AMSAT-NA The Four Seasons of AO-13

by William Roth, N7RYW. from the AMSAT Journal, March/April 1995.

For the last few months, Oscar 13 has had a bad attitude. Most of us have heard the reasons why, that the sun angle is bad because of the orbit. You might have also heard that the orbit is changing and will lead to a serious case of burnout late next year. The details, however, are not very easy to understand so I will see what I can do to make the information useable. I will also cover an area of continual concern, and that is Transponder Loading and Mode Schedules.

So, why is it that the satellite has these problems? Why do the command stations tell it to point off into the distance half the year? Why does the Sun not shine on it the way it did before? Why am I asking you, when I am supposed to be the one providing the answers!?

The most helpful thing for me to gain a real understanding of the elliptical orbit was for me to stop using references to any points on the Earth. When I pictured the Earth as a featureless sphere, it all came much more into focus. So, I'll ask that you do just that, too. Picture a ball (the Earth) and nothing else. Then we will add the sunlight and AO-13's orbit to the picture. This is how it looked a few years ago:

As you can see, the orbit is rather sideways to the sunlight. This makes it easy to get the solar panels lit, since they are on the sides, and which is why this type of orbit (high inclination) is used. As we have heard, there have been changes to this orbit. These changes will eventually mean that the satellite will get real warm at the end of next year. The changes to the orbit are much more than just getting a little closer to the atmosphere as time goes by. The orbit from the same view would now look like:

Now, that's quite a change! As you probably know, the best pointing angle for communications is 180/0, or at the center of the Earth at apogee. This is also straight along the centerline of the orbit. Unfortunately, this means that the Sun is right behind the bird, where there are no solar panels to keep the batteries charged.

One more aspect to know about is the centerline of the orbit. It is stationary, not in relation to the Earth or Sun but to the Galaxy! There is a slight amount of drift, but it is very small, and we don't need to account for it here. Since the Earth orbits the Sun, the result is that the sun is aligned with the orbit centerline twice each year. Again, a picture is worth a lot of words:

AO-13's solar panels are around the sides of the spacecraft. This means that the Sun needs to be off to the side (or 90 degrees from the antenna pattern) for the batteries to be charged. With the attitude of 180/0, you can see that the Sun angle is good at the top and bottom of Figure 3. At the left and right, however, the Sun would be shining on the antennas and motor of the spacecraft, and no light would shine on the solar panels. So at these times the spacecraft is commanded to change its attitude to point the antennas away from the Earth at apogee. This has been to 230/0 lately, which allows for at least some communications.

Also note on the right that at the highest point of the orbit, the satellite would be in darkness from an Earth eclipse. At these times, the transponders are turned off to keep from discharging the batteries.

In this figure, the top depiction is if the satellite were to be at 180/0, with the Sun shining on the motor. The bottom one is at 230/0, and there is some light on the panels, while the antennas are pointed somewhat away from the Earth at perigee. This is shown from above the orbital plane, not from the Equator.

The changing of the orbit continues. If you want, you can watch the Argument of Perigee parameter in the keps as time goes by, and you will see that the perigee of the orbit keeps moving north. It passed the Equator a few months ago. This means that our neighbors in the Southern Hemisphere will be getting better and better pointing angles, while we in the North will be getting worse. It used to be that we could not communicate with Australia very well, but we now can. I have had talks with Graham, VK5AGR on Mode S and I must say that I like the extra opportunities.

Speaking of Mode S! The change in attitude to 180/0 will be coming up shortly, and I want to make an important point or three. I'll relate what happened the last time...

When AO-13's attitude was put back to 180/0 in late December, I jumped back on Mode S, hoping to do some real talking with clear signals. I'm not into static, so I cherish the 2.4 GHz talks on Mode S, especially the long, clear ones. With a 2 meter dish and automatic tracking, the quiet mode is really sweet. Or is it?

I tuned around, only to find multiple dead carriers sweeping the band with FAR too much power. One bloke was clearly holding the Dits on his paddle on the same frequency, trying to tune around with his receiver. The effect on the transponder was disastrous. It sounded like a machine gun, as the AGC clamped at every dit, and let go between. Everyone's signals were hammered, as there is only one AGC circuit that covers the entire uplink passband.

At first, I suspected that this CW signal was the result of someone not knowing the mode schedule. At times, we have heard about the importance of checking the mode schedule before transmitting to AO-13. It seems that not all have heard this request, or don't understand it, or simply ignore it. We have also been in "Groan" mode while the bird's attitude (and therefore our own) is off in a bad direction. So, it seems that it is time for an explanation of just why we are asked to check the schedule.

"Why check the schedule? Why can't I just hold my key down and see if I can find my signal? " It would seem that you could do just that, as the uplinks for B and S are separate. There are a couple of good reasons why not. First, there is a problem with the bird. The Mode B uplinks are mixed in with the Mode S uplinks, then both are transmitted on 2.4 GHz. This is true even when Mode B is off! The only way around this is for ALL users to look before they jump! Check the schedule! This is good common sense anyway. It is very impolite (to put it too nicely) to just start transmitting on top of someone and hope you can find the signal.

Checking the schedule is the first thing I do when I see AO-13 is up. The current schedule is available in many places. If you have Internet, both the amsat-bb mailing list and rec.radio.amateur.space have regular postings, as well as the www.amsat.org web page. Don't have Internet yet? Packet bulletins are sent regularly. Don't have Packet? The AO-13 schedule is sent on AO-13 in CW, RTTY, and 400 bps psk on the beacons.

Speaking of beacons, there may be some confusion as to what mode is operating with what beacon. Some are hearing the 145.812 MHz beacon and assuming Mode B is working. Not true! Due to a design factor, the 145 MHz beacon (instead of 2400 MHz) is operating while Mode S is on. This is why just hearing the B beacon is not enough. It can be on while the B transponder is off. Again, check the schedule!

Not checking the Mode Schedule is not the only problem, however. For the entire time that AO-13 was at 180/0, a CW station was up hammering away at Mode S. Many of us tried to ask them to reduce their power, but to no avail. I suspect that they were unable to hear a voice signal. So, it seems that there needs to be a bit of educating on just how the AGC works on Mode S. This is from direct observation over a 3 month period.

The Automatic Gain Control circuit comes into play when a signal (any signal) received is excessive. If the receiver hears 6dB too much, it applies about 6dB of attenuation to the entire passband. So, I will apply these figures to an example of what happens...

A station is running about 25 watts to a good uplink antenna. His downlink, though, is poor, and they can't hear it very well. So, using the normal HF procedure when there's not enough signal, the amplifier is switched on and 100 watts is produced. This is 6dB more signal. Since 25 watts is the max before the transponder attenuates, there is 6dB too much uplink, and the signal is reduced by ~6dB at the satellite.

However, the received signal is perceptably better! How can that be, you ask? Well, the attenuator works on everything that the transponder receives, including noise. The result is that the original signal strength is maintained on the downlink, but that there is 6dB less noise through the transponder. Hence, the signal is 6dB more out of the noise.

This sounds great, and it would be if there was only one user on the transponder. There are others who use it, however, so the result is one signal that sounds great, and every other one is 6dB down. A very poor situation indeed!

There is one more factor to consider for CW operators. If your power is set to 25 watts, you will probably activate the AGC on CW, bot not on SSB. SSB rarely actually hits its peak power. If it does, it is for an extremely short period of time. Morse code, on the other hand, hits peak power on each and every element for the duration. So, even though the signal is just fine (or even a little weak) on SSB, it could very well be hurting on CW.

The result of too much CW power is quite dramatic. The downlink signals (except for the offender) are all chopped up. If you have a good receive setup, you can actually hear the code in the noise over the entire bandpass as the attenuator applies and releases on each element! This can sound somewhat like spin modulation, but I never heard a spin calling CQ endlessly. I did a bit of experimenting, and 10 watts into my KLM antenna was PLENTY of uplink for Morse Code.

You can easily find out if you have too much uplink power. Turn on your Mode-S equipment a little earlier, and listen for the beacon. It is on for a few minutes before the transponder comes on. The reason it is on is so that you can get a look at what your downlink signal level should be AT THE MOST. Get your antennas pointed, and then note what the signal level is, and please don't exceed it when you transmit. The offending signals I have heard have been a full S-unit above the beacon and more.

All of this goes to show that there is absolutely no substitute for good receiving equipment. If you can't hear the beacon well, there's no reason to expect to hear other stations. Just using CW does not make up for a lack of receive. If you can't hear an SSB signal, you can't hear the desperate pleas to reduce your power! If you have trouble hearing yourself at 10 watts, instead of using more power please try working on your receive setup, and we can all use the "Silent Mode".

One big problem in getting tuned up is finding your own signal. Using the memories in my FT-736R has helped me to greatly reduce the tuning time (and therefore QRM) to get set up on Oscar 13. With the memories set, you will be within a very few kilohertz of your signal to begin with on Mode B. No more searching the whole band, looking for your signal and interfering with others! You can pull the needed frequencies from the list below and set up your memories for the modes you use.

It's true that you cannot use the VFO knob in memory mode on an FT-736R, but did you know that you can transfer the Memory contents to a VFO without changing what's in the memory? Some of us have done it using the VFO<&>&MEM and then the VFO>&MEM keys, but you can do it also by pressing the F and then VFO keys while in Memory Recall. This is a real help, as you can store combinations of the frequencies in memories and go right to them. I just wish Yaesu would put this feature in the manual!

Once you have a frequency pair ready, be sure to tune around and see if there is anyone on the transponder before you transmit that carrier! If you don't hear anyone, double check the schedule and the date. It may have changed. If the transponder is on, try to find a spot where there are a few kHz of open space to prevent QRM while you are tuning up. Once you are ready to find your own signal, make sure that you vary your TRANSMIT frequency. This is so that you are not sitting on top of someones conversation with a steady carrier while you tune your receiver looking for it.

Now, this all may seem like a lot of work just to say "Hello" to someone on a satellite. At first, it is. But with a little practice, it is a LOT easier, certainly better than tuning the whole passband trying to find your own signal! With your help, Mode S can again be a LOT of fun and Mode B will be much improved as well.

For all you Mode S stations, how many of you have tried mode BS? Not the content, the mode. With the mixing I mentioned earlier, you can talk to someone who is on Mode B without all that VHF static and intermod! How? Transmit to them on the normal Mode B frequency, but listen on Mode S. You will hear both them and yourself as usual. This takes a very good understanding of the frequency relationships between the transponders. Hint: You'll need LSB on both VFOs, and the signals are non-inverting. I made up a cross reference for myself to keep it all straight. I based this on a 145.900 downlink, which is a good reference. All of these frequencies are equivalents between the modes, and were tested with < 100 Hz doppler on 2400.750 MHz.

Mode B Tx	Mode S Tx	Mode B Rx	Mode S Rx
435.497.1	435.620.0	145.900.0	2400.745.7 (144.745.7)

Your mileage may vary due to Doppler, individual radio tuning, and especially 2400 MHz conversion differences. Mine is the Down East Microwave unit, which I calibrated to within 100 Hz (warm) at the time of the test. The S uplink does not, as far as I can tell, get downlinked on B. Be sure to let your converter warm up for a while before trying this. Using this cross-mode, I was able to talk to a group of friends one night, and they couldn't even tell what I was up to. The S passband is narrower than the B, so there are limitations.

That's a lot to digest all at once, so I'll stop here. Have fun on AO-13!


Original article by William Roth, N7RYW (n7ryw@amsat.org). Hypertext conversion by KB5MU.

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