SO-50 vs AO-91: Which Satellite Should You Start With?

When new satellite operators ask me where to start, the answer is almost always one of two birds: SO-50 or AO-91. Both are FM voice repeater satellites accessible with a basic dual-band handheld, both have healthy user communities, and both are reliably active. But they have meaningful differences that make one or the other a better fit depending on your situation.

The Quick Comparison

SO-50 (SaudiSat-1C, NORAD 27607) launched in 2002 and has been one of the most popular amateur satellites ever flown. It operates as an FM repeater with a 10-minute on-timer activated by a specific tone sequence.

• Uplink: 145.850 MHz with 67.0 Hz CTCSS
• Downlink: 436.795 MHz
• Activation tone: 74.4 Hz (to wake the satellite if it's sleeping)
• Mode: FM cross-band repeater

AO-91 (Fox-1B, NORAD 43017) launched in 2017 and is one of AMSAT's Fox-1 series satellites. It's a more modern design with a linear power control system and operates only in sunlight.

• Uplink: 145.960 MHz (no CTCSS required)
• Downlink: 435.250 MHz
• Mode: FM cross-band repeater
• Power: solar only — inactive in eclipse

Access Requirements

SO-50 requires a 67.0 Hz CTCSS tone on the uplink — without it the satellite's squelch won't open and it won't hear you. This is the single most common reason new operators have trouble with SO-50. Check your radio is actually transmitting the tone before the pass, not just that it's programmed. Some radios need CTCSS enabled separately from simply having the tone value entered.

SO-50 also has a 10-minute activity timer. If the satellite has been idle, it goes to sleep. To wake it, transmit a brief burst with 74.4 Hz CTCSS before your main transmission. You'll know it's awake when you hear the downlink squelch tail. If you're hearing other stations through the satellite already, it's obviously awake and you can skip the activation step.

AO-91 is simpler in this respect — no CTCSS required on the uplink at all. Just transmit on 145.960 MHz and listen on 435.250 MHz. If the satellite is in sunlight and your signal is adequate, you'll get through.

Verdict for beginners: AO-91 is slightly easier to access — fewer things to configure on the radio. But SO-50's activation procedure becomes second nature very quickly.

Sunlight Dependency

This is the key practical difference. AO-91 runs entirely on solar power and shuts down in eclipse. SO-50 has battery backup and operates through eclipse periods.

In practice this means you need to pay attention to whether AO-91 passes are in sunlight or shadow. A pass that looks ideal on the predictor — high elevation, good timing — may produce nothing if the satellite is in the Earth's shadow at that moment. Ham Sat Tracker doesn't currently flag eclipse passes, so for AO-91 you need to use a tool like Gpredict or check the pass geometry manually if you're not getting a signal.

A rough rule of thumb: AO-91 is more likely to be active during daylight passes and during the summer months when the orbital geometry keeps it in sunlight longer. Evening passes in winter are more likely to catch it in eclipse.

SO-50 doesn't have this issue. Any pass where you can hear it has the potential to work.

Verdict: SO-50 wins on reliability — you don't have to worry about eclipse.

Signal Characteristics

Both satellites produce a clear FM downlink when you're in range with a decent signal. SO-50 has been in orbit for over 20 years and its audio quality remains good — a testament to the design. AO-91 is a newer, more modern transponder and sounds crisp when active.

SO-50 tends to get busier during peak hours simply because it's been around longer and more operators know it. AO-91 often has slightly less traffic, which can actually make it easier for a new operator to get a call in.

Both satellites have automatic level control on the transponder, which means a stronger signal doesn't necessarily dominate over a weaker one the way it might on a terrestrial repeater. Good news for QRP operators.

Which One First?

If you want the simplest possible first attempt — fewest variables, no activation procedure, no eclipse concerns on a clear day — start with AO-91 on a daytime pass with good elevation. Program 145.960 simplex transmit and 435.250 simplex receive, apply Doppler correction from Ham Sat Tracker, and see what happens.

If you want the satellite that's been a community favourite for two decades, has passes at all hours, and rewards a bit of technique — start with SO-50. The activation tone procedure is a rite of passage for satellite operators and you'll learn it in one pass.

Realistically, work both. They're both in the free tier of Ham Sat Tracker for exactly this reason — they're the two birds every FM satellite operator should know. Once you've worked both comfortably, you'll have the confidence and technique to start exploring SSB satellites like FO-29 and RS-44, where things get genuinely interesting.

73 de VE3AKK