Working AO-51 Calls for Courtesy, Common Sense

NEWINGTON, CT, Sep 1, 2004--Mike Kingery, KE4AZN, of the AMSAT OSCAR Echo (AO-51) command team is challenging users of the new satellite to stick to one contact per pass that they initiate with a call. The suggestion is aimed at reducing continued congestion on the satellite and to allow more operators to make contacts through Echo.

"If someone calls you, then by all means reply and complete that contact," Kingery advises. "Then go back to listening." Among other recent suggestions is that stations trying to access AO-51 run the lowest power necessary to reach the satellite--since the receiver already is quite sensitive--and avoid the temptation to fire up the high-power amplifier brick in an effort to dominate the passband.

Another "Experimenter's Wednesday" starts September 1 at 0215 UTC and continues until September 2 at 0310 UTC. This week, AMSAT will conduct a Mode L/S FM repeater text on AO-51. The L-band uplink frequency will be 1268.700 MHz FM voice, and no 67-Hz CTCSS tone will be needed during the test. The S-band downlink will be 2401.200 MHz. This test will mark the first time that the AO-51 command team has powered up the S-band transmitter and the SQRX receiver at the same time for an extended period

As usual, ground controllers will check the whole orbit data after the first Experimenter's Wednesday pass, and if all is well, L/S FM repeater mode will continue for the remainder of the test.

During Experimenter's Wednesday, digital transmitter TXA on 435.150 MHz will be off, except to download data over a command station. FM transmitter TXB on 435.300 also will be off during the L/S test.

At the conclusion of Experimenter's Wednesday, ground controllers will return the satellite to V/U FM repeater mode: 145.92 up, with 67 Hz CTCSS tone, and on TXB 435.300 MHz down. The BBS will be off.

AMSAT's Bruce Paige, KK5DO, recently offered some additional pointers--for which he credits Emily Clarke, W0EEC--to help operators to work Echo when its in V/U FM repeater mode. When using a handheld transceiver, open up the squelch, he advises. "The weak signal from the satellite will probably never break your squelch setting for a local repeater, so open it all the way," he says. "You will get a lot of static but you will hear the satellite."

Users also might want to use headphones. "It will cut out some of the background noise, and two ears are better than one for picking out some of the weak signals," he says.

He also recommends that if you're using just the built-in "rubber duckie" or even a larger telescoping antenna to position it somewhere between aiming at the ground to pointing up 45 degrees. "Adjust it for the elevation of the satellite, so that your antenna is about perpendicular to it," he suggests. Handheld users also might want to try holding the antenna above the hood or trunk of a vehicle, or near the wheels, he says. "You'll be surprised at the help you get from some of these reflections."

Jim White, WD0E, of the AO-51 command team points out that stations must transmit the 67 Hz CTCSS tone continuously in order to be heard through the FM repeater. Unlike previous similar satellites, Echo's tone decoder turns on the downlink transmitter and gates the audio path. "The tone must be present on a signal for the audio to get through," White notes. No audio will get through if a signal without the CTCSS tone captures the receiver, so the satellite will end up transmitting a carrier but no modulation. Should that happen for more than 10 seconds, the transmitter will turn off until it hears a signal with the correct CTCSS tone.

White also notes that multiple uplink signals can mix. When this happens, it's possible that the satellite receiver's decoder cannot detect a clean CTCSS tone. The result is the same as no tone, and no audio will be repeated.

"What has been reported as a 'dead carrier' is usually a result of this condition," White points out. If this happens enough, it can suck down the power available in the satellite. As White puts it, "it is a zero-sum game, one orbit at a time."

The idea behind the CTCSS tone is to keep the transmitter off when it's not in use so that it can run at higher power when it is in use. The transmitter has been running on the order of a half-watt on recent passes.

AMSAT President Robin Haighton, VE3FRH, says he was at the AMSAT-UK Colloquium in England when AO-51 first opened for general use earlier this summer. "However the uplink was so crowded that it took some effort to make a contact," he said. He was eventually successful, however.

Some $7300 remains to meet AMSAT's $110,000 Echo launch cost. AMSAT-NA invites tax-deductible donations to complete its fund-raising effort. Visit the Echo Project page on the AMSAT Web site for information.

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