Last week, I wrote about new ideas for finding the lost. One I've done some follow-up on is the cell phone approach. While it's not hard to design a good emergency rescue radio if you are going to explicitly carry a rescue device when you get lost, the key to cell phones is that people are already carrying them without thinking about it -- even when going places with no cell reception since they want the phone with them when they return to reception.

Earlier I proposed a picocell to be mounted in a light plane (or even drone) that would fly over the search area and try to ping the phone and determine where it is. That would work with today's phones. It might have found the 3 climbers, now presumed dead, on Mt. Hood because one of them definitely had a cell phone. It would also have found James Kim because they had a car battery, on which a cell phone can run for a long time.

My expanded proposal is for a deliberate emergency rescue mode on cell phones. It's mostly software (and thus not expensive to add) but people would even pay for it. You could explicitly put your phone into emergency rescue mode, or have it automatically enter it if it's out of range for a long time. (For privacy reasons you would want to be able to disable any automatic entry into such a mode, or at least be warned about it.)

What you do in this mode depends on how accurate a clock you have. Many modern phones have a very accurate clock, either from the last time they saw the cell network, or from GPS receivers inside the phone. If you have an accurate clock, then you can arrange to wake up and listen for signals from rescue planes at very precise times, and the planes will know those times exactly as well. So you can be off most of the time and thus do this with very low power consumption. It need not be a plane -- it's not out of the question to have a system with a highly directional antenna in some point that can scan the area.

If you don't know the exact time, you can still listen at intervals while you have power. As your battery dies, the intervals between wakeups have to get longer. Once they get down to long periods like hours, the rescue crews can't tell exactly when you will transmit and just have to run all the time.

If you know the exact time a phone will be on, you can even pull tricks like have other transmitters cut out briefly at that time (most protocols can tolerate sub-second outages) to make the radio spectrum quieter.

At first, you can actually listen quite often. The owner of the phone, if conscious might even make the grim evaluation of how long they can hold out and tell the phone to budget power for that many days.

When the phone hears the emergency ping (which quite possibly will be at above-normal power) it can also respond at above normal power, if it feels it has the power budget for it. It can also beep to the owner to get input on that question. (Making the searcher's ping more powerful can actually be counterproductive as it could make the phone respond when it can't possibly be received. The ping could indicate what its transmit power was, allowing the phone to judge whether its signal could possibly make it back to a good receiver.)

Of course if the phone has a GPS, once it does sync up with the picocell, it could provide its exact location. Otherewise it could do a series of blips to allow direction finding or fly-over signal strength location of the phone.

In most cases, if we know who the missing person is we'll know their cell phone number, and thus their phone carrier and in most cases the model of phone they have. So searchers would know exactly what to look for, and whether the phone supports any emergency protocol or just has to be searched for with standard tech.

I've brought some of these ideas up with friends at Qualcomm. We'll see if something can come of it.

Update: Lucent does have a picocell that was deployed in some rescue operations in New Orleans. Here's a message discussing it