Step 03 — Frequencies
Frequencies & Doppler
Every amateur satellite publishes a nominal uplink and downlink frequency — but a satellite traveling at 7.8 km/s means those frequencies shift as it approaches and recedes. Here's how to understand and correct for it.
What is Doppler shift?
A LEO satellite moves at roughly 7.8 km/s (17,400 mph). As it approaches your location, the radio waves are compressed — the received frequency appears higher than nominal. As it recedes, the waves stretch and the frequency appears lower.
For a 145 MHz downlink, the total shift from horizon to horizon is typically ±3–4 kHz — small enough that an FM radio's capture range handles it automatically. For a 435–437 MHz downlink, the shift is ±9–11 kHz, which FM radios still manage but SSB operators must tune manually.
Practical rule for FM satellites: set your radio to the nominal frequency and leave it. Your 5 kHz FM deviation captures the Doppler swing without tuning.
Practical rule for SSB/linear satellites: start your receive VFO about 10 kHz above the published downlink center at AOS, and tune down through the pass, arriving roughly 10 kHz below at LOS. Software like Gpredict can drive your radio's VFO automatically via CAT control.
Doppler rule of thumb
CTCSS tones for FM sats
Some FM satellites require a CTCSS (PL) tone to access the transponder. Without the correct tone, the satellite ignores your uplink.
Doppler shift calculator
Enter your satellite's downlink frequency and expected pass elevation to see the expected shift range.
Doppler Shift Calculator
At AOS/LOS (max shift)
±3.3 kHz
145796.7–145803.3 kHz
Mid-pass (typical)
±1.6 kHz
145798.4–145801.6 kHz
At overhead
±0.0 kHz
145800.0–145800.0 kHz
For FM voice (wide-band), your radio will often capture the signal across the full Doppler swing. For SSB/CW linear transponders, you must manually tune 6.6 kHz across the pass.
Frequency reference
Nominal frequencies — apply Doppler correction as described above. Verify operating status with AMSAT before each pass.
| Satellite | Lic | Uplink | Band | Mode | CTCSS | Downlink | Band | Mode | Doppler ± | |
|---|---|---|---|---|---|---|---|---|---|---|
| ISS (ARISS) Voice25544 | Tech | 145.990 | 2m | FM | — | 145.800 | 2m | FM | ±3.5 kHz | TLE › |
| ISS APRS Digipeater25544 | Tech | 145.825 | 2m | APRS | — | 145.825 | 2m | APRS | ±3.5 kHz | TLE › |
| SO-50 (SaudiSat-1C)27607 | Tech | 145.850 | 2m | FM | 74.4 Hz burst67 Hz | 436.795 | 70cm | FM | ±9.5 kHz | TLE › |
| AO-91 (RadFxSat)43017 | Tech | 435.250 | 70cm | FM | 67 Hz | 145.960 | 2m | FM | ±9.5 kHz | TLE › |
| AO-92 (Fox-1D)43137 | Tech | 435.350 | 70cm | FM | 67 Hz | 145.880 | 2m | FM | ±9.5 kHz | TLE › |
| PO-101 (Diwata-2)43678 | Tech | 437.500 | 70cm | FM | 141.3 Hz | 145.900 | 2m | FM | ±9.5 kHz | TLE › |
| AO-73 (FUNcube-1)39444 | Tech | 435.150 | 70cm | USB | — | 145.935 | 2m | LSB↕ | ±9.5 kHz | TLE › |
| FO-29 (JAS-2)24278 | General | 145.950 | 2m | USB | — | 435.850 | 70cm | LSB↕ | ±9.5 kHz | TLE › |
| AO-7 (Phase 2B) — Mode A7530 | General | 145.850 | 2m | USB | — | 29.450 | 10m | LSB↕ | ±3.5 kHz | TLE › |
| AO-7 (Phase 2B) — Mode B7530 | General | 432.125 | 70cm | USB | — | 145.975 | 2m | LSB↕ | ±8 kHz | TLE › |
↕ = inverting transponder. Transmit the opposite sideband from what you want to receive.
Operating tips
On linear transponders, you should hear your own signal in the downlink. If you hear yourself sounding "clean," your transmit power is appropriate. If you hear nothing, you are likely off-frequency. If you're loud and distorted, reduce power — you're stealing bandwidth from other operators.
AO-7, AO-73, and FO-29 all use inverting transponders. Transmit USB on your uplink and you'll receive LSB on the downlink — as shown in the ↕ column of the frequency table. Your downlink tuning moves opposite to your uplink VFO. Gpredict handles this automatically.
FM satellites are shared resources. 5 watts into a Yagi is typically more than enough. Cranking to 50 W can desensitize the satellite's receiver and block other operators. If the sat is quiet, lower power first, not higher.
Many satellites transmit a CW beacon on their downlink continuously. You can receive and decode these without a license as listening-only practice. Great way to verify your antenna is pointing correctly before transmitting.
Next step
Find passes for your location — when to point your antenna and where.