The 20,000km view: How GPS works
summary
Explains that GPS measures time-of-flight from satellites at 20,000 km altitude moving ~14,000 km/h, requires relativity corrections, and recovers signals that start as 500 watts -> -18 orders magnitude when they arrive by using direct‑sequence spread spectrum and PRN codes. Navigation data is only 50 bits per second nav message so cold starts take ~12.5 minutes while ephemeris downloads give warm starts in 18–48 seconds; assisted GPS and open SDR hardware (a GPS RF board prototype for rockets) are used to get fast, hackable fixes.
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chapters (15)
- 0:00Intro
- 0:41Overview
- 1:32What is GPS
- 6:28How far is GPS
- 8:21Spread spectrum
- 12:33History of GPS
- 16:20Transit
- 21:17Can we trust GPS
- 22:43GPS modernization
- 27:41Avocation message
- 29:28Ephemeris
- 31:03Lowassisted GPS
- 32:17Accuracy
- 34:32Openness
- 39:41Summary
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