[Namaste-dev] Re: Fw: Modulation Baseline

Phil Karn karn at ka9q.net
Mon Jun 16 14:27:47 PDT 2008


I'm finally plowing through the Namaste-dev mailing list...

> 1. Why hop the uplink?  Up to now, I've thought we were in a noise limited
> environment.  Is there the expectation of narrow band interference?

I am not saying we're not in a noise-limited environment, at least not
yet. I just wanted to get the alternative on the table.

> 2. It really isn't FDMA if you are hopping, but FH-CDMA, right?

Yes.

> 3. Who's looking at control requirements - user identification, user to user
> signaling, hop sequence assignment, timeslot to FH matching, duplex vs.
> simplex, maybe power control, group calling and so on?

We all are...

> 4. Do you envision the uplink hop rate and data rates to be variable to
> match the modulation alternatives on the downlink?

The purpose of a "smart" transponder is to disconnect the uplink and
downlink requirements. There is no reason for them to use the same
speed, modulation, coding and multiple access. In fact there are very
good reasons for them to be different, as they are very different problems.

> 5. LPI advantaged modulation on the uplink may give regulatory authorities
> in some countries pause.  I'd guess the capability for monitoring by a
> regulatory authority will be a minimum requirement, assuming the use of the
> equipment is permitted.

Maybe. In the US the FCC is on record that we can do whatever we like as
long as we document it and don't deliberately encrypt.

> 6. The fallback safe mode is pre-deployment or dynamic after launch?

Every spacecraft needs a basic instinct for self-preservation. It should
not be possible to command the spacecraft to commit suicide. (I exclude
propulsion systems that might cause reentry, impact or earth escape if
used incorrectly. A deliberate re-entry at end of life may be desirable.)

At the top of the list is maintaining a positive power budget. It's like
breathing. Otherwise it dies as soon as its batteries do.  Heavy loads
that threaten to deplete the batteries must be dumped automatically. If
the spacecraft is covered with panels, then keeping them in sunlight is
fairly easy. Steerable panels may need an autonomous capability to find
and orient towards the sun.

Next is thermal management, so it doesn't burn up or freeze. Most
satellites in low earth orbit can use passive thermal control, meaning
it's enough to apply the proper thermal blankets and coatings before
launch. There's a regular day-night cycle. There's a lot of radiation
coupling with the earth, which covers nearly half the sky at a nice
moderate temperature.

High altitudes are trickier. You tend to stay in the sun for a long
time, though you may have long eclipses at certain times (e.g,.
geostationary satellites during spring and fall). You can't rely on the
earth as a thermal buffer. Three-axis spacecraft of any size usually
require active heat transport from the hot side to the cold side. This
can be done with passive heat pipes. Otherwise you'll need active
thermal control ranging from a simple "barbecue roll" as done on Apollo
to actively controlled louvers and heaters.

One trick used by most AMSAT satellites is using solar photon pressure
to provide a torque to keep the satellite in a slow roll, so thrusters,
momentum wheels and magnetorquers aren't necessarily required. Care is
needed here; at low altitudes the solar torque is eventually balanced by
hysteresis drag in the earth's magnetic field. At high altitude the
spacecraft might speed up without limit.

Third on the list is maintaining ground command capability. That means
keeping the command receiver on and an antenna in position to hear
earth. If the altitude is low enough and/or enough EIRP is available on
the ground, an omni may do. If a directional antenna is required to make
the links work, autonomous attitude determination and control may be
needed. (An increasingly viable alternative is an array of medium gain
antennas covering the whole sky, each with its own command receiver. Or
maybe a phased array of low gain antennas.)

If the command demodulator feeds a computer, then there needs to be a
reliable hardware reset mechanism and either a basic boot monitor or
some sort of bootstrap to get the computer reloaded with software and
running again.


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