Satellite Tracking

The "where is the ISS / NOAA-19 / SO-50 / a Starlink train right now and when will it pass over me?" tier. Powered by NORAD Two-Line Elements (TLEs) updated daily from celestrak.org / space-track.org. Used for visual passes (binoculars + iPhone), photographic passes (long exposure of ISS transit), ham-radio satellite QSOs (linear-transponder cubesats, FM birds), and ground-station automation (rotor + radio Doppler correction).

For pro-am ground-station networks like SatNOGS see SatNOGS & Pro-Am; for weather-satellite imagery decoding see Weather Satellites; for SDRs that go on the antenna see SDR Receivers; for non-ham satellite-pass apps see Astro Cloud & Weather; for the planetariums that show passes see Planetariums.

Free / OSS desktop trackers

• ★ ★ Gpredict — GPL, Win/macOS/Linux. The OSS pass predictor: live tracking, Doppler frequency calculation, antenna rotator control via Hamlib's rotctld, multiple TLE sources, scheduled-pass tables. The default for ham-radio satellite ops on Linux/Mac. Pi-friendly.
  • http://gpredict.oz9aec.net
• ★ Stellarium with NORAD TLE plug-in — free; built in; auto-updates TLEs. The visual / photographic pass tool. See Planetariums.
• Orbitron (Sergey Stoff) — freeware Win. Older but rock-solid; many ham operators still prefer it. Closed binary.
• PreviSat — free Win/Linux; multi-language; clean.
• PyEphem / Skyfield — Python libraries; free; for scripting custom pass workflows. Skyfield is the modern pick.

Web / mobile

• ★ Heavens-Above — see Astro Cloud & Weather. The classic; visual pass forecast, ISS / Hubble / Tiangong / Starlink trains.
• N2YO.com — free web; live tracking maps.
• ★ ISS Detector — free + paid; Android/iOS; push alerts for ISS / brighter sats / iridium remnants.
• GoSatWatch — paid iOS/Mac.
• AmsatDroid Free — Android; ham-focused.
• SatSat — iOS; ham-focused.

Antenna rotor / radio control

• ★ Hamlib's rotctld — free; the universal rotator daemon; Gpredict, SatNOGS, Predict all talk to it. Supports Yaesu G-5500 / SPID / RAS / Alfaspid / Easycomm.
• PstRotator — paid Win; closed; very popular in ham community; integrates with N1MM, HRD, Gpredict.
• GS232 / Easycomm — protocols spoken by most rotators.
• SatPC32 — paid Win; another ham favorite.

Doppler correction

LEO sats sweep ±10 kHz on 2 m, ±25 kHz on 70 cm, ±70 kHz on S-band; correcting for that doppler in real time is a basic requirement. Gpredict + rigctld (Hamlib) talks to ICOM IC-9700 / Yaesu FT-991A / Kenwood TS-2000 directly; SDRs (HackRF / SDRplay / RTL-SDR) via gqrx-frequency or SoapySDR.

Visual / photo specifics

• ISS transits across Sun / Moon — apps like Transit Finder (free web by ISS-Transit.com) compute centerline + path; Stellarium can plot.
• Starlink trains — Heavens-Above and Stellarium both show the train; "find the launch yesterday" is the request that made these apps viral 2019–22.
• Iridium flares — gone; the original satellites de-orbited 2018–19. Iridium-NEXT replacements don't flare.

Ham-radio satellite specifics

• ★ AMSAT — global ham-satellite organisation; cube-sat builders; sat news; ARISS school contacts. Free membership info.
• Hams.at (Mike, KE8DDV) — newer "satellite social network": who is operating which satellite right now, log a contact, see leaderboards.
• AMSAT-DL — German AMSAT; especially active on Es'hail-2 / QO-100 (geostationary linear transponder).
• Active FM birds (May 2026): SO-50, AO-91 (limited), PO-101, IO-117 (digital), Tevel-2 cubesats.
• Active linear transponders: RS-44, FO-29 (occasional), JO-97, AO-7 (heritage).

TLE sources

• ★ Celestrak (celestrak.org) — free; the canonical public mirror.
• ★ Space-Track (space-track.org) — free with registration; authoritative US-government source; required for non-public sats (some classified, some "elsets withheld").
• AMSAT TLE distribution — focused on amateur sats; updated daily.
• TLEs go stale within 1–2 weeks for LEO; refresh nightly for any automation.

Ground-station automation

For full unattended decoding of cubesat downlinks see SatNOGS & Pro-Am; for the Pi at the rotator see Pi-based Astro Control for adjacent patterns.

What's changing in 2024–2026

• Starlink dominance — ~6000 active sats; passes are constant; visual hobby is now "go AWAY from the Starlinks".
• QO-100 / Es'hail-2 — geostationary ham transponder; the big story since 2019; consistent SSB / digital activity.
• Cubesat explosion — many new amateur sats; SatNOGS network captures the rest.
• Smartphone TLE apps — Stellarium Mobile, SkySafari Plus, ISS Detector all do passes well now.

License / pricing summary

• Free / OSS: Gpredict, Stellarium, PreviSat, PyEphem, Skyfield, Hamlib, Heavens-Above, N2YO, Celestrak, Space-Track.
• Free + paid: ISS Detector, AmsatDroid Pro.
• Paid Win-only: PstRotator, SatPC32, Orbitron (free but closed).
• Paid mobile: GoSatWatch, ISS Detector premium.

Pick this if…

• Default OSS desktop: Gpredict.
• Visual / photographic passes: Stellarium.
• One free mobile app: ISS Detector.
• Web, no install: Heavens-Above.
• Ham-radio sat operating, Win: SatPC32 or PstRotator + Gpredict.
• Rotator automation, OSS: Gpredict + Hamlib rotctld.
• Build it into a script: Skyfield (Python).