Gear Review: ACR PLB vs. SPOT

ACR SARlink 406
ACR SARlink 406

I own an ACR SARlink 406. I have used a Spot in the past, which in my opinion is the cheapest most useless piece of trash ever marketed.

Bottom line – like many things get the equipment for the intended use. If you want to send messages to family, friends, & facebook – go with a Spot. If you are looking for a last line in an emergency go with an ACR. The article below, along with a lot of other research is what helped me make up my mind – hope it helps you as well.

Note that on a premise, the ACR is much more expensive than a spot, but then again, you get what you pay for. There are no monthly fees with an ACR, so over time the economics are pretty simple.

Below is a comment posted at the following url:
http://desertexplorer.wordpress.com/2009/11/10/the-spot-messenger-and-personal-locator-beacons/

There are a few more critical emergency signaling differences between the SPOT and ACR SARLink:

Power and Frequency Differences:
Power and Frequency are two key areas to consider when researching a life saving device. Satellites are thousands of miles away from earth, so your beacon’s signal needs to have enough power to travel that far and be able to go through anything between you and the satellite (trees, weather, out of slot canyons, etc.).

POWER: SPOT is powered by 400 milliWatts while ACR 406 MHz PLBs and EPIRBs use 5 Watts. You would need 12.5 Spot units to equal the power of one ACR PLB or EPIRB! When your signal has to travel 22,000 miles to reach a satellite, you want to make sure you have more than enough power to get it there!

 

ACR SARlink 406
ACR SARlink 406

FREQUENCY: The basic principles of frequency are that the lower the frequency, the easier it can penetrate buildings, trees and meteorological activity that appear between the transmitting device (PLB, EPIRB or SPOT) and the receiving device (the satellites). FM radios and TV channels work on a lower frequency which is why they can penetrate buildings and the environment pretty easily. Now think about radar which uses a really high frequency. Radar works by hitting an object and bouncing off, that’s how radar knows where to place an airplane on the radar screen. So the higher the frequency, the less likely it can penetrate things in between, the lower the frequency, the easier it can penetrate. 406 MHz PLBs and EPIRBs use a dedicated frequency set up by the search and rescue community that is in the same range as UHF TV stations. SPOT uses the 1.6GHz frequency which is four times higher in the frequency spectrum. This means SPOT’s frequency is four times less likely to go through an object or weather than the lower 406 MHz frequency.

Considering power, combined with frequency, ACR’s 406 MHz beacon stands head and shoulders above satellite messenger systems like SPOT. ACR has 12.5 times more power and is four times more likely to penetrate objects in between the beacon and the satellite than SPOT.

Redundancies: PLBs and EPIRBs have multiple ways to contact SAR in order to get you rescued. SPOT has one single transmission method. PLBs and EPIRBs can contact SAR via 406 MHz, which locates your beacon using Doppler Shift, it can contact SAR using GPS data and it also has a 121.5 MHz homing frequency so when SAR forces get a few miles away from you, they can home in directly on your beacon and find you faster. SPOT uses GPS only to send its location. If you cannot download GPS, SAR will have no idea where you are!

Coverage: EPIRBs and PLBs use the Cospas-Sarsat satellite system which contains 2 different satellite systems (LEOSAR – Low earth orbiting and GEOSAR -Geostationary). These two systems cover every inch of the planet. SPOT tracker is part of Globalstar and uses the Globalstar (Nasdaq: GSAT) satellite system which only covers a certain percentage of the earth, but has quite a few locations that are undetectable.

The SPOT is an excellent tool for personal tracking and checking in with friends and family to say I’m Okay. However, the SPOT should be used purely for those tasks, not for emergency use. For emergency use you want to ensure you have a device that is certified to the highest standards, is fast, and provids more than just GPS signaling, for these reasons I carried an ACR SARLink GPS equipped PLB.

More info:
– The SPOT unit will still send out a distress message even if it doesn’t have any GPS data. However this distress transmission will be a simple ‘help me’ without any geo-location, hence not very useful for SAR. In comparison, a 406MHz PLB distress signal (also without any GPS data) still identifies your location to within 5km using Doppler triangulation (LUTs detecting the non-geostationary satellites of the Cospas-Sarsat system interpret the Doppler frequency shift heard by LEOSAR and MEOSAR satellites as they pass over a beacon transmitting at a fixed frequency. The interpretation determines both bearing and range). Of course if the PLB is able to get a GPS-lock then your location is even more accurate. The reason I feel this distinction is important is because the GPS-lock is the least reliable part of the distress system. The upper limit of a GPS cold-lock is around 20min (regardless of whether you’re using a SPOT or a PLB, they both rely on the same GPS satellite network). Furthermore, a GPS-lock under canopy or in a slot canyon is very unreliable, because the GPS unit requires a minimum of four visible satellites). SPOT relies entirely on this GPS system to acquire coordinates. With PLBs, as soon as you press the distress button, you will be able to immediately send out your location to within 5km using a powerful 5 Watt blast of the 406MHz signal, then if a GPS lock is possible the following transmissions (every 50 seconds) will include the GPS coordinates.

– As I mentioned previously, the SPOT unit uses the Globalstar communications satellite network, which is not designed for SAR and does not have dedicated channels listening for SAR distress signals. This means that your SPOT, similar to a Sat Phone, needs to first listen in order to find a free slot before it can transmit your distress signal to a Globalstar satellite (acquisition time). This introduces extra delay into the system. Comparatively, the PLBs Cospas-Sarsat system is dedicated to 406MHz distress signals, so a PLB can begin transmitting distress signals immediately, and they will be picked up by the satellites immediately as well. In short what I’m trying to convey is that the Cospas-Sarsat system has been designed for the ground up specifically for just SAR distress signaling. The SPOT unit uses a commercial communications satellite network which is not dedicated for SAR nor specifically optimized to handle SAR distress calls.

On the subject of battery life, the much shorter (35hr) transmission time of the ACR PLB is because the ACR unit transmits at a much much higher power (minimum 5 watts, typically 6.3 watts), hence drains its batteries quicker compared to the weaker 400 milliwatts SPOT signal. PLB’s transmit every 50 seconds during that 35hr period, while the SPOT unit does not guarantee any transmission rate (only that most likely something will be sent within 20 minutes). The 35hr time window is definitely more than enough to relay the message to the satellite. Plus, I’d much rather have a shorter transmission time of a powerful signal rather than a longer transmission time of a weak signal that has a lower chance of actually reaching the satellites.

Another thing to consider when comparing battery life is to look at the different testing and certification requirements between the two units: Personal Locator Beacons, like EPIRBs, must be submitted to an independent test lab that verifies the frequency, operating life, testing in extreme temperatures, environmental testing, etc. From there, the product must go to Cospas-Sarsat, USCG and finally the FCC (or other comparable agencies in each country) for approval to certify that the EPIRB/PLB meets the standards for Radio Technical Commission for Maritime Services (RTCM) and that it exceeds required operating and mechanical conditions that will appear in the marine environment. PLBs and EPIRBs are rigorously tested by governing agencies to confirm they will work properly in the environment. This is a big difference from SPOT which only has to self-test and self certify that it meets Part 15 FCC. So the ‘up to 7 days’ that SPOT reports in its spec sheet might not be a very well grounded statistic.

One note on the GPS coordinates for the SPOT and GPS equipped PLBs such as the ACR SARLink: I’m not sure about the exact GPS chipset used in either of the units, but on average autonomous civilian GPS horizontal position fixes are typically accurate to about 15 meters (45ft). From my experience the SPOT unit is consistent with these figure. HOWEVER, any PLB or EPIRB is only accurate to 100m a much bigger error range! The reason is actually not the inferiority of the unit’s GPS chipset itself but because the space in the hex message protocol (set by Cospas-Sarsat) for position information is limited, so transmitted accuracy is approximately +/- 125 meters. In my opinion 100m range accuracy is plenty enough for SAR purposes, however might not be accurate enough if you wish to use the device for non-emergency use (to plot trail heads, etc). The SPOT is by design a great GPS tracker, accurate to 15m on average, however I still feel that for emergency signaling use PLBs are far superior by design.

ACR SARlink 406
ACR SARlink 406
ACR SARlink 406
ACR SARlink 406

 

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