mgrant0 wrote [posted by Geoff G.]:
> I was at Telecom99 last week and I had the opportunity
> to use Iridium, Globalstar and Inmarsat. In short, I
> was not impressed with either Globalstar or Iridium.
>
> I was expecting the phones to look exactly like GSM
> phones. They're not! All the phones are huge and have
> towering antennas which must be extended above your
> head in order to work. Furthermore, you really need
> to be outside in the open.
Here's a thought: do you think that maybe, just maybe,
that the handsets work just fine, but that this person
has formed unrealistic expectations of the service?
Why was this person assuming they would look just like
GSM phones? He's darn right they're not, they operate
using satellites some 2000 - 4000 km away! Realizing
this, I would *expect* they handset to have rather
large antennas. It's unrealistic to assume otherwise.
Note, the author expresses no concern about Inmarsat
phones being as big as a briefcase! Why? Because he
doesn't have any other expectation out of them. So,
from the outset, he's already formed the opinion that
it's okay for Inmarsat phones to be huge, but it's not
okay for LEO phones to be slightly larger than GSM
phones. hmmmmm . . . .
> I then proceeded to try out the 2 other phones that
> were floating around for G*, all of them were just
> as bad. The call dropped at least once while standing
> still. I write that off to an incomplete
> constellation though.
>
> Later in the day, I called a friend of mine in Boston.
> The connection was so doppler shifted that he refused
> to believe who he was talking to!
This really puzzles me. G* uses a CDMA signal that is
digital. How can a digital signal be Doppler shifted?
The RF signal from the satellite is analog, but when
received by the handset, it is digitally decoded. So,
the digital signal is either decoded correctly or not
at all - meaning the conversation would be clear or
choppy. There is really no way a digital signal can
have a warbling Doppler shift.
Furthermore, if Doppler were a problem (which again,
I'm pretty sure can't happen) it would most certainly
happen on *every* call, since the satellites are always
moving, regardless of what the person with the handset
is doing. The observation that some calls "had
Doppler shifting" and others didn't cannot be correct.
Lastly, I've used a G* phone. I got to use it for a
whole week. Yes, it did drop calls from time to time,
but it never had any "Doppler shifting". The only
imperfections in the call quality I observed were
short choppyness in speech. And when I say "choppy",
I mean losses in speech on the order of miliseconds.
As a whole, the voice quality was great, and most of
the time, the party on the other end did not even
know it was a satellite call until I informed them.
> Iridium is available everywhere on the planet. It
> will switch a call from one satellite to another
> then eventually down to a ground station. Globalstar
> is only available in certain places where the
> satellite can see a ground station, it does not switch
> between satellites, only up and then down again.
> Globalstar's coverage is ok if you're on land but
> they don't cover much of the ocean at all.
Again, what is the author's expectation of "ocean
coverage"? G* will cover oceans. In some places, it
can cover several hundred miles offshore. Can you
say the same about GSM? How often does the average
person travel more than 500 miles offshore?
In other areas, an entire ocean will be covered.
The entire North Atlantic shipping route from North
America to the British Isles can be covered. The
entire Yellow Sea will be covered. So will the
Adriatic and Mediteranean. So will the East China
Sea, the North Sea, etc., etc.
The statement that G* won't "cover much of the
ocean at all" is not based upon any coverage maps.
It's a personal statement showing his lack of
geographic knowledge, or ability to think on a
order-of-magnitude scale.
> Just for fun, I also test drove Inmarsat. Inmarsat
> is GEO based. Their system has been fully functional
> for years. They don't have hand-held telephones but
> they do have briefcase models, most notably the
> mini-M which I had the pleasure to use. The call was
> crystal clear, went through in seconds and the delay
> was was actually less noticeable than the LEO systems!
Again, it was a "pleasure to use" a briefcase phone, but
not a phone slightly larger than a GSM phone? Was it
a "pleasure" to set up the briefcase phone and make sure
it was pointed south, or was that already done in advance
for him?
And a voice delay less than a LEO with a GEO? Without
getting into a discussion of orbital altitudes and the
speed of light, that statement is complete bunk.
----
For the record, I am very interested in hearing the pros
and cons of G*, but I think it needs to be done in
context of the specifications of the system. Yes, the
LEO phones are bulky compared to GSM. Yes, the Inmarsat
system works well, but not with its own limitations.
But, if you're going to compare systems (G* and Inmarsat),
compare them *both* to GSM, not just one!
It's okay to be a fan of Inmarsat, but it seems unfair
to bash LEO systems by comparing them to a different
standard, and advance your argument by making up facts.
-Dr.E. |
