[gps-talkusers] how to deal with multi path in cities
- From: Michael May <mikemay@xxxxxxxxxxxxxxxx>
- To: "GPS-talkusers-freelists.org" <GPS-talkusers@xxxxxxxxxxxxx>
- Date: Tue, 27 Jun 2006 09:19:49 -0500
There is one minor down side to the latest super sensative GPS receivers. When
you are among tall buildings, it is amazing how well these receivers, like the
Holux, report several satellites. The problem is that the quality of those
signals is degraded because they are bouncing between the buildings. this is
called multi path. The good news is that your position can be tracked but the
bad news is that your accuracy and particularly your heading is degraded by
this multi path.
You'll know this is happening when you find your direction of travel swinging
all over the place. Walk in a constant direction and keep hitting the H key for
heading and see if there is any consistency or not. If you see 90 degree shifts
in your heading, you are experiencing multi path.
So I ask myself, "Self, what can I do about heading swings?" First, note that
the compass direction for your destination will not shift as the compass
reading is by definition constent. If your destination is reported as
Northwest, that will not change no matter what your relative heading says. You
must then rely on this compass direction by either using an actual tactile or
audio compass to verify your real heading, or determine your compass direction
from the GPS when you are not among tall buildings and keep track of that
heading once you are among the buildings. In other words, if the GPS heading is
going to be flakey, it is up to you to know the compass direction.
If you know the compass direction, you can then utilize the other information
the system is giving you to navigate a route or to use the getting warmer
method to get to your destination.
Besides the compass direction, you will want to rely heavily on the distance to
your destination or waypoint decreasing. Even if your heading is reported to be
South and your Destination is north, if the distance to the destination
decreases, don't turn around. A decreasing distance to your destination is
something you can depend upon.
This multi path is not something that will vary among receivers or systems. We
have tested the $900 Garmin Nuvi receiver against the Holux in this kind of
situation and there was no difference.
Until you get out of the tall buildings, pay close attention to that compass
direction and to the distance to destination.
Mike
Michael G. May
CEO Sendero Group
Developers and distributors of BrailleNote GPS
Distributing BrailleNote, VoiceNote, Talks, Miniguide, The Tissot Silen-T
tactile watch, and the ID Mate bar code reader
MikeMay@xxxxxxxxxxxxxxxx
http://www.SenderoGroup.com
1-888-757-6810, Fax (530) 757-6830, Mobile (530) 304-0007
Sendero Group, LLC
1118 Maple Lane, Davis, CA 95616-1723, USA
Latitude, 38 33 9.239 North
Longitude, 121 45 40.145 West
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