Documentation and FAQ updates.

3 files changed, 25 insertions, 3 deletions

diff --git a/TODO b/TODO
index fb6bbd8c..eaca870b 100644
--- a/TODO
+++ b/TODO
@@ -31,6 +31,10 @@ will typically cause extra work once per seven days.  The benefit is that
 we can compare received week to the compiled in week and if the latter
 is less know that a rollover has occurred.
 
+**** Maybe apply wraparound compensation ****
+
+See <http://www.febo.com/pipermail/time-nuts/2013-August/079271.html>
+
 **** Track gpsd's confidence in the GPS time it's seeing
 
 This would increase when we see a leap-second offset or get GPZDA from
diff --git a/www/faq.html.in b/www/faq.html.in
index 552549fd..119e12e4 100644
--- a/www/faq.html.in
+++ b/www/faq.html.in
@@ -554,6 +554,24 @@ to get a new satellite <i>almanac</i> to do its job. The satellites
 broadcast this information very slowly (at 50bps) on a fixed schedule,
 and it can take up to 20 minutes.</p>
 
+<p>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.)</p>
+
+<p>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.</p>
+
+<p>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.</p>
+
 <p>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?</p>
 <h2 id="fifo">Setting FIFO threshold to 1 to reduce jitter in serial-message times</h2>
 
 <p>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.</p>
 
@@ -1373,8 +1373,8 @@ device UART to 1 using TIOCGSERIAL/TIOCSSERIAL.  This would, in
 effect, disable transmission buffering, increasing lag but decreasing
 jitter.</p>
 
-<p>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
+<p>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.</p>
 </ol>