As an embodiment of the present invention, in a navigation system using an acceleration sensor, when position information cannot be obtained from a GPS processing section, a velocity detecting unit performs an operation using detected acceleration α G, a measurement time mt, a velocity V0 at a time t0, gravity acceleration g and an amount of height change Dh, according to Expression (11). By using the relationship among a gravity acceleration component gf, the gravity acceleration g, the amount of height change Dh and distance Dm shown in Expression (4), the gravity acceleration component gf can be offset by the amount of height change Dh. Therefore, velocity V can be calculated with high accuracy without receiving the effect of the gravity acceleration component gf.

Apparatus and method for providing the apparatus, the apparatus comprising: a duct; and a non-return valve (18, 24); wherein the duct comprises a first portion and a second portion; the first portion has a first end (28, 30) and a second end; the first end (28, 30) is open to a fluid (e.g. air); the second end is connected to the second portion such that fluid is permitted to flow between the first portion and the second portion; the non-return valve (18, 24) is positioned at or proximate to the second end; and the non-return valve (18, 24) is arranged such that, upon application of a suction force to the first portion at the first end (28, 30), fluid flows into the first portion via the non-return valve (18, 24) from outside the duct, and through the first portion from the second end to the first end (28, 30).

A multi-function probe system (14, 16, 18, 50, 70) that provides redundancy for measurements and compensates for effects of sideslip of an aircraft (10) includes at least two probes (14, 16) that are symmetrically located on the opposite sides of the aircraft, and a third probe (18, 50, 70) mounted on the centerline (20) of the aircraft (10) positioned to directly measure local sideslip, in one form (18, 50) of the third probe with ports (60, 62) that are positioned on opposite sides of the center plane of the aircraft. Each of the probes (14, 16, 18, 50, 70) includes self contained instrumentation (14D, 16D, 18D, 38) for providing signals indicating various pressures. The local angle of sideslip sensed by the third probe (18, 50, 70) is used as a compensation for pressure readings at either of the other probes (14, 16) for determining actual angle of attack, and static pressure.

A variometer for use in a glider to indicate the rate of lift or sink during flight. An audible signal is generated variations in which correspond to variations in the output of a differentiation circuit receiving signals from an electrical pressure transducer, the audible signal so providing an indication of the rate of lift or sink thereby relieving the pilot of an otherwise visual task and hence simplifying the procedure of flying the glider. The audible signal may vary in pitch between a level indicating a desired lift or sink and another level indicating the actual lift or sink. Alternatively a pulsed signal is used in which the repetition rate of pulses is indicative of the rate of lift or sink. Also the audible pitch of the pulsed signal may be changed to indicate whether the glider is flying at less than or greaterthan a predetermined sink rate.