Method and apparatus for determining altitude

• US 6,216,064 B1
• Filed: 02/23/1999
• Issued: 04/10/2001
• Est. Priority Date: 02/24/1998
• Status: Expired due to Term

• Current Assignee: Alliedsignal Inc. (Honeywell International Inc.)
• Original Assignee: Alliedsignal Inc. (Honeywell International Inc.)
• Inventors: Muller, personal representative, by Beat, Muller, Hans Rudolph, Johnson, Steven C.
• Primary Examiner(s): Cuchlinski, Jr., William A.
• Assistant Examiner(s): Hernandez, Olga
• Application Number:
  US09/255,670
• Time in Patent Office: 777 Days
• Field of Search: 701/4, 701/1, 701/216, 701/213, 701/214, 701/224, 342/462, 244/76.R
• US Class Current: 701/4
• CPC Class Codes: G01C 5/005 altimeters for aircraft G01C5/02, G01C5/06 take precedence
• CPC Class Codes: G01S 19/14 specially adapted for specific applications
• CPC Class Codes: G01S 19/39 the satellite radio beacon positioning system transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO

Abstract

Methods and apparatus for determining altitude, specifically altitude in an aircraft, and an estimated error of the altitude. The altitude determination preferably uses a first altitude based on hydrostatic calculations, including local pressure and temperature, as well as a second altitude which is preferably a GPS altitude. A radio altimetry can also be used instead of or to complement the GPS altitude. Other sources of altitude determination can be used in the equation for the calculation of the final altitude. Each of the sources of altitude determination is provided with a complementary estimated error. In the final determination of the probable altitude, each source of altitude information is preferably accorded a weighting according to the estimated error of the altitude source. For global positioning altitude, the final combination of the altitude sources uses a complementary filter which takes into account the selective availability of the GPS altitude. This accounts for the long-term accuracy but short-term inaccuracy of GPS altitude. Corrections are provided to account for horizontal changes in pressure gradient as the aircraft moves from an origin to a destination. The invention further provides for the altitude to be corrected based on non-standard atmospheric temperature (ISA) variations. The apparatus includes a computer processor and memory configured to receive the altitude information and make the necessary calculations to result in an estimate of the current altitude which is then displayed on an output display. The apparatus includes inputs to receive sources of altitude information.

Claims

1. A method for determining a modified aircraft altitude signal, said method comprising:

a. receiving an air temperature signal, a pressure altitude signal, a first validity signal associated with the pressure altitude signal, aircraft altitude and position signals, and an error signal and a second validity signal associated with the aircraft altitude and position signals;

b. if the aircraft is below a predefined altitude, receiving a radio altimeter generated altitude signal and a third validity signal associated with the radio altimeter altitude signal;

c. generating a hydrostatic altitude signal based on the received pressure altitude signal and the air temperature signal;

d. correcting the received barometric altimeter signal based on the received air temperature signal and a flight crew setting;

e. initializing the hydrostatic altitude signal;

f. calculating an error signal for the corrected barometric altimeter signal based on a predefined basic error factor, a predefined atmosphere instability factor and a predefined airport database;

g. calculating an error signal for the received hydrostatic altitude signal based the initialization of the hydrostatic altitude signal;

h. calibrating the hydrostatic altitude signal based on the received aircraft altitude signal, the at least one error signal associated with the aircraft altitude signal and the calculated error signal for the hydrostatic altitude signal;

i. if the aircraft is below a predefined altitude, calibrating the hydrostatic altitude signal based on the received radio altimeter generated altitude signal and a predefined terrain elevation database;

j. calculating an error signal for the aircraft altitude calibrated hydrostatic altitude signal based on the most recent calibration of the hydrostatic altitude signal and calculating an error signal for the radio altimeter calibrated hydrostatic altitude signal based on the most recent radio altimeter calibration of the hydrostatic altitude signal;

k. determining a fourth validity of the aircraft altitude calibrated hydrostatic altitude signal based on said first received validity signal of the received pressure altitude signal and said second validity signal associated with the received aircraft altitude signal and determining a fifth validity of the radio altimeter calibrated hydrostatic altitude signal based on said first validity signal of the received pressure altitude signal;

l. determining reasonableness of the generated hydrostatic altitude signal, the aircraft altitude calibrated hydrostatic altitude signal, the radio altimeter calibrated hydrostatic altitude signal and the corrected barometric altimeter signal; and

m. generating a modified aircraft altitude signal based on the reasonableness determinations, the fourth and fifth validity determinations, the first, second, and third received validity signals, the aircraft altitude signal, the calculated and received error signals, the generated hydrostatic altitude signal, the aircraft altitude calibrated hydrostatic altitude signal, the radio altimeter calibrated hydrostatic altitude signal and the corrected barometric altimeter signal.

2. A computer program product for performing the method of claim 1.

4. The method of claim 1, wherein c. calculating an error signal for the received pressure altitude signal comprises:

(i). receiving an aircraft position signal;

(ii). determining nearest runway based on received aircraft position signal and a predefined database of runways;

(iii). determining distance to and height above the determined nearest runway based received aircraft position signal and received aircraft altitude signal; and

(iv). calculating the error signal for the received pressure altitude signal based on the received pressure altitude signal, a predefined altimeter error factor and the determined distance to and height above nearest runway.

5. The method of claim 4, wherein c.(iv). calculating the error signal for the received pressure altitude signal based on the received pressure altitude signal, a predefined altimeter error factor and the determined distance to and height above nearest runway, further comprises:

A. predicting current instability of the atmosphere; and

B. calculating the error signal for the received pressure altitude signal based on the predicted the current instability of the atmosphere.

6. The method of claim 1, wherein g. generating a modified aircraft altitude signal further comprises:

(i). if said first validity signal of the received pressure altitude signal indicates valid and said second validity signal of the received aircraft altitude signal indicates invalid, outputting the received pressure altitude signal as the modified aircraft altitude signal;

(ii). if said first validity signal of the received pressure altitude signal and said second validity signal of the received aircraft altitude signal indicate valid, generating a modified aircraft altitude signal based on the received pressure altitude signal and the calibrated pressure altitude signal; and

(iii). if said first validity signal of the received pressure altitude signal indicates invalid and said second validity signal of the received aircraft altitude signal indicates valid, outputting the aircraft altitude signal as the modified aircraft altitude signal.

7. The method of claim 2, wherein g.(ii). generating a modified aircraft altitude signal based on the received pressure altitude signal and the received aircraft altitude signal comprises:

A. determining the lowest value of the calculated error signal for the received pressure altitude signal and the calculated an error signal for the calibrated received pressure altitude signal; and

B. selecting the received pressure altitude signal or the received aircraft altitude signal with the determined lowest value as the modified aircraft altitude signal.

8. The method of claim 6, wherein g.(ii). generating a modified aircraft altitude signal based on the received pressure altitude signal and the received aircraft altitude signal comprises averaging the received pressure altitude signal and the received aircraft altitude signal.

9. The method of claim 6, wherein g.(ii). generating a modified aircraft altitude signal based on the received pressure altitude signal and the received aircraft altitude signal comprises:

A. generating a weighted value for the received pressure altitude signal and the calibrated received pressure altitude signal based on the calculated error signal for the received pressure altitude signal and the calculated error signal for the calibrated received pressure altitude signal, respectively; and

B. combining the received pressure altitude signal and the calibrated received pressure altitude signal based on the generated weighted values.

18. The method of claim 8, wherein c. calculating an error signal for the received pressure altitude signal comprises:

(i). receiving an aircraft position signal;

(ii). determining nearest runway based on received aircraft position signal and a predefined database of runways;

(iii). determining distance to and height above the determined nearest runway based received aircraft position signal and received aircraft altitude signal; and

(iv). calculating the error signal for the received pressure altitude signal based on the received pressure altitude signal, a predefined altimeter error factor and the determined distance to and height above nearest runway.

19. The method of claim 18, wherein c.(iv). calculating an error signal for the received pressure altitude signal based on the received pressure altitude signal, a predefined altimeter error factor and the determined distance to and height above nearest runway, further comprises:

A. predicting current instability of the atmosphere; and

B. calculating the error signal for the received pressure altitude signal based on the predicted the current instability of the atmosphere.

20. The method of claim 18, wherein the received aircraft altitude and position signals are generated by a satellite navigation system.

3. A method for determining a modified aircraft altitude signal, said method comprising:

a. receiving a pressure altitude signal, a first validity signal of the received pressure altitude signal, an aircraft altitude signal, and at least one error signal and a second validity signal associated with the aircraft altitude signal;

b. initializing the pressure altitude signal;

c. calculating an error signal for the pressure altitude signal based on the initialization of the pressure altitude signal;

d. calibrating the received pressure altitude signal based on the received aircraft altitude signal, the at least one error signal associated with the aircraft altitude signal and the calculated error signal for the pressure altitude signal;

e. calculating an error signal for the calibrated received pressure altitude signal;

f. determining a third validity of the calibrated received pressure altitude signal based on the received said first validity signal of the received pressure altitude signal and the received said second validity signal of the received aircraft altitude signal; and

g. generating a modified aircraft altitude signal based on the determined said third validity of said calibrated received pressure altitude signal, the received aircraft altitude signal and the calibrated pressure altitude signal.

10. A apparatus for determining a modified aircraft altitude signal, said apparatus comprising:

a. a receiving component for receiving pressure altitude signal, a first validity signal of the received pressure altitude signal, an aircraft altitude signal, and at least one error signal and a second validity signal associated with the received aircraft altitude signal;

b. an initializer for initializing the pressure altitude signal;

c. a pressure altitude error calculator for calculating an error signal for the pressure altitude signal based on the initialization of the pressure altitude signal;

d. a complementary filter for calibrating the received pressure altitude signal based on the received aircraft altitude signal, the at least one error signal associated with the aircraft altitude signal and the calculated error signal for the pressure altitude signal;

e. a calibrated pressure altitude error calculator for calculating an error signal for the calibrated received pressure altitude signal;

f. a validity determiner for determining a third validity of the calibrated received pressure altitude signal based on said first received validity signal of the received pressure altitude signal and said second received validity signal of the received aircraft altitude signal; and

g. an aircraft altitude generator for generating a modified aircraft altitude signal based on the determined said third validity of the calibrated received pressure altitude signal, the received aircraft altitude signal and the calibrated pressure altitude signal.

11. The apparatus of claim 10, wherein the aircraft altitude generator further comprises:

(i). a first component for outputting the received pressure altitude signal as the modified aircraft altitude signal, if said first validity signal of the received pressure altitude signal indicates valid and said second validity signal of the received aircraft altitude signal indicates invalid;

(ii). a second component for generating a modified aircraft altitude signal based on the received pressure altitude signal and the calibrated pressure altitude signal, if said first validity signal of the received pressure altitude signal and the said second validity signal of the received aircraft altitude signal indicate valid; and

(iii). a third component for outputting the aircraft altitude signal as the modified aircraft altitude signal, if said first validity signal of the received pressure altitude signal indicates invalid and said second validity signal of the received aircraft altitude signal indicates valid.

12. The apparatus of claim 11, wherein the second component comprises:

A. a determiner for determining the lowest value of the calculated error signal for the received pressure altitude signal and the calculated an error signal for the calibrated received pressure altitude signal; and

B. a selector for selecting the received pressure altitude signal or the received aircraft altitude signal with the determined lowest value as the modified aircraft altitude signal.

13. The apparatus of claim 11, wherein the second component comprises an averaging component for averaging the received pressure altitude signal and the received aircraft altitude signal.

14. The apparatus of claim 11, wherein the second component comprises:

A. a weight generator for generating a weighted value for the received pressure altitude signal and the calibrated received pressure altitude signal based on the calculated error signal for the received pressure altitude signal and the calculated error signal for the calibrated received pressure altitude signal, respectively; and

B. a combiner for combining the received pressure altitude signal and the calibrated received pressure altitude signal based on the generated weighted values.

15. The apparatus of claim 10, wherein the pressure altitude error calculator comprises:

(i). a second receiving component for receiving an aircraft position signal;

(ii). a runway determiner for determining nearest runway based on received aircraft position signal and a predefined database of runways;

(iii). a proximity determiner for determining distance to and height above the determined nearest runway based received aircraft position signal and received aircraft altitude signal; and

(iv). an error calculator for calculating the error signal for the received pressure altitude signal based on the received pressure altitude signal, a predefined altimeter error factor and the determined distance to and height above nearest runway.

16. The apparatus of claim 15, wherein the error calculator further comprises:

A. an atmosphere predicting component for predicting current instability of the atmosphere; and

B. a subcomponent for calculating the error signal for the received pressure altitude signal based on the predicted the current instability of the atmosphere.

17. A method for determining a modified aircraft altitude signal, said method comprising:

a. receiving a pressure altitude signal, a first validity signal of the received pressure altitude signal, an aircraft altitude signal, and at least one error signal and a second validity signal associated with the aircraft altitude signal;

b. initializing the pressure altitude signal;

c. calculating an error signal for the pressure altitude signal based on the initialization of the pressure altitude signal;

d. calibrating the received pressure altitude signal based on the received aircraft altitude signal, the at least one error signal associated with the aircraft altitude signal and the calculated error signal for the pressure altitude signal;

e. calculating an error signal for the calibrated received pressure altitude signal;

f. determining a third validity of the calibrated received pressure altitude signal based on the received said first validity signal of the received pressure altitude signal and the received said second validity signal of the received aircraft altitude signal;

g. determining reasonableness of the received pressure altitude signal and the calibrated received pressure altitude signal; and

h. generating a modified aircraft altitude signal based on the determined reasonableness, the determined said third validity of the calibrated received pressure altitude signal, the received aircraft altitude signal and the calibrated pressure altitude signal.

21. The method of claim 17, wherein h. generating a modified aircraft altitude signal further comprises:

(i). if the received pressure altitude signal was determined reasonable and the calibrated received pressure altitude signal was determined unreasonable, outputting the received pressure altitude signal as the modified aircraft altitude signal, the calculated error signal for the received pressure altitude signal and the received said first validity signal of the received pressure altitude signal;

(ii). if the received pressure altitude signal and the calibrated received pressure altitude signal were determined reasonable, generating a modified aircraft altitude signal based on the received pressure altitude signal and the calibrated pressure altitude signal; and

(iii). if the received pressure altitude signal was determined unreasonable and if said second validity signal of the received aircraft altitude signal indicates valid, outputting the aircraft altitude signal with corresponding error signal.

22. The method of claim 21, wherein h.(ii). generating a modified aircraft altitude signal based on the received pressure altitude signal and the received aircraft altitude signal comprises:

A. determining the lowest value of the calculated error signal for the received pressure altitude signal and the calculated an error signal for the calibrated received pressure altitude signal; and

B. selecting the received pressure altitude signal or the received aircraft altitude signal with the determined lowest value as the modified aircraft altitude signal.

23. The method of claim 21, wherein h.(ii). generating a modified aircraft altitude signal based on the received pressure altitude signal and the received aircraft altitude signal comprises averaging the received pressure altitude signal and the received aircraft altitude signal.

24. The method of claim 21, wherein h.(ii). generating a modified aircraft altitude signal based on the received pressure altitude signal and the received aircraft altitude signal comprises:

A. generating a weighted value for the received pressure altitude signal and the calibrated received pressure altitude signal based on the calculated error signal for the received pressure altitude signal and the calculated error signal for the calibrated received pressure altitude signal, respectively; and

B. combining the received pressure altitude signal and the calibrated received pressure altitude signal based on the generated weighted values.

25. An apparatus for determining a modified aircraft altitude signal, said apparatus comprising:

a. a receiving component for receiving pressure altitude signal, a first validity signal of the received pressure altitude signal, an aircraft altitude signal, and at least one error signal and a second validity signal associated with the aircraft altitude signal;

b. an initializer for initializing the pressure altitude signal;

c. a pressure altitude error calculator for calculating an error signal for the pressure altitude signal based on the initialization of the pressure altitude signal;

d. a complementary filter for calibrating the received pressure altitude signal based on the received aircraft altitude signal, the at least one error signal associated with the aircraft altitude signal and the calculated error signal for the pressure altitude signal;

e. a calibrated pressure altitude calculator for calculating an error signal for the calibrated pressure altitude signal;

f. a validity determiner for determining a third validity of the calibrated received pressure altitude signal based on said first received validity signal of the received pressure altitude signal and said second received validity signal of the received aircraft altitude signal;

g. a reasonable determiner for determining reasonableness of the received pressure altitude signal and the calibrated received pressure altitude signal; and

h. an aircraft altitude generator for generating a modified aircraft altitude signal based on the determined reasonableness, the determined said third validity of the calibrated received pressure altitude signal, the received aircraft altitude signal and the calibrated pressure altitude signal.

26. The apparatus of claim 25, wherein the aircraft altitude generator further comprises:

(i). a first component for outputting the received pressure altitude signal as the modified aircraft altitude signal, the calculated error signal for the received pressure altitude signal and the received said first validity signal of the received pressure altitude signal, if the received pressure altitude signal was determined reasonable and the calibrated received pressure altitude signal was determined unreasonable;

(ii). a second component for generating a modified aircraft altitude signal based on the received pressure altitude signal and the calibrated pressure altitude signal, if the received pressure altitude signal and the calibrated received pressure altitude signal were determined reasonable,; and

(iii). a third component for outputting the aircraft altitude signal with error signal, if the received pressure altitude signal was determined unreasonable and if said second validity signal of the received aircraft altitude signal indicates valid.

27. The apparatus of claim 26, wherein the second component comprises:

A. a first subcomponent for determining the lowest value of the calculated error signal for the received pressure altitude signal and the calculated an error signal for the calibrated received pressure altitude signal; and

B. a second subcomponent for selecting the received pressure altitude signal or the received aircraft altitude signal with the determined lowest value as the modified aircraft altitude signal.

28. The apparatus of claim 26, wherein the second component comprises an averaging component for averaging the received pressure altitude signal and the received aircraft altitude signal.

29. The apparatus of claim 26, wherein the second component comprises:

A. a weight generator for generating a weighted value for the received pressure altitude signal and the calibrated received pressure altitude signal based on the calculated error signal for the received pressure altitude signal and the calculated error signal for the calibrated received pressure altitude signal, respectively; and

B. a combiner for combining the received pressure altitude signal and the calibrated received pressure altitude signal based on the generated weighted values.

30. The apparatus of claim 25, wherein the pressure altitude error calculator comprises:

(i). a second receiving component for receiving an aircraft position signal;

(ii). a runway determiner for determining nearest runway based on received aircraft position signal and a predefined database of runways;

(iii). a proximity determiner for determining distance to and height above the determined nearest runway based received aircraft position signal and received aircraft altitude signal; and

(iv). an error calculator for calculating the error signal for the received pressure altitude signal based on the received pressure altitude signal, a predefined altimeter error factor and the determined distance to and height above nearest runway.

31. The apparatus of claim 30, wherein the error calculator further comprises:

A. a predicting component for predicting current instability of the atmosphere; and

B. a subcomponent for calculating the error signal for the received pressure altitude signal based on the predicted the current instability of the atmosphere.

32. The apparatus of claim 30, wherein the received aircraft altitude and position signals are generated by a satellite navigation system.

33. A method for determining a modified aircraft altitude signal, said method comprising:

a. receiving an air temperature signal, a pressure altitude signal, a first validity signal associated with the pressure altitude signal, aircraft altitude and position signals, and at least one error signal and a second validity signal associated with the aircraft altitude and position signals;

b. generating a hydrostatic altitude signal based on the received pressure altitude signal and the air temperature signal;

c. initializing the hydrostatic altitude signal;

d. calculating an error signal for the received hydrostatic altitude signal based the initialization of the hydrostatic altitude signal;

e. calibrating the hydrostatic altitude signal based on the received aircraft altitude signal, the at least one error signal associated with the aircraft altitude signal and the calculated error signal for the hydrostatic altitude signal;

f. calculating an error signal for the calibrated hydrostatic altitude signal based on the most recent calibration of the hydrostatic altitude signal;

g. determining a third validity of the calibrated hydrostatic altitude signal based on the received said first validity signal of the received pressure altitude signal and the said second validity signal associated with the received aircraft altitude signal;

h. determining reasonableness of the generated hydrostatic altitude signal and the calibrated hydrostatic altitude signal; and

i. generating a modified aircraft altitude signal based on the determined reasonableness, the determined said third validity of the calibrated hydrostatic altitude signal, the received aircraft altitude signal, and the calibrated hydrostatic altitude signal.

34. The method of claim 33, wherein f. calculating an error signal for the calibrated hydrostatic altitude signal comprises:

(i) determining the difference between the last received error signal associated with the last received aircraft altitude and position signals and the most recently received error signal associated with the most recently received aircraft altitude and position signals;

(ii) if the determined difference is below a predefined threshold, the calculation of the error signal for the calibrated hydrostatic altitude signal is based on predefined error values, changes in time, distance and altitude since the last calibration with a determined difference below the predefined threshold, and the error signal associated with the received aircraft altitude and position signals; and

(iii) if the determined difference is above a predefined threshold, calculating the error signal for the calibrated hydrostatic altitude signal is based on the predefined error values, on changes in time, distance and altitude since the last calibration with a determined difference below the predefined threshold and the error signal associated with the aircraft altitude and position signals received at the last calibration with a determined difference below the predefined threshold.

35. The method of claim 33, further comprising replacing the generated hydrostatic altitude signal with the received pressure altitude signal, if the air temperature signal becomes lost or invalid.

36. The method of claim 35, wherein d. calculating an error signal for the received hydrostatic altitude signal performs calculating an error signal for the pressure altitude and comprises:

(i). determining nearest runway based on the received aircraft position signal and a predefined database of runway elevations;

(ii). determining distance to and height above the determined nearest runway based received aircraft position signal and received aircraft altitude signal; and

(iv). calculating the error signal for the received pressure altitude signal based on the received pressure altitude signal, a predefined altimeter error factor and the determined distance to and height above the nearest runway.

37. The method of claim 33, wherein c. initializing the hydrostatic altitude signal is performed on the ground prior to takeoff or is performed in flight, if a power up is required in flight.

38. The method of claim 37, wherein d. calculating an error signal for the hydrostatic altitude signal is based on predefined error values and changes in time, distance and altitude since most recent initialization.

39. The method of claim 38, wherein d. calculating an error signal for the hydrostatic altitude signal is further based on a predefined value, if the most recent initialization was performed on the ground prior to takeoff;

or based on the error signal associated with the received aircraft altitude and position signals, if the most recent initialization was performed in flight.

40. The method of claim 33, wherein i. generating a modified aircraft altitude signal further comprises:

(i). if the generated hydrostatic altitude signal was determined reasonable and the calibrated hydrostatic altitude signal was determined unreasonable, outputting the calculated hydrostatic altitude signal error and the generated hydrostatic altitude signal as the modified aircraft altitude signal;

(ii). if the generated hydrostatic altitude signal and the calibrated hydrostatic altitude signal were determined reasonable, generating a modified aircraft altitude signal based on the generated hydrostatic altitude signal, the calibrated hydrostatic altitude signal and the received aircraft altitude signal; and

(iii). if the generated hydrostatic altitude signal was determined unreasonable and if said second validity signal of the received aircraft altitude signal indicates valid, outputting the aircraft altitude signal with error signal.

41. The method of claim 40, wherein i.(ii). generating a modified aircraft altitude signal based on the generated hydrostatic altitude signal, the calibrated hydrostatic altitude signal and the received aircraft altitude signal comprises averaging the generated hydrostatic altitude signal and the generated calibrated hydrostatic altitude signal.

42. The method of claim 40, wherein i.(ii). generating a modified aircraft altitude signal based on the generated hydrostatic altitude signal, the calibrated hydrostatic altitude signal and the received aircraft altitude signal comprises:

(i). generating weighted values for the generated hydrostatic altitude signal and the calibrated hydrostatic altitude signal based on the calculated error signal for the generated hydrostatic altitude signal and the calculated error signal for the calibrated hydrostatic altitude signal, respectively; and

(ii). combining the generated hydrostatic altitude signal and the calibrated hydrostatic altitude signal based on the generated weighted values.

43. The method of claim 40, wherein i.(ii). generating a modified aircraft altitude signal based on the generated hydrostatic altitude signal, the calibrated hydrostatic altitude signal and the received aircraft altitude signal comprises:

(i). determining the lowest value of the calculated error signal for the received hydrostatic altitude signal and the calculated an error signal for the calibrated received hydrostatic altitude signal; and

(ii). selecting the received hydrostatic altitude signal or the received aircraft altitude signal with the determined lowest value.

44. The method of claim 40, further comprising:

j. receiving a radio altimeter generated altitude signal and a fourth validity signal associated with the radio altimeter altitude signal;

k. if the aircraft is below a predefined altitude, calibrating the hydrostatic altitude signal based on the received radio altimeter generated altitude signal and a predefined terrain elevation database;

l. calculating an error signal for the radio altimeter calibrated hydrostatic altitude signal based on the most recent radio altimeter calibration of the hydrostatic altitude signal;

m. determining a fifth validity of the radio altimeter calibrated hydrostatic altitude signal based on said first received validity signal of the received pressure altitude signal and said fourth validity signal associated with the received radio altimeter altitude signal; and

n. determining reasonableness of the radio altimeter calibrated hydrostatic altitude signal;

wherein generating a modified aircraft altitude signal is further based on the radio altimeter calibrated hydrostatic altitude signal, the determined reasonableness of the radio altimeter calibrated hydrostatic altitude signal and the calculated error signal for the radio altimeter calibrated hydrostatic altitude signal.

45. The method of claim 44, further comprising calculating an accuracy value for the predefined terrain elevation database based on a predefined database cell size and the received aircraft position signal.

46. The method of claim 45, wherein l. calculating an error signal for the radio altimeter calibrated hydrostatic altitude signal is based on predefined error values, changes in time, distance and altitude since most recent radio altimeter calibration of the hydrostatic altitude signal, predefined radio altimeter accuracy values and the estimated accuracy of the predefined terrain elevation database.

47. An apparatus for determining a modified aircraft altitude signal, said apparatus comprising:

a. a receiving component for receiving an air temperature signal, a pressure altitude signal, a first validity signal associated with the pressure altitude signal, aircraft altitude and position signals, and at least one error signal and a second validity signal associated with the aircraft altitude and position signals;

b. a hydrostatic altitude generator for generating a hydrostatic altitude signal based on the received pressure altitude signal and the air temperature signal;

c. an intializer for initializing the generated hydrostatic altitude signal;

d. an hydrostatic altitude calculator for calculating an error signal for the received hydrostatic altitude signal based the initialization of the hydrostatic altitude signal;

e. a complementary filter for calibrating the hydrostatic altitude signal based on the received aircraft altitude signal, the at least one error signal associated with the aircraft altitude signal and the calculated error signal for the generated hydrostatic altitude signal;

f. a calibrated hydrostatic altitude calculator for calculating an error signal for the calibrated hydrostatic altitude signal based on the most recent calibration of the hydrostatic altitude signal;

g. a validity determiner for determining a third validity of the calibrated hydrostatic altitude signal based on said first received validity signal of the received pressure altitude signal and said second validity signal associated with the received aircraft altitude signal;

h. a reasonable determiner for determining reasonableness of the generated hydrostatic altitude signal and the calibrated hydrostatic altitude signal; and

i. an aircraft altitude generator for generating the modified aircraft altitude signal based on the determined reasonableness, the determined said third validity of the calibrated hydrostatic altitude signal, the received aircraft altitude signal, and the calibrated hydrostatic altitude signal.

48. A computer program product for performing the method of claim 47.

49. The apparatus of claim 47, wherein the calibrated hydrostatic altitude calculator comprises:

(i) a difference determiner for determining the difference between the last received error signal associated with the last received aircraft altitude and position signals and the most recently received error signal associated with the most recently received aircraft altitude and position signals;

(ii) if the determined difference is below a predefined threshold, the calibrated hydrostatic altitude calculator performs the calculation of the error signal for the calibrated hydrostatic altitude signal based on predefined error values, changes in time, distance and altitude since the last calibration with a determined difference below the predefined threshold, and the error signal associated with the received aircraft altitude and position signals; and

(iii) if the determined difference is above a predefined threshold, the calibrated hydrostatic altitude calculator performs the calculation of the error signal for the calibrated hydrostatic altitude signal based on the predefined error values, on changes in time, distance and altitude since the last calibration with a determined difference below the predefined threshold and the error signal associated with the aircraft altitude and position signals received at the last calibration with a determined difference below the predefined threshold.

50. A computer program product for performing the method of claim 49.

51. The apparatus of claim 47, further comprising a default component for replacing the generated hydrostatic altitude signal with the received pressure altitude signal, if the air temperature signal becomes lost or invalid.

52. A computer program product for performing the method of claim 51.

53. The apparatus of claim 51, wherein the hydrostatic altitude calculator comprises:

(i). a runway determiner for determining nearest runway based on the received aircraft position signal and a predefined database of runway elevations;

(ii). a runway proximity determiner for determining distance to and height above the determined nearest runway based received aircraft position signal and received aircraft altitude signal; and

(iv). an error calculator for calculating the error signal for the received pressure altitude signal based on the received pressure altitude signal, a predefined altimeter error factor and the determined distance to and height above the nearest runway.

54. A computer program product for performing the method of claim 53.

55. The apparatus of claim 47, wherein the initializer performs the initialization on the ground prior to takeoff or in flight, if the apparatus requires a power up in flight.

56. A computer program product for performing the method of claim 55.

57. The apparatus of claim 55, wherein the calculation performed by the hydrostatic altitude calculator is based on predefined error values and changes in time, distance and altitude since most recent initialization.

58. A computer program product for performing the method of claim 57.

59. The apparatus of claim 57, wherein the calculation performed by the hydrostatic altitude calculator is further based on a predefined value, if the most recent initialization was performed on the ground prior to takeoff;

or based on the error signal associated with the received aircraft altitude and position signals, if the most recent initialization was performed in flight.

60. A computer program product for performing the method of claim 59.

61. The apparatus of claim 47, wherein the aircraft altitude generator comprises:

(i). a first component for outputting the calculated hydrostatic altitude signal error and the generated hydrostatic altitude signal as the modified aircraft altitude signal, if the generated hydrostatic altitude signal was determined reasonable and the calibrated hydrostatic altitude signal was determined unreasonable;

(ii). a second component for generating the modified aircraft altitude signal based on the generated hydrostatic altitude signal, the calibrated hydrostatic altitude signal and the received aircraft altitude signal, if the generated hydrostatic altitude signal and the calibrated hydrostatic altitude signal were determined reasonable; and

(iii). a third component for outputting the aircraft altitude signal with error signal, if the generated hydrostatic altitude signal was determined unreasonable and if said second validity signal of the received aircraft altitude signal indicates valid.

62. The apparatus of claim 61, wherein the second component comprises:

(i). a weight generator for generating weighted values for the generated hydrostatic altitude signal and the calibrated hydrostatic altitude signal based on the calculated error signal for the generated hydrostatic altitude signal and the calculated error signal for the calibrated hydrostatic altitude signal, respectively; and

(ii). a combiner for combining the generated hydrostatic altitude signal and the calibrated hydrostatic altitude signal based on the generated weighted values.

63. A computer program product for performing the method of claim 62.

64. The apparatus of claim 61, wherein the second component comprises an averaging component for averaging the generated hydrostatic altitude signal and the generated calibrated hydrostatic altitude signal.

65. A computer program product for performing the method of claim 64.

66. A computer program product for performing the method of claim 61.

67. The apparatus of claim 61, wherein the second component comprises:

(i). a first subcomponent for determining the lowest value of the calculated error signal for the received hydrostatic altitude signal and the calculated an error signal for the calibrated received hydrostatic altitude signal; and

(ii). a second subcomponent for selecting the received hydrostatic altitude signal or the received aircraft altitude signal with the determined lowest value.

68. A computer program product for performing the method of claim 67.

69. The apparatus of claim 61, further comprising:

j. a second receiving component for receiving a radio altimeter generated altitude signal and a fourth validity signal associated with the radio altimeter altitude signal;

k. if the aircraft is below a predefined altitude, a radio altimeter calibrator for calibrating the hydrostatic altitude signal based on the received radio altimeter generated altitude signal and a predefined terrain elevation database;

l. a radio altimeter error calculator for calculating an error signal for the radio altimeter calibrated hydrostatic altitude signal based on the most recent radio altimeter calibration of the hydrostatic altitude signal;

m. a radio altimeter validity determiner for determining a fifth validity of the radio altimeter calibrated hydrostatic altitude signal based on said first received validity signal of the received pressure altitude signal and said fourth validity signal associated with the received radio altimeter altitude signal; and

n. a radio altimeter reasonable determiner for determining reasonableness of the radio altimeter calibrated hydrostatic altitude signal;

wherein the aircraft altitude generator performs generating the modified aircraft altitude signal further based on the determined reasonableness of the radio altimeter calibrated hydrostatic altitude signal and the radio altimeter calibrated hydrostatic altitude signal.

70. A computer program product for performing the method of claim 69.

71. The apparatus of claim 69, further comprising an accuracy calculator for calculating an accuracy value for the predefined terrain elevation database based on a predefined database cell size and the received aircraft position signal.

72. A computer program product for performing the method of claim 71.

73. The apparatus of claim 71, wherein the radio altimeter error calculator performs calculating an error signal for the radio altimeter calibrated hydrostatic altitude signal based on predefined error values, changes in time, distance and altitude since most recent radio altimeter calibration of the hydrostatic altitude signal, predefined radio altimeter accuracy values and the estimated accuracy of the predefined terrain elevation database.

74. A computer program product for performing the method of claim 73.