What does the future hold....


Alan Sewards <alan.sewards@...>
 

I am wondering what the outcome of this Dissemination Trial will be. It
could have a big impact on the attitude to the provision of high resolution
data, but on the other hand, commercial considerations may prevail. Let me
explain what I mean.

Early weather satellites (Tiros series) were polar orbiters, and carried
their own means of dissemination with them, largely because there was (and
is) no reasonable alternative for such satellites. The main data channel was
HRPT, but for the benefit of people without the resources to install a full
HRPT ground station, a low resolution signal was also produced and
transmitted using the APT channel, which could be received by a relatively
low cost receiving setup. (I am writing here about the situation when the
Tiros series was designed.) When a geostationary weather satellite was
designed, this same principle was carried through, in the GVAR and WEFAX
channels, and WEFAX was sensibly designed to be similar enough to APT that
much of the same equipment could be reused. Of course, in the case of the
GOES and Meteosat geostationary satellites, alternative means of
dissemination could have been used, but the authorities responsible for
these satellites decided to keep control of the whole thing and added
bent-pipe transponders to them which served as the dissemination channels.
The important thing to note here is that the high resolution channel carries
a much higher cost penalty for the user than the low resolution one,
although the differential has reduced a lot since the early days.

What the serendipitous loss of the SSPA on MSG-1 has showed is that this
high vs. low resolution differential is now meaningless from the technical
viewpoint of the communications channel. It is just as easy to disseminate
files containing HR data as LR data over the TV satellite channels now being
used for the trial. From this viewpoint, LR data, that is data whose
resolution has been artificially reduced so that it can be transmitted over
a lower bandwidth and hence lower cost channel, is quite unnecessary and
could be dispensed with.

This conclusion is reached from the viewpoint of the communications, but
there are also commercial aspects to be considered. From the Eumetsat
viewpoint, HR data is probably more valuable than LR data, and users may be
prepared to pay more for it - if this is believed to be the case, the
distinction between HR and LR data may be continued, even though the
original technical reason for the difference has vanished.

Will MSG-2 have an SSPA allotted to the job of data dissemination? Will the
SSPA be omitted or used as a backup for the really important jobs of
satellite control, telemetry and sensor downlinks? Is the TV satellite
dissemination approach the next generation, or will it be quietly dropped
after MSG-1 as it potentially upsets the charging basis for the data?

Alan Sewards
email: alan.sewards@computer.org
web site: http://asewards.free.fr

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