> Can you measure *road* speed using GPS? On a straight road, does
> the software do a speed calculation?

JT> Yes, it takes two consecutive point reads (and times), and
JT> calculates the horirzontal speed. It does not cater for slopes,
JT> but on the road, you don't get enough of a steep slope to make
JT> a significant enough difference to note. 

 That's terrific! It's an absolute *mongrel* trying to do real road
speed.

JT> Things to look out for, the fixes end up being about 1-2
JT> seconds behind real life, so for fluctuating speeds it's
JT> useless, on the freeway, where you are steady for longer
JT> periods of time, it's quite right. Some GPS recievers can
JT> average the speeds over a number of seconds, so it has even a
JT> slower update, but it's closer to the mark. (only small
JT> differences in this case though) 

 A freeway is ideal... the problem is measuring the actual distance.

> Timing a measured mile is usless.. I've tried it.

JT> Well, yes. It only measures your speed _average_ over a mile.
JT> Which is useless. 

 That's right... unless you can come into the measured section at the
speed you carry thoughout. I went through all this when I hotted up my
Bellet back in 1960, and all I could do was "*relative* acceleration
figures unless I calibrated the speedo first. It was virtually
impossible to get within 10% on the road.

> Frequency meters measure *both* period and frequency.

JT> There are hardware implications if doing both, (easy to do, but
JT> still adds a little to hardare costs). If you're using a
JT> microprocessor, there are no differences in hardware, but major
JT> differences in software. 

 There is a hardware variable. You need to time a CYCLE which needs
an accurate level-sensing switch, or count cycles in a fixed TIME,
which means the switch is less stringent. An ordinary Schmitt Trigger
is good for 1000:1 or 0.01%. If you count 1000 cycles you get 1 ppm.
That's why they prefer to count cycles rather than microseconds (or
whatever).

> It's insane to measure frequency rather than time at that speed,
> especially when you only need three digits. I despair of
> enginerring in Australia today...

JT> It's cheaper. (in engineering costs, not hardware- hardware
JT> costs the same either way).

 It's not. The problem is accuracy, and three digits is shit-easy.
That's only 0.1% and the cheapest-crap Schmitt can do 0.01% (80dB
S/N). What is is...is plain stupidity, and there's a lot of it about.

JT> If you were to measure period, you would need to number crunch,
JT> as well as have a separate timer to handle the update rate. (so
JT> the update rate is a constant, separated from the current
JT> speed).

 The update rate is a human-thing. Constantly changing numbers
confuse us because our brain actually runs a little behind real-time. 
How does it cost any extra? In a micro, there is the clock and a
timer. It's just a matter of finding a convenient pulse at 100ms or
so... and update the count on that. It's just a matter of whether you
count known short-pulses in the interval between speed-pulses, or
count speed-pulses in a long known interval. You just switch them
around in the counter!

 The number-crunch is just an inversion which can be done at 12MHz or
something... really fast!

> How did they get engineers for a *really* complex problem like a
> train? 

JT> University graduates. Sometimes you get lucky and find someone
JT> who knows the difference between the colour of their bodily
JT> fluids, but usually, you get someone who just did it because it
JT> sounded like a cool job. 

 A graduate is not an engineer. An engineer is someone who can solve
a problem cheaply and reliably.

JT> Can anyone say "millenium train"?

> The fuckers don't work...

JT> I'm not so sure it's as simple as that. As far as I can tell
JT> the only ones left are the ones who are geek die-hards who
JT> wouldn't be caught doing anything else because they love it too
JT> much, or the wanna-bes, who are doing it because they are happy
JT> to take the shit pay for a job that requires extensive training
JT> and experience. I know plumbers who were quite cluey engineers.
JT> Plumbing pays much better though.

 KI was talking to the guy in the fish shop last week, he's building
a house, and he pays carpenters $40/hr! That's mad! You can get an
engineer for that!

 The problem with engineering is that it is not *just* engineers
coming out of university. Young engineers need support - a production
line to show them how it works in the real world when they fuck up,
technicians to show them how to fix it, older engineers to show them
the guts it needs to get it right, first - and a management to kick
the shit out of them when they cost them money. All we have left are a
few techs and universities. It can't *possibly* work. Australia is now
a banana repulblic.

Regards,
Bob


   




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