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

Mike, if the heading varies during signal degradation, would the location of what ever you are looking for vary as well?

Dave

taxation WITH representation isn't so hot, either!

----- Original Message ----- From: "Michael May" <mikemay@xxxxxxxxxxxxxxxx>
To: "GPS-talkusers-freelists.org" <GPS-talkusers@xxxxxxxxxxxxx>
Sent: Tuesday, June 27, 2006 7:19 AM
Subject: [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
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