Some technical considerations related to Spot tracking performance

In another thread today, the issue of Spot tracking performance came
up. In particular, it was pointed out that several pilots at the
ongoing Senior's contest were experiencing long periods of missed
tracking reports. This is an area that I have personally investigated
recently. I gave a presentation at a recent Arizona Soaring
Association meeting on the subject. As Tuno pointed out in that other
thread, my presentation is available for download at my Wing Rigger
web site at this page:

http://www.wingrigger.com/wingrigger5_005.htm

Unfortunately though, that presentation does not inform well without a
lot of words to go with it. With that in mind, I will summarize some
of the key points here. I did a bit of analysis and a lot of long
duration experiments to arrive at some conclusions which I think are
pertinent to our application in sailplanes.

First, I think it is useful to understand that Spot is using the
Globalstar constellation of 40 satellites orbiting in low Earth orbit
- like 800 miles up. At any given point on the earth's surface at any
given time, there will likely be two or three satellites in view and
sometimes only one. Statistically, you must expect that the
satellites that you need at any given time are going to be very near
to the horizon. It would actually be quite rare to have a Globalstar
satellite in a 45° cone overhead for example. If you want to have
really good Spot performance, you must be thinking foremost about
having a good line of sight to the horizon -- the whole horizon. This
is not easy to achieve in a glider cockpit. The unit should be
located high in the cockpit in order to have a good view of most of
the horizon.

It is also useful to understand a little bit about the antenna in the
Spot unit. It is a patch antenna oriented horizontally as the unit
sits flat on a table. This antenna transmits preferentially into its
upper hemisphere and only weakly in the downward hemisphere
direction. In long duration ground-based testing on top of the gazebo
in the back yard of my house, I was able to demonstrate about 99%
throughput performance with the unit oriented flat, logo side up.
However, with the Spot unit turned upside down, throughput performance
drops to about 65%. This gives an indication of the relative
performance through the back lobes of the antenna. Ideally, you would
always want to keep your Spot unit perfectly horizontal.

Of course, gliders don't stay horizontal -- you have to bank them a
good portion of the time. If you think about a Spot unit banked at
45°, what that really means is that about half of the horizon will be
visible through the main lobe of the Spot patch antenna and about half
of the horizon is viewed through its back lobes. Furthermore, when
banked, a good portion of that weak back lobe energy will need to be
transmitted through portions of the glider structure which will
attenuate the RF energy even more. Carbon structures are much worse
than fiberglass in that regard.

This would certainly lead one to expect that a properly oriented and
well-placed Spot unit would perform much better while cruising as
opposed to tharmalling. To investigate that premise I analyzed two of
my recent five hour flights. I used SeeYou to examine whether or not
my glider was cruising or circling at Spot track transmission time
stamps (which occur every 10 minutes) as well as whether I was
cruising or circling at missing transmission times. That analysis
showed that for my mounting position, I am getting 98% message
throughput performance in cruise and 80% throughput while banked in
climb. For a typical flight of 70% cruise, my aggregate throughput
performance is about 93%. My mounting location is high behind the
headrest in my glider. This was the best location that I could come
up with from the point of view of track mode throughput.

My mounting position does not give me access to the unit during
flight; but that suits me fine. My concept is to turn it on and
forget it. I have better things to worry about than pushing Spot
buttons while I'm flying. I realize that many users are considering
that it could be important to have the Spot unit mounted on their
parachutes, and it might be. However, I don't suspect that there is
any good place to put the gadget on a parachute without significantly
diminishing the track mode throughput performance. I say this for
three reasons: it will be very hard to maintain the Spot unit
horizontally while attached to a parachute; spot unit will be
necessarily lower in the glider cockpit so that there will be more
airplane structured to attenuate signal; and the human body is a very
good absorber of RF energy at Spot's 1.6 GHz transmit frequency, so
close to your body will mean that a good portion of the horizon would
become blocked out.

As I weigh the likelihoods and the relative benefits of having spot
with me during a parachute jump as compared to the benefits of having
strong tracking mode performance, the latter wins hands-down. Having
reliable tracking mode performance means that I can expect a real ELT-
like benefit of having a feasible area to search for my glider should
the day ever come that I'm not heard from. Actual ELTs don't work
most of the time in crashes whereas Spot really will work. However,
if I were having 30 minute Spot gaps, then the potential search area
would become hopelessly large. It is primarily for that reason that I
consider it to be especially important to have good track mode
performance.

On Mar 9, 9:33*pm, Steve Koerner wrote:
In another thread today, the issue of Spot tracking performance came
up. *In particular, it was pointed out that several pilots at the
ongoing Senior's contest were experiencing long periods of missed
tracking reports. *This is an area that I have personally investigated
recently. *I *gave a presentation at a recent Arizona Soaring
Association meeting on the subject. *As Tuno pointed out in that other
thread, my presentation is available for download at my Wing Rigger
web site at this page:


I have just joined the SPOT community and have done some ground
testing and one flight. I received my SPOT the same day as I attended
Steve's interesting presentation so that was good timing.

Has anyone found a good place to mount SPOT in the cockpit so it has a
good view of the sky, is out of the way, not an obstruction to outside
vision, and easy to configure before flight. Like Steve, I don't mind
if I can't reach it in flight, but unlike Steve I can't mount it on
the headrest as it would be under the turtledeck as my seat back is
fully aft. Although there is a glass RF window in the turtle deck of
the 28 I think the front arch is carbon.

For my first flight SPOT was clipped to my parachute chest strap and
was not in an optimum orientation. As expected I lost many reports.

I am wondering if it would work under the glareshield between the
panel and the nose. That position would be high up but it would have
to transmit and receive through a layer of glass. The forward
position should mean there would be less blocking by the wings when
thermalling than for a headrest mounting.

So what has been tried and what works?

Andy

On Mar 10, 2:41*pm, Andy wrote:
So what has been tried and what works?

On my ASH-26E, I installed the spot on the canopy rail right next to
my right shoulder. It's got a good view of most of the sky, and I can
verify the LEDs are blinking. So far, I get good tracking results. I
removed the belt clip and attached a little bit of heavy duty hook and
loop material to keep it attached.

This should work on all recent Schleicher gliders as they have that
nice wide sill back there.

-Tom

On Mar 10, 4:50*pm, 5Z wrote:
On Mar 10, 2:41*pm, Andy wrote:

So what has been tried and what works?

On my ASH-26E, I installed the spot on the canopy rail right next to
my right shoulder. *It's got a good view of most of the sky, and I can
verify the LEDs are blinking. *So far, I get good tracking results. *I
removed the belt clip and attached a little bit of heavy duty hook and
loop material to keep it attached.

This should work on all recent Schleicher gliders as they have that
nice wide sill back there.

-Tom

Wow - thanks to everyone, especially Steve, for this very detailed
overview.

FWIW, on my LS8, I could easily mount the spot similar to an old
Camera (remember those) using whole pre-drilled into the canopy frame
and a small piece of bent aluminum. I expect many other ships would
be like that...

On Mar 10, 1:50*pm, 5Z wrote:
On Mar 10, 2:41*pm, Andy wrote:

So what has been tried and what works?

On my ASH-26E, I installed the spot on the canopy rail right next to
my right shoulder. *It's got a good view of most of the sky, and I can
verify the LEDs are blinking. *So far, I get good tracking results. *I
removed the belt clip and attached a little bit of heavy duty hook and
loop material to keep it attached.

This should work on all recent Schleicher gliders as they have that
nice wide sill back there.

-Tom

I fly with my SPOT on the canopy rail of my 26E, more forward than
Tom's. Just on top of the rail behind where the canopy open/eject
levers are. Held on with 3M adhesive "mushroom" tape. It is handy to
be able to check the LEDs and confirm it is indeed in tracking mode
and you just did not accidentally send a single OK message (i.e. 20 or
so minutes later the green LEDs are still blinking in unison). In the
Duo Discus I attach it to the rear instrument pedestal. I get reliable
SPOT tracking in both cases.

In general I'd want the SPOT where I can reach it, to check on it's
operation, and if needed to send a help or 911 message.

Those folks with them on their parachute harness with problems with
missed tracking points need to move them (or live with the trade of
that having it mounted there may cause lots of missed tracking points.

Remember that unlike other SPOT messages a tracking (aka SPOTcast)
messages is *not* resent, it is a one-shot attempt every 10 minutes.

Darryl

On Mar 10, 5:05*pm, Papa3 wrote:
On Mar 10, 4:50*pm, 5Z wrote:

On Mar 10, 2:41*pm, Andy wrote:

So what has been tried and what works?

On my ASH-26E, I installed the spot on the canopy rail right next to
my right shoulder. *It's got a good view of most of the sky, and I can
verify the LEDs are blinking. *So far, I get good tracking results. *I
removed the belt clip and attached a little bit of heavy duty hook and
loop material to keep it attached.

This should work on all recent Schleicher gliders as they have that
nice wide sill back there.

-Tom

Wow - thanks to everyone, especially Steve, for this very detailed
overview.

FWIW, on my LS8, I could easily mount the spot similar to an old
Camera (remember those) using whole pre-drilled *into the canopy frame
and a small piece of bent aluminum. * I expect many other ships would
be like that...

That should read "using the holes pre-drilled into the canopy frame"...

Andy:

My opinion is that immediately under the glare shield probably would
perform well.

It just seems like that would be annoying as far as getting at it to
turn it on and off. Maybe if you made a cut out in the glare shield
so that the unit would mount to the underside of the cut out plate and
would therefor just drop into place. Of course that would require
some type of fastening for the cut out plate. Maybe the cut out plate
would be hinged along the forward edge??

I like mine attached to me in case I have to bail.

http://picasaweb.google.com/lh/photo...eat=directlink


"Steve Koerner" wrote in message
...
Andy:

My opinion is that immediately under the glare shield probably would
perform well.

It just seems like that would be annoying as far as getting at it to
turn it on and off. Maybe if you made a cut out in the glare shield
so that the unit would mount to the underside of the cut out plate and
would therefor just drop into place. Of course that would require
some type of fastening for the cut out plate. Maybe the cut out plate
would be hinged along the forward edge??

I have mine on my parachute chest strap and I am seated reclined, so
the front face is about at a 45 degree angle. I get pretty good
tracking results.

You said you tested it with the logo side up and down. I assumed that
the device was designed with the belt strap on the back side so that
the logo would face horizonally / the horizon. It appears as they
intended it to hang vertically. Did you test it in this orientation?

Chris

On Mar 10, 7:56*pm, wrote:
I have mine on my parachute chest strap and I am seated reclined, so
the front face is about at a 45 degree angle. *I get pretty good
tracking results.

You said you tested it with the logo side up and down. *I assumed that
the device was designed with the belt strap on the back side so that
the logo would face horizonally / the horizon. It appears as they
intended it to hang vertically. Did you test it in this orientation?

Chris

The SPOT messenger is a fantastic product, but because of it's
internal technology, and in spite of it's poor industrial design.
There must have been a disconnect somewhere between the product
management, the ID team/design house and engineering. And it's likely
literally this type of issue as the engineering is all done by Axonn a
separate company than SPOT and Globalstar and I'm sure SPOT or Axonn
contracted out the industrial and mechanical design work.

So for the antennas (both the GPS and Globalstar) a planar antenna
pointing at the horizon is far from optimal, half the usable
beamwidth would be lost in obstructions below the horizon. It is
actually worse than that simple model because of satellite orbit
geometry and satellite antenna footprint. So the belt clip is designed
to hold the unit, but not reallly while it is in use. Pretty stupid
huh.

The lack of though about how to mount the unit on a car dashboard or
other location, the overloading of the button functions, the
overloading of the dual blinking LEDs, different LED patterns that
mean things that are not clearly explained are other examples of poor
design.

However the SPOT messenger is a great device (I really like mine) and
hopefully SPOT will correct some of the ID snd usability weaknesses in
future products, and/or provide products more tailored to different
uses/markets.

Darryl