Re: Details on the New SPOT 2

On Aug 5, 10:53 pm, Darryl Ramm <darryl.r...@xxxxxxxxx> wrote:
On Aug 5, 10:30 pm, Eric Greenwell <flyguy...@xxxxxxxxxxxxx> wrote:



Martin Gregorie wrote:
On Wed, 05 Aug 2009 11:11:47 -0700, Andy wrote:

On Aug 5, 10:10Â am, MarkHawke7 <markhaw...@xxxxxxxxx> wrote:
I'm trying to get a direct answer from my contacts at SPOT
Mark,

Since you have contacts there I'd be interested to know why they changed
from 2xAA to 3xAAA cells.  Was case size the only consideration, or
perhaps the increased voltage increases the uplink TX power?  In any
event I'd far rather have the longer life of AA cells than a smaller
case size.

I suspect we're in the minority and Joe Hiker would far rather have small
and light than long battery life.

Hopefully the Mk 2 will provide the best of both worlds, which would be
either:
- add an external power socket while retaining the 3 x AAA internal
  cells

- replace the AAA cells with a 1500 mAh LiPoly rechargable and
  a mini-USB charging socket. Alkaline AAA cells are 1250 mAh.

My SPOT will provide over 250 hours of tracking with the two AA cells,
which is more than a years flying for me. How many hours will the new
SPOT last?

Frankly, I don't care how many batteries they use or what the amphour
capacity is, as long they last a "long" time and it warns me when there
are 20-30 hours life left. I have no interest in external power or
rechargeable batteries, because that would not be simpler, more
reliable, or cheaper than the present system. Adding altitude and
cutting the subscription costs are the only priority items for me.

Eric Greenwell

Eric

I expect cutting subscription costs and adding altitude are mutually
exclusive.

Reporting ~2-3x (depending on how they do this) the data in track
mode, as the new model will (repeat sending the past two position
reports) , may reduce the battery life, so also having a reduced
battery capacity compared to the first generation is worth noting.
They also do things that may lower power consumption, so the only take
away is this is something again we'll want to check out.

I would like to see tracking at reporting frequencies on 1-2 minutes,
that combined with reporting additional altitude data would again
reduce battery life. At that point I'd expect a device that was
primarily powered from the ships power would make sense.

Darryl

OK I've better modeled power consumption of the SPOT messenger. I
measured the actual current draw of my first generation SPOT messenger
in track mode.This was done with a logging multimeter which can see
the LED power consumption spikes and they are averaged out. In track
mode the messenger looks like it is in a dormant state for most of the
10 minutes before sending a message, it wakes up some time before the
message is ready to be sent and runs the GPS to get a fix, how long
the GPS stays on varies but it looks around 20 seconds on average.
Presumably if it has problems getting a fix it will run for longer.
With the GPS running the consumption is about 35mA. Actually sending
the message over the Globalstar modem consumes ~530 mA for about 1.5
seconds. All these numbers are rough, I'm not interested in being too
precise since there is a lot of hand waving involved.

So it is easy to model what power consumption would be for sending a
message very 5, 2, and 1 minute by just assuming the GPS and
Globalstar loads occur more frequently at those periods. To model also
sending altitude data I ignore any additonal processing or longer fix
time that may bre required by the GPS engine and just assume the only
cost is doubnling the Globalstar modem load per message (since it
likely must send two packets of data to encode location and altitude
as I've mentioned in a separate post). Crunch those numbers and I end
up with.

Guesstimated Track Battery Life (DAYS): (SPOT "1" with 2 x AA
Batteries)

Track Reporting Period 10 min 5 min 2min 1min
Position 14.2 10.6
5.6 3.1
Position+Altitude 8.3 5.6
2.7 1.5

Of course the only number here that is meaningful today is the 14.2
days of battery life for transmitting position every 10 minutes, which
is all that the first generation SPOT messenger actually does
(Convenient how I fudged the numbers to give 14.2 days, SPOT claims 14
day battery life for the first generation SPOT messenger in track
mode).

Now that was all assuming battery capacity and GPS power load etc. of
the first generation SPOT Messenger. As discussed before the new SPOT
messenger has a different GPS chipset, although I suspect it has the
same Axonn STX2 Globalstar Modem. So going out on a limb, if you
assumed the same GPS and STX2 power consumption for the new model
messenger and factored in the fact that the STX-2 in the new model
messenger has to transmit three location packets with each position
report and it has a reduced battery capacity 60% for the change from 2
x AA to 3 x AAA lithium batteries. The above table would look like...

Guesstimated Track Battery Life (days): (SPOT "2" with 3 x AAA
Batteries)

Track Reporting Period 10 min 5 min 2min 1min
Position 3.3 2.1
1.0 0.5
Position+Altitude 1.3 0.8
0.3 0.2

Again the only number that makes real sense today is whether the
actual track battery life of the second generation SPOT unit will
really be around 3.3 days or not. These numbers may be significantly
off, I hope it is better than this. The messenger may be significantly
more power efficient than I've assumed here. Axonn (who design the
actual hardware) may well have reduced power consumption from the GPS
chipset, Globalstar modem and other components. If somebody want to
send me their brand new second generation SPOT messenger I'll be happy
to pull it apart and have a poke around.

But the point is for people who want high frequency reporting, say
once every few minutes and altitude reporting I think you are in an
expected battery lifetime where you are going to want SPOT to design
the device for external power. The reason for that is not battery cost
(getting as high as several dollars per day of tracking time for the
worse numbers above) it is the "oh crap" cost of forgetting to check
the battery and replace them or forgetting to carry spare batteries.
We'll see far too many of these failures if the battery life is a
total of a few days.

Anyhow that's the back of my envelope and I'm sticking to it.

Darryl

.

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