Damon,

They can certainly be frustrating when you have all these lids calling CQ and keying up who obviously cannot hear the satellite, but there is a ton of popular demand for FM satellites. I had a lot of people ask me what FM satellites were working at Dayton and at the hamfest I did a demo at in February. There are also a ton of callsigns that haven't been heard from on any satellite since the demise of AO--51.

I've worked several new grids and states on FM. I have two states only on FM - AR and DE. I've never heard anyone on SSB/CW from either state. I also wouldn't have worked Dominica without an FM satellite. They are a very useful tool!

Now, as far as the bad behavior on SO-50. At least 75% of that is due to people who can't hear the thing, but the Fox satellites are going to be 13 dB stronger than SO-50 (9 dB advantage due to the 2m downlink, about 4 dB due to the 750 mW power output compared to SO-50s 250 mW), so hopefully things will a bit more civilized when everyone can hear the satellite.

Of course, we should be encouraging those interested in satellite communications to add linear transponder capability to their stations. There are quite a few stations that never miss an SO-50 pass. Why not get on the linear birds? It's not that difficult or expensive at all and it's a lot of fun!

73,

Paul, N8HM

On Tue, Jul 22, 2014 at 10:49 AM, [email protected] wrote:

I hope someone can launch a 100 of them, single channel ,pileup ,total chaos ,someone keying down on the entire pass what more would you want. All we need to bust a crazy pile up is a 300 watt mfj amp right? Lets all work to the good of the satellite part of the hobby. SSB/CW birds are the only way to go and if you build it they will come . Now if someone can build a multi channel FM bird then do it. Lets face it guys ,everyone got mad trying to work AO51 and AO27. Was it fun making a call to a grid you wanted only to have someone key down on him coming back to you, Is this what you want , Then go to 20 meters DX. Single channel FM birds are obsolete, Damon _______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb

On 07/22/2014 07:49 AM, [email protected] wrote:

SSB/CW birds are the only way to go and if you build it they will come .

Think about what it would be like to tune for Doppler while working SSB through a LEO satellite on microwave.

Before you say "computer control", work out the effect of even a tiny error in your clock or orbital elements on a high elevation pass and remember that SSB has no carrier or pilot tone for closed-loop frequency tracking.

Sure, you could have a second receiver track the satellite's beacon. But if you're going to all that trouble, why not just have it track a single high speed data stream from the satellite, one that's much less affected by Doppler in the first place?

Tracking doppler on 2m and 70cm was hard enough at field day. I like 2GHz and up, but do not want to even think about the doppler.

In the KITSAT days, I used an IC-751 as the receive IF. It has a true FM detector, with discriminator centered at 0-volts. It steps in 1 or 10 Hz increments, even on FM. I wrote a simple radio-control program which would take data from InstantTrack, calculate frequency from that, and set the radio on the expected frequency at the beginning of the pass. Didn't matter if there was a little error, once AOS happened, the AFC put the receiver dead-on, and I got good data for the entire pass -- hands free. It was a joy to watch. And that was a simple digital mode.

The other thing that is exciting about going digital, is the idea of CAT-5 (or fiber) to the box at the antenna, not coax.

Digital will enable so much . . . .

73, Jim [email protected]

On 7/27/2014 1:24 AM, Phil Karn wrote:

On 07/22/2014 07:49 AM, [email protected] wrote:

...

SSB/CW birds are the only way to go and if you build it they will come .

Think about what it would be like to tune for Doppler while working SSB through a LEO satellite on microwave.

Before you say "computer control", work out the effect of even a tiny error in your clock or orbital elements on a high elevation pass and remember that SSB has no carrier or pilot tone for closed-loop frequency tracking.

Sure, you could have a second receiver track the satellite's beacon. But if you're going to all that trouble, why not just have it track a single high speed data stream from the satellite, one that's much less affected by Doppler in the first place? _______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb

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On 07/27/2014 09:57 AM, Jim Sanford wrote:

Tracking doppler on 2m and 70cm was hard enough at field day. I like 2GHz and up, but do not want to even think about the doppler.

At least not with analog modes.

High speed digital is very well suited to microwave in LEO as the link budgets generally support high data rates even with small antennas and low powers. And it's the ratio of the Doppler to the symbol rate (channel bit rate) that matters.

Doppler correction of analog signals is inherently difficult even with computer tuning for the following reasons:

1. Inexact knowledge of satellite position, especially for high-drag situations (small cubesats in low orbits).

2. FM can be closed-loop DC tracked if there no low frequency modulating components, but SSB has no pilot or carrier for closed-loop tracking.

3. Hard-to-model ionospheric delay variations.

Computer tuning of standard analog radios is problematic for the following additional reasons:

1. Little standardization (none across vendors) in radio control interfaces. Most seem to still use slow RS-232 type links.

2. No standardization in how the radios respond to a tuning command. How long does it take to settle on the new frequency? What trajectory does the instantaneous frequency take to get there?

3. If the signal is an appreciable fraction of the (usually narrow) receiver bandwidth, frequent retuning is necessary, aggravating the problems of #2.

A wideband software-defined-radio (SDR) front end operating on a well-designed digital signal is much better, especially when the A/D bandwidth is significantly greater than the signal bandwidth (as it usually is). Then only an occasional and relatively large tuning step need be made as necessary to keep the signal within the passband.

Most SDRs have fast USB or Ethernet interfaces so the demodulator can time each retune to occur when it will do the least damage, such as between frames after frame sync has already been acquired. Even if some damage is done to the signal, it can usually be corrected by the FEC.

Most SDRs produce a "complex" (two channel, quadrature, I&Q) digital stream that allows the signal to be converted directly to a near-zero-frequency IF without zero-frequency folding or aliasing. (This was a common problem with old analog direct-conversion receivers.) I.e., quadrature sampling distinguishes negative and positive frequencies.

The SDR doesn't need to actually keep the incoming frequency exactly centered at 0 Hz; it only need be somewhere within +/-0.5 times the sampling rate. For example, the Ettus USRPs being used at Arecibo and Bochum to receive the ISEE-3 telemetry signal use an effective complex sampling rate of 250 KHz. As long as the signal is somewhere between -125 kHz and +125 kHz, nothing is lost.

Like the vast majority of deep space probes ISEE-3 transmits residual carrier PM, so my first step is to locate the carrier with a FFT. It is typically around +16 kHz in the Arecibo signals, varying slowly due to Doppler as the earth turns. I then "spin down" the signal frequency so that the carrier is at exactly 0 Hz and on the I (inphase) signal axis. The BPSK data lands on the Q (quadrature) axis, with the sidebands symmetrical around 0 frequency. I do this by multiplying the input samples by a complex sinusoid, all in software, set from the results of the FFT. This gives me extremely precise control of the instantaneous carrier phase and frequency, with no uncontrolled jumps.

I included a hook, as yet unused, to "chirp" the digitally-generated carrier precisely in frequency to track slowly varying Doppler. Although I'm not actually using it yet, I could generate this chirp from calculations of what the Doppler should be (subject to the above limitations on knowledge of the orbit, the effects of the ionosphere, etc).

Not only is this a much better way to demodulate satellite signals, but it can be considerably cheaper too when you eliminate all the unnecessary components of a typical "conventional" receiver.

The ironic thing is that many modern ham rigs work much like this internally, but they were designed specifically for the traditional analog (and slow digital) modes and most don't give the user any easy way to gain access to the internal DSP mechanisms. They try hard to make their radios look purely conventional, and they succeed.

Phil