History or image based methods for altitude determination in a radar altimeter

Methods and apparatus for determining an altitude with an altimeter is provided. One method includes transmitting a signal having a fixed modulation period towards a ground target and then detecting reflected signals off the ground target. The method then implements a single Fast Fourier Transform (FFT) on the detected signals for each modulation period that computes all possible altitudes in real time. A short history of the real time altitude calculations is collected and then the altitude based on the short history of the real time altitude calculations is determined.

BACKGROUND

Aircrafts employ altimeter systems to determine the altitude of the aircraft above terrain. Typical frequency modulation-continuous wave (FM/CW) radar altimeter systems use a single servo loop that attempts to detect a leading edge of a single first return and adjust the parameters of the radar altimeter modulation so as to maintain a constant input frequency to signal processing equipment of the altimeter. In traditional systems only one potential altitude detection is made, ignoring the possibility that the earliest reply is not the ground but some other intervening object such as a tree top, building top, another plane, large machinery top such as construction cranes, rain, etc. This causes the altimeter to display altitudes that are too low and sometimes dangerously too high.

Current altimeter designs also attempt to smooth output altitude by averaging many sequential altitude measurements. Unfortunately this method permits one or more erroneous or large change measurements to skew the final computed value. In particular, prior art altitude tracking schemes revolve around “instantaneous detection” of the “leading edge” or “mean of the return spectrum” of return radar signals on a modulation to modulation period basis. Typically for each modulation period, the computed altitude is averaged with the succeeding period of N periods. This tends to cause any “outlying data points” to pull or skew the computed result. Many existing altitude tracking schemes utilize a suite of system loops that adjust intermediate frequency (IF) automatic gain control (AGC) and the modulation rate which in turn causes the altitude resolution of the altimeter to vary inversely to the altitude—(i.e. less resolution at greater altitude) in addition to the basic altitude tracking loop.

Moreover, existing radar altimeters are frequently based on FM/CM or pulsed modulation. These altimeters compute only a limited altitude extent of range gates centered around where the altimeter is either seeking or tracking ground reflections. These limited altitude extents can mislead the tracking algorithms or incur delays in acquiring or following rapidly changing topology. The tracking loop may be adversely impacted if the rate of change of altitude is greater than the tracking window altitude extent. Algorithms have been demonstrated to indicate greater or lesser altitude than in actually the case because of the need to adjust the limited signal processing extent over constantly changing reflection amplitude and complex ground structures.

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 present specification, there is a need in the art for Radar Altimeters that are immune to the effects of a) step change in ground altitude, b) brightness variation in ground reflections, c) rain echo effects, d) aircraft below own aircraft and E) snow/rain/dust covered runways.

SUMMARY OF INVENTION

The above-mentioned problems of current systems are addressed by embodiments of the present invention and will be understood by reading and studying the following specification. The following summary is made by way of example and not by way of limitation. It is merely provided to aid the reader in understanding some of the aspects of the invention.

In one embodiment, a method of determining an altitude with an altimeter is provided. The method includes transmitting a signal having a fixed modulation period towards a ground target and then detecting reflected signals off the ground target. The method then implements a single Fast Fourier Transform (FFT) on the detected signals for each modulation period that computes all possible altitudes in real time. A short history of the real time altitude calculations is collected and then the altitude based on the short history of the real time altitude calculations is determined.

DETAILED DESCRIPTION

Embodiments of the present invention use history or image based processing of ground returns collected by the radar altimeter to reduces the effects of a) step change in ground altitude, b) brightness variation in ground reflections, c) rain echo effects, d) aircraft below own aircraft and E) snow/rain/dust covered runways. Moreover, embodiments provide a means to verify the persistence of a measurement over time before including its value in the measurement of altitude. Embodiments also provide a means to compute all possible altitude values from minimum to maximum possible altitude. A signal processor then selects the leading edge of the most distant object as the true altitude of the aircraft, thereby ignoring all intermediate results caused by rain or other aircraft.

One embodiment is implemented by using a frequency modulated (FM)/continuous wave (CW) radar altimeter with a fixed modulation rate and fast fourier transform processing of each individual modulation period. The amplitude of each possible altitude value is threshold detected and saved in a history. The history of each altitude value is then evaluated by counting the number of detections made for each column of the history. An altitude detection is declared when the criteria for the minimum number and distribution of detections is found. Having collected all possible altitude detections, the leading edge of the most distant object is reported. A history of these reported altitudes may be in turn averaged and evaluated on other criteria to arrive at a final reported altitude at the required output data rate.

Referring toFIG. 1A, an example of a radar system100of one embodiment is illustrated. In this embodiment, the radar system is a FM/CW altimeter100. As illustrated, the altimeter100includes a transmitter circuit106that transmits a signal at a given frequency towards the ground surface103via Tx antenna102. In embodiments, the transmitter circuit106transmits at a select fixed modulation rate. The signal is reflected back off the ground surface103and picked up by Rx antenna104and passed to the detection circuit108which includes a receiver. The transmitted signal of FM/CW altimeter varies linearly with time, allowing the distance to the ground to be determined by taking the instantaneous difference between the currently transmitted signal and the received signal. In particular, the detection circuit108receives a sample of the transmitter signal106and subtracts the reflection signal form the ground to produce a frequency that is directly proportional to altitude. In one embodiment a single antenna system130is implemented in the radar system. In this system130, as illustrated inFIG. 1B, the transmitter132and the receiver133use a single antenna136. A circulator134, a three port device, is used to couple the respective transmitter132and receiver133to the antenna136.

Referring back toFIG. 1A, in this embodiment, an automatic gain control loop in the intermediate frequency (IF) subsystem is not used. Instead, a high pass filter (HPF)118that is designed to match the lowest and highest frequencies in the IF that corresponds to the highest and lowest altitudes. The filter118instantaneously eliminates all variation in amplitude due to change in altitude with no latency over or undershoot. Therefore, a fixed reflector size would not appear to vary regardless of distance. Only the amplitude variation due to reflectivity of a ground target will remain. In one embodiment, a 6 dB per octave HPF is used. As further illustrated inFIG. 1, an analog to digital (A/D) converter122is included in the detection circuit108. The A/D converter122is selected to match the dynamic range of the reflectivity. Accordingly, the control processor implements a threshold detection system that only requires a fixed minimum signal to noise ratio to declare a detection of a valid range signal. Therefore an automatic gain control system is not needed.

Altimeter100ofFIG. 1A, implements a FM/CW modulation rate that produces a constant altitude resolution at all altitudes. In one embodiment a dedicated FFT processor114computes a single fast fourier transform (FFT) for each modulation period that computes all possible altitudes in real time. A controller110generates and stores a short history of the real time altitude calculations in a two dimensional rolling memory112that is capable of holding history. In embodiments, a threshold detection algorithm120running either in the FFT processor114or the controller110is used by the controller110on the computed altitudes. The threshold detect algorithm120is used by controller110on the computed altitudes and as a result assigns pixels in an image of a 1 or a 0 that corresponds to the threshold state of each FFT spectral line which is equivalent to each altitude range gate. The controller110determines the altitude based on the short history or image of the threshold data and retains the later fraction of the history as the beginning of the next new history (i.e. push the old history up in the memory stack of a rolling memory112). The controller110computes new FFTs in real time for another history period and fills in a new image.

Controller110is capable of performing image processing steps such as passing a “window” over the formed altitude history images while counting and associating pixels with altitude gates. The rolling memory112permits retaining a fraction of a previous collected set of threshold detected gates with a fraction of new gates such that trends over time can be recognized. An optional dual processing chain that can provide a self monitoring (2 different solutions) of computed altitude for critical safety of life applications is contemplated and is employed in one embodiment. An example of this embodiment is illustrated in the flow diagram500ofFIG. 5.

Embodiments compute all possible altitude values at all times and maintains a constant modulation rate from the radar altimeter such that the only variation in measured altitude is caused by aircraft motion and objects on the ground, not caused by changes in the modulation of the altimeter. The algorithms used in embodiments provide the means to determine a distribution of reflections and determine via required persistence over time what values are valid measurements of altitude.

As discussed above, in one embodiment, a FW/CW waveform is used that is processed by using a FFT to produce altitude “gates” or “bin” that correspond one for one with an FFT spectrum line. FM/CM radars encode range to a reflection linearly in frequency wherein a near return produces an output in the FFT at a low frequency spectrum line that typically corresponds to the range/altitude resolution of the radar. Although, the algorithm discussed above is discussed in relation to a FW/CW waveform, the algorithm is not restricted to a FW/CW waveform. For example, a time based pulse system could also be used to form range/altitude gates that would be processed identically after initial amplitude detection. Similarly, any modulation scheme chosen can be used to create a set of range/altitude gates and those gates recorded as a history to create an image are contemplated and included in the present invention.

An image is formed by the controller110by collecting range gates for each modulation period. The range gates are threshold detected. Each gate or bin that has an amplitude exceeding a defined threshold is assigned a “1” or an ON pixel for that gate. Each gate that has an amplitude under and including the defined threshold is assigned a “0” or OFF pixel for that gate. A history of N modulated periods is collected in the memory112and the “1”and “0” pixels create an image.

Referring toFIG. 2, an altitude detection method200of one embodiment is illustrated. As illustrated, the method200starts by first setting a threshold X dB that is greater than a noise level (202). Ranging waves are transmitted and return signals are subtracted from a transmitter reference in the receiver to create an IF that is scaled and recorded in the hamming window A/D step (204). The hamming window is a multiplication factor applied to each A/D sample of the IF frequency output. The FFT of the scaled and recorded IF is computed at (206). A threshold detect is applied to the computed FFT and the FFT bins in a history image table are set to a 1 if they cross the threshold or a 0 if they do not (208). A row of the image table of the detected FFT bins above a previous row is saved (210). It is then determined if N rows have been saved (212). If n rows have not been saved (212), the process continues at step (204). Once N rows have been saved (212), each FFT bin column in the multi-row image is summed up (214). The column having the maximum number of summed detections is determined (216). Of the columns with the sum of detections equal to the maximum number of detections, the lowest column index is determined (218). The altitude associated with determined column is then determined (220). The determined altitude is then output (222). All the columns in the image224is set to zero (224), and the process continues at step (204).

An example of an image300is illustrated inFIG. 3. This example of an image300is made up of 6 rows and 9 columns. Each square in the image300represents a FFT bin. FFT bins that were above the threshold are indicated by the reference number304. These FFT are assigned a 1 value in one example. FFT bins that were not above the threshold are indicated by the reference number306. These FFT bins are assigned a 0 value in one embodiment. Each column is associated with a given altitude. Each row indicates a modulation period of detected FFT bins. A FFT bin column index308sets out the column numbers. The sum detections in each FFT bin is illustrated in the index of peak detection312. As illustrated, column 4 has the maximum number of FFT bins (6 in this example) above the threshold. The altitude is then determined based on column 4. An example of an algorithm to determine the altitude is as follows:

In one embodiment the altitude constant is equal to 3.088 Ft/bin. Using the example illustrated inFIG. 3, the altitude=3.088×((4−1)+(6/(6+2)))=11.58 ft. In one embodiment, a sub-resolve method is used to determine the true altitude when multiple columns of detections are made. For example, inFIG. 3, bin4has the maximum possible 6 detections and therefore the altitude may be constructed to be equal to 3.088 FT/bin×4 bins=12.352 ft. But, asFIG. 3illustrates, column number5has 5 detections and column6has 3 detections. Accordingly, the true altitude is actually slightly to the left of the position between column4and column5since bin5has one less detection. Had columns4and5had the same number of maximum detections, the true altitude would be halfway between them or 3.088×4.5=13.896.

In some embodiments, averaging algorithms are used to average a plurality of altitude determination. An example of one averaging method400is illustrated inFIG. 4. In this example, the method is stated by first setting a count (N) and a summation of computed altitudes (SumAlt) to zero (402). N is then incremented at (404). The SumAlt is then determined by adding a determined altitude to an accumulated altitude (406). The altitude is computed using an altitude algorithm such as the altitude algorithm as set out above (408). It is then determined if N is equal to the total number of determined Altitudes (M) (410). If N is not equal to M (410), N is incremented at (404) and process continues. Once N equals M at (410), the averaged, or smoothed) altitude is determined by dividing the summation of the computed altitudes by N (412). The smoothed altitude is then output at (414). As illustrated, the process continues at (402).

In one embodiment, a self monitored altitude algorithm500that has two different solutions is applied. This is illustrated inFIG. 5. As illustrated, this method starts by hamming weight A/D data (502). An Example of hamming weight is illustrated in respect toFIGS. 6A and 6B. Data collected by the A/D converter is successfully multiplied by hamming weight according to a well known hamming window600illustrated inFIG. 6A. For example, in one embodiment 4096 A/D samples are collected for use by the FFT. According to the weighing schedule603of the hammering window600, the first A/D sample is multiplied by 0.1, while sample N/2 or in this case 4096/2=sample 2084 the recorded A/D sample is multiplied by 1 and the last sample 4096 is multiplied by 0.1. This process is done to reduce the effects of a limited number of data sample points that would otherwise generate “side-lobes” as shown inFIG. 6B. In particular,FIG. 6Bshows a hamming window frequency response in an FFT602that compares the hamming window607to a “rectangular” window. In the rectangular window all weights are equal to 1.0. A hamming window600is one example of a data window function. Other optimal functions are contemplated and the present invention is not limited to a hamming window.

Returning back toFIG. 5, once the hamming weight is applied (502), a fast fourier transform (FFT) of a modulation period is computed at (504). It is then determined if the modulation period is an up chirp (going from a low frequency to a high frequency) or a down chirp (going from a high frequency to a low frequency) (506). If it is an up chirp (506), the altitude is computed at (512). If it is a down chirp (506), the altitude is computed at (514). The average of the computed algorithms from (512) and (514) are computed at (516). A result of the computed algorithms is stored in a first altitude history (520). As also illustrated, an average of 1 up and 1 down FFT is computed at (508). The altitude of the average 1 up and 1 down FFT is computed at (510). This computed altitude (510) is then stored in a second altitude history (518). The first and second altitude histories (520) and (518) are then compared at (522). If the altitudes match up (524), the determined altitudes are verified as being ok (526). If the altitudes do not match up (524), a fail warning signal is generated (528).