My DVB-T and DVB sat reception scrpts

Its 2D accuracy is much the same in the air as on the ground, i.e. +/- 2m or so, but the height is never better than about +/-5m. I'm told thats because the 2D angles between you and the 3-5 satellites your receiver sees the GPS constellation tend to be acute angles and changing quite fast while the vertical separation is a lot less because all GPS satellites are on roughly the same spheroid which makes compressed differences in their distance and bearing.

I suspect Druck knows a lot more about this than I do: as a glider pilot I carry three GPS-based systems but am purely a user of this kit, both for safety and navigation in the air and, post flight, to analyse how I handled the day. The systems I carry in the glider are:

- a FLARM unit. This is GPS-based and is a short range electronic conspicuity and collision avoidance system used by almost all gliders in the UK and quite a lot of GA aircraft too.

- a flight recorder. This uses GPS and barometric pressure to record a 3D trace of my flight, with a 3D fix recorded every second.

- a GPS-based moving map navigation system. As well as a decent terrain map, this also holds restricted airspace data for the whole UK and the day's plottable NOTAMS. It only shows restricted airspace and NOTAMS if I'm close to them within (500 ft vertically and 60 seconds flying time horizontally) so the screen stays uncluttered.

If I'm going cross-country, I'll enter my task for the day as well.

That will vary with how many satellites it can see. Not sure how accurate it is for altitude. Vertical accuracy is worse than horizontal accuracy by a factor of 2 or 3.

I usually only see that if its a newish section of road that has been displaced to one side of the old road and where the satnav map hasn't yet been updated.

As far as I'm concerned FLARM is it - we've only had it since about 2006, but I don't think many would fly without it now. All my club's gliders carry FLARM. A side benefit is OGN - the Open Glider network. The UK is pretty much covered with amateur-run OGN receivers at 30-50km spacing. The movin maps it supports maps are accessable over the 'net from pretty much any browser and there's a feed to Flight Radar 24. The OGN receivers are RaspberryPis with enough add-on kit to let the combination run as a web-connected software-defined radio transceiver. They can also deal with FlightAware traffic - much the same as FLARM but more oriented to GA aircraft. The OGN network can pass FLARM traffic to FlightAware but not the other way: FLARM units only receive from each other and anyway, since all glider electronics runs off one or two 7AH SLA batteries, unlike GA planes we don't have the power to stay on communication with OGN units. FLARM is short range - typically 4km or so in the air, which is find for what it does - resolve conflicts between aircraft with closing speeds of

Go and look at OGN by all means, but don't expect to see much outside wave soaring localities (Scotland, Wales, Pennines and the Yorkshire hills) until mid April because most of our gliders are in winter storage until the weather warms up and thermal soaring is possible again.

yes, but Iptables rejects that an thats the most effcient way

-- "Corbyn talks about equality, justice, opportunity, health care, peace, community, compassion, investment, security, housing...." "What kind of person is not interested in those things?"

"Jeremy Corbyn?"

If "lots of satellites" includes the WAAS (Wide Area Augmentation System) network, you can get down to 6m or better.

If your GPS is equipped with WAAS, there are LPV (localizer performance with vertical guidance) approaches which you can fly just like an ILS, using the same panel indicators, down to minimums almost as good as ILS. These approaches have been set up at many airports that have no ground-based navaids.

It's a wonderful time to get an instrument rating. :-)

I imagine if you do gliding you'll know Sutton Bank airfield. We live fairly near there and sometimes we've stopped and walked along the footpath along the cliff edge and watched the gliders taking off and landing, I bet it's a fantastic sight to look out across the Vale of York, and to see the White Horse and the cliffs at Sutton Bank as you're coming back.

On Wed, 12 Dec 2018 22:36:49 -0000, "NY" declaimed the following:

For ground speed, not air speed... Having a 200kt ground speed with a

I believe convention is to assume vertical error is 150% of lateral. Your phone is also most likely using just the C/A signal; airlines may be able to get decryption keys allowing use of the precision signal. 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.

Dennis Lee Bieber

Yes true, but pilots are well aware of wind speed and direction, meteo. Simple addition is not too much for them..

So A GPS SOG (speed over ground) can be used to verify pitot tube type sensors.

I have done some research or better say experiments myself making ultrasonic air speed sensors, basically for boating, some I posted about in sci.electronics.design. These days you can buy those for maybe a hundred Euro or so.

All those methods combined is the LEAST that should be mounted in a many millions dollar worth of plane.

Yes, for absolute position. These days however units with GPS, GLONASS, Beidu are just 25 $. Put 3 in a box and you can perhaps get smaller errors, Galileo is coming too, if not already here.

All is relative, here I fly my Hubsan drone with 100% GPS controlled auto-pilot to a few centimeter accuracy:

asm code is here:

I mean even for a pressure based altimeter you need to calibrate at ground. Maybe it is not so hard to (for example via radio) calibrate the on board GPS relative to one on the landing strip, basically what I do here. Over short time scales the GPS position does not normally wander that much. The math is very simple, see the C source code for composing the flight path on same page: fly_waypoints-0.4.tgz

I encountered the opposite on a recent flight: air speed 100kt, ground speed 50kt.

Or even vector addition, using the good old E6B that dates from WWII. (No batteries to go dead, either.)

More or less. Weather forecasting isn't perfect...

Oh, I'm sure it's all there. Tying it all together in the Flight Management System with properly designed (and documented!) software is another matter.

I have an ancient Garmin 48 handheld GPS which usually gets altitude within a couple of hundred feet.

That's old hat. The standard today is WAAS:

You might be thinking about differential GPS, where a receiver on the ground compares its position as reported by GPS to its known (accurately surveyed) position and transmits a correction signal to nearby receivers. The difference between this and WAAS is that WAAS uses a handful of receivers scattered all over North America, while DGPS is designed as a short-range solution for a specific location.

A friend recently retired from Nav Canada, where his job was calibrating ground-based aeronautical navigation aids. With a properly set up DGPS unit, he was able to measure positions with 1cm accuracy.

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?

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.

Of course, but detecting a defective pitot tube and using SOG and wind to land instead is a good emergency strategy.

Oh I dunno, been writing software now for ages, and designing stuff. It is not that hard.

WAAS is GPS only AFAIK. We need the other systems too. Likely for example China and Russia have their own by now? So on a flight from the US to say China the system needs to support both - or soon both, systems. Oh and Europe will not stay behind.

Yes that is differential GPS, in my case the GPS in the remote is the ground station and the GPS in the drone the other one. They call that 'follow me' mode, follow the remote, is what it does,

Right.

There is also altitude radar, this guy build his own:

Do I see a raspi opportunity there ?

His explanation of GPS and GLONASS is the best I have found so far, including the decoding hardware, he did that way before you could buy those 25 dollar GPS thingies,

Yes as we all know, wind speed and direction for all locations and altitudes is precisely as given the briefing charts 6 hours earlier. Gusting, windsheer, updrafts, all figments of the imagination.

FFS, I'm stopping reading before I encounter any more idiocy. I suggest you stop writing it, unless you think there is anyone not yet convinced that you don't know what you are talking about.

---druck

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

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.

Indeed. Those poor Lion Air buggers might have had half a chance.

Not for you and me, perhaps. Big bureaucratic outfits are a different matter, though. And don't get me started on Microsoft.

Satellite-based augmentation systems (SBAS, the ICAO generic term for WAAS-like systems) exist or are being developed on other continents. A box that works with all of them might be tricky. Damn that NIH syndrome...

Cool. I suppose you could call that relative position rather than absolute position since you're carrying the ground station around with you rather than fixing it at an accurately-surveyed location. But it must be great for drone operation.

I don't know of any, but they probably exist. Still, for sheer simplicity (i.e. reliabilitly) it's hard to beat a pitot/static system (heated to resist icing over) driving a pressure sensor.

Airspeed indicators are good to within a few knots, and altimeters to

Sea-level air pressure typically varies between 29.5 and 30.5 inches of mercury (with occasional excursions beyond that). Part of preparation for flight is to set your altimeter for the current air pressure - otherwise it could be out by 500 feet or more.

The formula models what's called a "standard atmosphere": at sea level this assumes a pressure of 29.92 inches and a temperature of 15 degrees Celsius. Pressure decreases exponentially, but below 10,000 feet it's about one inch of mercury per 1000 feet. Temperature decreases by 2 degrees Celsius per 1000 feet up to the tropopause (typically at about

Yes.

Not that long. In mechanical instruments the inertia of the parts provides some damping, but rapid fluctuations will show up.

I've flown at Sutton Bank a few times: an interesting, if sometimes scary, site to fly from. A tow westwards toward the Pennines can put you in wave when there's a westerly with the right vertical velocity profile and then the sky really is your playground.

OTOH landing in a northerly means finals are over the White Horse, often in horrible sink. I remember being warned not to turn base at less than

This is nothing like my home field in flat Cambridgeshire where no part of the county is more than 260 ft above sea level.

OTOH winch launches at Sutton Bank seldom if ever exceed 400 ft - just lobbing the glider over the cliff edge - while at home a winch launch to

Not that I'm aware of.

The only thing better AFAIK is differential GPS (DGPS) which works over small areas by putting a GPS receiver at a known 3D location and connecting it to a transmitter that broadcasts the difference between its known position and where GPS says it is. This difference signal can be used by specialised GPS receivers to give a corrected 3D position to well under a meter.

ASIs are calibrated (and annually checked) to be within +/- 2 knots of TAS (True Airspeed).

Base is 1013 millibars. The normal range in the UK is about 990 to 1035 millibars but storms may have lower pressures.

Fairly much. Otherwise a pressure altimeter would be telling lies.

Difficult to say because GPS altitudes are generally within +/- 5m for an uncorrected GPS receiver but can be considerably more accurate if its a DGPS receiver and reasonbly close to a DGPS transmitter (see above).

No. Normally there's no damping. Its not needed in normal conditions. An ASI is an undamped pressure sensor that shows the pressure difference between the static vent and the pitot but is calibrated in knots rather than pressure units.

An altimeter shows the undamped pressure at the static vent but is calibrated in feet or metres of altitude rather than pressure units .

Pilots of aircraft that take traffic services set their altimeters to the base pressure supplied by the controller they're taking a service from. As a UK glider pilot I never use traffic services. I zero my altimeter (to airfield height) before launch but, if I'm talking to air traffic control and they want to know my altitude I reset my altimeter to the base pressure they supply - this means my pressure altitude is directly comparable with that of other aircraft in the area: if ATC has radar (not all do) it may be just a 2D system rather than 3D.

High point of Cambridgeshire is 479 ft (146m) near Great Chishill. See

Charlie Gibbs wrote

I have some air pressure chips here, hardly any delay, very accurate, you can see the difference between on the floor and on the table.

This is a screenshot of my 'xgpspc' system running on a raspberry pi:

Idea for a drone anti-collision system, air traffic monitoring via a rtl_sd USB stick and the 'dump1090' program, air pressure is bottom right, Added some calculations to get true direction to plane and true elevation. The yellow 'boat' (or drone) is at the center.

xgpspc can also display pressure of the last 24 hours:

And display ships of course AIS:

Old source is here:

There are likely much better systems around like opencpn etc but this is my own experiment creation, contains features others do not have. Every now and then I add some things,

It is actually IAS (indicated airspeed), which matches the true airspeed at standard pressure and temperature.

The scale on altimeters is usually 950 to 1050 millibars. I have once (midsummer day 1987) been sitting in the cockpit and wondering how to set the altimeter to 1053 mb.

This is what is called the barometric formula. It is not strictly the correct geometric altitude at different conditions, but it is accurate enough to guarantee altitude separation between aircraft flying on similar altimeter indications.

The ATC secondary radar (SSR) using the aircraft transponders has a mode to interrogate the altimeter in the aircraft. The altitude encoder is set for standard pressure (QNE), 1013 millibars, regardless of the real air pressure on ground.