Just ran across this fascinating study: http://www.backcountrychronicles.com/plb-slot-canyon-test/

Well worth reading- PLBs don't work all that well in slot canyons in Zion National Park. I particularly appreciate the final sentence in the article...

As always, technology can't replace good planning and training. Technology is a backup to your other plans.

Although I will say that I tested the Delorme InReach in some canyons in Northern Arizona last year and had 100% communications with it, but they would not be considered slot canyons. This was only the satellite communications, not the PLB signal of course.

The InReach does not use COSPAS/SARSAT for emergency signaling -- it uses the Iridium satellites for all communications functions. Anywhere it works for text messaging it will work for emergency signaling, as long as it can obtain an accurate GPS fix and GEOS (the private RCC) is working.

Very interesting indeed!

COSPAS/SARSAT was originally designed to fix beacon locations without encoded GPS signals -- two satellite passes were required at a minimum. With GPS location data encoded in the signal the precision and time to locate were significantly improved, proving to be a tremendous boon to rescuers.

When comparing InReach to a PLB, it might be useful to know if the Iridium satellites have the capability of triangulating locations where there is no encoded GPS signal. I suspect that the answer to that is "no."

I didn't know anything about the Iridium satellite constellation so I checked http://en.wikipedia.org/wiki/Iridium_satellite_constellation to get an idea. The satellites orbit at an altitude of approx 483 miles.

It seems to my non-communications engineer brain that the communications would need to process the doppler out to get a stable comm signal. If true they should be able to use that doppler to pinpoint a location if the network software supported. If a signal was flagged as a SAR priority, Iridium may be able to use more than one satellite to obtain that geographic fix. I'd be surprised if the InReach folks hadn't considered that contingency. Something to ask InReach.

Russ,

I am not a communications engineer either, but satellite overhead is moving almost perpendicular to the line of sight therefore there is minimal to no doppler shift unless the satellite is near the horizon. I would expect that the phone is usually using the closest satellite (near overhead) and the comm should not need much correction. It would be hard to get a location in a slot canyon from doppler.

Respectfully,

Jerry

I did some more research and Iridium can provide a location based on the "spot beam" -- it uses directional antennae for terrestrial communication so the system knows which "spot" a user is in. However, the spot beam is 250mi in diameter. As the satellites move a ground station will move out of one spot and into another, so it's safe to assume that the location data will be more precise than that over time.

What I still don't know is if the satellite software allows for Doppler triangulation like Russ suggests.

Jerry -- If the satellite constellation were in a geo-sync orbit there would be little to no doppler, but the Iridium satellites appear to be in a polar orbit at 483 miles altitude with a velocity of 16,689 mph; I can't imagine an Earth based Iridium Sat-Phone that wouldn't have doppler coming and going (Up/Down). If the specific satellite you are talking through is at a particular moment in time straight overhead, a moment before it was either North or South of your location. The moment of no doppler would be when you are on a line perpendicular (E-W) to the path of the satellite (N-S).

The more I think about it, I'd be very surprised if the Iridium system doesn't track every Iridium call location. Considering the size of each antenna footprint, it seems to me that hand-off from one antenna to the next and from one satellite to the next would be enhanced by knowing where each caller was located.

Russ,

I will calculate the numbers in the morning, but the speed of the satellite is only .00249% of the speed of light. The doppler shift is going to be VERY small even if you were in orbit and it was moving directly away from you.

Respectfully,

Jerry

US Navy NAVSAT of the 1970's used observed doppler shift to provide location information to vessels. The rate shape of the frequency shift curve (it would be an S from high to low like listening to a locomotive horn going past you) reveals the distance from the observer to the ground track of the satellite. (still have to choose which side you are on from it)

So the concept is sound. But if you can only see a small window straight up it isn't likely to work very well. Need the speed IN the line of sight to get a good shift.