On Thu, 13 Dec 2018 19:12:53 -0000, "NY"  declaimed the
following:

>
>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.
>

 At the time of my books (over a decade old now -- primarily GPS
Satellite Surveying 2nd Ed [Alfred Leick, 1995 John Wiley & Sons]), GPS
signals are sent on L1 and L2 (frequencies: 1575.42MHz, 1227.60MHz). L1
carries the C/A (coarse/acquisition) signal -- it is a ground control
option to also have C/A on L2.. L1 and L2 carry the precise position
signal. PPS is encrypted and requires receivers with government furnished
decryption keys. While the keys do contain indicators for service type,
allowing the government to disable, say, commercial air receivers without
affecting military air receivers, it still requires the receivers to be
periodically updated by someone with crypto clearance (I can just see the
hassle that would be for, say, a Chinese airline using a US-made FMS and
related equipment -- which is not that odd; GE Aviation provides the FMS
boxes for Boeing, Airbus, and I think is on contract for China's big
airliner)..

 The "chip rate" for PPS is 10 times that of the C/A signal (putting it
at ~10MHz; you can envision the processor needed to receive [and separate
out the various satellites -- they are identified by the pattern of
pseudorandom noise impressed on the data stream], decode [for PPS], and
apply computed corrections. C/A is ~1MHz data stream.

 I have vague memories that they were adding L3 and L4 to new generation
NavStar. If they also put C/A on L2, it would allow consumer gear to use L1
and L2 time differences to correct for ionospheric delays, while still not
requiring decryption equipment.  I suspect commercial aircraft may be
running gear that can detect/synchronize the PPS signal without decoding it
-- which would be sufficient to perform the ionospheric correction while
still using the C/A signal for navigation. Unless they are relying upon
fully automated landings, a 5-10m error circle is still sufficient to
navigate to a visual runway approach.


>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

 Ever watch a weather barometer? If a major storm front comes through,
the pressure can drop by an inch or more. Even for consumer (hiking) GPS
with barometric altitude the recommendation is to calibrate to surveyed
altitude (you do have proper map showing altitude for your starting point)
at the start of the trip, and periodically (like, stopping for lunch at an
identifiable point on the map).

>place, and does it drop as altitude increases by a consistent formula? Are

 It also varies with temperature. To compute altitude you need: sea
level pressure, pressure at altitude, and temperature.

https://www.mide.com/pages/air-pressure-at-altitude-calculator

 It used to be standard procedure to adjust the aircraft barometric
altimeter to use a standard pressure setting before making final approach,
either mean sea level pressure or a setting provided by the ATC for a given
runway. This ensured that all approaching aircraft were on the same
(relative) altitude readings, rather than having them using altitudes based
upon the pressures present at their departure runways. Some aircraft may
have radar altimeters, but those are going to be more erratic the closer to
the ground one becomes (at cruise altitude, the reflection can be averaged
over the ground, and landing altitudes, just crossing a warehouse would
jump the altitude by 20-30 feet)


>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.

 What needle 

 Modern flight decks are all digital. The sensor inputs would be
massaged by PID algorithms to produce controller outputs
https://en.wikipedia.org/wiki/PID_controller


--
 Wulfraed                 Dennis Lee Bieber         AF6VN
 wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/

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