Experiment

Satellite Radio SDR Capture

Can you capture a satellite radio downlink with a receive-only SDR at the predicted pass time, frequency, and sky direction the way orbital geometry predicts?

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Materials Needed

  • receive-only SDR such as an RTL-SDR or similar USB radio
  • simple VHF or UHF antenna matched to the target signal
  • laptop or phone software that shows a waterfall or spectrum display
  • predicted pass time and direction for a public weather-satellite or lawful amateur-satellite downlink
  • clock, observation notes, and optional recording software

Setup Steps

  1. Choose one known public or otherwise lawful satellite downlink you are allowed to monitor, such as a NOAA APT weather-satellite pass or a published amateur-satellite beacon. Keep the experiment strictly receive-only and follow your local radio rules.
  2. Use a pass predictor for your own location and record the expected rise time, peak time, set time, and approximate maximum elevation.
  3. Tune the SDR to the published downlink frequency a few minutes before rise and watch the waterfall while pointing or orienting the antenna toward the expected sky path as well as you can.
  4. Record when the signal first appears, when it is strongest, whether the frequency drifts slightly during the pass, and when it fades out again.
  5. If the signal type supports easy decoding, save one recording and decode it after the pass so you can compare the received data or image with the predicted pass window.

Expected Calculation

visible=elevation > 0° (line of sight)
pass window=rise → peak elevation → set
fobs=ftx(1 − vradial / c)
T=2π √(R + h)3 / GM

A real satellite pass is line-of-sight limited. The downlink should appear only after the satellite rises above your local horizon, usually strengthen near the highest elevation, and fade near set. Because the satellite is moving rapidly relative to the observer, many SDR captures also show a small Doppler-related frequency shift through the pass, especially on narrower signals.

Graphic

This visual shows the core geometry the experiment is testing before the real-world observation is made.

Pass geometryRadio evidenceRisePeakSetSDR + antennaHorizonStrongest near peakAppears near riseFades near setWaterfall can show Doppler driftMatch pass time, sky direction,and the limited receive window

What to Observe

Downlink137.100 MHz
ModeAnalog APT weather image
Band137 MHz weather-satellite downlink
What to listen forA bright analog weather-satellite signal that can be decoded into a line-by-line cloud image.
How to decodeRecord the wide-FM audio during the pass and feed it into an APT weather-image decoder after or during reception.
Example decoderAny NOAA APT image decoder that accepts SDR audio or WAV recordings.
Expected outputBlack-and-white weather image strips that combine into a cloud picture.

Good first SDR target. Expect a strong moving signal only during the local pass window.

  • The signal appears near the predicted rise time instead of at arbitrary times all day.
  • Reception is usually strongest near the highest-elevation part of the pass and weakest near the horizon.
  • The waterfall often shows a continuous moving trace over a limited time window rather than a stationary local transmitter.
  • On suitable signals, the center frequency can drift slightly during the pass in a pattern consistent with satellite motion.
  • If you decode a public weather-satellite image or telemetry beacon, the recording lines up with the same pass window and sky track prediction.

Interpretation

This is a stronger follow-up to simply seeing a moving light. You are matching a predicted orbit pass to a real radio signal with limited visibility, pass-timed appearance, and often measurable Doppler behavior. That combination fits an orbiting transmitter far better than a vague local-tower or balloon substitute story.

Sources

Evidence Card

Claim

Can you capture a satellite radio downlink with a receive-only SDR at the predicted pass time, frequency, and sky direction the way orbital geometry predicts?

Model Used

The experiment’s stated geometry or physical model with fixed inputs across comparisons.

Formula

visible=elevation > 0° (line of sight)
pass window=rise → peak elevation → set
fobs=ftx(1 − vradial / c)
T=2π √(R + h)3 / GM

Last Verified

2026-06-30

Assumptions

  • The protocol holds location, height, and distance inputs fixed while comparing observation to prediction.
  • A single ambiguous photo is not enough; repeatable measurements matter.
  • Primary-source references carry more weight than generic summaries.

External Sources

What Would Falsify This Page

A repeatable observation that matched the competing model better than this experiment’s stated prediction would count against this page.

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