Metric Instruments

On Thu, 25 Aug 2005 09:43:46 +0200, "Bert Willing"
wrote:

Well, even if there is only one owner involved, I'm not going to rely on
electrically power instruments only. Never.^

I still remember one competition back in 1991 when the US shut down
GPS and suddenly a gaggle of 30 standard class gliders lost their
navigation in the vicinity of Sobernheim (several airbases and
restricted airspace there). I got to know a new definition of the term
"confusion".





Bye
Andreas

Whatever technology is behind the panel (analog or digital), the human
interface for instruments where trends and relative magnitude are
important then the "analog style" gauge is far superior to a digital
readout. It is easily discerned for ballpark, trends up or down, and
actual value rather precisely. So for airspeed indicators, varios, and
the like, the "analog style" interface is the way to go.



For battery voltage levels, radio frequencies, etc. where precision is
more important than trends or "ballpark" then a digital readout is just
the ticket.



Regards,



Larry





"Bill Daniels" wrote in message
:

Does it strike some of the digerati here that expensive mechanical
altimeters with easily mis-read clock-like hands locked into either the
metric or US measurement systems are archaic?

GPS provides highly accurate, although not ATC compliant, altitude. Various
vendors provide electronic pressure altimeters with digital displays that
can be switched between meters and feet with the push of a button. Digital
pressure altitude sensors drive the "glass cockpits" of new GA aircraft.

I seems to me that clock-like altimeters designed 70 years ago and
maintained by watchmakers must be nearing their well-deserved retirement.

Bill Daniels


"Bert Willing" wrote in
message ...
Yes, they are.

--
Bert Willing

ASW20 "TW"


"Roy Bourgeois" a écrit dans le message
de
news: ...
This may be a silly question - but are all metric altimeters
configured with 'Zero at 6 O'clock' as I saw in France?
I did not have trouble converting to meters/kilometers
but I did have trouble quickly reading the altimeter
with the zero at the bottom of the instrument face
(especially on the little 57mm instruments). Just
curious.

Roy

Bill Daniels wrote:

snip

GPS provides highly accurate, although not ATC compliant, altitude.

I am afraid that the claim that GPS altitude is recorded "highly
accurately" in IGC files from IGC-approved GPS recorders, is
unfortunately not true.

The second part of the statment above IS true, that the GPS altitude
datum is not the same as the pressure altitude datum used worldwide in
aviation for altimeter settings for aircraft separation and for
controlled and restricted airspace.

In theory, due to the angle of cut of the lines-of-position from the
satellites, GPS altitude errors should be, on average, about 1.8 times
those for horizontal position or lat/long. Measurements over many
years by the IGC GNSS Flight Recorder Approval Committee (GFAC) show an
average lat/long error of 11.4 metres, taken from a moving vehicle at
surveyed points at about 51N 001W (Southern England, near Lasham
Gliding Centre). Going on this, an average GPS altitude error could be
expected as about 20.5 metres.

However, in a significant proportion of IGC-format flight data files,
there are significant anomalies in the GPS altitude figures that have
been recorded, in excess of the 20 metres mentioned above. Only today
I was commenting in another email on aspects of an IGC file from a
recent glider flight in the USA that had a 1500 foot overshoot in GPS
altitude (compared to the much more reliably recorded pressure
altitude) for reasons unknown.

The problem seems to be, particularly in low-cost GPS boards, that,
rather than processing a fix in three dimensions, it is processed
separately as lat/long and then separately as altitude. The algorithms
for lat/long and for altitude appear to be different, hence the regular
occurrence in IGC files or GPS altitude anomalies despite few lat/long
anomalies. Naturally, more attention seems to be paid by GPS board
manufacturers to lat/long rather than altitude.

In a survey made in year 2000 after the deliberate Selective
Availability error was removed from the GPS system by Presidential
Decree, no less than 27% of over 400 IGC flight data files analysed
from 7 countries in both hemispheres, had anomaliesof one sort or
another in the GPS altitude recorded in the file. From IGC files that
I have seen since, there is no reason to believe that this proportion
is much improved today. Just look at a large selection of IGC-format
flight data files and see for yourselves. In my database, I have
literally hundreds of IGC flight data files that show major anomalies
in recorded GPS altitude data. Fortunately, anomalies in lat/long data
in the same IGC files are very rare.

This is not an attack on the accuracy of the GPS system or even its
altitude recording capability. It is a reporting of results of GPS
altitude recording in IGC flight data files derived from a number of
low-cost GPS boards made by a number of different companies from
different parts of the world. I guess that in more expensive
"professional aviation standard" GPS boards, and in differential-GPS
systems with local beacons, the GPS altitude figures are more accurate
and with less anomalies. But such (expensive) systems do not apply to
the current 27 types of GNSS flight recorders that are IGC-approved
(from 11 manufacturers) and whose IGC-approval documents appear on the
IGC gliding/gnss web site:

http://www.fai.org/gliding/gnss/igc_approved_frs.pdf

Ian Strachan
Chairman IGC GNSS Flight Recorder Approval Committee

By my checking, WAAS enabled, dual antenna DGPS receiver boards are cheap -
on the order of $10US in OEM quantities. The specs say 1 meter RMS in Lat
Long and 6 meters RMS in altitude when a DGPS signal is available. Of
course, they probably aren't in approved loggers.

6 meters in altitude is a lot better than a barometric altimeter on a
non-standard atmospheric day.
I wouldn't want to change ATC's reliance on barometric altimeters. On a
hot day, they understate the real altitude, giving us western US guys
another 1000 or so feet to play in below the floor of Class A airspace.

I checked GPS altitude a couple of times by putting a hand held Garmin GPS
on a prime US Geodetic Survey marker. The marker said 10,346 feet. The GPS
said 10,350 feet + or - 70 feet. The + or - error estimate seemed pretty
pessimistic. Those are pretty typical numbers.

Bill Daniels

"Ian Strachan" wrote in message
ups.com...
Bill Daniels wrote:

snip

GPS provides highly accurate, although not ATC compliant, altitude.

I am afraid that the claim that GPS altitude is recorded "highly
accurately" in IGC files from IGC-approved GPS recorders, is
unfortunately not true.

The second part of the statment above IS true, that the GPS altitude
datum is not the same as the pressure altitude datum used worldwide in
aviation for altimeter settings for aircraft separation and for
controlled and restricted airspace.

In theory, due to the angle of cut of the lines-of-position from the
satellites, GPS altitude errors should be, on average, about 1.8 times
those for horizontal position or lat/long. Measurements over many
years by the IGC GNSS Flight Recorder Approval Committee (GFAC) show an
average lat/long error of 11.4 metres, taken from a moving vehicle at
surveyed points at about 51N 001W (Southern England, near Lasham
Gliding Centre). Going on this, an average GPS altitude error could be
expected as about 20.5 metres.

However, in a significant proportion of IGC-format flight data files,
there are significant anomalies in the GPS altitude figures that have
been recorded, in excess of the 20 metres mentioned above. Only today
I was commenting in another email on aspects of an IGC file from a
recent glider flight in the USA that had a 1500 foot overshoot in GPS
altitude (compared to the much more reliably recorded pressure
altitude) for reasons unknown.

The problem seems to be, particularly in low-cost GPS boards, that,
rather than processing a fix in three dimensions, it is processed
separately as lat/long and then separately as altitude. The algorithms
for lat/long and for altitude appear to be different, hence the regular
occurrence in IGC files or GPS altitude anomalies despite few lat/long
anomalies. Naturally, more attention seems to be paid by GPS board
manufacturers to lat/long rather than altitude.

In a survey made in year 2000 after the deliberate Selective
Availability error was removed from the GPS system by Presidential
Decree, no less than 27% of over 400 IGC flight data files analysed
from 7 countries in both hemispheres, had anomaliesof one sort or
another in the GPS altitude recorded in the file. From IGC files that
I have seen since, there is no reason to believe that this proportion
is much improved today. Just look at a large selection of IGC-format
flight data files and see for yourselves. In my database, I have
literally hundreds of IGC flight data files that show major anomalies
in recorded GPS altitude data. Fortunately, anomalies in lat/long data
in the same IGC files are very rare.

This is not an attack on the accuracy of the GPS system or even its
altitude recording capability. It is a reporting of results of GPS
altitude recording in IGC flight data files derived from a number of
low-cost GPS boards made by a number of different companies from
different parts of the world. I guess that in more expensive
"professional aviation standard" GPS boards, and in differential-GPS
systems with local beacons, the GPS altitude figures are more accurate
and with less anomalies. But such (expensive) systems do not apply to
the current 27 types of GNSS flight recorders that are IGC-approved
(from 11 manufacturers) and whose IGC-approval documents appear on the
IGC gliding/gnss web site:

http://www.fai.org/gliding/gnss/igc_approved_frs.pdf

Ian Strachan
Chairman IGC GNSS Flight Recorder Approval Committee

Bill Daniels wrote:
Does it strike some of the digerati here that expensive mechanical
altimeters with easily mis-read clock-like hands locked into either the
metric or US measurement systems are archaic?

I have both a mechanical and digital altimeter. When i want to check my
altitude, I tend to rely on the mechanical. Like a watch with hands, I
don'd read it as much as glance at it and I find that easier. YMMV.

Tony V.

"Bill Daniels" wrote in message
...
Does it strike some of the digerati here that expensive mechanical
altimeters with easily mis-read clock-like hands locked into either the
metric or US measurement systems are archaic?

GPS provides highly accurate, although not ATC compliant, altitude.
Various
vendors provide electronic pressure altimeters with digital displays that
can be switched between meters and feet with the push of a button.
Digital
pressure altitude sensors drive the "glass cockpits" of new GA aircraft.

I seems to me that clock-like altimeters designed 70 years ago and
maintained by watchmakers must be nearing their well-deserved retirement.

Bill Daniels

Yeah, now if they can just make them so they don't need batteries.

Tim Ward

"Tim Ward" wrote in message
ink.net...

"Bill Daniels" wrote in message
...
Does it strike some of the digerati here that expensive mechanical
altimeters with easily mis-read clock-like hands locked into either the
metric or US measurement systems are archaic?

GPS provides highly accurate, although not ATC compliant, altitude.
Various
vendors provide electronic pressure altimeters with digital displays
that
can be switched between meters and feet with the push of a button.
Digital
pressure altitude sensors drive the "glass cockpits" of new GA aircraft.

I seems to me that clock-like altimeters designed 70 years ago and
maintained by watchmakers must be nearing their well-deserved
retirement.

Bill Daniels

Yeah, now if they can just make them so they don't need batteries.

Tim Ward


What's the big deal with batteries? IMHO, batteries are at worst a minor
inconvenience easily worth enduring for the benefits of the technology they
make possible.

Every portable gadget uses them. Most folks have a cell phone, PDA,
portable GPS, digital camera, maybe a camcorder and who knows what else.
Even your car, tug or winch won't start without a battery. They're cheap
and they work fine with a little TLC and regular replacement.

My glider uses a standard 7.5 AH 12V SLA that now sits on a shelf connected
to a charger that quietly maintains the charge. I know for sure that it
will work at least 10 hours and still show more than 12.5 volts while
transmitting. It has a three year "replace by" date written on it whereupon
I will plunk down $20 for another at "Batteries-R-Us" even if it still seems
OK. I don't trust old batteries.

Bill Daniels

"Bill Daniels" wrote in message
...

"Tim Ward" wrote in message
ink.net...

"Bill Daniels" wrote in message
...
Does it strike some of the digerati here that expensive mechanical
altimeters with easily mis-read clock-like hands locked into either
the
metric or US measurement systems are archaic?

GPS provides highly accurate, although not ATC compliant, altitude.
Various
vendors provide electronic pressure altimeters with digital displays
that
can be switched between meters and feet with the push of a button.
Digital
pressure altitude sensors drive the "glass cockpits" of new GA
aircraft.

I seems to me that clock-like altimeters designed 70 years ago and
maintained by watchmakers must be nearing their well-deserved
retirement.

Bill Daniels

Yeah, now if they can just make them so they don't need batteries.

Tim Ward


What's the big deal with batteries? IMHO, batteries are at worst a minor
inconvenience easily worth enduring for the benefits of the technology
they
make possible.

Every portable gadget uses them. Most folks have a cell phone, PDA,
portable GPS, digital camera, maybe a camcorder and who knows what else.
Even your car, tug or winch won't start without a battery. They're cheap
and they work fine with a little TLC and regular replacement.

My glider uses a standard 7.5 AH 12V SLA that now sits on a shelf
connected
to a charger that quietly maintains the charge. I know for sure that it
will work at least 10 hours and still show more than 12.5 volts while
transmitting. It has a three year "replace by" date written on it
whereupon
I will plunk down $20 for another at "Batteries-R-Us" even if it still see
ms
OK. I don't trust old batteries.

Bill Daniels


Upon reflection, Bill, I'm sure that an instrument could be built that could
satisfy both of us.
Digital, easily scalable, there's no reason it can't have both an analog
display (or quasi-analog, with LCD) for trends, and a 5 digit display for
accuracy. It could have a lithium cell recharged by the expansion and
contraction of an aneroid . Several "perpetual clocks" have used that
scheme to drive mechanical gear trains with far smaller pressure changes
than you'll get going up and down in a glider or airplane.
An update rate of twice a second should be plenty fast enough. With LCDs,
and CMOS circuitry running a few microamps at two volts or so, it's probably
not impossible to build. You could probably build one with a primary lithium
battery that would only need to be changed once every ten years or so.
That would be the cheapest way to go.
What will it cost to get it approved? How many people are going to buy it?
In the small market that is aviation, what will it cost to build? If you
could sell it cheaper than a mechanical altimeter, you might have a shot.
In enough volume, you might be able to do that. I don't know if the
altimeter market is large enough for that to be possible. The combined
output of all the altimeter manufacturers is probably not as big as a run
of, say, a cheap DVD player.

I dunno. I'm afraid good enough is the enemy of best.

If you don't care whether or not it's approved, or whether it takes
batteries, then you should look at the Flytec hang glider varios. They have
a lot of options as to what they display and how they display it, and I
believe they'll display altitude both digitally and analog. They'd take up
a bit more space on a panel, though.

Tim Ward

I have to fall firmly and loudly into the "digital is good, electrical
insturments can be reliable, mechanical varios belong in museums"
group.

I would love to see a serious study that shows that classic analog
airspeed and altimeters (as used in gliders) are easier to read and
less susceptible to misinterpretation than a properly designed (but
unfortunately, theoretical) replacement digital airspeed and altimeter.
With the advent of Head-up-Displays (HUDs), fighter planes have moved
to almost completely digital displays of most values - only those where
trend is crucial, such as vertical velocity and radar altitude,
continue to have a companion analog display. Otherwise, its a straight
number, usually rounded off to the nearest knot and 10 feet. Works
fine in an F-15E, should work pretty good in an LS6

By comparison, trying to interpret a three-needle altimeter is like
trying to read sanskrit! And then there are 1 1/2 revolution airspeed
indicators!

If you have a PDA in your cockpit, try setting it up to have a nice big
font altitude (and speed, if available) display on it and try it - you
might find that it is really easy to glance at and read.

I have two seperated battery systems, and no mechanical vario. I'm
stuck with a "steam-gauge" airspeed indicator and altimeter, but what I
would really like is a digital airspeed, digital altimeter, and an
accurate AOA indicator. For tradition, I'll keep the vario needles -
since there I'm looking for trend (to provide a value to the audio),
and read a digital averager for real decision making.

Heck, last year I took off on a fine day only to find my airspeed inop
(bug in the pitot) - but that didn't prevent me from flying a nice
little 500+ k XC with some friends of mine. The only time I really
missed the airspeed indicator was in the pattern. Just flew it a bit
faster than usual (that AOA indicator sure would have been nice to have
then...).

Now the huge caveat - this is all fine in a private ship - I don't see
how a the average club ship would manage.

Kirk
66

In message .com,
writes
I have to fall firmly and loudly into the "digital is good, electrical
insturments can be reliable, mechanical varios belong in museums"
group.

Must admit that my beliefs a Digital is good - for somethings - bad
for others - analogue is good - for somethings - bad for others.

Digital is very good for getting absolutes - fly at a particular flight
level - a specific temperature - or a rate of something. Analogue is
very good for trends and similar, horses for courses. Most of what we
do flying we just want a trend or rough peak - analogue - say (in my
personal opinion) thermal centering.For saying that a particular thing
is better or good enough, say is that thermal good enough to stay with
or is it falling off so we want to go to another, digital in the form of
an averager is the absolute best.

We don't need absolute altitude in an altimeter. Flight Levels are in
500 ft increments. We do absolutes in loggers.

Mechanical can break so can electric. You can get many more functions
out of electric which is good. However I am fully in favour of separate
and different technology systems in case something goes pop.

My own experience in club equipment is that electric goes wrong many
times more often than mechanical and it is far easier to get a poorly
installed mechanical system working than an electronic.

I would love to see a serious study that shows that classic analog
airspeed and altimeters (as used in gliders) are easier to read and
less susceptible to misinterpretation than a properly designed (but
unfortunately, theoretical) replacement digital airspeed and altimeter.
With the advent of Head-up-Displays (HUDs), fighter planes have moved
to almost completely digital displays of most values - only those where
trend is crucial, such as vertical velocity and radar altitude,
continue to have a companion analog display. Otherwise, its a straight
number, usually rounded off to the nearest knot and 10 feet. Works
fine in an F-15E, should work pretty good in an LS6

Well known fact, much publicised by the ergonomicist who sits next to
me, is that three needle altimeters are pure trouble from a reading
point of view. ASIs are less prone to missreading but it does happen.
(She once borrowed one of mine for a lecture on the fact).

Very fast ships (F15s and the like couldn't use foot or even hundred
foot needles as they would spin so fast that they would fall off) need
different technology. Actually, the best (from my opinion) ASI was the
one used in the lightning which was a horizontal tape that wound across
the top of the instrument panel.

They are using analogue in the same way that we are but the low values
are inappropriate. For this they use digital which is easier to control
at fast fates of change.

As you say they are using needles for trends, we do the same. I kinda
think that to do our job properly we need both (needle and digital), the
argument between electric and mech is different but again I think we
need both from a safety point of view.

By comparison, trying to interpret a three-needle altimeter is like
trying to read sanskrit! And then there are 1 1/2 revolution airspeed
indicators!

If you have a PDA in your cockpit, try setting it up to have a nice big
font altitude (and speed, if available) display on it and try it - you
might find that it is really easy to glance at and read.

And see what happens when the software goes pling which happens with
even the best systems.

I have two seperated battery systems, and no mechanical vario. I'm
stuck with a "steam-gauge" airspeed indicator and altimeter, but what I
would really like is a digital airspeed, digital altimeter, and an
accurate AOA indicator. For tradition, I'll keep the vario needles -
since there I'm looking for trend (to provide a value to the audio),
and read a digital averager for real decision making.

Heck, last year I took off on a fine day only to find my airspeed inop
(bug in the pitot) - but that didn't prevent me from flying a nice
little 500+ k XC with some friends of mine. The only time I really
missed the airspeed indicator was in the pattern. Just flew it a bit
faster than usual (that AOA indicator sure would have been nice to have
then...).

Now the huge caveat - this is all fine in a private ship - I don't see
how a the average club ship would manage.

Kirk
66

Robin
--
Robin Birch