[amsat-bb] LilacSat-2 Transponder active over N America 13:15 UTC

Paul Stoetzer n8hm at arrl.net
Tue Jul 12 17:05:28 UTC 2016


I understand there have been some questions about how inverting vs.
non-inverting transponders and why it makes a difference with the
Doppler shift.

At AOS, the satellite is moving towards you. A 435 MHz downlink signal
for a LEO satellite is going to appear to be about 10 kHz higher than
the frequency at which the satellite is transmitting  (this does vary
based on orbital altitude, and thus velocity). At the same time, your
145 MHz uplink signal is going to appear at the satellite to be about
3.5 kHz higher than the frequency at which you're transmitting.

Let's look at an example for FO-29, an inverting mode V/U (J) linear
transponder satellite, based on these estimated numbers:

The convention for manual tuning for Doppler on amateur satellites is
to tune the highest of the tune frequencies and leave the other one
alone as much as possible, so someone calling CQ will generally leave
their transmit fixed and tune only their downlink on a mode V/U linear
transponder satellite. If you uplink at 145.925 MHz at your AOS, your
signal will appear to be about 145.928.5 MHz at the satellite, since
the transponder is inverting (uplinks from 145.900 MHz - 146.000 MHz
corresponding to downlinks from 435.900 MHz - 435.800 Mhz), FO-29's
transponder will retransmit your signal at 435.871.5 MHz. So at your
AOS, you will receive your signal at about 435.881.5 MHz. At your LOS,
with the satellite moving away from you, the satellite's downlink
frequency will appear to be about 10 kHz lower and your uplink
frequency will appear to be about 3.5 kHz lower at the satellite.
FO-29 will be receiving your uplink signal at 145.921.5 MHz and will
retransmit that at 435.878.5 MHz, which will appear to be 435.868.5
MHz at your LOS. The total Doppler shift encountered by the user is
thus 13 kHz. You'll note that this is the difference between the total
shift on the downlink (20 kHz) and the total shift on the uplink (7
kHz).

When the frequencies are inverted, the sidebands of a SSB signal are
naturally inverted as well. The convention is to always receive on
USB. Users therefore transmit on LSB.

Now, let's try this example on LilacSat-2's non-inverting mode V/U
linear transponder. The uplink frequencies for the transponder are
144.342.5 MHz - 144.382.5 MHz and the downlink frequencies are 437.180
MHz  - 437.220 MHz. If you uplink in the center of the passband, at
144.362.5 MHz, at AOS your signal will appear to be approximately
144.366 MHz at the satellite. LilacSat-2, with it's passband
non-inverted, will retransmit this signal at 437.203.5 MHz, and you
will receive your signal on the ground at 437.213.5 MHz. At your LOS,
your signal will appear to be approximately 144.359 MHz at the
satellite and it will retransmit this signal at 437.196.5 MHz, which
you will receive on the ground at 437.186.5 MHz. The total Doppler
shift encountered by the user is thus 27 kHz. You'll note that this is
the SUM of the total shift on the downlink (20 kHz) and the total
shift on the uplink (7 kHz).

Of course, when using a non-inverted transponder, the sidebands will
not be inverted, so you'll naturally transmit in USB to receive your
signal on USB as well.

By inverting the frequencies on the downlink, more than half of the
total Doppler shift is canceled out, making manual tuning much easier!
Mode A (V/A), K (H/A), and T (H/V) transponders have traditionally
been non-inverting due to the low total Doppler shift at those
frequencies, but those using higher frequencies have used inverted
passbands since the first one was flown on AO-7. The P4A analog
transponder on Es'HailSat-2 will be an exception, being non-inverting
despite the use of 2.4 GHz and 10 GHz, but that's because the
satellite will be nearly stationary relative to the ground users and
the total Doppler shift will be very minimal. I do not know why
LilacSat-2's SDR linear transponder was programmed to be
non-inverting, but based on the comments made on Twitter by a member
of the Harbin University team, it was added purely as an experiment
(after all, when you're using an SDR, different modes are just lines
of code, no new hardware is needed). The transponder also lacks
automatic gain control, which is critical for maintaining the
linearity of downlink signals on a linear transponder. Strong signals
on the transponder may cause distortion of downlink signals. As
always, it's important to keep your ERP at a reasonable level.

73,

Paul, N8HM

On Tue, Jul 12, 2016 at 9:36 AM, Paul Stoetzer <n8hm at arrl.net> wrote:
> I hope to be able to catch it at some point. I was on the 09:51 UTC
> pass over the Atlantic, but only heard the familiar telemetry bursts
> of the satellite in it's default mode.
>
> Keep in mind that this transponder is entirely done in software - the
> second software defined linear transponder to fly after the ARISSat-1
> transponder. Being non-inverting adds a bit of a challenge for manual
> operators, but nothing that can't be handled.
>
> LilacSat-2 is a neat satellite. When turned on it's been in FM
> transponder mode most often, but the on-board SDR also has programming
> for an APRS digipeater as well as a linear transponder as we've seen
> the past couple of days. Unfortunately, it suffers from periodic
> resets and resets to a telemetry only mode, so you never really know
> whether or not it will be on when it comes over.
>
> 73,
>
> Paul, N8HM
>
> On Tue, Jul 12, 2016 at 9:18 AM, KO6TZ Bob <my.callsign at verizon.net> wrote:
>> Clayton,
>>
>> Thanks for the "Heads-Up" worked perfect.  A couple of taps on the "-" key
>> and SatPC32 tracked the satellite perfect.  Had a QSO with Glenn.  Nice
>> sounding transponder.
>>
>> KO6TZ
>> Bob
>>
>>
>> I should have been a little more clear in my email earlier.  The
>> transponder on LilacSat-2 active today was not the normal FM one but
>> rather a SSB transponder.  It is experimental and has no AGC according
>> to the reports from Twitter.  I'd suggest paying attention to the
>> AMSAT OSCAR status page (http://www.amsat.org/status) and following
>> BG2BHC on Twitter for operational updates.
>>
>> I was using manual control but you can add this line to your
>> DOPPLER.SQF.  It will need some minor calibration:
>>
>> LILACSAT-2,437200.0,144362.0,USB,USB,NOR,0,0,SSB Transponder
>>
>> 73
>> Clayton
>> W5PFG
>>
>> On Mon, Jul 11, 2016 at 9:26 AM, Clayton Coleman <kayakfishtx at gmail.com>
>> wrote:
>>> This morning, Glenn AA5PK, and I had a QSO via the experimental
>>> transponder activated on LilacSat-2.  It was very interesting and fun.
>>> A non-inverting mode J transponder was new to me.
>>>
>>> Using manual tuning to adjust for Doppler shift proved to be a small
>>> challenge as I am more accustomed to operating on a transponder like
>>> FO-29.  Running SDR in parallel to my Icom IC-9100 gave me a good
>>> visual representation of the transponder pass band.
>>>
>>> You can listen to a brief clip of our QSO here:
>>>
>>
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