Google is our friend:
http://www.advrider.com/forums/showthread.php?t=282391&page=4
read the posts from Spotmaker
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Hi Adventure Riders!
I work for Axonn LLC, the company that designed and manufactures the SPoT. I am one of the engineers that designed it. I'm loving seeing the response that it is getting! Lots of folks seem to be excited about it.
Here are a few tips for it's use:
The "check in" message actually transmits THREE TIMES. It transmits in the following pattern: As soon as the SPoT has a GPS fix it transmits, then 5 to 10 minutes later (randomly dithered) it transmits a second time, then 5 to 10 minutes after that (random again), it transmits a third time. These three transmissions all have the same internal packet ID which causes the SPoT back office to throw out the duplicates and only send one to the emails and cel-phones on your check-in list.
As a result, this coverage map:
http://www.findmespot.com/explorespot/coverage.aspx
Shows the probability of a check-in message getting through!
How do you know when it's done sending all three messages? Simple, the "ok/check" LED stops blinking! So, it's best to send check-in messages from your base camp, or when you've stopped for a few minutes to admire a view. Activate check-in mode and leave the SPoT sitting with it's face to the sky until the ok/check LED stops blinking.
The coverage map also shows the probability of at least one out of any three consecutive help or 911 messages getting through (at 5 minute intervals, help and 911 will have transmitted 3 or 4 times in 20 minutes, depending on when you start measuring the time). Each help, 911, or track message is only sent once, but a new one, with fresh GPS data and a new packet ID (causing the SPoT back office to pump all of them through), is sent every 5 minutes for help/911 or every 10 minutes for track.
Offroute is exactly right in his assumptions as to what it means that the satellites are moving around in the sky. For a message to go through, at least one satellite has to see your SPoT, and see one of the SPoT ground stations! What this means is: if you've got a great horizon-to-horizon view of the sky, and you're not too close to the edge of the dark orange area in the coverage map, almost 100% of your "track", "help" or "911" messages will get through. If you have only a partial view of the sky, or you're starting to get near the edge of the dark orange area of the map, the condition of a satellite being in view of both your SPoT and the ground station happens somewhat less often, so fewer of your messages will get through.
But: since the satellites are moving around, time is your friend. If at one moment in time, there is no satellite in a "good" position for you, the odds are very good that 5 – 10 minutes later one will be! So, since the check-in message is actually transmitted three times, we've measured that throughout most of the US and Canada the probability that the check-in message will get through is around 99.99% (you'll miss something like 1 in 10,000). The likelihood that at least 1 out of any given 3 consecutive help, 911, or track messages will get through is about the same.
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n order to make the unit as 1)small, 2)light, 3)inexpensive and 4)power efficient as it is, some design choices were made that traded off raw acquisition time for smallness, price, and improved battery life.
There's also an internal delay of a total of 30 seconds after an actual GPS fix is obtained before we transmit for power management reasons. (So, when you see it transmit, it actually had a GPS fix 30 seconds prior.)
The satellite transmitter within the unit is a major power sucker when the messages are being transmitted to the satellite. This resulted in the other parts of the unit having to be extraordinarily power efficient to compensate.
The GPS subsystem in the SPoT uses an infinitesimal fraction of the power that the equivalent subsystem in a hand held GPS navigator uses, for example.
So, raw dead cold start on a SPoT is a little longer than the best of the newest GPS navigators.
What's a "dead cold start"?
That's when:
* The unit hasn't obtained new almanac data from the GPS satellites for more than 30 days or,
* The unit has been moved more than about 600 miles since the last time it obtained a fix.
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etc…
well a last tidbit which answers my one question
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As a result, that antenna is optimized for it's radiation pattern on the transmissions to Globalstar's network (otherwise, they wouldn't have certified it) and sacrifices a little bit of pattern symmetry on the GPS (compared to a dedicated GPS antenna). Interestingly, GPS gain is biased slightly in the direction of the buttons and LED's, ..
The transmit pattern, on the other hand, is an even broad cone-shaped pattern. It's very close to Globalstar's ideal example of what a simplex transmit antenna is supposed to do on their network.
Maybe, but be careful not to overdo it. The radiation pattern is very broad. With the unit flat on it's back, the pattern is quite even in a cone from about 10-15 degrees from horizontal to 90 degrees straight up. So, if you tilt it more than 10 – 15 degrees, you're doing little more than sending potentially useful RF energy into the ground.
Safest bet for optimum transmission performance is always to have the unit laying flat on its back. However, if you're at a latitude greater than 45 degrees north or so, a clear view of the southern sky is important. The further north you go, the more important a clear view to the south is and less important the view to the north is.
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The unit puts the GPS chipset into a "standby" mode between transmissions to save battery power. In this standby mode the Real Time Clock still runs and the ephemeris and almanac data are retained. As a result, after a shut off of only 10 minutes, the restart is what is known in the GPS world as a "hot start", and assuming the GPS signal strengths are at all trackable, it re-acquires them in about 1 – 5 seconds.
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