{
  "$type": "com.whtwnd.blog.entry",
  "theme": "github-light",
  "title": "X-band radios... check!  Ready for first light next week!",
  "content": "Today we tested our X-band radios and they are working fine.  That is a huge relief.\n\nWe have two separate radios on each of four spacecraft: an S-band transmitter (2-4 GHz, roughly the same band as a microwave oven) and an X-band transmitter (7-11 GHz, i.e. about two octaves higher).  We use the S band for commanding and for basic telemetry, but we're limited to about 5 Mbps over that radio.  The X band gives us a higher data rate, about 25 Mbps.  That's important, because it means we can downlink a whole day's worth of data in a single ground station pass.\n\nDuring commissioning, we've been sighting in exactly how to track the spacecraft.  NORAD provides tracking information (via space-track.org; you can also see those data as visualizations of every orbiting object, at n2yo.com), but we produce even better tracking/prediction information ourselves, from on-board GPS receivers on each spacecraft.  NORAD is good to a few hundred meters.  GPS is accurate to within a few meters, which is pretty spiffy in itself -- but also gives the best possible prediction for where each PUNCH spacecraft will be, in the sky.\n\nTracking information is important, because ground stations have to know pretty much *exactly* where to look for each satellite as it goes overhead.  They have to \"paint\" the spacecraft with a tiny pencil beam.  We use roughly 7 meter ground station dishes, so the diffraction limit (1.22 λ/d) for the S band is about 15 arcmin -- or something like 5km wide at PUNCH.  The X band is four times narrower -- just over 1km wide at orbit!\n\nFor a while our GPS tracking data were wildly inaccurate: instead of a few meters, it was a few *kilometers* off.  That boiled down a to timestamp issue: we were using local computer time instead of the reported \"GPS time\".  Sometimes those were off by up to a half-second.  At an orbital speed of about 7 km/sec, that's a big deal.  It meant that we could easily lock on with S band, but often missed the spacecraft entirely with X band, and those ground passes would be \"missed\".\n\nSo tracking was a really big deal.  Last week, we sorted that out and started using better GPS timestamps, and our tracking errors dropped by a factor of 1,000.  Today, we tried several downlink passes with the X-band radios, and they worked like a champ.  Phew.\n\nWe're doing a little more shakeout Thursday and Friday, and we're planning on opening our first doors on Monday the 14th.\n\n\n",
  "createdAt": "2025-04-10T03:47:50.593Z",
  "visibility": "public"
}