Since Aron Ralston was pinned by a rock in a slot canyon near Moab, Utah in 2003, there have been questions about the ability of SARSAT satellites to receive a distress calls from a PLB that is deep inside a slot canyon, especially canyons that are oriented north-south. For more background information about Personal Locator Beacons, please refer to our articles, PLBs In-Depth and McMurdo vs ACR PLB Comparison.
The International Search and Rescue system has satellites in two types of orbits and altitudes; Low-Earth Orbiting Search And Rescue (LEOSAR) Satellites and Geostationary Orbiting Search And Rescue (GEOSAR) Satellites. The GEOSAR satellites orbit the Earth at different longitudes, but all are stationed over the equator, so there is only a small chance that they could receive a signal from inside a slot canyon in the Northern Hemisphere, unless the slot canyon had open sky toward the South.
The LEOSAR satellites orbit the earth from pole to pole about every 100 minutes and since the Earth rotates about 25° each orbit, the entire Earth is covered twice per day by each satellite. Since they pass from North to South or from South to North, there is a good chance that they can pick up a signal from a slot canyon with an East-West orientation, but there is less chance that they line up directly over a North-South slot canyon.
PLB Test in Slot Canyons at Zion NP
The Search and Rescue personnel at Zion National Park conducted a test in 2008 to see if PLBs could be detected in 10 areas of the park, including eight sites that were in slot canyons that were deliberately chosen because they could not acquire GPS locations.
Example of a Slot Canyon in Zion National Park
PLB Test in Open Areas
Two open areas were tested with unobstructed views of the sky, where GPS data could be gathered. The good news first, the distress signals were detected by both GEOSAR (High orbit Geo-stationary) and LEOSAR (Low Orbit, Pole-pole orbits) satellites. The GPS location data was received within five minutes and six minutes respectively. The SARSAT system worked exactly as planned, hoped for and advertised. Good news.
PLB Test in East-West Slot Canyons
Four east-west oriented slot canyons (3 east-west and 1 northeast-southwest) were tested and the signals were detected as fast as one minute and as slow as 19 minutes. No GPS signals were detected at any slot canyon site, no signals from any slot canyon was detected by a GEOSAR satellite. LEOSAR satellites were able to resolve locations to about one mile in as fast as 10 minutes at one sites to three miles in 24 minutes at two sites. Three of these tests were conducted for only two hours, but one east-west canyon test was 22 hours long. The Doppler location estimate improved from 10 miles from that actual site after nine minutes, to about five miles in five hours, to about three miles in nine hours, to less than two miles error after 17 hours. All four slot canyons (100%) with east-west or northeast-southwest orientations were detected by LEOSAR satellites and Doppler locations were determined within three miles of the actual sites (100%).
PLB Test in North-South Slot Canyons
Four slot canyons were tested with north-south orientations. Satellites failed to record any signal from one canyon. Signals were detected at two other canyons in nine and 10 minutes. The start time for the test was not recorded for the fourth canyon. As with the East-West Slot canyons, no GPS signals were detected, but the satellites were able to resolve locations to about two miles in nine minutes at one canyon and to about six miles in the canyon where start time was not recorded.
All hope is Not Lost
No signals from any of the slot canyon test sites were detected by GEOSAR satellites and no GPS data was transmitted from any of the units, but that does not mean that all hope is lost. Three of the four slot canyons (75%) with north-south orientations were detected by LEOSAR satellites and Doppler location was determined within two miles of one canyon (25%). The Doppler location was determined within six miles for another north-south canyon (25%). No Doppler location could be determined for two north-south canyons (50%) and the signal from one canyon (25%) was not detected by any satellite.
Keep in mind, that most of these tests were only two hours in duration. The test signal from the canyon with no satellite detection had only two very low angle LEOSAR satellite passes (11° & 14°) during those two hours. It is likely that the signal would have been detected on a higher angle satellite passes within a few hours. If you’re interested, here is the full report (pdf file) of the PLB test in Zion National Park.
A PLB Should be Part of Your Safety Net
The SARSAT system has saved thousands of lives since 1982 and the relatively new PLBs have saved hundreds of lives since 2003. This test shows that there are certain areas, specifically deep canyons and probably dense forests in mountainous areas where the SARSAT system may have difficulty receiving and determining the location of PLB distress signals. If you were alone and seriously injured in a remote slot canyon, rescuers will have a hard time locating you in a two or three mile circle, because if your PLB can not see the sky, SAR will not see you from the air.
If you are injured or lost in remote areas that have only a partial view of the sky, a PLB will not guarantee your rescue. You would have to be able to do something to help SAR find you. This may still be a better situation than if nobody even knew you were missing at all. Another good reason to hike with a buddy. A buddy could apply first aid, go get help or put the PLB in a better location to contact satellites, even if in the bottom of a slot canyon. It may not be high tech or even a new idea, but the best backup safety plan may simply be to tell someone where you are going and when you expect to be back.