Low-Cost Device to Detect and Locate Cell Phones in Signal Dead Zones for SAR Use?

Hi:
I work in search and rescue and we're increasingly getting incidents where the person lost or overdue is in a cell signal dead zone -- that is, their cell phone may be working, but is unable to hit a cell tower and so be located. This often happens in rural areas or large recreation areas (e.g. National Parks).

Is there a low-cost device -- or can one be built -- that detects a cell phones outbound ping and gives a direction? I'm imagining a gizmo that can be moved around and triangulate the phone and would work like an avalanche transceiver (a passive receiver).

I'm aware of the Stingray and it's capability to emulate a cell tower. I believe that can work, but you've got to go through law enforcement to get ahold of one. This is not necessarily a problem -- all ground SARs are run by law enforcement agencies in the US. I'm working on a proof of concept demo with an agency now, but wondering if there's something easier out there.

I'd imagine it being used where there's no active ping detectable by a tower, but the person is known to have a cell phone. SAR would use GIS to map the cell tower dead zones and, assuming the person is in one because there's no active signal, those would become the higher priority search areas. Using the gizmo either on board a helicopter or vehicle, you'd move around looking for a ping within those areas and narrow it down by triangulation.

Ideally, you'd be able to narrow it down by EIN or cell number but that might present legal problems in using it. Can that information be derived from the cell outbound ping? That sort of information is obtained from the family and exigent circumstances covers getting cell companies to check their cell logs, but this would be another method.

Comments and ideas are welcome. I know almost nothing about this stuff and am just trying to come up with more effective ways of searching. I'm aware of 3 searches in just the last 2 weeks where this sort of technology might help expedite finding the people.

Many thanks,

George

This is WAY out of my area of expertise, but I have an idea that may be worth sharing:

It's not true of every cellphone, but most also ping for WiFi. The signal isn't as strong, but from a helicopter, in most cases, it will be line of site. So, if the former problem is unsolvable, this may be a backup strategy -- or even an interim strategy.

Another possibility is a cell phone repeater. Having one of those in the helicopter, won't allow the rescuers to detect a cellphone, but it might give the lost person cell service so they can call for help. In fact, how about making it standard procedure to set up temporary, portable celphone repeaters around the area the person is believed to be.

Ah, both good suggestions. What's the distance a wifi-acquiring signal would travel and is there a gizmo that would give you direction? Would it only be recent phones that do that? I actually have service with republic wireless, which favors wifi-only calls.

I've actually heard of cell phone repeaters. Are they portable? Would you be able to get a ping off one and/or call the cell? Again, you could move them into dead zones for increased coverage if that would work.

thanks,

george

Well, like I said, not my area -- hopefully someone else will chime in and answer these questions.

Certainly the triangulation technology is well-established with things like Lo-Jack, and Ham Radio operators have been doing "hidden transmitter hunts" for well over half a century, with increasing sophistication. A cellphone will be operating at its maximum power (about 500mW-ish) when it gets no reply while looking for a tower, improving odds of detection at distances of 1-2 miles. But transmission times are short and infrequent to save battery power.

The suggestion to use a mobile cell repeater seems a good one, though I don't know anything about them personally. Even if the "lost" person can't use the phone, it will acknowledge receipt of signal, which is the first requirement for triangulation.

I did some Googling on Cell Phone Repeaters in an attempt to determine if there is any indication of cell phone presence. I couldn't tell for sure. So, I suggest you contact the various manufacturers and tell them about your unique application and see if they have any solutions or suggestions. Also, ask then about range (i.e. how close would the repeater need to be to the lost person's cell phone, for it to "light up". And, ask what the "detection timing window would be". In other words, how long would you need to be in range of the lost person's cell phone before a detection would occur--i.e. worst case. This would, probably, have everything to do with how often the cell phone pings for a cell tower.

Here's what I'm thinking: If the range is, say, 3 miles, line-of-sight, without any obstructions (not unreasonable from a helicopter), then that is a sphere (or "bubble") with a radius of 3 miles. While flying over, you would have to be within that sphere long enough for a detection to occur. A more rapid ping rate would mean a wider search grid or a faster speed along the same search grid. And, it probably wouldn't have to be a grid, but more like "gridiron" lines, because you would only need to fly through the bubble once.

If you guys have a budget for it, perhaps one of the cellphone repeater manufacturers will modify some units for you (i.e. modify them to behave like cellphone detectors -- and consider that if it merely detects the presence of a cell phone signal and provides no other information (such as location, or conversation content), then it might be legal. I mean, where does the line get drawn between a "repeater" and a full on cell phone conversation/location eavesdropping system? Also, you might find a manufacturer who would be willing to donate some equipment -- perhaps in trade for the publicity.

If you can find such a thing as a repeater that indicates cellphone activity (or can have one or more made), then you could fly over the area(s) where the person might be, until a cell signal is detected, and then concentrate your search there. In other words, even if triangulation isn't feasible, then it might work to use it as a proximity detector.

Consider, also, that there might be "false positives". In other words, it will indicate for anyone who is NOT lost, but merely hiking, camping, working, etc. in the area.

In my Googling, I hit on various "bounty hunter" sites and learned about another technique (that may or may not be a myth--and if real, is, likely, illegal): the technique seen in various movies and TV shows where someone's phone is "pinged" which causes it to send it's GPS location. Of course, this will only work if their phone has it's GPS system running, and if the phone is in contact with enough GPS satellites, but if that is a separate function from the "phone" part of the phone, then it might be an option even when phone reception is absent [like I said, this ain't my area, but I think like an engineer, so I'm offering that service--even if it's in an "information vacuum" [BTW: in my youth I pursued Radio Communications as a career--even got my 2nd class Radio Telephone license--but soon decided to go in a different direction. But it did give me some understanding of RF. I'm just not up on modern technology in that area]

I first hit on this with the following search phrase:
"how often does a cell phone ping the cell tower"

But, the phrase "cell phone pinging" really gets it (check out the "Pursuit Magazine" article [and especially the comments]).

Steve & Mark:

Many thanks for weighing in on this problem. One of our guys also, this very morning, came up with the idea to use a portable cell repeater, so we're going to look into that.

As a side note, SAR often uses cell pings to locate people -- or narrow the search area -- by going through the phone company of the towers in the area. That's been in use for a number of years and is pretty successful, though dependent on the type of tower. Nationally, one of our people has created a web site where, if the user is in range and conscious, we send an SMS message. They reply, giving permission to activate their phone's GPS and the location is sent to our server in the reply.

As noted, the problem we're working on now is finding an active phone that is unable to hit a cell tower. You're right that the potential of false positives could be a problem. In a wilderness setting, it might not be a major problem though. If the accuracy is high, a return location in an anomalous location (e.g. just off a highway, indicating an accident; or in a closed area -- road gate closed because of snow -- that would help).

Anyway, keep those thoughts and suggestions coming. I'll post any solutions we come up with. I'll also talk to our local cell tech who's been pretty helpful on other problems.