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#11
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SPOT Versus ELT Analysis (Long)
The problem I have with this is it is a false paradox, you don't need to choose between a PLB/ELT or a SPOT. Especially since a SPOT is cheap enough (think the cost of a few tows, a long aeroretrieve, a lot cheaper than annual glider insurance... and so on) let alone the worry of friends and relatives or the hassle of people trying to find you if you land out etc. I've used a spot for about 9 months now, from when they first became available. There are quite a few people I fly with now who also use a SPOT, universally the interest in them is for tracking (aka SPOTcasting). This allows just tracking people out of interest, facilitating easier aero and ground retrieves and hopefully being able to find somebody in a crash or similar should the SPOT still be transmitting after the crash or at least being able to have a good last know location (and likely some direction intelligence) if the SPOT is not transmitting after a crash. I've also flown with a PLB on my parachute harness for several years. My sentiment is that if I'm in serious trouble then I do want a full PLB with 121.5 MHz homing beacon etc. for local SAR resources to find me with. Also since for best GPS reception and GlobalStar transmission the SPOT unit is sitting on the canopy rail on my glider and I have the PLB on my parachute harness so it goes with me in case of a bail out. But anyhow a couple of corrections to the original post - ELTs and PLBs transmit a homing beacon on 121.5 MHz (not 121.6 which I assume was a typo - although you'll see 121.6 MHz mentioned as a test frequency by ELT/PLB manufactures their distress beacon is always on 121.5Mhz). Comparing internal antennas on SPOT vs. external anennas on PLBs or ELTs is nonsensical. The beacon anennas on PLBs and ELTs have to be large becasue of the 121.5MHz beacon frequency. All current PLBs with GPS use a planar GPS antenna (GPS is ~1.5 GHz), SPOT uses a similar GPS antenna. The GloabalStar L-band antenna in SPOT operates on a similar frequency to GPS and uses the same planar antenna technology. The antenna are different because they operate on significantly different frequencies. PLBs and SPOT use planar and not helical coil antennas (which would also work at these frequencies) for various technical reasons and practical ones including compactness and robustness. The 1 year life for SPOT batteries vs. a shelf life for PLB batteries also is not a valid comparison. SPOT claims a ”Powered On unused” battery life of one year. That means the unit is turned on by the user but not sending any messages. SPOT claims that batteries installed but with the device turned off the batteries will “last several years” (see the SPOT User's Manual) . The Energizer e2 Lithium (non- rechargeable Li-FeS2) batteries SPOT uses have shelf lives of more than a decade and are the same basic chemistry as the batteries used in the best PLBs and ELTs. PLB battery packs however are larger than SPOT because a PLB draws more power than SPOT does in any of it's operational modes. The sealing and construction of battery packs for PLBs is more impressive than for a SPOT messenger, but the battery compartment seals and construction on SPOT appear more than adequate for our uses. SPOT messengers seem to give ample warning of battery exhaustion, and it is easy to carry a spare set of batteries and replace them if needed (tool less access to a well built, sealed battery compartment). I'd actually be more worried about PLB batteries because people don't test their PLB, so how confident should they be that the actual device really will work if needed and the batteries really are OK?). Bottom line is that batteries and SPOT just do not seem to be an issue. Not all 406MHz ELTs or PLBs transmit a GPS location. If I was buying a PLB today I'd only look for one with a GPS and I'd look for one with the longest battery life (I care about the built in 121.5 MHz homing beacon lasting as long as possible so the local SAR organization can home my location if needed). Effectively all current 406 MHz ELTs that you would install in a glider do not encode a GPS location. --------------------- While the SPOT service may be a black box in terms of how things operate, ELTs and PLBs are not themselves magical devices and knowing how they work in some detail might help in emergencies, so at the risk of drifting off topic and getting boringly long... A common misconception about 406MHz devices is they just transmit on 406Mhz - all of these also transmit a 121.5MHz homing beacon. PLBs typically put out a less powerful 121.5Mhz beacon than an ELT and (in the USA) also encode a morse code "P" (dit dah dah dit) about every 50 seconds on the beacon to signify it is a PLB. So if you suspect somebody is missing and may have activated a PLB listen for their 121.5Mhz beacon just like you would with an ELT. This is also the reason that 406 MHz ELTs and PLBs have long antennas - they are sized for the 121.5 Mhz signal (some high end ELTs likely intended more for corporate/jet aircraft do have separate 406 MHz and 121.5 MHz antennas). 121.5 MHz ELT beacons can be Doppler located by low earth orbiting (LEOSAR) satellites within the COSPAS-SARSAT network. It takes several satellite passes to determine the approximate position and due to how this is done at 121.5Mhz and the poor frequency stability specs or these ELTS you end up with something like typically 4 hours needed for a final position estimate and a position uncertainty of something like up to 15 nautical miles (over 100k acres to search). It is this COSPAS- SARSAT monitoring of 121.5MHz beacons that is going away in Febuary 2009. But airlines will still monitor 121.5 MHz, SAR organizatons like CAP and the Coast Guard will still be able to DF/home on them etc. The 406 MHz ELT/PLB beacons transmit a much more stable signal, and the signals are handled by the LEOSAR satellites differently and a fix can be determined within about an hour to within a few miles uncertainty, say typically less than a 1,000 acres to search. The 406 Mhz signal is transmitted in pulses roughly every 100 seconds, the higher power during the pulse, the improved frequency stability specs and different signal handling in the satellites allow this faster and more precise Doppler location. Now if the 406 MHz ELT (most glider ones won't today) or PLB (try to buy one with GPS) also optionally encode GPS coordinates in the 406 MHz signal then the geostationary (GEOSAR) COSPAS-SARSAT satellites can relay the location very accurately within a few minutes of activating the ELT/PLB - it may take several minutes (up to 10 minutes on older units) for the GPS unit in the PLB/ELT to obtain a cold GPS fix. The other thing the GEOSAR satellites always do is pick up a 406 MHz ELT/PLB signal almost instantly and that allows the coordination/SAR organizations to identity through the digital serial number who the beacon is registered to. The SAR coordinators can attempt to contact the owner or emergency contacts and quickly try to determine if this is a real emergency or not. That's a big problem with 121.5 MHz ELTs, there are just a huge number of false alarms. BTW there is a significant fraction fo false alarms with 406MHz beacons, I don't have the numbers handy, but the ability to identify the owner an contact them helps quickly resolve those. Many of those false alarms have historically come from 406 MHz marine beacons accidentally triggered during testing/inspection (I believe due to having to remove units from their shipboard mounts to read labels and thereby accidentally activating it - a pretty amusing design mistake). If the GPS enabled ELTs or PLBs can't get a GPS fix then they revert and behave like a non-GPS unit. Just let it sit there and be Doppler located in case there is a problem with the GPS (know what any GPS status LEDs on the device mean), also you can verify the 121.5Mhz beacon is working by listening with the glider VHF radio (or the backup handheld you carry in your survival kit). The GPS enabled ELTs and PLBs are not designed to provide continuous position tracking, in fact they specifically avoid this for signal qualiy reasons so thigns like obtain a location fix and continue to transmit that location for a while. The exact behavior will depend on what spec the device is complaint with but it may fix obtain a GPS fix and transmit that location in every 406 Mhz pulse for 20 minutes or so or longer. So you could imagine corner cases where this could create some problems (like ideas people might have for trying to trip a PLB in flight if they think they are in trouble then managing to flying some way before crashing...) - but that hopefully should be rare wacky stuff and ultimately that last location stuff can often be helped by SAR using the 121.5Mhz homing beacon. BTW a SPOT messenger that can't receive a GPS signal will transmit the unit ID information and "911" or "Help" message type (but apparently not an OK or SPOTcast message?) without the GPS coordinates and so the same thing can apply there with people at least able to try to contact the messenger owner or emergency contacts. Like many PLBs the SPOT messenger also shows via LEDs wether it has a GPS fix or not. ---------------- SPOT is a tremendous tool for tracking gliders, it is the only practical technology that I can think of that can do this. Like all technology it is not goign to wakr always, but my experience with it has been very impressive. It provides "I'm OK" or "I've landed out and need a retrieve" type messages. The only other technology that has anything as broad coverage and reliable as this would be an Iridium satelite phone where you can call your crew once on the ground (practically impossible in flight becasue of the large fold out antenna--unless you have an external Iridium antenna installed on the glider). There is a big piece of mind as well in flying with a SPOT with tracking and knowing people have been able to watch your tracks – which is my experience so far. Yes there are times when people have seen problems with this - it would be useful to know if this was really just the shared pages, whether it affects the private account pages as well or whether position reports were really never received. SPOT is so convenient for tracking for lots of reasons and is such low cost that people should just get one, if you also fly in desolate areas (most pilots I know do), fly by yourself a lot, etc. want a back- up for real emergencies then I would also carry a PLB on your parachute harness as well. If I have no interest in tracking or non- emergency retrieve messages etc. and only wanted a distress beacon then I'd buy a PLB - but I don't know of any glider pilot who want a SPOT to use just as a distress beacon (i.e. in "911" mode). Darryl |
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