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authorEric S. Raymond <esr@thyrsus.com>2005-07-04 04:49:48 +0000
committerEric S. Raymond <esr@thyrsus.com>2005-07-04 04:49:48 +0000
commitab049fc32676041a7fe2a16abd6cc9dea9f49255 (patch)
tree003bbbe01727127715ebcc40738aec2404de251d
parent6cecdba6513d00b119fff4070582c02bb5c86b45 (diff)
downloadgpsd-ab049fc32676041a7fe2a16abd6cc9dea9f49255.tar.gz
Fold in some advice from Wolfgang Rupprecht.
-rw-r--r--HACKING35
-rw-r--r--gpsd.c4
-rw-r--r--gpsd.xml2
3 files changed, 38 insertions, 3 deletions
diff --git a/HACKING b/HACKING
index 181b7fc4..24d78c90 100644
--- a/HACKING
+++ b/HACKING
@@ -388,6 +388,41 @@ which we round up to 2.
Due to multipath uncertainty, Carl says 4.1 is too low and recommends
a non-DGPS UERE estimate of 8. That's what we use.
+** Impact of DGPS on error estimates
+
+Wolfgang S. Rupprecht:
+> In any case, the 2 meter number almost certainly is for a differential
+> reference station that is located very close to the GPS in question.
+> The whole differential hack assumes that the paths and angles between
+> the reference station and the mobile GPS unit are similar enough that
+> one can simply subtract the path-length errors the reference station
+> is seeing from the path the mobile GPS had calculated. If the angles
+> to the satellite are vastly different (as in the case of dgpsip from
+> across the country) one can start adding errors. In the days of SA
+> the largest component of error was due to SA, so miscorrecting the
+> paths hardly mattered at all. Even cross-country dgpsip was
+> worthwhile and improved accuracy. I doubt that one would see any that
+> 2meter accuracy if one were more than a 100 miles away.
+>
+> Just for completeness, there are other issues too, like different
+> ionospheric and tropospheric pierce points for the rays going to the
+> reference and mobile units. The speed of light is slightly lowered by
+> going through the atmosphere. The amount it slows down is enough to
+> add a few meters (very roughly, up to 15 meters) of path-length error.
+> The ionosphere and troposphere aren't a constant height. They are
+> substantially pumped up by sunlight, so taking a reading from one
+> location in heavy sun and applying it to a distant area with a lower
+> sun angle could make things quite a bit worse. (There may also be
+> roughness associated with the layer thickness that defies modeling
+> with a simple sun-angle model. I don't know enough about that sort of
+> thing.)
+>
+> In summary, while I have no problem believing the SiRF engineer that
+> you can get 2 meters accuracy from their chips when using a very close
+> reference, I fear that the average user will be getting the same 8m or
+> worse accuracy by using dgpsip.
+
+
** Adding new GPS types
Almost all GPSes speak NMEA 0183. However, it may occasionally be necessary
diff --git a/gpsd.c b/gpsd.c
index 5b981945..56234fde 100644
--- a/gpsd.c
+++ b/gpsd.c
@@ -1312,10 +1312,10 @@ int main(int argc, char *argv[])
subscribers[cfd].active = now;
}
} else if (subscribers[cfd].device == NULL && timestamp() - subscribers[cfd].active > ASSIGNMENT_TIMEOUT) {
- gpsd_report(1, "client timed out before assignment request\n");
+ gpsd_report(1, "client timed out before assignment request.\n");
detach_client(cfd);
} else if (subscribers[cfd].device != NULL && !(subscribers[cfd].watcher || subscribers[cfd].raw>0) && timestamp() - subscribers[cfd].active > POLLER_TIMEOUT) {
- gpsd_report(1, "client timed out on command wait\n");
+ gpsd_report(1, "client timed out on command wait.\n");
detach_client(cfd);
}
}
diff --git a/gpsd.xml b/gpsd.xml
index dfeaef75..c6556bd9 100644
--- a/gpsd.xml
+++ b/gpsd.xml
@@ -594,7 +594,7 @@ computations at sufficient accuracy that it will add no measurable
position error of its own.</para>
<para>DGPS correction will reduce UERE from roughly 8 meters to
-roughly 2 meters, provided you are within 1000 kilometers or so of the
+roughly 2 meters, provided you are within about 100mi (160km) of a
DGPS ground station.</para>
<para>On a 4800bps connection, the time latency of fixes provided by