PARROT REPEATER OPERATIONS
Introducing "Poly" the ORB Parrot Repeater
The "parrot repeater" on board ORB is similar to terrestrial simplex repeaters.
Basically, it listens for a period of time and records its receiver audio
digitally into the repeaters RAM. After recording, the stored audio is played
back, 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. Operation of the
parrot repeater makes it possible for an ordinary handheld/rubber duck station
to work it with good results.
Now, onto the specifics. Poly, runs with closed squelch, carrier detection is
used to trigger the repeater and run according to a simple cycle. The cycle is
outlined in the table below:
1 Receive Cycle - 30 second record cycle - Ground stations can Tx.
2 Transmit Cycle -Stored audio is played back - Ground stations listen.
3 Single tone - End of playback.
4 Cycle restarts, back to #1
Problems with Parrot Operation.
Parrot repeater operation has a few problems which make especially tricky, some
of which are similar to what packet radio operators experience when using digi-
peaters, as well as the "unnatural" feel of the mode. The major problems with
parrot operation are as follows:
Hidden transmitter issues (same as those which plague terrestrial packet radio).
Lack of control from the operators' perspective. Most operators' lack of experience
with simplex repeaters in general.
"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 balloon 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.
The lack of control referred to here is that Poly's timing is the _only_ thing
which determines whether it 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 Parrot's timing sequence to have any chance of
being relayed.
The effects of the above problems tend 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 Poly, and also
increase as the proportion of inexperienced operators is increased. However,
a number of techniques can be used to maximise the QSO throughput.
Taming Poly.
Orderly operations on Poly require an understanding of the underlying problems.
Use of efficient and courteous techniques by individual operators is one.
Addressing the second two issues raised previously, namely arming the operator
with knowledge to work in with Poly's cycles, and to take the quirks of simplex
repeaters into account. The hidden transmitter problem cannot be addressed at
all with this strategy. 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 Poly rests with all the operators present. I found
the following strategy successful.
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*** IMPORTANT - READ THIS ***
Initially, it is necessary to transmit blindly into an uplink timeslot. However,
the normal rules of "Polyetiquette" apply in a sense, and if the previous 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 Poly seems to be fewer, from experience.
If you're successful, you will hear yourself come back down (retransmittesd).
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, Poly 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:
When you key your mike, wait about a half second before you talk. There is
a slight lag before Poly starts to record. Brevity is especially important on
Poly, 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.
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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 (Poly's status is in braces):
Put out a brief call (Record Cycle)
Hear own audio (Playback Cycle)
WAIT! Do not Tx! (Record Cycle)
Listen for responses (Playback Cycle)
Ack responding stations - sig report, grid sq. etc (Record Cycle)
Hear own transmissions (Playback Cycle)
WAIT, Do not Tx! (Record Cycle)
Hear other stations close remaining QSOs (Playback Cycle)
* Note how every second uplink window is spent standing by.
In busy regions, the sheer number of stations means that even with good operating
practices, there may still be several stations on the uplink, and then again,
there will be a significant number of inexperienced stations to add to the mayhem.
:-)
Conclusion:
Poly the Parrot Repeater on ORB is a unique mode of operation with its own
benefits and challenges. When properly used, it can be an excellent introduction
to balloon 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.
Good luck and Have Fun!
73
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Harry - KC5TRB - Tulsa
Oklahoma Research Balloons
http://members.cox.net/hhm_74775/orb/
http://groups.yahoo.com/group/oklahoma_research_balloons/join
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Adapted from VK3JED's proceedure for Satellite SO-35
http://www.qsl.net/vk3jed/parrot.html
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