Taming the SO-35 Parrot

In late 1999 and during UN Space Week in October 2000, SO-35 was operating in a unique mode as a simplex or "parrot" repeater. This new mode of operation opened SO-35 up to more amateurs across the world. However, the unique properties of this mode also mean a higher degree of operator skill is required to work this mode, especially under busy conditions.

The "parrot repeater" on board SO-35 is similar to terrestrial simplex repeaters. Basically, it listens out for a period of time and records its receiver audio digitally into the spacecraft's RAM. After recording, the stored audio is played back into one of SO-35's transmitter, usually on the same frequency as it was recorded from. This means that an ordinary FM transceiver in simplex mode can be used to work the parrot, particularly on 2 metres, where the Doppler shift is small enough to not require any compensation for the entire pass. Operating the parrot repeater on a 2 metre frequency also allows both uplink and downlink to take advantage of the lower free space losses on this band, making it possible for an ordinary handheld/rubber duck station to work the bird with good results. In addition, some areas don't have many or even any 70cm capable stations, but are more likely to have stations with 2 metre capabilities. The parrot opens up SO-35 and FM satellite operation up to these countries, meaning more DX opportunities.

Now, onto the specifics. Because SO-35, like all FM satellites currently active, runs with an open mute, carrier detection can't be used to trigger the parrot repeater. Instead, it runs according to a simple, but strictly timed cycle, which is controlled by the satellite. The cycle is outlined in the table below:

Stage	Duration (sec)	Satellite activity	Meaning
1	1	Single tone on downlink	Start of uplink window.
2	10	No signal (satellite in receive mode)	10 second uplink window - ground stations can Tx.
3	1	Two tone (like Morse "M") on downlink	Start of downlink period
4	10	Stored audio is played back	Downlink period - stations heard on downlink.
5	-	Cycle restarts, back to stage 1

Parrot repeater operation has a few problems which make especially tricky, some of which are similar to what packet radio operators experience when using digipeaters, as well as the "unnatural" feel of the mode. The major problems with parrot operation are as follows:

"Hidden transmitter", simply means that not all of the uplink stations can hear each other directly (if they could, there'd be no point to having a repeater on the satellite in the first place!). This means that stations may inadvertently interfere with each other on the uplink, due to not being aware of others transmitting at the same time. This isn't as big a problem for the more common duplex satellites, as it's possible to hear uplink activity in real time in a full duplex system and avoid "doubling".

The lack of control referred to here is that the satellite's timing is the _only_ thing which determines whether the satellite is transmitting or receiving at any given time. Contrast this to most amateurs' experiences with terrestrial repeaters, where they have indirect control of when it keys up by sending a carrier. This means operators must work within the satellite's timing sequence to have any chance of being relayed by the satellite. Similarly, simplex repeaters require slightly different operating techniques that amateurs are used to.

These effect of the above problems tends to be that stations transmit on top of each other, and few QSOs succeed, due to the chaotic conditions. The problems increase with increasing number of stations attempting to use the satellite, and also increase as the proportion of inexperienced operators is increased. However, a number of techniques can be used to maximise the QSO throughput of the satellite.

Orderly operations on the parrot repeater require an understanding of the underlying problems. There are at least two strategies that can be applied to minimise the effects of the limitations of the system. They are:

Strategy 1 addresses the second two issues raised previously, namely arming the operator with knowledge to work in with the satellite's cycles, and to take the quirks of simplex repeaters into account. The hidden transmitter problem cannot be addressed at all with this strategy.

Strategy 2 aims to minimise the impact of hidden transmitters, by using a net control station to direct a small proportion of the ground stations to transmit, while everyone else stands by. This is very similar to procedures used by emergency service organisations when coordinating large ground based radio nets.

For areas with relatively low levels of satellite activity (e.g. Australia/NZ), there is no need for a net controller, it is possible for individual stations to maintain a reasonable throughput, and live with the hidden Tx issues. The responsibility for maximising the use of the satellite rests with all the operators present. I found the following strategy successful.

Initially, it is necessary to transmit blindly into an uplink timeslot. However, the normal rules of "satiquette" apply in a sense, and if the previous 10 second downlink had a half finished QSO on it, it's better to wait a cycle. When uplinking, the best times to aim for are the first couple of seconds of the timeslot (most people seem to take a while to realise it's time to Tx!), or the last 3 seconds, after most people have finished. During these times, the number of stations attempting to access the satellite seems to be fewer, from experience.

If you're successful, you will hear yourself come back down in the following 10 seconds. Whether you hear yourself or not, DO NOT TRANSMIT during the next uplink period, unless there were NO successful calls heard on the downlink. The reason for this wait is simply to give ground stations a chance to respond to your call. Transmitting now may wipe out the very reply you're waiting for, if you have a good station! If all goes well, you will hear a reply or two in the next downlink period. If they're directed to you, you can respond in the next Tx window. If not, stand by and let the successful station complete their QSO, before having another attempt. The key here is that no one station should transmit more frequently than every SECOND uplink cycle, or the system will not work properly, and QSOs in progress should be allowed to complete.

Sometimes, stations in the same geographic region can hear each other directly. On some occasions, this has been known to cause confusion as the stations attempt to work each other in a normal simplex fashion. This only causes problems. Remember, the satellite controls the timing, and at all times, you need to operate as though you can't hear anyone directly, and wait for the correct uplink slot before responding.

Note that brevity is especially important on the parrot repeater, and also, acknowledging 2 or more stations in the same cycle can save precious transponder cycles, and give more people a chance to work someone.

One of the dangers of the above procedure is that it works!! Someone who carries it out successfully tends to get a lot of QSOs and other operators look out for them, because they're effective in getting through, so it's necessary to pass it around a bit, and wait out several cycles, if you've had your fair share of the activity. The bare minimum number of cycles to wait is 3, preferably 4 or more, to gauge how the activity is before having another go. A sample sequence for a successful call would be like this (satellite status is in square brackets):

In busy regions (e.g. Europe, North America), the sheer number of stations means that even with good operating practices, there may still be dozens of stations on the uplink, and then again, there will be a significant number of inexperienced stations to add to the mayhem. :-) To cope with these extreme conditions, Bob Burninga, WB4APR, suggested that a net controller be agreed upon before the pass arrives (e.g. via the AMSAT mailing lists). The net control station, in contrast to normal "satiquette", should run an efficient antenna system AND high power to ensure they're heard under all conditions. Due to the length of a pass, it may require two net controllers - one for the first half, one for the latter half - to cover the entire pass. Ideally, they should be sited in a location where few (if any) stations can hear them directly. During the pass, the net control station calls in a subset of the stations present (e.g. by callsign suffix, geographic region, class of station - handheld, mobile, etc), and invites them to work each other. On each cycle, the subset chosen is different. On the next available pass, a different way of dividing up the stations is used, to give everyone a chance to work different stations. The net effect is that during the course of a pass, everyone gets a chance to attempt a QSO, and over several days of parrot passes, hopefully a chance to work everyone else. To my knowledge, controlled net operations have not been attempted on the parrot repeater, so at this stage, the finer points of the technique haven't been worked out.

More details and discussion about controlled net operation can be found in the amsat-bb mailing list archives at the AMSAT-NA site. Look around early October 2000.

The parrot mode on SO-35 is a unique mode of operation with its own benefits and challenges. When properly used, it can be an excellent introduction to satellite operation, especially for stations with limited equipment. However, it requires special techniques to be used for the parrot repeater to work effectively. The techniques outlined here can make a huge difference to the overall experience.