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Keep the User in the LoopUser feedback is critical. Originally in Embedded Systems Programming, September, 1999. Want to increase your team's productivity? Reduce Bugs? Meet deadlines? Take Jack's one day Better Firmware Faster seminar. You’ll learn how to estimate a schedule accurately, thwart schedule-killing bugs, manage reuse, build predictable real-time code, better ways to deal with uniquely embedded problems like reentrancy, heaps, stacks and hardware drivers, and much, much more. Jack will be presenting this seminar in Chicago (April 23, 2008), Denver (April 25) and London, UK (May 19). Want to be your company’s embedded guru? Join us! More info here. For hints, tricks and ideas about better ways to build embedded systems, subscribe to The Embedded Muse, a free biweekly e- newsletter. No advertising, just down to earth embedded talk. Click here to subscribe.
Position 19° 21 North, 61° 40 West, 140 nautical miles from Antigua, 1600 miles since leaving Baltimore. Long time sailing buddy Scott and myself have been pushing Voyager, my 32 foot sailboat, towards this small Caribbean island for two weeks. We expect to arrive in another day or two. The first week’s frustrating light winds depleted our diesel supply, so we diverted to Bermuda for fuel, staying in that beautiful island for a mere 13 hours. After topping off the tanks nature rewarded us with nonstop strong southeasterlies; the engine has been off other than an hour or two a day for battery charging. 25
years in the embedded industry has taught me to try and separate professional
from personal activities. Once nothing seemed cooler than creating an electronic
solution to a boat problem; now I find simplicity much more satisfying, and have
worked hard to eliminate excessive technology from my sailing life. Yet here I
am, 1000 miles off the US coast, propelled by the wind, but typing into a 200
MHz laptop. Voyager exists in an odd mix of the traditional and the transistor
age; her two masts and dacron sails keep us going using the technology of Noah.
We use archaic navigational tools, relying on a sextant to find position from
the sun, stars and planets, but dual GPSes hide in the chart table for use on
cloudy or seasick days, and for finding difficult harbors. 15
miles out of Bermuda I powered a GPS up for the first time on this voyage. The
resulting latitude and longitude matched our celestial position closely, but as
I watched the unit the displayed latitude jumped by two degrees – 120 miles,
making a mockery of GPS’s vaunted 100 meter accuracy. Scott, also an old
embedded hand, cut to the heart of this technology failure as he showed the unit
just did not get enough signal strength while below deck. In the cockpit, its
unobstructed view of the sky yielded good fixes. I
was aghast. Shouldn’t the box show clearly
that the displayed data was in error – or at least suspect? We eventually
found a signal strength page hidden deep in the box’s menus, but to my mind,
no embedded system should ever lie to the user. Every embedded system should tell the user when the data is unreliable. GPS
has taken the navigational world by storm, due to its high accuracy, ease of
use, and low cost. Until the relatively recent invention of GPS navigation was
always more art than science. Early sailors were uneducated seamen, plowing the same short routes across the Mediterranean. Rarely long out of sight of land, they depended mostly on piloting - the art of figuring out where you are by looking at features ashore, mostly without instruments. By Columbus’s time even the compass was little more than a magnetized needle floating on a cork. The
ancient Arabs learned to find latitude by observing the stars. It stands to
reason that as you head north any specific star will appear lower and lower over
the horizon. Arab astronomers invented the astrolabe to measure the stars'
angles above the horizon. Some incorporated crude ephemeris data (the position
of the stars based on season) into intricate rings inserted into the
instruments. Published
ephemeris data became available during the great age of discovery that began
around the time of Prince Henry. Finding latitude by observing the sun at
meridian transit (when it is due south of the ship) became one of the expected
arts of the sailing master. Longitude remained a thornier problem. Longitude
is easy to find if you know the time. In effect all one has to do is note when a
star passes due south; if that time is 0000 GMT, and if the star's position in
the universe is cataloged with an "hour angle" of 0 degrees, then you
are standing on the prime meridian. If this same star transits your local zenith
at 0100 GMT your longitude is the distance the earth rotates in one hour to the
west of the prime meridian: 15 degrees. Prince
Henry's sailors did not have reliable clocks, so had no way to find longitude. A
sandglass is not terribly accurate to start with. Clocks built prior to the
middle of the 18th century mostly relied on a pendulum and counterweight,
mechanical analogs of the sandglass with all of its shortcomings. As
the English started to dominate the sea lanes, Parliament realized that they
were a nation of seafarers who were all but lost on every long voyage. In 1714
the desperate ruling body enacted a law offering 20,000 pounds for the first
clock that lost or gained no more than two minutes after a round trip to the
West Indies. The
prize went unclaimed until 1761. John Harrison’s temperature compensating
chronometer used a spring instead of a weight and passed the test well within
specifications. Though decades passed before chronometers became cheap enough so
every ship carried one, accurate clocks had solved the problem of computing
position anywhere on the globe. Now
sailors could observe the angle of two or more celestial bodies above the
horizon with their sextant, note the time of each observation, and by not
terribly difficult mathematics compute their position. Officers guarded the
secrets of navigation so a mutinous crew would be lost, creating an almost
priesthood-like mystery shrouding it that persists to this day. The
heaving deck of an ocean-going vessel is far from the ideal platform for making
sextant observations, as an error of a single minute of arc (1/60th
of a degree) yields a mile of error in position. Because the earth rotates 15
minutes of arc per minute of time, noting the sight’s exact time is just as
important. A 4 second error also creates a mile of position error. Worse,
because it’s hard to see the horizon and more than one body at a time (e.g.,
during the day the sun is usually the only visible celestial object) each
sextant sight generates not your position, but merely a line that the vessel is
on. Cloudy days means no observations are possible. Clearly, determining
position via celestial sights is tedious, error-prone, and subject to nature’s
whims. Yet, until the late 60s virtually every ship in the world navigated by
nothing more than a sextant. Various
forms of electronic navigation offered simpler ways to fix position. Loran A
used phase differences broadcast by multiple shore stations to fix position to
an accuracy of a mile or better. I loved Loran A. The user interface was an
oscilloscope display; the navigator rotated a knob to align pulses from two
shore stations on the scope. With practice one learned to tell the quality of
the fix by looking at the strength of each pulse; you could even tell sky waves
from ground waves by the displayed signals. The user got two sorts of data:
position, and an assessment of the quality of the fix. Loran
C replaced the A version in the 80s; it offered higher accuracy, and was the
first system to rely on microprocessors for data reduction. Loran C receivers
entirely removed users from the loop; the units’ CPUs computed and displayed
nothing more than latitude and longitude, exactly what the navigator needs. But
Loran C illustrated the peril of insulating users from the physics that underlie
any application. Sure, we really only want final, smoothed, accurate, data in
intuitive units. But incorrect data is
worse than no data, a lesson even today too many systems seem to ignore. On
a sail to Newfoundland in the early 90s the Loran’s output just did not jibe
with my dead reckoning track. We learned later that off Cape Race some weird
atmospheric effects drove Loran positions off by 20 miles, enough to have put us
on the rocks if we had chosen to believe the displayed data. Part of me is wary
of electronic data –perhaps those of us who grew up with slide rules rather
than calculators tend to always question data – so I’ve always used multiple
navigational inputs rather than just believing the box. GPS
uses its own constellation of electronic stars - satellites that broadcast radio
signals to a computer in the user's receiver - and produces positions accurate
to better than 100 meters. GPS gives worldwide coverage and requires no
expertise. A nice sextant – hard to use, suffering from all sorts of accuracy
problems, and requiring high levels of expertise – might cost $2000. A GPS
runs for under $100. The electronic solution clearly wins any way you compare. I
love electronic toys, but am appalled at the unspoken philosophy of GPS which is
"be happy, don't worry". The sailor determining position from a GPS
looks like a worshipper at Sunday services. There he stands, hanging on the lip
of the chart table nervously watching the LCD readout. Will the system acquire
satellites today? He bends over to press a button. Suddenly a position appears!
The miracle has repeated itself once again! And
it is indeed a miracle. Arthur Clark, writing about advanced aliens visiting our
backwater planet, commented that any sufficiently advanced technology appears to
be magic. The sailor shuffles off, content that somehow this group of numbers
simply must be right - the little box
told him so. This
effect is symptomatic of modern society. Our technology is so advanced we've
fooled ourselves into thinking it's all magic. How many people understand why
that suspension bridge stands up? Who cares - it's a miracle! Few bother to
learn about the bridge's delicate web of forces engineered to cancel each other
out. What about the MRI machine that saves your life, or the TV set far too many
remain glued to? Insatiable curiosity was never important before the industrial
revolution since there wasn't that much to know. Now I'm afraid it's been all
but bred out of us. GPS,
like many modern technologies, uses a very complex constellation of satellites,
astonishingly intricate algorithms, and possibly intermittent earth downlinks.
Clearly things can go wrong. Any time we designers produce a result – like a
lat/lon display – we have a responsibility to our users to qualify that result
in some manner. If the signal level is too low for accurate readings, blank the
display, or somehow make it clear that the data is less than pristine. As I
mentioned, off Bermuda we saw readings jump 120 miles in a matter of seconds,
probably due to low signal levels. A much better design would recognize the
inadequate amplitudes and blank the display. Bad results are worse than no
results. When
a navigator computes position using a sextant sources of errors abound. Small
plotting errors create chaos. My middle-aged eyes have trouble focusing on
tables of numbers, so all too often I’ll miscopy a critical parameter. But the
errors don’t necessarily matter, as the process of navigating using these
atavistic tools includes cross checks and common sense evaluations. I plot the
sun’s line of position, but find it’s 20 miles off of my “dead
reckoning” (nav by computing course and speed) position, so I go back over the
numbers until the mistake stands revealed. The
magic of technology is it frees us from the tedium of older solutions. Punch a
button, get a number, but give me some sort of cross-check as well. The
repressed Luddite in me wants more than a result. Spreadsheets
create a similar frustration. Change one number and a thousand computations
alter an entire matrix of results. I use spreadsheets all day long, but have
learned that a side effect of computing is that we can now make thousands of
mistakes per second, instead of one or two. Long before computers accelerated
our production of bad numbers, accounts invented the double-entry ledger to
cross check all of their results. All of my important spreadsheets emulate this
accounting trick, creating a column or row of numbers computed as a cross check
on my main results. Tedious – sure! But it’s the price of technology. Another
embedded system on Voyager, one that suffered from none of these user interface
problems, is the single sideband radio (SSB), a 100 watt transceiver that we use
for chatting with other boats, friends in distant places, getting weather
information, and simply listening to BBC for entertainment (after 17 dys at sea
entertainment becomes important!). SSB radios long predate the embedded
revolution, but adding processors both enhances signal reception (a DSP reduces
noise and boosts the signal), as well as controls the operation of the radio.
The micro helps modern SSBs synthesize frequencies from digital counters, rather
than rely on far too many crystals. LED readouts – driven by a processor –
give the user a complete view of the system’s operation. My
radio shows frequency, mode, and all important parameters on the display. It
will even show the match to the antenna, a critical indication of how well the
unit is transmitting. The
processors enhance the features and functionality of the radio, without hiding
any important operational details. This, to me, is a well-designed embedded
product. The
boat’s VHF (short range) marine radio is also nicely implemented. Simple
controls give access to all system features. Though a micro is not needed for
basic VHF communications, adding one allows features like dual channel
monitoring. Though some VHFs now use arrow keys to control everything
from working channel to volume, I prefer the pot-like knobs that some other
vendors still provide. Twist the volume control to set audio level in one quick
motion, instead of holding an arrow key and watching a display for some
interminable time. Voyager’s
autopilot is equally well designed. A flux gate compass sends heading to a
paperback-sized control unit. The micro therein computes the difference between
the boat’s real course and the one entered into the unit by the helmsperson,
sending the correction needed to a motor that drives the wheel. In the awfully
wet salt environment of an ocean-going boat keypads and other button-rich
interfaces are a constant source of leaks. Instead, to set the course we steer
the boat in the direction we want to go, and press a single “auto” button.
What could be simpler? The truly important operational parameters I can see by
glancing at the wheel, and watching how the unit steers. If the wheel is hard
over one side all of the time, that tells me it’s time to balance the sails
better. If for any reason it cannot
steer the desired course, an alarm sounds. So, even when singlehanding (in July
I’ll sail alone from the Virgin Islands to Bermuda to meet a friend), I can
sleep below safe in the knowledge that an alarm will wake me if we’re off
course. The
designers tailored the unit’s features and design around the real needs of
sailors, subordinating technical features and details to ease of use
considerations. Voyager’s
radar detector also provides a great mix of embedded smarts with an almost
perfect user interface. The unit wakes me when a ship comes within 10 miles or
so. (Collision at sea is a constant worry.) The micro monitors RF circuits to
detect nearby X or S band radar. If the signal matches parameters characteristic
of marine radar sets, it sounds an alarm. Though an interface this simple
provides adequate functionality, it resembles the GPS in distancing the user too
much from the physics of the acquired data. To overcome this, the unit includes
a “listen” mode, which disables the alarm and dumps the raw radar signal
into a speaker. When the alarm sounds I switch to “listen” and hear the
signal itself, which might consist of a main pulse with two sidelobes. With a
bit of practice it’s not too hard to separate commercial radar sets from
military units, and to even get a vague feeling for the distance off of the
radar source. The
designers provide the user with a wonderful mix of sophisticated digital signal
processing and an analog output that lets me use the wetware computer between my
ears to do further processing. As
we slide into the close of the 20th century many of our electronic
gadgets seem designed for people with the most minimal cognitive skills. Me, I
figure that the brain is still a useful adjunct to life, and am not yet ready to
delegate all thought to my electronics assistants. Back to home page. The Ganssle Group
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