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Head up Display Symbol Generators

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Head up Display Symbol Generators

Old 9th Aug 2013, 12:01
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Head up Display Symbol Generators

Looking at the latest HUD systems, the symbol generator is a briefcase sized unit mounted on the cockpit ceiling. can anyone explain how they work and the reason for the size. What is the impediment/technology breakthrough required to make them much smaller where they could be fitted to any aircraft regardless of size. General Motors have a unit for their cars that's about the size of an orange.
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Old 9th Aug 2013, 14:18
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Looking at the latest HUD systems, the symbol generator is a briefcase
sized unit mounted on the cockpit ceiling. can anyone explain how they
work and the reason for the size. What is the impediment/technology
breakthrough required to make them much smaller where they could be
fitted to any aircraft regardless of size. General Motors have a unit
for their cars that's about the size of an orange.
Aviation technology often lags the state of the art by a considerable
margin, because the design process and lifetimes can run to decades.
This is also dictated by the approvals process, which tends to favour
well proven technologies with good track record.

Modern HUD systems probably use a raster scanned display, much like a
computer crt monitor and there's nothing particularly special about
the electronics side of the technology. Small projection quality crt,
symbols in non volatile storage, medium performance embedded processor
and graphics hardware, comms interface and power supply. You could
think of it as a rather special dedicated function pc, but built to a
far higher standard using proven technologies run well within their
ratings.

Small briefcase sized box sounds about right. A couple of decades ago,
it would mnost likely have been several medium sized boxes, with a
separate box just for the symbol generator...
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Old 9th Aug 2013, 14:55
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Whilst aviation technology is not always at the forefront of developments there are significant differences in quality and reliability with the car industry.
HUD more often uses vector graphics; raster is used in some synthetic vision applications. Aviation has to cope with a much wider range of ambient light conditions and meet stringent integrity requirements – no hazardous misleading information (10-9?). These requirements more often require dual inputs, processing, and monitoring, which increase the capacity of processing systems, but not necessarily size.
The most significant size issue is usually the optical / projection system. Modern ‘glass’ combiners use holographic techniques which enable larger fields of view, which in turn require larger display mounts.
The optical size depends on display position – distance from the eye. Military systems can be very small, and ‘Google Glass’ concepts would be feasible if head position with reference to the real world was not required – e.g. flight path vector, true horizon, which may not be the case in cars.
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Old 9th Aug 2013, 15:28
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In the last aeroplane I flew with a HUD, the symbol generator was a very small part of the instrumentation display.

The whole computer system was way smaller than a briefcase - and that was in 1978!
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Old 9th Aug 2013, 17:53
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HUDs use raster displays

I think not. I believe they use symbol generators because they are vector displays and not raster. However, I'm an aero engineer not an avionics man so i could be wrong.
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Old 9th Aug 2013, 19:59
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I think not. I believe they use symbol generators because they are
vector displays and not raster. However, I'm an aero engineer not an
avionics man so i could be wrong.
I did say probably :-). The very first and older HUD's did use
vector graphics, but that technology dates back to the 1960's and
the days of early graphics terminals such as Tektronix. It was the
only way to build hi resolution graphics displays in those days
and it took multiple boards of electronics using hundreds of ic's.
The problem with vector is that the hardware is more complex and
thus less reliable. Raster scan is also cheaper and with the move
to more cots (commercial, rather than mil spec) equipment it
wouldn't surprise me to see raster scan used as the default method
where possible.

The vector approach drives the screen as an x/y plotter, with lines
continuously drawn between two screen coordinates. Well, not quite
true in that those lines will be stored as discreet points along the
path between the two coordinates. Raster scan, on the other hand paints
a dot at the point on the line and builds up the line as a series
of dots. In the early days, vector would have provided the highest
resolution, but raster scan should be capable of similar or equal
performance now.

I think Smiths Industries developed the very first Head Up Display
system. Yet another excellent uk high tech company given away to
the US for a pittance because we couldn't be bothered anymore ...

(Correction: RAE developed the first HUD, with Cintel manufacturing,
but Smiths were heavily involved with the technology)

Last edited by syseng68k; 9th Aug 2013 at 20:19.
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Old 9th Aug 2013, 21:04
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Apparently modern HUGS use both vector symbology and raster image display, for example for FLIR or synthetic vision. There is quite a bit of information on the rockwell collins webpage and i'm sure there are probably other companies in the field as well.
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Old 9th Aug 2013, 22:21
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This is the HUD used on Boeing 737NG (700-900)

The projector is shown in this picture.

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Old 10th Aug 2013, 03:14
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HUD used on Boeing 737NG
Looks like the Rockwell Collins HGS (Heads-up Guidance System). According to their brochure, this is a raster system. They aren't clear on the projector technology beyond this. But the example views all seem to show monochrome green. Even the terrain view examples. This makes me think its CRT technology. Old school.

The state of the art for high contrast, high brightness projector display is DLP (digital light processor) technology. This is 'light source agnostic' in that one can put an HID lamp (with color wheel) LEDs solid state [email protected], etc. behind the light gate. And they can be made pretty small. A system that can be used in a lighted conference room is about the size of a hardback book (well, maybe War and Peace).
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Old 11th Aug 2013, 00:20
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Flew a HUD from late 1971 to 1984. Both were vector symbology and both Marconi. Basic CRT's and monochrome - green, due to our human eye sensitivity. Also had to account for lottsa sun above, as we were in open canopies that allowed the sun to shine directly on the CRT and combiner glass.

Then saw the first raster scan displays in mid-80's, and by then the resolution and frame rates were super. The projection still used a CRT, but the symbology was all digital and very high resoulition.

The vector symbology we had back then had to use discrete chips or firmware for each symbol. The raster symbology was lots easier to implement/modify according to mission needs.

With the current DLP and LED projection systems, the HUD should easily be able to show the relevant symbology regardless of ambient light and so forth. The boxes are also a hundred times smaller that what I flew with.

Okie is close WRT history. Both the one I used in the A-7D and the Jag were virtually identical - both Marconi. So figure late 60's technology. Then the Viper about ten years later, also Marconi.

BTW, to see the state of the art in HUD and other displays, go search the F-35 stuff. The thing doesn't even have a fixed, back-up HUD or gunsight. All the mission data and flight path data is projected on a "Darth Vader" helmet that is cued to the aircraft attitude and the real world. The jet also has sensors that allow the pilot to look "thru the floor" or low and behind or........
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Old 11th Aug 2013, 02:41
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Way of the future?


Last edited by Brian Abraham; 11th Aug 2013 at 02:43.
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Old 12th Aug 2013, 13:36
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I think Smiths Industries developed the very first Head Up Display
system. Yet another excellent uk high tech company given away to
the US for a pittance because we couldn't be bothered anymore ...
Possibly, but they have invested millions at the site and continue to employ a couple of thousand people in the aviation sector, which has to be better than closing the site completely as per the car industry!
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Old 12th Aug 2013, 16:11
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OK465

I believe current HUD displays are monochrome because any color reflected back to the pilot's eyes by the combiner is also degraded externally when entering thru the combiner.

You could have a multi-colored SVS HUD, and then you could actually eliminate any windows on the flight deck, not much light coming thru anyway.
Right. There are some good reasons for choosing monochrome and green. The combiner can be optimized to reflect one color and only attenuate that color coming through the windscreen. Green is a good choice for night vision anyway. But when you step up to an SVS system with a monochrome projector, you end up with green symbols on top of green video. And that can look a bit jumbled.

The original logic behind selecting green was that for vector displays, the refresh rate varies based upon the number of symbols. Past a certain point, the image will flicker unless high persistence phosphors are used. Same thing holds true with old oscilloscopes with slow sweep speeds and slow refresh computer CRTs (all old tech). Green was the easiest color to work with.
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Old 15th Aug 2013, 16:10
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Possibly, but they have invested millions at the site and continue to
employ a couple of thousand people in the aviation sector, which has
to be better than closing the site completely as per the car industry!
I'm not being critical of the present owners, just wondering why we
didn't have the vision and balls to keep it uk owned. The buyers
(Was it Honeywell ?) obviously saw value and synergy in the aquisition,
so why couldn't we ?. If you look at older issues of Flight, the UK was
teeming with advanced avionics companies and all that went with it, but
now a pale shadow of it's former self. I know there's been a lot of
rationalisation with the end of the cold war, which probably explains
some of it. Also economies of scale, critical mass etc that make it
very difficult to develop some types of high tech without a lot of
prior experience over a very wide range of skills and knowledge base,
but Smiths were a first class company, with some very bright people
and it deserved better.

Slight interest in that I worked at the Basingstoke site for a couple
of years and still have great respect and good memories. What is it
now ? a cathedral to consumerism, a branch of tesco of all things ...

Last edited by syseng68k; 15th Aug 2013 at 16:12.
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