receiver; and a processor coupled to the reflected power monitor via a first analog-to-digital converter, the processor configured to compute and track reflected power measurement statistics from data generated by the reflected power monitor and provide a performance output indicating when one or more of the reflected power measurement statistics exceed a predetermined deviation threshold. 
    
    
     
       DRAWINGS 
       Embodiments of the present invention can be more easily understood and further advantages and uses thereof more readily apparent, when considered in view of the description of the preferred embodiments and the following figures in which: 
         FIG. 1  is a block diagram illustrating a radar altimeter of one embodiment of the present invention; and 
         FIG. 2  is flow chart illustrating a method of one embodiment of the present invention. 
     
    
    
     In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize features relevant to the present invention. Reference characters denote like elements throughout figures and text. 
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of specific illustrative embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense. 
     Embodiments of the present invention utilize characteristics of voltage standing wave ratio (VSWR) estimates to monitor for the occurrence of static and dynamic events to evaluate the operational status of a single antenna radar altimeter (SARA). Dynamic event are characterized as relatively transient in nature and include external events such as sprays of water, rain, snow, sand, and the like falling upon the SARA antenna dome. Static events are characterized by relatively longer term events such as ice accumulation on the SARA antenna, water intrusion of the SARA electronics and other long term antenna degradations. 
     VSWR is understood in the art as a ratio of two values. Technically VSWR=(1+β)/(1−ρ) where ρ is the Reflection Coefficient that is defined as the ratio of the Voltage Transmitted to the Voltage Reflected. In the embodiments present below, an estimate or proxy for a VSWR measurement is obtained by measuring only the voltage reflected from an antenna without computing the entire calculation for VSWR. Detecting antenna degradations can be accomplished by knowing what a nominal reflected power range for a SARA&#39;s antenna should be and what reflected power is not acceptable. By monitoring this one portion of the overall measurement embodiments of the present invention can make antenna degradations determinations as discussed below. 
       FIG. 1  is a simplified block diagram of a single antenna radar altimeter (SARA)  100  of one embodiment of the present invention. SARA  100  comprises a single antenna  150  (typically housed within a radar dome  151 ), a transmitter  156  and a receiver  158  each coupled to a circulator  152 . In this embodiment, circulator  152  provides coupling of transmitter  156  and receiver  158  to antenna  150  while providing isolation between transmitter  156  and receiver  158 . In this embodiment, operation of transmitter  156  is driven by a programmable logic device  166 . The receiver  158 , is coupled to an analog-to-digital converter (ADC)  160  which provides digital samples of the signal received by receiver  158  to an external component  132  (such as a navigation processor, for example) for further signal processing. Specific details regarding the operation of single antenna radar altimeters in general are available to those of ordinary skill in the art, and for that reason are not repeated. See, U.S. Pat. Nos. 7,161,527 and 7,239,266, herein incorporated by reference. 
     As mentioned above, embodiments of the present invention introduce the concept of utilizing characteristics of reflected power measurements (as a proxy for obtaining actual VSWR values) to identify, characterize and respond to event that affect operation of a SARA. Accordingly, SARA  100  further includes a reflected power monitor  110  coupled between circulator  152  and receiver  158 . The output of reflected power monitor  110  is coupled to Reflected Power ADC  112 , which in turn in coupled to a processor  114 . 
     Although circulator  152  isolates receiver  158  from directly receiving signals transmitted from transmitter  156 , receiver  158  will still receive a certain amount of such transmissions as reflected power from antenna  150 . That is, because antenna  150  is not 100% impedance matched with the atmosphere, some small percent of the transmitted signal&#39;s power will not be propagated into the atmosphere but will instead be reflected back towards circulator  152 . Since circulator  152 &#39;s function is to pass signals received from antenna  150  to receiver  158 , the transmitted signal&#39;s reflected power is received by receiver  158 . Reflected power monitor  110  observes this reflected power and outputs a reflected power measurement. In the embodiment of  FIG. 1 , the reflected power measurement is an analog signal which is digitally sampled by Reflected Power ADC  112 . The digital samples produced by Reflected Power ADC  112  are processed by processor  114  as discussed in greater detail below. In one embodiment Reflected Power ADC  112  is a 6 bit converter which provides a resolution of 64 possible output values which represent a range from −20 dBm to −2 dBm (a −18 dB dynamic range). In other embodiments, other resolutions can be used. 
     When antenna  150  is operating normally, the amount of power reflected back toward receiver  158  should be a consistent reflected power level (as measured in dB) less than what was transmitted by transmitter  156 . When fluids, sand or ice accumulate on, or impinge on, antenna  150 , that changes antenna  150 &#39;s ability to transmit a signal into space, which affects the proportion of the transmit signal power that is reflected back by antenna  150 . With embodiments of the present invention deviations in reflected power level are analyzed to identify faults or degradations that can affect SARA  100 &#39;s ability to provide reliable altitude measurements. 
     In one embodiment, in operation, processor  114  collects samples of reflected power dB measurements into a sample group. The sample group comprises a group of reflected power measurement samples that are collected over a sampling period that is equal to the radar modulation period used by transmitter  156 . For example, where the radar modulation period is 1 mSec, processor  114  collects samples over that 1 mSec period as a sample group. In one embodiment, Reflected Power ADC  112  is clocked to produce 256 samples of the reflected power measurements for every 1 mSec period (that is, each sample group would include 256 samples of reflected power measurements) which is a sampling rate of 256 KHz. In other embodiments, Reflected Power ADC  112  can operate at other clock speeds. The clock speed for obtaining samples of reflected power can be readily determined by one of ordinary skill in the art upon reading this specification based on the particular design of the SARA to which embodiments of the present invention are being utilized. 
     Because the both the shape and the power level of a signal transmitted by transmitter  156  over the modulation period is known, and because the reflected power from antenna  150  under non-event conditions is known (that is, the reflected power should vary in a predictable manner as a function of the transmit signal over the modulation period), it possible to collect baseline reflected power dB power statistics for SARA  100 , which are stored in memory  116  of processor  114 . In one embodiment, the collected baseline reflected power dB power statistics over a sampling period for a baseline sample group include a maximum reflected power in dB, a minimum reflected power in dB, a Δreflected power max-min  in dB and an average reflected power in dB. Subsequently, when SARA  100  is in operation under field conditions, real-time reflected power dB power statistics are computed and tracked and compared to the base line sample group to identify anomalous operating conditions. 
     For example, in one embodiment in operation, processor  114  computes and tracks maximum reflected power in dB, minimum V reflected power in dB, ΔV reflected power max-min  in dB and average reflected power in dB for sample groups over time. For this example, the expected normal Δreflected power max-min  in dB for baseline conditions is 1.5 dB with an average reflected power of −15.5 dB. As long as the real-time reflected power dB power statistics are within predetermined deviation thresholds for these values, antenna  150  is presumed to be dry and the altimeter readings provided by SARA  100  are presumed valid. 
     Dynamic events such as fluids impacting antenna  150  will not significantly affect the average reflected power measurement, but are detectable from a sudden increase in Δreflected power max-min  as well as changes in Δreflected power max-min  from one measured sample group to the next. When random patterns of fluids impact antenna  150 , the fluid will cause and improvement in the impedance matching of the antenna  150  with the atmosphere for one moment (reducing reflected power), and reduce the impedance matching of antenna  150  at another moment (increasing the reflected power). For this reason, the Δreflected power max-min  within a sample group can be expected to increase under such conditions with respect to the baseline, and the Δreflected power max-min  from one sample group to the next will also fluctuate with respect to each other. In one embodiment, when the Δreflected power max-min  exceeds a predetermined threshold (such as 4.5 dB for example), without deviation in average reflected power beyond a threshold value, processor  114  flags the event as a media impact on the antenna event. Alternately, can utilize maximum reflected power instead of, or in addition to average reflected power. For example, in one embodiment, when the Δreflected power max-min  exceeds a predetermined threshold, without an increase in the maximum reflected power beyond a threshold value, processor  114  flags the event as a dynamic media impact event. Processor  114 , in one embodiment, reacts to the dynamic media impact event by altering its signal processing to accommodate the dynamic data. If the condition persists over a predetermined time period, processor  114  will output a No Computed Data (NCD) signal. Once the event conditions have passed (i.e., the reflected power statistics return to within threshold values), processor  114  then returns normal processing. 
     Unlike dynamic events, static event such as water intrusion or ice buildup on antenna  150  (or dome  151 ) can be expected to significantly affect the average reflected power measurement and maximum reflected power measurements, but not necessarily cause an increase in Δreflected power max-min  with respect to baseline data. For example, as ice builds up on the antenna  150 , the reflected power increases as the antenna&#39;s ability to propagate power to the atmosphere decreases. Thus both the maximum and average reflected power can be expected to be increased over baseline values for as long as the condition exists. A flooded radar antenna dome will be characterized as a flat reflected power response over the modulation period, as opposed to a Δreflected power max-min  increase over baseline. In addition, because of the reduced ability to transmit power to the atmosphere with water inside the radar antenna dome the maximum and average reflected power measurements will increase over baseline. In one embodiment, when one or both of the maximum and average reflected power exceed a threshold, processor  114  generates a failure warning (FW) override signal. 
     In an alternate embodiment, the additional monitoring of transmitted power can provide measurements to augment the reflected power information. As illustrated in  FIG. 1 , in one embodiment, SARA  100  further includes a transmitted power monitor  120  coupled to a transmit power ADC  122  which is coupled to processor  114 . In addition to the conditions discussed above, conditions suggesting additional fault modes can be identified when transmit power measurements are available. For example, if the reflected power measurements should collapse, a loss of power at transmitter power monitor  120  can confirm a transmitter  156  failure. A loss of reflected power measurements when power at transmitter power monitor  120  is normal can indicate a circulator  152  failure (i.e., the signal path between antenna  150  and receiver  158  has failed). An increase in power at reflected power monitor  110  when power at transmitter monitor  120  is normal can also indicate a circulator  152  failure (i.e., circulator  152  no longer inhibits leakage current from transmitter  156  to receiver  158 ). 
       FIG. 2  is a flow chart illustrating a method of one embodiment of the present invention. The method begins at  210  with measuring a reflected power from an antenna of a single antenna radar altimeter. As discussed above, by obtaining reflected power measurements and monitoring them over time, identifying the occurrence of static and dynamic events can be used to evaluate the operational status of a single antenna radar altimeter (SARA). The method proceeds to  212  with computing and tracking reflected power measurement statistics from the reflected power from the antenna. In one embodiment, the reflected power measurement statistics include a Maximum reflected power, a Minimum reflected power and a Δreflected power max-min . In one embodiment, as the reflected power raw data measurements and statistics are computed by a processor and stored in memory so that changes in reflected power can be monitored over time. 
     In one embodiment, “computing and tracking” at block comprises collecting samples of the reflected power measurements (in dB) as a sample group over a sampling period that is equal to the radar modulation period used by SARA. In that case, Maximum reflected power, a Minimum reflected power and a Δreflected power max-min  statistics each describe those statistics with respect to a particular sample group. For example, where the SARA&#39;s radar modulation period is 1 mSec, “computing and tracking” at block  212  comprises collecting samples over that 1 mSec period as a sample group. In one embodiment, an analog to digital converter coupled to a reflected power monitor is clocked to produce 256 samples of the reflected power measurements for every 1 mSec period (that is, each sample group would include 256 samples of reflected power measurements). In other embodiments, other clock speeds can be used as mentioned above. 
     Because the both the shape and the power level the transmitted signal over the modulation period is known, the pattern of reflected power under normal conditions from the SARA antenna is also known. The reflected power will vary in a predictable manner as a function of the transmit signal over the modulation period establishing baseline reflected power dB power statistics used for identifying anomalous conditions. In one embodiment, collected baseline reflected power dB power statistics include a maximum reflected power, a minimum reflected power, and a Δreflected power max-min  (which in one embodiment are each measured in dB). In alternate embodiment, other statistics such as an average reflected power can also be computed and tracked. 
     The method proceeds to  218  with evaluating changes in reflected power measurement statistics to identify a static event failure condition. Where a failure condition is identified (at block  220 ), the method proceeds to  222  with providing a failure warning override response. Unlike dynamic events, static event such as water intrusion or ice building up on the SARA antenna will significantly increase the Maximum reflected power measurements because reflected power from the antenna will increase as the antenna&#39;s ability to propagate power to the atmosphere decreases. Note that such static event will not necessarily cause an increase in Δreflected power max-min  because both the minimum and average reflected power can also be expected to be increased over baseline values for as long as the condition exists. For example a flooded radar antenna dome housing will be characterized as a flat reflected power response (i.e., having a small Δreflected power max-min  over the modulation period). 
     Accordingly for some embodiments, evaluating changes in reflected power measurement statistics is based on increases in either Maximum or average reflected power of a sample group as compared with previous sample groups. Then, when one or both of the maximum and average reflected power exceed a threshold a failure warning (FW) override signal is generated. 
     The method proceeds to  224  with evaluating changes in a Δreflected power max-min  measurement statistic to identify a dynamic event condition. Where a failure condition is identified (block  226 ), the method proceeds to  228  with providing a no computed data (NCD) override. Dynamic events such as fluids impacting a SARA&#39;s antenna will not significantly affect the average reflected power measurement, but are detectable from a sudden increase in Δreflected power max-min  and by observing variations in Δreflected power max-min  from one measured sample group to the next. As mentioned above, this is because, random patterns of fluids impacting the antenna will alternate between causing improvements and reductions in the impedance matching of the antenna with the atmosphere. The Δreflected power max-min  within a sample group can be expected to increase under such conditions with respect to the baseline, and the Δreflected power max-min  from one sample group to the next will also fluctuate with respect to each other. In one embodiment, if the dynamic event condition persists over a predetermined time period, then the No Computed Data (NCD) signal is generated. 
     For example, in one embodiment a normal Δreflected power max-min  for a SARA is less than 1.5 dB. A dynamic event such as fluid impact on the antenna dome causes the Δreflected power max-min  to exceed 4.5 dB. Where fast Fourier transforms are being used to evaluate the reflected power measurements, Δreflected power max-min  needs to be evaluated over corresponding samples of a sample group when comparing the current Δreflected power max-min  with historical values. For comparison purposes, a Maximum reflected power measurement persistently greater than 6 dB above a threshold maximum value (which can be based on the SARA&#39;s baseline statistics) would be an indication of a static event such as fluids penetrating the antenna dome. These values are for illustrative purposes only. One of ordinary skill in the art upon reading this specification would be able to determine baseline and/or threshold values for implementing embodiments of the present invention based on the particular design parameters of the SARA. 
     Several means are available to implement the systems and methods of the current invention as discussed in this specification. These means include, but are not limited to, digital computer systems, microprocessors, application-specific integrated circuits (ASIC), general purpose computers, programmable controllers and field programmable gate arrays (FPGAs), all of which may be generically referred to herein as “processors”. For example, in one embodiment, signal processing may be incorporated by an FPGA or an ASIC, or alternatively by an embedded or discrete processor. Therefore other embodiments of the present invention are program instructions resident on computer readable media which when implemented by such means enable them to implement embodiments of the present invention. Computer readable media include any form of a physical computer memory device. Examples of such a physical computer memory device include, but is not limited to, punch cards, magnetic disks or tapes, optical data storage system, flash read only memory (ROM), non-volatile ROM, programmable ROM (PROM), erasable-programmable ROM (E-PROM), random access memory (RAM), or any other form of permanent, semi-permanent, or temporary memory storage system or device. Program instructions include, but are not limited to computer-executable instructions executed by computer system processors and hardware description languages such as Very High Speed Integrated Circuit (VHSIC) Hardware Description Language (VHDL). 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.