Ship to Ship
The Intergalactic "Aldis Lamp"*
Check Our Progress
We, the passengers of earth are drifting alone in a very dark sea of stars. Our nature is to shine a light into the dark, to call out when we feel alone. When we next gather in the desert, we will send our voices on a flashing light to the stars. Each will say what they will, and with the approval of three, we will send our voices through the sky on the light of a billion candles.
The "Ship to Ship" project is about creating the experience of sending a message to another planet. This is an experience in three parts, creating, approving, and sending. A message needs to be recorded somewhere, so we will have recording booths at several key locations. You would think that sending a message to another planet might require some "official" approval, so we will have a master control room where three (random) people must vote to authorize transmission. If you were sending a message through space on a light beam you’d expect it to be an impressive beam. We will have that!
The basic message will be a standard media format file captured by any PC audio recorder or web cam package. Messages will be limited to 30 seconds or less. The message file will be uploaded to a database server with a standard web interface. Recording stations could be located near Playanet kiosks or remotely located further out on the playa and linked with wireless networking. The server site will allow selecting a stellar object from the standard star charts as the destination of your message. When a message is received on the server it will be put in the "earthside" queue to be processed. Remote web access could also upload messages. It will be interesting to see what people say to the stars, will it be serious, more serious, or something else...
This is a tent in a high traffic location. Inside is a video projector showing the messages that have been stored in the "earthside" queue. There are three special places to stand with two buttons at each location. The buttons are labeled "send" and "trash" The messages are shown one at a time on the screen. The buttons may be pushed while a message is being played. If all three "send" buttons are pressed together, the message is moved to the "starside" queue and will be transmitted when it’s star is above the horizon. If all three "trash" buttons are pressed together, the message will be deleted.
There is a dilemma facing the creator of interactive public art, how do you allow free expression from the public when a few people will try to dominate and control the resulting resource. It’s important that any public resource have some basic "yahoo control". But the intent is really not about passing judgment on the quality of the messages. The goal of making message sending into a process is to allow the sender to have the feelings that an artist experiences with any creation. Each creation goes through a time of growing in its creator’s mind before it is expressed into the outside world. After being expressed it has value to its creator, but when others validate it, a whole different, larger feeling becomes attached to the creation. When the creation moves into the public view, yet another set of feelings arise. These feelings are unique to the person doing the creation, but whatever the feelings are, it represents a growth and learning experience about the processing of being creative. The three stages of sending a message into space will create a sequence of feelings about being creative, about daring to broadcast something personal and unique to the greater world. Maybe people will want more…
The beam is the most impressive part of the project. No wimpy laser here, we going for a fat white searchlight beam. A 7000(**) -watt xenon short-arc bulb will be modulated to produce peaks of a billion candlepower. (This is more light than a WWII carbon arc searchlight.) The beam modulation will be obvious, a simple mapping of a person’s voice directly to the brightness of the beam. It can be received with nothing more complex than a solar cell connected to a speaker. The human eye loves flashing lights, and this will be the mother of all flashing lights.
The beam projector will be mounted on a dual axis platform that can accurately direct the light toward chosen stars. Standard software for directing telescopes will be used to control the platform drives. The projector will move quickly from one star to another. It will lock in the location (making an appropriate noise while moving) and then pump up (maybe enlarging or swelling a section) before flashing out enough light to distract astronauts.
The projector will cycle through the "starside" queue, picking locations that are far enough above the horizon to be illuminated safely. It will rotate through the possible targets, selecting different messages during the night as stars rise and fall. The oldest messages will be deleted if a section of sky gets too full of targets for the projector to sight them all.
A receiver may be located in the head of the man, thus giving him a voice. The voice of his people calling out in the dark…
It matters not who sees, or hears, or cares about our show.
We shout at the sky to say, "I’m here!",
Because that’s all we really know…
*For centuries the navies of the world have been using and still use flashing lights to send messages from one ship to another. The flashing light has a high wattage bulb, is directional, and operates via a set of louvers. The louvers are attached to the lever seen on the left side of the signal lamp and open when the lever is pushed, emitting light.
** Why 7000? Because 10,000 is twice as much money and harder to get.
One of the things that started this was a thought on how to modulate a Xenon arc light. I have a few surplus xenon- mercury bulb to experiment with. These are a lot harder to modulate than pure xenon, because the gas has to stay hot enough to keep the mercury vaporized. I currently (12/13/01) have a working prototype of the driver for these lamps and I can modulate the arc in analog mode upto several kHz.
The good thing about this modulation is you could decode it with nothing but a solar cell and a speaker. (Received energy would directly translate to the original waveform).
It should be possible to make receivers out of very few parts, so there could be a few around. Maybe a cheap telescope with a phototransistor and a audio amp that could be pointed at the beam from the side so you could hear the messages as they are sent.
The 7000 watt lamp current is around 50 volts at 150 to 200 amps, (less power than a electric car controller, and I know how to build that...)
Xenon discharge tubes are not bad in terms of efficiency. They have a lot of nice emission peaks in the infrared and convert about 10% of the electrical power into useable light power. That means they are about 20 times more efficient than infrared LEDs. But, most lamps are designed for exciting lasers or taking photographs and not for communications. To get the most from the lamp you need the have the space between the electrodes small. Keeping the electrode gap narrow helps reduce the inductive reactance of the gas plasma. The idea is to pulse the discharge tube with narrow current pulses, at low duty cycles. According to some information I got from EG&G, the shortest practical pulse for xenon is about 1us. EG&G and some other manufactures do have some lamps with narrow gaps that can dissipate about 30 watts of average power. These lamps use tungsten electrodes and a quartz envelope. If you can keep the discharge pulses to about 1us and have a repetition rate of about 10,000 pulse per second (1% duty cycle) (good for voice audio communications) then you should be able to put about 3,000 electrical watts into the lamp for each pulse and get about 300 watts of light out. With a narrow gap, the light emitting area would be small and you should be able to use some inexpensive flashlight mirror reflectors to produce a tight exit angle. With a good reflector and 300 watts you should be able to go all the way to an orbiting satellite some 150 miles away. But, if you want higher data rates you will have to sacrifice range. With pulses as short as 1us you can go as high as 500,000 pulses per second (50% duty cycle). But, a 30 watt lamp would not be able to launch more than about 6 watts of light at such a pulse rate. Still, 6 watts will go 10 miles easily.
There are many ways to modulate the lamps. If you can stand a higher average power dissipation, a simmer circuit works best. The simmer circuit is a low power discharge that is maintained through the lamp continuously. A few hundred volts at a few milliamps usually will do the trick. A little neon gas added to the xenon gas inside the lamp helps lower the simmer current needed. The discharge provides a continuous ionization path that a high current pulse can ride on. Since there is already plenty of ions from the simmer discharge there is no pre-ionization time delay. Therefore, the optical rise time when pulsed at high currents is fairly fast. But, it does take some time for the higher ionization to die down after the current pulse. This "long tail" effect is lessened with narrow electrode gaps. The disadvantage of the full simmer circuit is that it might double the average lamp dissipation, thereby lowering the overall efficiency.
To increase the efficiency, a "pseudo-simmer" circuit could be used. It is a little more complicated than the continuous simmer circuit but it can drop the average power dissipated. The pseudo-simmer circuit works the same was as the full simmer circuit except the low current discharge is turned on just prior to the big power pulse and is not turned on again until the next pulse. However, at high pulse rates I think the pseudo-simmer method would need a third trigger terminal on the xenon lamp. External triggering through the glass envelope using several thousand volts would not be practical at high pulse rates.
To my knowledge nobody has built a communications system using a xenon source
Yet.