Satgen408  Iridium Mobile Phones  by GM4IHJ   (BID SGEN408)  17 Jan 97

Presently delayed by a minor fault until after 19 Jan 97. The first 3
mobile phone satellites of the anticipated 60+ IRIDIUM constellation ,
should go into space soon. The Iridium system calls for the satellites
to be in 750 km altitude, 99 minute period low earth orbits, using the
frequency band 1616 to 1626.5 Mhz , for both up and down links. Voice
2.4 Kb/sec and, Data at 2400 bps QPSK modulation, will be accomadated
using both Frequency and Time Division Multiplexing , to separate the
many concurrent users.
There are a large number of problems which must be solved by the
Motorola Iridium designers if this satellite network is to succeed.
Claims for it have been very ambitious, and includes seamless
switching and passing of calls from one satellite to another ,with
transfers between satellites occuring automatically as one satellite
goes out of the users range and another satellite comes into range,
plus daisy chaining of several satellites to carry a call from the
limited service area of any one satellite ( 2900 kms radius )through
several other satellites to a distant recipient. All this being
achieved despite 70 Khz + of doppler shift and doppler rates of nearly
20 khz/minute , at closest approach. The solutions to these problems
could give pointers to the path amateur radio satellites will follow
in the near future.
What has emerged so far , is not exactly promising. Prices being
touted for the mobile phone handsets are around $3000 per unit. Very
different from the "free phone if you sign up for our terrestrial
mobile phone network", that folks here are used to. Clearly at a price
greater than that for 5 large colour TV sets, the handset is complex
and shows a clear departure from normal practise  of making each of
the millions of handsets , cheap, and putting all the complex
expensive electronics in a few costly satellites.
Has this situation been forced on the manufacturers by the practical
realities of operating digital equipment in an unfriendly, radiation
hazardous space environment ? Amateur radio experience with digital
satellites over the past 13 years, suggests that the weak link is the
large number of microchips required to operate the satellites complex
protocols , whilst the satellite is being subjected to frequent
radiation induced soft hits, corrupting onboard memory. Iridium is
likely to require a very complex protocol to handle hundreds of users
, sat to sat transfers and satellite chains for long distance calls.
Have the designers had to compromise and put a lot of the software and
associated memory into the mobile phone itself, rather than risking it
in the much more demanding satellite environment ?
Time will tell (via Iridiums Mean Time Between Failures) , just
whether or not they have got it right. Meanwhile OrbCom the Store and
Forward commercial system seems to be in limbo. Two satellites
launched 18 months ago, but no more have followed to complete the
forecast 26 satellite fleet, and still further back in time scale the
rivals to Iridium (Globalstar,Odyssey, Aries, Ellipso and Teledesics),
all of whom propose , what look like, simpler systems than Iridium.
Have yet to announce firm launch dates.
No doubt they , and many radio amateurs will watch Iridiums progress ,
with interest.