Speed Cameras

in the fast lane....

 

Can you be fined for going to slow in the slow lane?

Nev..

 

Are you trolling? Or just make a habit of failing to grasp the
salient issue?

The point isn't prosecution of readings off the trailers, but of the
prosecutions resulting from the use of the same core technology;
being simple Doppler-shift in the microwave band; which fails not
only to clearly identify a particular target, but also fails to take
into account the limitations of that type of measurement.

And there are _millions_ of prosecutions every year in Australia,
based upon the presumption of accuracy of such equipment. That may
be right 95% of the time; but the other 5% is a significant burden
on justice; for the sake of administrative efficacy.

 

It'd be a very heavy truck, on a very long freeway grade.
FWIW: it's not usually the trucks causing the problem, but the
clueless non-merging idiots who "merge" at 40 km/h with traffic
trying to do 100.

You may have noted that I drew readers' attention to the fact the
German (specifically Bavarian) Police are encouraging an increase in
the minimum power requirement for heavy vehicles using the Autobahn
because of the congestion and crashes that are caused by heavy
vehicles which cannot maintain 80 km/h.

Back-of-the-envelope calculation says that'd be about 6 kW/tonne on
the road, allowing for 2% grade and a fairly low coefficient of
rolling resistance.

Lane discpline is actually tangible on the Autobahn, which is why
traffic moving at speed of over 200 km/h can safely coexist with
such slow vehicles in light traffic. As traffic density increases,
the speed of the faster vehicles drops rapidly as it adapts to keep
the speed differential between "full" lanes quite low (<50km/h).

The "friction" caused by very slow vehicles on the Autobahn is very
noticeable.

 

Who's failing to grasp the issues? I asked for examples of prosecutions and
you responded with a pedantic comment about who owns the trailers...

Nev..

 

Pedantic comment about who owns the trailers? It's YOUR diversion to
consider the _trailer_ the issue, instead of the "speed detection"
issue.

Which bit of "The same technology is used for prosecution." do you
not comprehend?

You honestly believe that the _trailer_ is the issue?

 

No I believe it was YOU that was educated but have obviously forgotten that
the lunar module did in fact use radar in order to land.

http://www.clavius.org/gravdust.html

Fig. 1 - The lunar module's landing radar antenna, normally mounted on the
underside of the LM descent stage. The angled panels send out beams for
differential range altitude and attitude sensing. APOLLOSATURN.COM


The landing radar (Fig. 1) is used to to determine altitude and rate of
descent. Normal aircraft altimeters use differences in air pressure to
measure altitude, but obviously that won't work in a vacuum. So the only way
to measure the distance and descent rate is by radar, which was accurate to
about 15 feet (5 meters). But the descent rate was already fixed before
switching to instruments, so it was just a matter of riding it down until
the contact light illuminated.

 

A check of sources closer to the event are worth reading:

"Complete" transcript and subsequent comments at
http://www.phrd.ab.ca/Schools/LJHS/APOLLO/ap11jnl1.htm

They landed *despite* the radar which overloaded the LEM computer,
after initially giving misleading altitude readings (53,000 feet vs
actual 50,000).

[Armstrong - We measured the position of certain prominent
landmarks at a couple of points prior to ignition and then
after ignition. Once that was finished, we did a 180-degree
yaw (to the face-up position) so that the radar could see
the surface. My recollection is that the landing radar
wasn't really reliable above something like 30,000 feet.
They had tried it on Apollo 10 (at 50,000 feet) to see if
they could get altitude readouts, and we did try to use it
(at that altitude) to get some measurements. We wanted to do
that while we were still up at a reasonable altitude to help
the guidance equation converge. (The timing of the
180-degree yaw) was a balance between seeing the surface and
getting the radar information in the guidance solution."]

....

102:38:11 Aldrin: Yeah. Altitude light's out. (Pause) Delta-H is
minus 2,900.
[They have a radar return from the lunar surface and the
radar altitude is 2900 feet lower than the PGNS altitude.]

Further reading:
http://www.exn.ca/apollo/interviews/kranz.cfm

Apollo 11 Flight Director Gene Kranz:
Well there's one where we were working with the back up
crew, where we started off on our trajectory, higher than we
should've been to start the descent but we're still within
the box to get down to the surface of the moon. And then we
had a radar data discontinuity with radar showing a major
difference between true altitude and what we provided the
computer. ...

... So all of a sudden the problems are mounting, the
communications problems, we got navigation system, we had a
minor electrical problem and then very shortly thereafter,
as we're preparing to accept because we have to get radar
data that tells us our true altitude, and we have to make a
determination whether we're going to accept this into the
computer or not. And about the time this is occurring, a
series of program alarms come on and the program alarms are
telling us that the space craft computer is too busy to do
all of the things we're asking it to do. ...

[snip]

Method of operation is completely different to the mis-named "radar"
used for "speed detection".

The "speed detection" units transmit continuously. With continuous
wave, it's impossible to determine distance.

It is necessary to pulse the signal and measure the echo's _delay_
to determine an estimate of the distance.

The moon is also a significantly-larger target to hit from 50,000
feet, than a car from 100 metres. And when you are one of only two
moving objects of significant RCS within about 1 light-second,
target identification isn't very difficult. At least much easier
than determining which of the dozen or so intermittent returns from
CW is actually "tracking" a particular target, or a reflective
phenomenon between objects moving at slightly-different speeds.

 

Does it matter if there's an error in distance calculations? As long as the
error is relatively consistent the speed measurement will be accurate won't
it?..

Nev..

 

Not really, if speed is a measurement of distance/time, and we have an
inaccurate measurement of the distance travelled over that time, how do we
get an accurate speed?

If the reading says 90m and its really 100m then the calculated distance
over time will be 10% out.

 

But I said what if the error is consistent... the distance travelled will
remain relatively accurate won't it, and presuming the timing is accurate,
wouldn't the speed calculation be relatively accurate too?

Nev..

 

Depends...

Keep in mind, the speed measurement relies on _2_ distance measurements.

If by "consistent", you mean "it _always_ reads 3000 feet too high",
then yeah, the speed accuracy would not be affected. If it was "it
always reads plus or minus 8%" or whatever, then no, sometimes you'll
get a "plus 8%" error reading followed by a "minus 8%" reading, and
you'll get an incorrect speed. (especially if the distance errors are
large compared to the speed).

big

 

It does if you switch off your main retro-thruster 3000 feet too
early!

Did you miss the bit about the USD$30million equipment?

Did you miss the fact that the altitude sensing is entirely
different to that used for "speed detection"?

If NASA can't get it right for USD$30 million (in 1965 value) with
dozens of pairs of eyes and minds in constant supervision, then
what hope has a AUD$10,000 "speed detection" digital moron operating
in a much less-controlled environment with multiple targets and
reflective objects at different velocities?

NASA never used the "same technology" to land anything on the moon.
Armstrong landed because the signals from the entirely-different
type of radar were _ignored_.

The technology used is not the "same technology" as used for "speed
detection"; not by a long shot. The technology used for speed
detection is more like what your mum uses to heat your dinner when
you come home late after school: Continuous microwave emissions.

"Speed" measurement by Doppler shift is obtained by measuring the
"beat frequency" (frequency difference) between the continuously-
transmitted microwave signal and return echo. There is _no_
distance measurement taking place.

The measuring device doesn't have a clue where the target is
actually located as it has no means of determining distance by
Doppler shift alone. The strength of the return is no clue
whatsoever as equivalent echoes may be from a distant, good
reflector, or a close, poor reflector.

Or it may be from a microwave phenomena such as scintillation; or
multi-path with a nett shift at the receiver in excess of that
corresponding to any target that's illuminated.

 

Interesting question.

If you're *unnecessarily* obstructing the flow of traffic in the left-most
lane, I would expect you could be fined.

I shouldn't expect that a heavily loaded truck climbing a steep hill would
be fined, as it's in such a condition whereby obstructing traffic is rather
unavoidable. And on freeways, where signs specifically direct trucks to use
left lanes, this is obviously taken into account.

J.

 

Heres an idea, so you wont get pinged by the possibly inaccurate speed
camera...

DONT SPEED...

Howzat!!!

 

Which part of "inaccurate" don't you understand?

If the "speed detection" device _exaggerates_ the vehicle's actual
speed it can still get booked when it's not actually exceeding the
speed limit.

The claimed accuracy is nothing like the practical accuracy of the
devices. Multanova claims +-1% yet independent field trials have
shown repeatable errors exaggerating vehicle speed by more than 5%
under "normal", controlled traffic conditions as well as the
tendency to identify the incorrect target as being the offender.

 

yes, that is what I meant...

Even then I think the unit would probably discard the results. I'm sure
several consecutive matching measurements would be required before a speed
calculation would pass internal error correction.

Nev..

 

So lets see... RADAR technology was first implemented in the early 1940s by
the Poms, NASA had a version of RADAR which it used to land lunar modules in
1965, and you are of the firm belief that in the 39 years since then there has
been no significant improvement in the implementation and accuracy of radar
distance measuring devices. Fair enough. I suppose you still think that $30
million dollars will buy a computer which will run at an astounding 2MHz with
a whopping 64KB of RAM and let you store information on 720KB 8inch magnetic
disks.

Nev..

 

"Nev.." wrote

No way Nev. 8 inch floppies came a lot later. And in 1965 our mainframe was
running at 0.16MHz. We didn't dare dream about 2MHz. We got 1MHz in 1968.

Theo
Who still has nightmares about head crashes on the 4MB hard drives in 1969.

 

Unfortunately the Victorian experience seems to show otherwise, with the
speed cameras happily accepting 170kmh 1979 Corollas and 140kmh uphill
trucks...

There's no doubt that radar systems _can_ be engineered with very high
degrees of accuracy, but remember these were almost certainly contracted
out to the lowest bidder...

big