On 13.12.18 21:12, NY wrote:
> "Dennis Lee Bieber"  wrote in message
> news:g1751edssbc3h2lpgoh3qag0cl9gmjm97h@4ax.com...
>> For ground speed, not air speed... Having a 200kt ground speed with a
>> 100kt tail wind means an air speed of only 100kt.
>
> Is there an electronic means of measuring airspeed, which doesn't
> involve measuring air pressure as the plane flies forwards and
> subtracting the current static air pressure, corrected for altitude?
>
>>> My phone varies between about 20m and 5m radius of error for lateral
>>> movement. Not sure how accurate it is for altitude. I sometimes find
>>> that I
>>
>> I don't even know if any consumer level GPS is able to make use of
>> dual frequency
>> signals yet -- the precision signal is sent over two (or now, maybe four)
>> frequencies, allowing the receiver to correct for ionospheric delays. C/A
>> traditionally was sent on one frequency only.
>
> Yes I was implying that if a consumer-grade GPS receiver can get
> position information to +/- a few metres, then professional-grade
> equipment (which I imagine aircraft would use) would be able to give
> more precise readings; I'm surprised that any non-GPS equipment could be
> that accurate.
>
> What is the typical +/- for air-pressure airspeed and altitude readings?
> How much does ground-level air pressure vary from time to time or place
> to place, and does it drop as altitude increases by a consistent
> formula? Are such errors in altitude readings smaller than you'd get
> with consumer or professional GPS?
>
> I presume both airspeed and altitude gauges include a fairly long
> time-constant in the needle damping to avoid gusts of wind causing
> apparent fluctuations in speed and altitude.


Much of your air pressure questions can be answered with the
barometric formula, google for it.

In ICAO standard atmosphere, temperature is 15 degrees C, pressure
1013.2 hectopascals (or millibars) and density 1.29 kg/m3. The
temperature drops by 6.5 degrees (Celsius or Kelvin) per km rise.
This is valid up to the tropopause (usually 10 - 12 km).

The altitude change per 1 mb pressure drop is about 8.1 m near the
surface.

The response speed of the gauges is enough to follow the movement
of the aircraft with no noticeable lag.

You should remember that the main use of the airspeed gauge is
to get a good feeling of the expected aerodynamic forces on the
airfoils. All pilots know that the speed on the gauge may be
off from the real speed of the airflow.

--

-Tauno Voipio, MSEE, avionics engineer and instructor pilot

--- SoupGate-Win32 v1.05