Measuring and testing – Barometer – Aneroid
Reexamination Certificate
2002-06-05
2003-12-16
Chapman, John E. (Department: 2856)
Measuring and testing
Barometer
Aneroid
Reexamination Certificate
active
06662652
ABSTRACT:
TECHNICAL FIELD
The present invention pertains to devices for measuring the altitude of an aircraft, and in particular to an altimeter having a digitizer correctable to barometric pressure.
BACKGROUND ART
Air traffic control systems utilize transponders installed within an aircraft to monitor the position and altitude of aircraft flying within controlled airspace. The transponder is a transceiver which, when interrogated by an air traffic control radar station, replies with an identification code input by the pilot and the pressure altitude of the aircraft referenced to sea level (29.92 inches of mercury). This pressure altitude information is provided by an altitude measuring and reporting device. Three fundamental types of altitude reporting devices are in use today: (1) air data computers, (2) encoding altimeters, and (3) altitude digitizers. The air data computer is a device which uses various sensors and micro processing techniques to determine altitude, airspeed, position and numerous other parameters which affect aircraft performance. Air data computers are highly accurate, extremely sensitive, and very expensive to purchase and maintain.
Encoding altimeters are much less expensive.
FIGS. 1-3
illustrated cutaway side elevation, end elevation, and opposite end elevation views, respectively, of a conventional prior art encoding altimeter, generally designated
500
. Altimeter
500
is an electromechanical device which uses a pressure sensitive mechanical movement in the form of an aneroid
502
to sense outside air pressure. Aneroid
502
is in a sealed housing
515
connected to a static pressure line through a port
512
. The aneroid
502
drives a series of gears which in turn drive a pointer
504
and a numbered counter drum
503
which provides the pilot with an altitude reading. Pointer
504
makes one revolution for each 1000 feet of altitude. Counter drum
503
indicates the altitude of the aircraft to the nearest 100 feet. Pointer
504
and counter drum
503
are both used to read altitude. Counter drum
503
provides the most significant digits and pointer
504
provides the least significant digits. The aneroid
502
is also mechanically linked
506
to a shaft angle encoder
508
which provides a digitized representation of the aircraft's altitude to a transponder via a connector
510
.
Altimeter
500
has two counter drums
505
and
509
which the pilot sets to the barometric pressure provided by air traffic control over the radio. Counter drum
505
indicates sea level barometric pressure in isobars and counter drum
509
indicates sea level barometric pressure in inches of mercury with 29.92 inches being nominal. When the pilot hears the barometric pressure over the radio, he looks to see if the readings on the drums
505
and
509
are correct. If not, he uses a barometric pressure reference correction control in the form of knob
507
to set the readings to the barometric pressure. When knob
507
is turned, gears inside mechanically move the readings on the drums
505
and
509
. In
FIG. 1
, the barometric pressure is represented as being 29.92 inches of mercury and the altitude is 5460 with 5400 feet shown on drum
503
and 460 feet shown by needle
504
.
FIG. 4
illustrates the changes which result when altimeter
500
is set to a different barometric pressure. In
FIG. 4
, the pilot has moved knob
507
to increase the setting of barometric pressure on drum
509
to 30.00 inches of mercury. Indicated altitude is changed thereby to 5532 feet with 5500 feet shown on drum
503
and
532
shown by needle
504
. Counter drum
505
showing isobars has also moved to 1016. Thus a change of 0.08 inches of mercury in barometric pressure results in a 72 foot correction in indicated altitude.
It should be noted that this altitude correction is not made in the shaft angle encoder
508
. This is because air traffic control prefers to receive a raw altitude indication from aircraft instead of one which is subject to errors created by the pilot. Air traffic control adjusts the raw altitude indication provided by the transponder from the shaft angle encoder using the barometric pressure at the location of the aircraft to establish the true altitude of the aircraft.
FIGS. 5 and 6
show another form of prior art encoding altimeter generally designated
600
, which is a Series 5035 encoding altimeter sold by United Instruments, Incorporated, 3625 Comatara Avenue, Wichita, Kans. 67226.
FIG. 5
is a front, right side, exploded perspective view and
FIG. 6
is a front elevation view. This encoding altimeter has a frame
602
which carries an altimeter section
604
and encoder
606
and rotates inside a housing
608
. The previous encoding altimeter of
FIGS. 1-4
has multiple gears and shafts for adjusting the setting of the barometric pressure. The present encoding altimeter has only two gears: a ring gear
610
at the perimeter of the frame
602
and a driver gear
612
connected to knob
614
. When the pilot turns the knob, the ring gear turns the frame in relation to a face
616
which is fixed to the housing
608
. A pressure setting dial
618
fixed to the front of the frame shows through a window
620
in the face to show the pilot the barometric pressure setting.
The encoder
606
is not adjusted by the pilot when he moves the knob
614
. As in the previous encoding altimeter, the encoder
606
only provides a raw pressure indication to the transponder which is interpreted by air traffic control using the barometric pressure at the location of the aircraft.
Altitude digitizers may also be completely independent of the altimeter and may either be electromechanical or solid state in nature. The FAA requires that such an altitude digitizer be calibrated to within ±125 feet of the primary altimeter viewed by the pilot. U.S. Pat. No. Re. 29,436 illustrates an electromechanical digitizer mechanically linked to an aneroid. A shaft angle encoder converts angular position into a digital code representing altitude. Here again, this device is expensive and difficult to calibrate. Solid state digitizers are disclosed in U.S. Pat. No. 4,106,343, and Model SSD 120 Altitude Encoder/Digitizer, sold by Trans-Cal Industries, Inc., 16141 Cohasset Street, Van Nuys, Calif. 91406. These devices have a solid state pressure transducer which converts air pressure to voltage which is then converted to a digital code representing altitude. These devices are much less expensive than electromechanical shaft angle encoding digitizers.
FIG. 7
is a block diagram of a solid state digitizer in a typical aircraft system. The digitizer and altimeter are connected to different portions of the static pressure line. The output of the solid state digitizer is routed in parallel form to the aircraft's transponder, and in serial form to a Global Positioning System (GPS) navigational computer, where it serves as a backup altitude signal should the GPS solution become degraded when less than four satellites are available for determination of an accurate altitude
DISCLOSURE OF INVENTION
The present invention is directed to an improved altitude measuring device including an altimeter having a digitizer output corrected for barometric pressure. The present invention has the following advantages over previous devices:
since the digitizer output is corrected for barometric pressure, the output can be used by each of the receiving navigation system devices without the pilot having to adjust the barometric pressure on each device; and,
the digitizer altitude output is the same as the indicated altimeter altitude.
In accordance with a preferred embodiment of the invention, the altimeter having a correctable digitizer includes a conventional altimeter of the aneroid type, having a housing, a pressure sensitive mechanical movement disposed within the housing, and a barometric pressure correction control, such as a knob, disposed outside the housing. A digitizer is also located within the altimeter housing and generates an aircraft altitude output value based upon sensed air pressure. An input d
Ferrero John
Smith Hugh W.
Chapman John E.
Freilich Hornbaker & Rosen
Masters Ted
Trans-Cal Industries, Inc.
Tyson Timothy Thut
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