From d20910b125a707b831171a03095cd73be3024f66 Mon Sep 17 00:00:00 2001
From: "Eric S. Raymond"
Without an almanac, your GPS has trouble finding the satellites. +The satellites broadcast on a known frequency, but they are moving, +and that gets shifted all over the place by the Doppler effect. ("All +over" means a big shift relative to the bandwidth of the signal.)
+ +If you have a recent almanac and you know the date/time and location, then +you can compute the Doppler and look in the right frequency and find the +satellites quickly. In this context, "find" means hearing a signal at an +expected frequency. If you don't hear anything where you expect it, then you +get to check nearby frequencies. If you don't find anything nearby, you get +to give up and start searching the whole Doppler range. This is the difference +between warm start and cold start.
+ +Once you do see one or more satellites, you can figure out the +date/time and location and after a while get a new almanac. This will +be stored in non-volatile memory in your devices and make subsequent +satellite acquisitions faster, until it gets stale.
+Warm start on a modern GPS with a good skyview (4 or more sats visible) normally takes about 30 seconds. (Vendor spec sheets fib by quoting this time only, leaving out the cold-start lag to fetch diff --git a/www/hacking.html.in b/www/hacking.html.in index 9b95f497..0ddcf648 100644 --- a/www/hacking.html.in +++ b/www/hacking.html.in @@ -1364,7 +1364,7 @@ suppose the control string were a baud-rate change?
When using gpsd as a time reference, one of the things we'd like to do -is make the amount of lag in the message path from GPS to GPS small +is make the amount of lag in the message path from GPS to GPS as small and with as little jitter as possible, so we can correct for it with a constant offset.
@@ -1373,8 +1373,8 @@ device UART to 1 using TIOCGSERIAL/TIOCSSERIAL. This would, in effect, disable transmission buffering, increasing lag but decreasing jitter. -But it's almost certainly not worth the work. Rob Janssen, our timekeeping -expert, reckons that at 4800bps the UART buffering can cause at most +
But it's almost certainly not worth the work. Rob Janssen +reckons that at 4800bps the UART buffering can cause at most about 15msec of jitter. This is, observably, swamped by other less controllable sources of variation.
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