[gps-talkusers] how to deal with multi path in cities

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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