Abstract:
An altitude sensing arrangement which combines the advantages of a barometric altimeter with a radar altimeter. The apparatus monitors the radar altimeter&#39;s associated radar validity signal and selects the radar altimeter&#39;s reading when the validity signal indicates a valid condition. Alternately, when the validity signal does not indicate a valid condition, the invention computes the difference in barometric altitude since the last valid radar altimeter reading and sums this difference with the last valid reading from the radar altimeter to produce a combined altitude reading.

Description:
RIGHTS OF THE GOVERNMENT 
     The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
    
    
     BACKGROUND OF THE INVENTION 
     Broadly speaking, this invention relates to altitude sensing means suitable for use with altitude control systems or with pilot warning systems for aircraft. More particularly, in a preferred embodiment, this invention relates to a barometric altimeter combined with a radar altimeter. 
     A common type of altimeter is the pressure responsive altimeter which includes an aneroid barometer arrangement having expansible bellow means. Other types of altimeters have been developed based upon the principle of radar which utilizes reflected signals from the surface of the earth. These radar altimeters have means for sensing and measuring the absolute altitude above the earth&#39;s surface. 
     A barometric altimeter measures the pressure of atmospheric air which can be used for determining the true altitude above sea level but cannot detect actual height above terrain. In contrast, a radar sensing device accurately measures the actual height above the terrain but cannot provide reliable readings during some flight attitude conditions. For example, it is well recognized that sharp banks, dives and climbing attitudes of an aircraft will cause unreliable altimeter signals from a radar unit due to the aircraft&#39;s pitch and roll angles being greater than the radar altimeter&#39;s cone of operation. Because of this, radar altimeters generally include an indicator of whether the reading is valid or invalid. Furthermore, a failure in radar signals will cause a radical change in an altitude control system, especially during a low altitude, terrain following flight. 
     It has been proposed that barometric and radar altitude sensors be combined so that the advantages of both can be utilized while the disadvantages of the individual sensors are neutralized. For example, U.S. Pat. No. 3,140,483 combines barometric and radar altimeters using complex analog devices. While this patent is good for its purpose of generating an altitude error signal for driving an aircraft control surface, it does not provide a true and absolute reading of altitude above ground. 
     SUMMARY OF THE INVENTION 
     One of the principal objects of this invention, therefore, is to provide an altitude sensing arrangement which provides an output reading of the absolute altitude above the earth&#39;s surface, even during periods when the radar altimeter output is not valid. 
     A feature of the invention relates to an arrangement incorporating a barometric altitude sensing system with a radar altitude sensing system such that the barometric sensor provides an alternate altitude sensor to update the altitude reading in the event that the radar sensor fails to deliver a valid signal. In this manner, the output continues to indicate the altitude above the earth&#39;s surface relative to the surface level at the last valid radar altimeter output. 
     Another feature of the invention is the provision of an altitude sensing system having in combination a barometric pressure altimeter and a radar altimeter with digital logic to compare a pilot selected altitude with a true altitude above the earth&#39;s surface. 
     Yet another feature of the invention is the provision of an apparatus to trigger an electrical warning signal whenever the true altitude falls below the pilot selected altitude. 
    
    
     DESCRIPTION OF THE DRAWING 
     The single FIGURE is a functional block diagram of one embodiment of the radar-barometer altimeter circuit according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The drawing illustrates the preferred embodiment of the invention. Radar altitude as a linear DC analog signal and an associated validity sign are provided by a commercial or military radar altimeter 1, for example, a Honeywell type AN/APN-194 or Sunstrand type AVH-5 radar altimeter. If necessary the DC analog voltage may be scaled by a potentiometer 2 in order to make the signal compatible with a connecting analog-to-digital converter 4. Analog-to-digital converter 4 converts the scaled analog altitude signal into a digital representation upon a convert command at time t 1 . At the same time, t 1 , the validity status of the radar altimeter is stored in a D-type flip-flop 6. 
     A barometric altimeter indicated at 8 may be of the type widely used on aircraft, such as an ARINC 565, which provides a synchronous, 3 wire, AC analog signal. This signal is applied to a corresponding synchronous motor attached to the cockpit altimeter instrument to provide precise meter movement. In the invention, the three synchronous, AC analog signals are connected to a synchro-to-digital (S/D) converter to provide a digital representation of barometric altitude. The S/D converter (a specialized A/D converter) may be of any suitable commercially available design, such as, for example, the Computer Conversion Corp. Model SDC 40. The digital representation is then fed to a memory (A) where it is continually updated. 
     A clock 12 and a ring counter 14 provide clocking pulses t 1 , t 2 , t 3 , and t 4  on four strobe lines which are pulsed at times separated by the clock period. Typically, a clock frequency would be on the order of 1 KHz which would result in the strobe pulses being separated by 1 millisecond. Although other values may be used, the strobe separation must be sufficient to prevent accepting or changing data prematurely. 
     When the ring counter 14 initializes time t 2 , a memory (C) stores the digital representation of radar altitude provided by the analog-to-digital converter 4. Memory (A) stores the digital representation provided by the synchro-to-analog converter 10 at the time of t 2  plus a finite delay time imposed by a delay circuit 19. Any delay is acceptable so long as it does not extend to time t 3 . Also at time t 2 , a memory (B), connected in series with memory (A), stores the barometric altitude which was stored in memory (A) during the previous time cycle. 
     The output of memory (A) through signal line A, together with the inverted output of memory (B) through signal line B, are inputted to an adder 26. The adder 26 is connected to a positive source voltage, V+, and is electrically grounded in such a manner to provide the proper sign bit. As a result, the function of adder 26 becomes subtraction of memory (B) contents from memory (A) contents by making memory (B) contents appear as a negative one (-1&#39;s) complement. The output of adder 26 is, therefore, the -1&#39;s complement of the barometric altitude change over the time between two t 1   strobes. As will be discussed in greater detail below, the barometric altitude change will be made available for addition to the radar or radar/barometric combined altitude. 
     The information contained in memory (C) is fed into data selector 24 which passes the information to memory (D) when the trigger signal from the D type flip-flop 6 applied to data selector 24 is &#34;high&#34;. A &#34;high&#34; signal is the proper output of the D type flip-flop 6 when the rader altimeter validity signal is &#34;high&#34; (i.e., valid) at time t 1 . 
     The data from the data selector is stored in memory (D) at time t 3 . The same data is transferred directly to memory (E) at time t 4 . Two memories are utilized to prevent the data being stored by memory (E) from changing when the output of memory (D) changes. 
     If the radar altimeter validity signal is &#34;low&#34; (i.e., not valid) at time t 1 , the D type flip-flop 6 will not trigger data selector 24 to pass the contents of memory (C) on to memory (D). Instead, the output of adder 26, which represents the barometric altitude change since the last time t 1 , is fed through input signal line F to another adder 28. Information stored in memory (E), which contains the digital representation of radar altitude is fed through signal line E to adder 28 and summed with the output of adder 26. Thus, the resultant output of adder 28 (indicated in the drawing as signal line G) is the barometric altitude change added to the last valid radar altitude. This combined radar/barometric altitude is fed into data selector 24 and is the information stored in memory (D) when the radar altimeter validity signal caused the D type flip-flop 6 to generate a &#34;low&#34; trigger signal to data selector 24. Since memory (E) is directly connected to memory (D), the combined radar/barometric altitude information will be transferred to memory (E) at subsequent time t 4 . As a result, the signals on the output bus of memory (E) could contain either combined radar/barometric altitude data or solely radar altitude data, either of which would be fed into adder 28 for updating during the next time cycle. 
     Memory (E), which contains the digital representation of radar/barometric altitude data, is directly connected to digital-to-analog converter 42. Digital-to-analog converter 42 converts the digital altitude information into an analog DC output, which is then fed into amplifier 44, where the analog signal is buffered and scaled producing a final output signal. The scaling parameter is determined by the altitude indicator utilized, such as a cockpit altitude indicator or other system such as a ground proximity warning system (GPWS). 
     The output of memory (E) is also connected to comparator 38, together with a signal from pilot selectable digital switch 36. The pilot, using digital switch 36, selects the altitude which he desires to maintain. Comparator 38 compares the combined altitude stored in memory (E) with the altitude selected by the pilot using digital switch 36. Comparator 38 will output a &#34;high&#34; signal if the selected altitude is higher than the radar and barometric combined altitude. The comparator 38 output signal is connected to a D type flip-flop 40, which is clocked at time t 4 , and delayed by time delay circuit 32. Any delay time may be used so long as it does not extend the time to t 1 . A delay is needed to insure that the data of memory (E) is stabilized. The resulting output of the D type flip-flop 40 will be high and will trigger a cockpit warning signal if the combined radar/barometric altitude is below the pilot selected altitude. 
     Delay circuits 19 and 32 could be implemented by pairs of inverters, or by one shot multivibrators. Adders 26 and 28 can be implemented by four-bit adders, while the memories could utilize Hex or Quad D flip-flops. The data selector can be implemented by a Quad two-line to one-line data selector. 
     In summary, the output signal of amplifier 44 is an analog signal of height above terrain, which is solely radar altitude if the radar altimeter signal is valid. When the radar altitude signal becomes invalid, the last valid radar altitude, which is stored in memory (E), is added to the change in barometric altitude since the last valid radar altitude was stored. This combined altitude is then stored in memory (E), and the output signal of amplifier 44 then becomes radar and barometric combined altitude. 
     Thus, while preferred constructional features of the invention are embodied in the structure illustrated herein, it is to be understood that changes and variations may be made by the skilled in the art without departing from the spirit and scope of the invention.