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A Concise Guide to Working the Easy Sats



I hope this document serves to answer the many messages filling my
mailbox these past few days.  If you are not interested in this
document, my apologies for the bandwidth.

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A Concise Guide to Working the Easy Sats

This document should be considered a WIP (Work In Progress).  In
reaction to many questions regarding how to work the easy sats, this
effort was born.    

Mike Gilchrist, KF4FDJ -- September 25, 1999 -- Fort Myers, FL



THE DOPPLER EFFECT

Since LEO (Low Earth Orbit) satellites travel at high rates of speed,
radio signals arriving at and moving away from the satellite are subject
to a perceived change in frequency.  This phenomenon is called Doppler
shift.

Much as a passing car with horn blaring will seem to change pitch at the
moment it passes an observer, satellite communications are subject to
the same effects.  While listening to a signal from a satellite that is
approaching your position, you will receive the signal slightly higher
than the actual transmitted signal.  As the satellite moves away from
you, the received signal will be found lower in frequency than the
actual transmitted frequency. 

The opposite effect is seen on an uplink signal to a satellite.  If the
bird is approaching you, your transmit frequency will be slightly lower
than the frequency the satellite receiver hears.  Likewise, as a bird
recedes from your position, you will need to transmit higher in
frequency to compensate for the Doppler effect.

Doppler shift is more pronounced, the higher the transmitted frequency. 
As a rule of thumb on the LEOs, the maximum 2-meter Doppler shift is
around 3.5 kHz, while on 70-cm, it may be close to 10 kHz.

Most readers of this document will be using a handheld scanner or an HT
to listen to, or work the easy sats.  Since these devices are generally
only capable of being tuned in 5 kHz increments, you will have to
program a series of frequencies into your rig.  Under each section for a
particular bird, I have included a list of frequencies tabulated,
mindful of this limitation.



THE ACTIVE BIRDS

Now that MIR is silent, there are currently 3 satellites that fit the
category of easy sats.  FO-29, when in digitalker mode, AO-27, for
daylight passes over the Northern Hemisphere, and SO-35, or SUNSAT, when
the operators turn the device on, which is usually weekend daylight
passes over Europe and North America.

The FO-29 digitalker transmits on a frequency in the 70-cm band.  

AO-27 is a mode J FM bird.  Mode J is 2-meters up, and 70-cm down.  

SO-35 is also an FM bird, and is usually operated in mode B, but can
also be operated in mode J.  Mode B is 70-cm up and 2-meters down.



WHEN AND WHERE TO POINT

Since all these birds are low earth orbit satellites, most passes will
be between 6 and 18 minutes in duration.  Timing is critical in working
any amateur satellite.

Since these birds are all considered weak signal, you must know when the
satellite will pass over your location, and where in the sky to point
your antenna.  You will need a program to generate predictions of when
the satellite is "visible" to you.  I believe WINORBIT 3.5 is one of the
better programs available for beginners.  

An excellent place to find downloadable software for real time tracking
of satellites, and other information on the amateur satellite program in
general, is:  

     http://www.amsat.org

Be sure to download the latest keplerian coordinates if you download a
tracking program.  Each prediction program needs these coordinates to
determine where the satellites are, and to help you determine when the
satellite will be over your horizon.  Another casual approach, is to use
an online prediction site, such as:

     http://acsprod1.acs.ncsu.edu/scripts/HamRadio/sattrack

This site will generate pass predictions for a selected satellite.



LISTENING TO THE FO29 DIGITALKER

The digitalker on FO-29 is a digital loop announcement, which makes the
same announcement over and over. You will hear, "Ho- ho'kke'kyo, this is
JAS two."   The first part is the song of a bush warbler.  JARL
(Japanese Amateur Radio League), plans to cycle digitalker, mode JA, and
mode JD.  The digitalker message is subject to change.  Check
announcements on the BB for a mode schedule and other details.

Since FO-29 transmits in FM mode while in digitalker, any FM or
multimode receiver capable of tuning the 70-cm ham band should be able
to detect the signal.

Digitalker operation dedicates most of the power budget of the satellite
to the FM transmitter, instead of spreading it across a mode JA
transponder. It is a very strong signal!  It is possible to hear the
satellite with a handheld scanner or transceiver, and a stock "rubber
ducky" antenna.  Most 70-cm base stations, or scanner listening posts,
with fixed antennas, should also be able to tune the signal.

If you are using a handheld device, you will need to rotate the rig (and
antenna) for the best orientation.  Experience shows that horizontal
polarization, with the axis of the antenna perpendicular to the
satellite gives the best reception.  Experiment!

The transmit frequency is 435.910.  Of course, you will have to
compensate for Doppler shift.  Just remember you will tune a little
higher in frequency as the satellite approaches your position, and a
little lower in frequency as it recedes.  You will always be tuning
higher in frequency when the satellite comes over the horizon.  Make
sure you have your radio set to tune small increments, as increases and
decreases in Doppler are gradual, and linear.

The digitalker presents an easy opportunity for hams and non-hams to
have their first taste of real time reception of signals from space. 
Use this opportunity to show a kid, or a terrestrial ham how easy it can
be to tune a satellite.  This opportunity is excellent for scout
meetings, monthly ham meetings, schools, or any place where inquisitive
persons congregate.  



WORKING AO27

As previously stated, Oscar 27 is a mode J satellite.  Your uplink needs
to be a low power 2-meter signal.  AO27 is at times a difficult bird to
hear.  You will need 70-cm FM receive capability.  Many operators use a
dual band HT to work this bird.  

A gain antenna for 70-cm is required; a stock rubber ducky will not be
adequate.  I have had limited success with a long, wispy dual band 'cat
whisker' type gain antenna.  If you use a car body or another piece of
metal as a reflector, you will increase your chances of working the
bird.  In any event, whichever antenna you use will have to be rotated
to best match the polarity of the bird.  Experience shows a vertical
orientation at the beginning of the pass, to nearly horizontal mid pass
works best.

Operators should understand that this satellite is seldom vacant, and if
you can't hear it, DON'T transmit.  You will only cause QRM to other
operators, and sully your reputation.  AO27 has a very sensitive
receiver, and I have managed to hit it with 100 mw of power at times.

Since AO27 is a very busy satellite, especially on holidays and
weekends, a portable operator will most likely have limited success,
unless they use a multi-element gain antenna for the uplink and
downlink.  I have also found use of a preamplifier beneficial for
hearing the bird well, and knowing when to jump in for a contact.

Most QSOs are short, contest style exchanges, including grid square,
first name, and city.  This sort of contact maximizes use of the bird
for the scores of operators who might be trying.  Weekend passes, to the
uninitiated, sound a bit chaotic.  There is a culture, which exists on
the bird, and my best advice is to listen to a couple passes, and model
your operation after the successful operators you hear.

AO27 is a full duplex bird, meaning that you can listen to the downlink
as you transmit.  If your rig is capable of full duplex operation, fine,
otherwise half-duplex operation works just fine on the FM birds if you
program the frequencies in your HT correctly.

You should program your HT with 5 frequency pairs for working AO-27. 
Start with the first pair, using a non-standard split, and tune to the
next one as the pass progresses.

Downlnk -  Uplink
_________________
436.805 - 145.845
436.800 - 145.850
436.795 - 145.850  middle of pass
436.790 - 145.850
436.785 - 145.855

The Doppler shift on the 70-cm downlink can be a little more than 10
kHz.  The 2-meter Doppler is close to 3 kHz.  

I seldom need to use the 5th pair, except on high passes, and only when
the TEPR keeps the bird on so far south like it is operating presently!



WORKING SO35 - SUNSAT

SO35 typically operates in a mode opposite AO27 -- mode B as opposed to
mode J.  Mode B is 70-cm up and 2-meters down.  While the 2-meter uplink
Doppler correction on mode J is not terribly critical, you must
compensate for Doppler on the 70-cm uplink while using this bird.  The
Doppler correction on this bird is counter-intuitive, if you are used to
working a mode J bird. 

While the downlink on AO27 is hard to hear, SUNSAT has a very strong
downlink signal, and is receivable with a standard rubber ducky antenna
atop a handheld scanner or HT.  The bird also seems to have a very
sensitive receiver, so low power HTs will work the bird if conditions
allow.

Since this satellite has only been active a few times, and it is active
only on weekend daylight passes; it is a very popular bird when it is
active.  As with all FM receivers or repeaters, the satellite is
captured by the strongest signal.  As a portable operator, you might
have only limited success working this bird, although you should be
quite successful receiving signals from the satellite.

As with AO27, short contest style contacts help facilitate more users
during a pass.  Common sense and courtesy must prevail.  As with any
amateur communication, use the minimum power necessary to complete the
contact.  

If you plan to use an HT to work SO35, here are 5 pairs of frequencies
you should program into your HT to help ensure you are close to the
proper uplink frequency.  You will have to program in a nonstandard
split.  Most HTs will only allow you to program on 5 kHz settings. 
Start with the first pair at the start of the pass, and sequence through
them as the bird passes.

Downlink       Uplink
145.830        436.280
145.825        436.285
145.825        436.290 (bird directly E or W of you)
145.825        436.295
145.820        436.300

The first and last pairs may not be required, depending on your
latitude.  The bird turned on well after the footprint was over Fort
Myers.


OTHER INFORMATION

One of my favorite web sites to send an inquiring person for a visit is:

	http://sulu.lerc.nasa.gov/rleonard/index.html#section1

A great place for kids to learn about space science is:

	http://www.nasa.gov/kids.html

Another site which all teachers, scout leaders, or any other person
wanting to bring the world of space science to others should bookmark
is:

	http://education.nasa.gov/


If you have any comments, additions, or modifications to this document,
please contact me directly.  We will attempt to post the latest version
of this document monthly.

This document is copyright © 1999 by Michael J. Gilchrist and may not be
reprinted in part or whole without permission from the author.

73, Mike KF4FDJ

-- 
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   [X]~~~|k|   * Fort Myers, FL  33902 ....... ARRL  member  #1781549 *
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