Laser PLB/ELT
 
(This was written so long ago I lost the information on when I wrote it.)
 
 
Laser PLB/ELT

 

By Peter Gransee 

 

A PLB (personal locator beacon) is a small device designed to help a person be located in an emergency.


 

Several portable PLBs already exist for hikers, sports, boaters, pilots, military, etc. The problem with these systems is either range, cost or portability. To maximize life saving potential, an ideal PLB would have global range, fit on your keychain or in your watch and cost less than $100. Anything other than this is certainly doable, but less effective. This proposal uses the advent of low cost, high power Infrared laser diodes to communicate with an overhead satellite.

 

--

Almost anyone can build a transmitter powerful enough to let people know that you are lost in the woods. The question; is the system practical enough so that the people who actually need this system the most will actually have it with them when the emergency occurs?

The system must be small, global and cheap. We don't usually plan on getting lost. Hikers don't like to carry a lot of gear. As the size of the gear increases, the reasons for carrying it diminish. Many tools that would have benefited a lost hiker are simply left at home. Call it stupidity or calculated risk based on the inconvenience of carrying the item.

The smallest satellite based PLB I have seen is the size of a cell phone and costs several thousand dollars. Too big, too expensive. People are going to die because the system is impractical. The reason these systems are as large as they are is because of the output powered required (~5w) to reach the satellite.

Another factor is cost. Breitling makes a PLB watch that can transmit about 100 miles LOS on the 125 MHz band. The watch costs $3500. So, only the rich get rescued. But that assumes that someone is within 100 miles line of site (reduced with terrain, etc), will actually be looking for you or happen across your signal by accident.

Another system I looked at was a watch designed for locating kids. It uses GPS and local cell towers to locate and track the wearer. Since it relies on the cellular network, it is unlikely to work out in the boonies (otherwise you could use your cell phone to call for help).

A satellite system would work almost anywhere and not require people to know you are lost. Activating the system will let them know. This means help will come sooner.

While we are at it, it would be nice if this marvelous system would let us know that our signal has been received and help is on its way.

When studying ways to do this, I first looked at RF solutions. But that had low chances of working. The problem is for my target size (a key fob or watch), the amount of RF power that could be generated and punch through the atmosphere was simply not enough. I looked at pulsed communications, etc. I consulted with a satellite communications expert and he said the possibility of any RF system in that form factor working was very slim at best.

The next candidate was a laser. Compared to RF, lasers pulse better and focus with less bulky collimation. Could it be done?

The earth's atmosphere varies opacity with frequency. See: Atmospheric Opacity

Notice that there is a nice window at about 1 micron. This is in the infrared band. The inexpensive 15mw green laser pointers you see on the market are fired by 250mw 808nm IR laser diodes. Reliability and cost are good.

Further research found that companies like Osram make even more powerful laser diodes and closer to 1 micron. Osram Semi

They have small (5mm diameter), inexpensive (less than $5), plastic-encapsulated, laser diodes that can output a 75watt pulse at 100ns. As the duty cycle is increased, the power level comes down. My goal is a couple of watts at 1ms.

For this application, the laser does not have to be right at 1 micron. Efficiency of the system drops as you move away from 1000nm but light does make it through. Using more laser power can compensate for the atmospheric attenuation.

The duty cycle for the laser can be very short. All you need is to fire the laser long enough to burst about 32 bytes of information. This can be done in less than 1 millisecond. Since our PLB has a small battery, laser power is provided by a capacitor. A capacitor is an electronic part that can slowly be charged, store the charge for a period of time and then dump it all at once into a laser diode. Capacitors are not generators of electricity, they merely store it. Their advantage over batteries is that they can unload their total energy very quickly.

Using a capacitor does not provide more energy than you start with. It is not a perpetual motion machine. A battery is used to charge the capacitor and energy is wasted in the process. The reason for even using a capacitor is that a battery can not provide all of it energy in a matter of milliseconds. Therefore a capacitor is used as an intermediary.

Capacitors are used in camera flash systems for example. In a camera, a 3v battery (2 AA cells in series) slowly charges the capacitor (usually in less than 10 seconds) through a step-up voltage converter. Once fully charged, the capacitor unloads all of its energy through a spark gap light source (the xenon flash tube). Light is produced by a large spark jumping between two electrodes. The energy in that single pulse is enough to melt the tip of a screwdriver and contains thousands of watts of power. The pulse is very short of course (less than 100ms). If the pulse is shortened further, the peak energy level can be increased further. Capacitors used in point and shoot size cameras are usually the diameter of an AA cell and about 2/3 the length. They usually cost less than $1 and are made of a wound tape of plastic and metal film.

To keep the size down, I recommend the use of a super capacitor like an Aerogel Capacitor.

Aerogel Cap

"The Aerogel Capacitor's breakthrough low resistance enables its use in pulse power and electronic circuitry applications that other types of super capacitors cannot address. For example, an Aerogel Capacitor in an "AA" size cylindrical package (A1450A100) can deliver up to 35 Amperes of peak current."

For the target power level, the AA sized super capacitor is not needed. A smaller size (size of pencil eraser) will probably be more than sufficient.

Laser diodes can handle high pulse currents for a brief period of time while maintaining sufficient beam quality, etc.

For the sake of discussion, I am thinking of powering the whole getup with a watch battery, charging a supercap with a step-up converter and unloading through a FET switch into a 980nm laser diode producing ~1 watt of laser energy for 1 millisecond or less. For use in a PLB, the system is only required to fire a couple of times in the entire lifespan of the device.

How is this detected by the satellite? Light Amplification. NVG (starlight amplifiers) are especially sensitive to IR bands. Typical civilian units can achieve 40,000x light amplification. Plate response time is within the data rate requirements (no slow phosphors are needed). So we have a small beach ball size satellite in LEO or MEO orbit with an IR amplifier/detector, etc. Cost of the satellite and launch? $5-10 million. Pricey yes, but amortize that cost over 3 years typical life and tens of thousands of PLB users.

Another compromise in the system is that fact it will probably not work during the day or in cloudy weather. You may have to wait until night fall or a day or two pass. This is still a lot faster than waiting for people to discover you are missing and start conventional search patterns.

Satellite LIDAR systems have been built that can bounce a laser beam off the surface of the earth and detect the faint echo. LIDAR systems must contend with a large percentage of light being scattered by oblique surfaces. This transponder system should be easier to detect because the signal strength from the laser diode is higher than a passive return echo. And this in spite of the fact that some of the energy is lost because the laser beam is spread out to about 90 degrees. This is required because the satellite is rarely directly overhead.

Another example of current satellites picking up faint light signals from the earth is those nighttime world maps taken with weather satellites. The pulse of a laser diode is much brighter from the sensor’s perspective than a street lamp.

Risk of eye damage. IR frequencies are less dangerous to the eye than visible frequencies because the cornea absorbs most of the energy at this wavelength. Damage type is cornea heating which requires sufficient power x time. The short duty cycle of the laser and the orientation required for link further minimize risk. The system also has an interlock that improves safety.

The software interlock prevents the laser from firing if it does not have an unobstructed view of the satellite. In some cases, this would prevent the system from firing if the operator was using the system incorrectly or holding the emitter close to their eye.

A simple tilt sensor could also contribute to the software interlock. The system would not arm unless the emitter is pointed away from the earth mass.

The largest safety effect comes from the fact that the pulse is so brief (1ms). Laser eye damage is a factor of wavelength, wattage and time.

Laser Eye risks

In spite of all these safety measures, there is a possibility of eye damage. This possibility is slight but most people would prefer to have the rescue option with some risks than no rescue option at all.

Because light is being used instead of radio waves, FCC approval should be simpler. The only complaint to this system may come from IR astronomers but the duty cycle of the satellite is so short and the location predictable.

The satellite would be in LEO orbit and pass overhead every 90 minutes or so. The satellite would transmit a constant, "I'm here" IR signal that would trigger the PLB to fire when the satellite is in the best position to receive the PLB signal. The transmit beam of the satellite would be focused into a narrow slit so it would scan all the earth below but not dwell on any area for long. This is sufficient to activate any PLB beacons below. The satellite would use a ~100w CW diode laser for transmitting. This should be sufficient for the PLBs to detect with their fisheye optics and photodiode detector. 

If it were determined that daytime transmission was usually ineffective, the system could sleep during daytime passes and wake on nightime passes.

Typical system sequence is:

1. PLB lens cap is removed and finger loop is pulled. This loop has a warning sticker over it. The cap protects the optics and is part of the waterproof design. The device is intended for one use only and then thrown away. This also minimizes abuse. The lithium battery inside has a 10 year shelf life.
2. The device enters "seek mode". The operator orientates the devices upright towards the sky. A slow flashing red LED indicates seek mode is activated. Seek mode will take a maximum of 90 minutes (one LEO orbit). If 90 minutes transpires and no satellite is seen, the red LED will flash rapidly. The operator should move the PLB to a better location. After 5 minutes, the unit will go back into seek mode.
3. When the satellite comes over the horizon and the beacon beam is detected, the PLB fires its data burst. During the burst, the indicator LED is dark. The burst is only fired if the satellite can be seen. This prevents firing of the laser if someone/something is blocking the beam or the device is not orientated correctly.
4. If the satellite gets the PLB signal, it quickly acknowledges by repeating the ID suffix and emergency type. The PLB then starts flashing a green LED to indicate your signal got through. This green LED can also help the ground crews locate you while walking the ground around your signal coordinates. If the PLB does not get an Ack, it goes back into seek mode. If this happens, the operator should move the device to a position with better sky view.
5. The satellite downloads the PLB data to the ground station with its next pass. Only one ground station is required since the comm traffic and mode is low bandwidth.
6. Ground station forwards data to dispatch.
7. Dispatch looks up the customer file which has phone numbers, address, medical conditions, etc. Dispatch then attempts to call those numbers and see if the person is indeed missing, etc.
8. If dispatch decides a legit emergency has occurred, positional coordinates of the PLB are sent to state rescue dispatch. The package will include name, coordinates, type of terrain, experience of person lost, medical problems, etc.

The burst transmission is very quick and repeated twice. It includes the unique ID of the device and the emergency type. Example emergency types include:

- general personal PLB beacon
- general vehicle PLB beacon
- medical emergency
- remote sensor
- stolen item
- Military type 1
- Military type 2, etc
- Short message upload (includes payload)
- PLB test

The switch would have a pull-pin on it to prevent accidental turn on. The fob could also have a beeper or test mode to indicate a low battery or transmitter fault. The fob would be powered by a lithium coin cell with a 10-year shelf life. Lithium cells also have excellent cold temperature resistance compared to alkaline.

To prevent misuse of the system, there would be fines for abuse and/or a service charge for each use. Part of the price of the FOB would pay for the satellite and ground control systems. These systems may also be government subsidized to some degree out of the (limited) S&R budget. This system could be standard equipment for military personnel. The fobs could be made in < xml="true" ns="urn:schemas-microsoft-com:office:smarttags" prefix="st1" namespace="">Asia to keep the cost down. The electronics would be shrunk to a single chip to lower costs. Typical production runs of 500k units would probably get the cost down to less than $100.

The stolen car locator version would require an optical window on the roof of the car. Remote sensors, etc would also have a top mounted optical window. The short message version would have either an optical package mounted on the lid of the laptop or a wire-connected external package for optimal mounting.

This system could also be packaged into a watch and combined with other sports/camping related features that would increase the chance that the kind of people who need a PLB would be carrying a PLB. When combined with a host device, the PLB function could share common parts such as a housing, battery, processor, indicator LEDs, etc.

Building the fob into a commonly carried item like a key chain, knife, lighter, watch, etc would increase the likelihood that it is carried at all times. The chip and protocol could also be OEM'd to other manufacturers and built into GPS, cell phones, FRS radios, aircraft avionics, cars, etc.

The biggest impediment to this system beyond if it can work is the cost of the satellite. This could be mitigated by using existing on-orbit LIDAR resources or pigbacking on another LEO asset. The military (like with GPS) may drive this technology for their own use. Since it uses an optical band, it would not tie up over-tasked RF bands.

 --

This idea is free from copyright and intended to be used to save lives. There are no royalties or fees associated with its use in a safety system or product. I do ask however that you let me know if you intend to use it.