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How to use a SPOT device

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Re: How to use a SPOT device

Postby CAMERONM » Sat Feb 24, 2018 6:15 pm

I have had the experience of pushing the OK button and leaving the SPOT untouched on a prominent rock until it stopped sending, in fact doing this twice, and both messages still did not get through.

I did a test of creating tracks on a local hike sent from a SPOT 3. During the three hours the SPOT could have theoretically sent 36 points, but in fact only 26 were received during this time. I overlaid these points on a map with the 1,262 points recorded by my iPhone 6 with GAIA. Every point from the two system overlaid each other x,y,z accurately, but one can see that the tracks from the SPOT are not nearly as detailed.
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Re: How to use a SPOT device

Postby oldranger » Sat Feb 24, 2018 11:51 pm

I had 2 failures to communicate with the first generation spot. 1 from the bottom of an e-w trending canyon and the other was totally operator error. Last summer I purchased a gen 3 Spot and it worked flawlessly in sending an ok message (every night I remembered to send an "I'm ok message") I think the key is realizing that anything that can interrupt a signal that basically goes n to s can do so. I have been amazed at how accurate the locations of my "OK" messages have been--litteraly with in 10 feet or so.
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Re: How to use a SPOT device

Postby longri » Sun Feb 25, 2018 12:05 pm

oldranger wrote:I think the key is realizing that anything that can interrupt a signal that basically goes n to s can do so. I have been amazed at how accurate the locations of my "OK" messages have been--litteraly with in 10 feet or so.


The GPS in the SPOT works as well as any good GPS. That's not the issue.

The problem is knowing when your message isn't transmitted successfully. The device won't always tell you. If you're in a deep canyon or under heavy tree cover the device will often signal failure. And it's worth remembering that in those situations the SOS feature won't work (whereas a PLB stands a better chance). But in other situations you are just left wondering.

I had failures at Kearsarge Pass, White Pass, Frozen Lake Pass, Knapsack Pass, Sapphire Lake, Elba Lake, and Seldon Pass. None of these locations had a completely unobstructed 360° view to the horizon, but they all easily got GPS lock quite quickly. One might hypothesize that the Globalstar satellites simply don't have sufficient coverage. Or maybe it's a different problem that causes messages to be dropped. One poster on backpackinglight.com claimed that for a message to be successful a satellite must be simultaneously within view of both the SPOT device and a response center.

Another possibility is that my unit was faulty, although I'm certainly not the only one to notice reliability issues with a SPOT messenger.
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Re: How to use a SPOT device

Postby CAMERONM » Sun Feb 25, 2018 1:16 pm

Regarding reliability, I believe that the satellite network is inferior, and Alan Dixon points out that the transmit wattage for the InReach is 4x higher than for the SPOT. Perhaps bad weather can be as much a culprit as obstructed views?
Last edited by CAMERONM on Sun Feb 25, 2018 8:21 pm, edited 1 time in total.
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Re: How to use a SPOT device

Postby longri » Sun Feb 25, 2018 3:06 pm

According to GlobalCom heavy cloud cover can be a problem. They recommend having an 80% view of the sky for using a satellite phone. The SPOT uses that satellite network but it isn't a phone.

I don't fully understand the physics but I know the there are a number of factors involved including transmission power, transmission frequency, satellite coverage, and satellite altitude. The SPOT devices, Inreach devices, and PLBs all differ with each of those factors.

The lower frequency is an advantage with PLBs with regard to transmission through clouds but it isn't clear to me if it is actually significantly better overall in terms of likelihood of reception as many people claim.
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Re: How to use a SPOT device

Postby bobby49 » Sun Feb 25, 2018 4:55 pm

longri wrote: I don't fully understand the physics


You guys are hitting all around the problems. Part of the problem is that each individual user must understand the particular strengths and weaknesses of his device and the supporting satellite network.

Different devices have different transmit powers. However, if you think about it, different constellations of satellites are at different altitudes, so that is where the power differences go.

I can't say that this operation is correct for all devices, but in general it is necessary for (1) the device has to get a GPS position lock first. Obviously that requires different degrees of a clear view of the sky. If the device has been running continuously or at least very recently, then it will already have the ephemeris data to speed things up. (2) Then when the user presses some button or otherwise commands a transmission, the signal has to get out to a different set of satellites, and the view of the sky there is different from in the first step. Different devices will "keep trying" to transmit for different amounts of time. Some make a poor assumption that a clear view of the sky for the first step will mean a sufficiently clear view of the sky for the second step. That is simply untrue. Therefore, the user can be lured into a false sense of security that the device is working as expected.

Consider that GPS satellites are flying oblique polar orbits and are overhead for a moderately long time. The Globalstar or Iridium satellites that handle transmitted messages are overhead for a relatively short time. GEOS is different yet.

In complete contrast, if you are using a two-way texting device, then you can transmit a message that is fairly likely to get through. When or if you get a reply, then that confirms that your message got through, but lack of a reply does not.
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Re: How to use a SPOT device

Postby longri » Sun Feb 25, 2018 6:07 pm

bobby49 wrote:I can't say that this operation is correct for all devices, but in general it is necessary for (1) the device has to get a GPS position lock first.


That's not true for any of the devices we've been discussing. Both the SPOT and InReach devices are capable of sending messages, including SOS, without a GPS lock. A PLB can also signal without a GPS lock. Older PLBs didn't even have GPS.
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Re: How to use a SPOT device

Postby bobby49 » Sun Feb 25, 2018 7:36 pm

longri wrote:
bobby49 wrote:I can't say that this operation is correct for all devices, but in general it is necessary for (1) the device has to get a GPS position lock first.


That's not true for any of the devices we've been discussing. Both the SPOT and InReach devices are capable of sending messages, including SOS, without a GPS lock. A PLB can also signal without a GPS lock. Older PLBs didn't even have GPS.


If you read the rest of the paragraph, you will see that I was referring to normal operation. The reason that they do require a GPS lock before transmission is that sending an SOS message or a normal message without valid GPS location is often meaningless. The inReach will not send a normal message before getting the GPS lock. The inReach will appear to start sending the message, but it won't make progress until that GPS lock has happened.

A modern PLB is kind of a different animal, since it is used only in the SOS situation. If it has a GPS lock, it can send the SOS. If it does not have a GPS lock, then it can get the SOS transmission started with the hope and expectation that it will have a GPS lock later. Also many PLB devices have a short range beacon, in the event that rescue is near.
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Re: How to use a SPOT device

Postby longri » Mon Feb 26, 2018 9:26 am

bobby49 wrote:If you read the rest of the paragraph, you will see that I was referring to normal operation. The reason that they do require a GPS lock before transmission is that sending an SOS message or a normal message without valid GPS location is often meaningless. The inReach will not send a normal message before getting the GPS lock.

Yeah, you're right. I could've sworn I'd read the opposite but InReach and SPOT do in fact withhold the message if there's no GPS lock. But I disagree that sending a message without location information is often meaningless. When I would send an OK message with my SPOT I didn't really care if it included my exact location. Similarly, the couple I encountered who told me of closing a real estate deal with their InReach while walking the JMT also wouldn't have needed to give precise UTM coordinates to their realtor. It's a bit of a moot point though since obtaining a GPS lock is easier than transmitting to a satellite.

bobby49 wrote:A modern PLB is kind of a different animal, since it is used only in the SOS situation. If it has a GPS lock, it can send the SOS. If it does not have a GPS lock, then it can get the SOS transmission started with the hope and expectation that it will have a GPS lock later. Also many PLB devices have a short range beacon, in the event that rescue is near.

You're forgetting that PLB location can also be obtained, albeit with far less precision, from the satellites via doppler. It's one advantage of a PLB, particularly over the SPOT. And, lacking a GPS lock in an emergency, it would be the perfect time for an InReach to allow you to send a custom message anyway. I wonder why they disallow it?
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Re: How to use a SPOT device

Postby bobby49 » Mon Feb 26, 2018 11:56 am

longri wrote: Yeah, you're right. I could've sworn I'd read the opposite but InReach and SPOT do in fact withhold the message if there's no GPS lock. But I disagree that sending a message without location information is often meaningless. When I would send an OK message with my SPOT I didn't really care if it included my exact location.


That was the design decision made by the satellite messenger device companies. Sending without location is not always meaningless, but often so.

bobby49 wrote:A modern PLB is kind of a different animal, since it is used only in the SOS situation. If it has a GPS lock, it can send the SOS. If it does not have a GPS lock, then it can get the SOS transmission started with the hope and expectation that it will have a GPS lock later. Also many PLB devices have a short range beacon, in the event that rescue is near.


longri wrote:You're forgetting that PLB location can also be obtained, albeit with far less precision, from the satellites via doppler. It's one advantage of a PLB, particularly over the SPOT. And, lacking a GPS lock in an emergency, it would be the perfect time for an InReach to allow you to send a custom message anyway. I wonder why they disallow it?


No, I did not forget. That form of PLB location is inferior (inaccurate) to GPS location, and that's why it existed in the very early days, but it has become increasingly irrelevant for modern devices.
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