Fix DGPS section.

Differential works by transmitting pseudorange errors, not position
errors.  While trying  to avoid too  much detail, make the point that
all unmodeled errors end up in the mix; DGPS doesn't specifically
estimate extra delay separately from clock errors and orbit errors.

1 files changed, 16 insertions, 10 deletions

diff --git a/www/gps-hacking.html b/www/gps-hacking.html
index 5de1320a..6827cf92 100644
--- a/www/gps-hacking.html
+++ b/www/gps-hacking.html
@@ -146,16 +146,22 @@ the signals.  The U.S. military knew the pattern, but nobody else did
 
 <p>Now that 'Selective Availability' is gone, the important limits are
 natural.  One is a variable amount of signal lag produced as the GPS
-signals pass through the ionosphere, which partly refracts radio
-waves.  This can be largely compensated for by a technique called
-"Differential GPS" or DGPS.  Ground stations are established in
-known fixed positions, and compare their calculated GPS locations
-with their known values.  Thus, they can measure for the
-ionospheric lag and tell nearby GPSes the corrections required.
-This information may be broadcast via radio (Ground Based Augmentation
-Systems), or via satellites (Space Based Augmentation Systems).
-Examples of SBAS are WAAS and EGNOS.  See <a href="#ggps">DGPS, WAAS,
-and EGNOS</a> for details on how this works.</p>
+signals pass through the ionosphere and troposphere, which partly
+refracts radio waves.  This can be largely compensated for by a
+technique called "Differential GPS" or DGPS.  Ground-based reference
+stations are established in well-surveyed locations, and compare
+measured ranges (pseudoranges to be precise) with their calculated
+values.  These errors account for unknown propagation delays, clock
+errors, and any other unmodeled errors.  The reference stations can
+then tell nearby GPS receivers the required corrections, which are
+then applied to observed pseudoranges before computing a position.
+This information may be broadcast via radio (called "Ground Based
+Augmentation Systems"), or via satellites (called "Space Based
+Augmentation Systems").  An example of GBAS is the National
+Differential GPS system (NDGPS) in the US, transmitting corrections
+around 300 kHz.  Examples of SBAS are WAAS and EGNOS.
+See <a href="#ggps">DGPS, WAAS, and EGNOS</a> for details on how this
+works.</p>
 
 <p>In practice, the most important limit on accuracy is the actual
 visibility of satellites.  A timing signal has to be fairly strong and