Stefano Rampazzo

@ibelings you rock !

I've been itching to dig into LTE physical-layer stuff for a while, and Starlink DTC turned out to be the perfect excuse to go end-to-end. So I threw together a little SDR pipeline from scratch on a Mac and pointed it at the Band 25 downlink to see if I could pull anything sensible out of the air. Setup was a USRP B210, a conical antenna, GPSDO for clock reference, and a window with a decent slice of sky. Record IQ at 7.68 MS/s around 1992.5 MHz, then a Swift app (with some coding help from Claude) that walks the samples through the usual LTE steps. The first goals were pretty straightforward: find which cells (and thus which satellites) are visible right now via their Physical Cell IDs, measure signal strength, estimate the leftover carrier-frequency offset after Starlink's Doppler pre-compensation, and match each detection back to a specific satellite using SGP4 against fresh Celestrak TLEs. The ladder from raw IQ to PCI is short but every step has its quirks: PSS, the Primary Synchronization Signal. Three possible Zadoff-Chu roots (25, 29, 34) on 62 subcarriers around DC. Matched filter against each and the strongest peak gives you timing plus N_ID(2). SSS right before it. Two interleaved m-sequences scrambled by N_ID(2). Correlate across the 336 possibilities to get N_ID(1) and the half-frame. PCI = 3 * N_ID(1) + N_ID(2). Done. One number from 0 to 503. With PCI known you can place the Cell-specific Reference Signals for RSRP, pull a coarse CFO from the PSS, and so on. Da​niel Estévez's LTE write-ups (destevez.net, April and May 2022) were pure gold here and are worth reading for anyone doing similar work. Then the fun part: group detections by PCI, fit the observed frequency offsets against the predicted line-of-sight Doppler from every candidate satellite using SGP4 + least squares. Best match wins, and the residual tells you how clean the pre-compensation actually is. Early results (taken with a big grain of salt): the pipeline reliably picks up multiple satellites per minute, often several different PCIs from the same bird during one pass. The trajectory fits look solid even with TLEs aged a day or two. On beam-center cells the residual Doppler is small enough that the linear trend is basically flat across the whole pass, instead of the tens of kHz you'd see from raw orbital motion. Off-center cells show bigger swings, which makes sense if each PCI is tied to a different spot beam. Beyond DTC itself, a coherent ground station at 2 GHz that can see every visible Starlink also opens doors to ionosphere phase work, bistatic passive radar with Starlink as illuminator, and time-transfer experiments. None of which I've tried yet, but the same pipeline opens the door to all of them. This is very much a side project and still a work in progress. The fun is in the building.

Just got my first iPad app running: full local ADS-B aircraft decoding on a 2020 iPad with direct-connected PlutoSDR. No internet needed. This is only the beginning of my iPad SDR plans. 🚀

@ibelings AirSpy and SDR++ should work on a Mac with a Silicon processor, but I don't have an AirSpy to test. SDRPlay and SDRConnect work without any problems.

@s_ramp I have an Airspy somewhere maybe a Discovery. Not sure if it works on a Mac

I want to upgrade my SDR to an RTL-SDR. Which one should I get these days?

Broadcast audio from your SDR receiver across your local network with VB-Audio Virtual Cable and VBANserver. Download and read the tutorial. sramp.com/products/vbans… #sdruno #sdrconnect #swl

SWLtools 1.3 has been released right now. Fixed several issues that recently arose due to conflicts between various settings in the audio section. Refactored the entire codebase to ensure greater app stability. apps.apple.com/us/app/swltool… #swl #bcl #ios

A NIGHT OF FIRE ON 60 METERS: 3X3A MAKES HISTORY FROM GUINEA! Île de Roume 5356 khz - 60m #ft8

SWLtools in action 11039 khz DWD 2 Pinneberg RTTY Display IQ streaming from the Belka Free download apps.apple.com/us/app/swltool… #swl #rtty #sdr #belka

Great free iOS app, Echo, can connect to over 2,000 remote SDR receivers (KiwiSDR, OpenWebRX, WebSDR, and FM DX) by Mark Garrison  apps.apple.com/us/app/echo-gl… #swl #bcl #hamradio #KiwiSDR #OpenWebRX #WebSDR #FM

Do you have a .ddd tachograph file and don't know how to read it? Extract the most important information into a clear, readable PDF report in seconds. Try the service here: tacho.sramp.com #trucking #tachograph #logistics #transport #trucks

Promo codes for SWLStation YR4XM3KPN967 9YE76XXPYH4P HEMRTYJM4R3K 4JH7P3YFATWX 9TERMJNXLPJ3 WJEXYHKMK6M3

SWLStation - SWLTools - SWList Three iOS app for swl #hamradio #swl #bcl

New free geoJSON viewer sramp.com/geojson/info.h… with automatic CRS conversion #geojson

With hamlib and VBAN listen to your sdr radio with your iPad. Read the setup guides here sramp.com #swl #swlstation #swlist #bcl #hamradio

Hai un file .ddd del tachigrafo e non sai come leggerlo? 🚛💨 Estrai le informazioni piú importanti in un report PDF chiaro e leggibile in un secondo. Prova il servizio qui: 👇 tacho.sramp.com #autotrasporto #tachigrafo #logistica #trasporti #trucks

6 more promo codes on request

📻 SWLstation is finally here! > A new iPad experience designed for shortwave listeners. Track stations, log your catches. I’m giving away 3 promo codes to the community: 🧵 (Link and codes in the first reply below! 👇) #swl #shortwave #hamradio #iosdev

apps.apple.com/us/app/swlstat… KHTPETE9E6W7 XFHHM3ER4YHF WHT479KRKPNP