The instruments. Three chains, three targets.
The rig is not one antenna. It’s three operating configurations of the same rooftop — a V-dipole on a pole for the polar pass, a one-metre offset dish aimed at GK-2A through the morning haze, and the same dish repointed south-southwest when Elektro-L5 decides to broadcast. Each configuration is its own instrument. Each instrument writes its own log.
137 MHz V-dipole
Target METEOR M2-4 (and M2-3)
A pair of crossed half-wave elements on a rooftop pole, listening for the LRPT downlink as METEOR sweeps overhead. The simplest instrument on the roof — and the one that has produced the only polar-orbit imagery in the register so far.
- Antenna
- Crossed V-dipole, linear polarisation
- Mount
- Rooftop pole, ~3.6 m, no rotator
- Frequency
- 137.900 MHz (LRPT primary)
- Sample rate
- 1.024 Msps (Nyquist for 1024 ksymbol/s QPSK)
- Gain
- RTL gain 25 dB (no physical LNA in chain)
- Decode
- SatDump on NAS, --dc_block=true mandatory for cu8
Operative note The V-dipole is linear and LRPT is circular, so there's an unavoidable ~3 dB cross-polarisation loss before anything else fails. The instrument's real failure mode isn't the polarisation though — it's the tuner gain. Pushing the R828D above 30 dB compresses the front end and raises the noise floor by 20+ dB; at 25 dB the rig is the quietest it has ever been measured.
L-band chain · GK-2A
Target GEO-KOMPSAT-2A Geostationary, 128.2°E
A one-metre offset dish on the rooftop, repurposed from a Ku-band TV setup, with a hand-wound RHCP helix at the focus and a SAWbird+ GOES doing the front-end work. Aimed thirty degrees up and 112° east of true. Decodes a full-disk Earth every ten minutes.
- Antenna
- 1 m offset parabolic (ex-Ku TV)
- Feed
- 3-turn RHCP helix, hand-wound
- Pointing
- az 112.6° true · el 29.6°
- Front-end
- Nooelec SAWbird+ GOES (LNA + SAW, ext. PSU)
- Frequency
- 1692.140 MHz (LRIT, BPSK 128 ksymb/s)
- Sample rate
- 2.4 Msps cu8
- Post
- Sanchez · geostationary projection · blue underlay
Operative note The chain is shared between GK-2A and Elektro-L5 — only the pointing changes. SAWbird+ GOES is externally powered; the SDR's bias-tee must remain off. Beamwidth at L-band with the 1 m dish is roughly 12°, so a re-aim for any non-GK-2A target is a deliberate manual operation, not a click.
L-band chain · Elektro-L5
Target Elektro-L5 Geostationary, 76°E · commissioning
The same one-metre dish and helix-and-SAWbird front-end as Instrument II, repointed south-southwest at the 76°E geostationary slot. Listening for Elektro-L5's sporadic LRIT broadcasts during its commissioning window — currently silent more often than not.
- Antenna
- 1 m offset parabolic (shared with II)
- Feed
- 3-turn RHCP helix
- Pointing
- az 190.9° true · el 58.0° (target 76°E equator)
- Front-end
- Nooelec SAWbird+ GOES (LNA + SAW, ext. PSU)
- Frequency
- 1691.000 MHz (LRIT)
- Schedule
- Sporadic — slots at :12/:27/:42/:57 UTC, mostly silent
Operative note L5 has been in commissioning since 2026-02-12, with broadcasts clustered around 08:00 UTC and rarely lit outside that window. The first-light attempt on 2026-05-17 produced zero CADU — Viterbi never synced, FFT confirmed no signal in band — leaving the unanswered question of whether the dish was on the bird, the polarisation sense was wrong, or L5 simply wasn't transmitting during that slot.
Standing notes
- Switching
- Only one chain is wired to the RTL-SDR at a time. The SPDT switch the rig was designed around is not yet installed, so moving between instruments is a manual coax swap and, for II–III, a physical re-aim.
- Cadence
- GK-2A transmits continuously; capture slots are scheduled on the hour and the half hour. METEOR is captured per-pass when the orbit is favourable. Elektro-L5 captures track its sporadic broadcast cadence, mostly within an 05:30–09:30 UTC window.
- Calibration
- The 137 MHz chain uses the always-on ORBCOMM constellation as a chain canary — if frames decode, the rig is alive. The L-band chains have no equivalent free signal; GGAK at 1693 MHz is a candidate but unverified on this rig.
- Retention
- Raw IQ from a successful decode is purged within an hour. Failed-decode IQ is held seven days in
/satellite/_raw-iq/needs-review/, then auto-deleted. - Provenance
- Every IQ pulled and processed is logged one JSON line at a time in
/satellite/.processed-iq-log.ndjson— keyed on filename and first-megabyte SHA-256, then projected to a canonical per-IQ state file the website reads from.