And, in a related question, wouldn't more proccessing demand more power from the batteries/solar panels?  I know my PC cetainly draws a lot more power when the CPO is working hard.  Just curiosity questions, I support digital, FM, and Linear satellites.  If it were not for FM sats, I would never have gotten interested in satelites at all.  The congestion on the FM sateliltes drove me to start using linear satellites.

Doug K9DLP

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From: Dave Marthouse [email protected] To: [email protected] Sent: Tuesday, July 22, 2014 10:41 AM Subject: [amsat-bb] Digital Satellites Question

I have been following the digital satellite discussion with great interest.  Both sides have valid points.

I would like to pose a strictly engineering question.  Keep in mind that I'm not an engineer but consider myself reasonably well read on the amateur satellite world.

A digital satellite would imply loads of processing power on the satellite.  I would assume that with this additional activity that there will be more hardware on the bird with more complexity as this won't be a bent pipe system.

With more physical hardware of much more complexity in orbit what about the radiation hazards to this more complex and physical hardware heavier system?  Wouldn't a software based system be prone to radiation induced hardware and software glitches? What about shielding etc?

Dave Marthouse N2AAM [email protected]

-- Dave Marthouse [email protected]

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On 07/22/2014 08:41 AM, Dave Marthouse wrote:

A digital satellite would imply loads of processing power on the satellite. I would assume that with this additional activity that there will be more hardware on the bird with more complexity as this won't be a bent pipe system.

Not necessarily. Yes, it will rely on onboard computers -- but so do the many analog satellites that use them for command and control. When the IHU memory on AO-10 failed after 3 years in a high radiation orbit, the analog transponder continued to operate but there was no way to repoint the antennas and solar panels or to change the operating mode or power system setpoints.

With more physical hardware of much more complexity in orbit what about the radiation hazards to this more complex and physical hardware heavier system? Wouldn't a software based system be prone to radiation induced hardware and software glitches? What about shielding etc?

Radiation usually isn't a serious problem in LEO, though relatively simple precautions like error-corrected memory are still a good idea. With 14-16 day-night cycles per day, the usual problem is battery wearout.

And for low altitude cubesats, atmospheric drag usually trumps everything else. Small objects tend to have small ballistic coefficients making them more susceptible to drag. Even ARISSat-1, which was larger and heavier than a cubesat, decayed in only a few months. Most cubesat launch opportunities are to low altitudes, partly because that's where the ISS flies, and partly because of the increasing pressure to minimize space debris by keeping them out of the longer-lived altitudes.

The biggest load in a communications satellite.... is almost always the downlink RF power amplifier(s)...

True for some, not true for others. The locations of ham operators are only about 10% of the earths surface. 90% of transmitter power can be saved if the transmitter is not left on 100% of the time.

An APRS satellite takes advantage of this by knowing not only are there users only 10% of the time, but even over a saturated ham area, the TX/RX duty cycle cannot exceed 40% or so. The result is that the transmitter is only on for 4% of the orbit. This means we can run a 5W transmitter to make the packets hearable on an HT with a stock antenna, but that the transmitter AVERAGE power is only 200 mW (less power than the receiver).

And by putting the downlink on 2m instead of UHF, there is another 9 dB advantage to OMNI anennas. Add that to the 10 dB advantage of running a 5W transmitter instead of 0.5W and you can see that an APRS cubesat can have almost 19 dB downlink advantage over the typical UHF 0.5W downlink. (theoretically)...

Many other factors of course are involved, but we need a mix of satellites and modes, but the above is why I like AX.25 for short bursty texting and data for a large number of people to share in a short period of time.

Bob, WB4APR