[amsat-bb] Saturday night fun with 70cm ISS packet

Patrick STODDARD (WD9EWK/VA7EWK) amsat-bb at wd9ewk.net
Sun Nov 27 16:48:00 UTC 2016


With the recent change to the UHF radio for the ISS packet digipeater,
this has disappointed some - and made others happy. I enjoyed working
stations through the 145.825 MHz digipeater, but working packet on
437.550 MHz has been fun, and bringing new challenges. Last night,
with 3 ISS passes over Arizona, was a great time for that...

The first of the 3 passes came just before 0130 UTC. Using my TH-D74A
HT and Elk handheld log periodic, I was able to get my position packets
retransmitted on this 9.9-degree pass to the southeast. Unfortunately,
I made no QSOs, and only heard one other station being retransmitted
before my LOS. Although this pass was a bust in terms of adding to my
logbook, the next couple of passes would be more successful. On to
the 0300 UTC pass...

The second of the three evening passes saw the ISS rise higher, to a
maximum elevation of just under 55 degrees. For those in California,
this was basically an overhead pass. After I worked a station on a
similar pass using my TH-D74A and a Diamond RH77CA long duckie antenna
just over a week ago, Mark KK6OTJ in southern California mentioned that
he wanted to try for a QSO where both of us use an HT and long duckie.
I suggested that this pass could work for us, and we went for it. Mark
has a TH-D72A HT, a BNC-to-SMA adapter on his radio, and a Diamond
RH77CA duckie antenna. Other than the radio, we had the same setup on
each end to make this attempt.

With the ISS coming up from the southwest, I thought we would have the
best chance to make this QSO earlier in the pass, certainly before the
midpoint of the pass. I prepared an APRS message for Mark's KK6OTJ-7
packet call sign with the text, "HT/duckie in DM43. QSL?" This is what
I started sending, once the ISS was up from the horizon. Watching the
clock and the AmsatDroid Free tracking app on my mobile phone, we went
through the first 5 minutes of the pass with nothing. Still sending my
APRS message with my grid locator, I saw an ACK come through the ISS
digipeater around 0307 UTC. Mark received my message, and his TH-D72A
sent an acknowledgement back to my TH-D74A. This was quickly followed
by Mark's APRS message, "QSL, Thanks & 73!" I sent a final "rgr" back
to Mark, completing the exchange and the QSO.

After the pass, Mark sent a photo of his TH-D72A's screen showing my
APRS message, and wondering if I had received his message. Apparently,
my radio's ACK for receiving his message didn't make it through the
ISS digipeater, but I had received his message. I tweeted a reply,
showing a listing of the messages - my message to him, then his message
to me, and the final message I sent him - along with a screenshot of
each message from that listing. Thanks for the QSO, Mark, and for the
challenge to do this.

After that high pass, there was still one more pass to work around
0440 UTC. Like the first pass of the evening, this was another shallow
pass - maximum elevation of just under 9 degrees, but to the northwest.
I was hoping to hear Kevin VE6QO in southern Alberta on that pass, and
maybe make a QSO with him. Kevin has been trying to work stations, but
the passes weren't helping me make a QSO. I have a line of thick trees
north of my house, which acts like a wall for the 437.550 MHz signals.
I went back to my Elk log periodic for this pass, knowing that the
Diamond duckie would not be a good antenna for a shallow pass. If I
was able to make an exchange with Kevin before the ISS was north of me,
it might be possible. And that's what we did...

I started sending my position packets once the ISS rose above the
houses west of me. I could see those packets being digipeated, and
then I saw "VE6QO" show up on the top of my TH-D74A's display. I knew
Kevin was there, even though his position wasn't being transmitted. I
made a very short APRS message to him - "dm43", my grid locator - and
started to send that. Kevin was also trying to send me his grid locator,
but the low pass was making this tough. While this was happening, I saw
two other stations showing up on my screen - VA7THO in British Columbia,
and W0JW in Iowa. Kevin and I kept at it, and finally I saw my "dm43"
APRS message being retransmitted. After that, I saw "TU and 73" come
from Kevin's end. I followed that up with a "73" to complete our QSO.
Thanks again, Kevin, for the QSO!

More stations are starting to find their way over to the 437.550 MHz ISS
digipeater. Unfortunately, this also includes the unattended beacons that
were all too common on 145.825 MHz. It appears we will have ISS packet
on 70cm for a while, given that a radio capable of VHF packet may not be
sent to the ISS until late next year. For many FM transceivers capable of
packet operation, using a group of 5 memory channels to compensate for
Doppler is the ticket to being successful working packet on 70cm. Think
of working the 70cm ISS packet digipeater like the combination of AO-85's
uplink and SO-50's downlink, all on one frequency. The uplink frequency
moves up as the downlink frequency moves down, and near the middle of the
pass the two frequencies are the same.

I posted on this list and tweeted the chart I used for programming my HTs
to work the 437.550 MHz digipeater. That chart is also available from

You can get to the chart with this direct link:


This document, along with K9JKM's guide on ISS packet operation and more,
is available from AMSAT's Station and Operating Hints page:


If you previously worked the ISS packet digipeater on 145.825 MHz, working
the 437.550 MHz is not that different. Dealing with Doppler is a must, and
the higher frequency could lead to issues if your QTH has trees in the way
of your antenna(s), but there are stations looking to do more than transmit
beacons automatically. If you would like to attempt a QSO with me using the
70cm ISS packet digipeater, please e-mail me directly. There should be some
options for QSOs with stations across much of the continental USA, as well
as with many parts of Canada and Mexico.


Twitter: @WD9EWK


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