[Rhodes22-list] Hanks & Daves GPS Compass Analysis

[Ronald Lipton]

The GPS knows it's location based on the relative time of reception
of signals from the various satellites.  It solves an equation like
(r-distance to satellite i) = speed of light (t - time of emission from 
satellite i)
so it can only determine it's position with respect to the constellation
of satellites.  Direction and speed are calculated from sucessive 
measurements.

The accuracy need to achieve on meter resolution is about 3 billionths of a
second.  The fact that all of this works depends on corrections due to
both special (the speed of the satellites) and general relativity (the 
graviational
field at the satellite radius.  So you all should thank Einstein the next 
time
you avoid a dangerous shoal.

Ron
----- Original Message ----- 
From: "Hank" <hnw555 at gmail.com>
To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
Sent: Thursday, April 07, 2005 3:01 PM
Subject: Re: [Rhodes22-list] Hanks & Daves GPS Compass Analysis


> Ed,
>
> Try this and see if it makes a difference.  Walk down the street with
> the compass pointed ahead of you and then walk the same path with the
> compass pointed to the side and see if the compass heading reads any
> different.
>
> Hank
>
> On Apr 7, 2005 3:55 PM, ed kroposki <ekroposki at charter.net> wrote:
>> Hank,
>>        You made me go back and look at the directions.  Yes, movement is
>> involved, but the heading is based on satellite fixes.  The north stays 
>> put
>> at north, the heading varies.  So yes you can figure out true north and 
>> set
>> a compass by it.
>>        The compass mode is not instantaneous but takes 15 to 20 feet to
>> tune in an accurate heading.  My driveway is about 10 degrees west of 
>> north.
>>
>> Dave,
>>        Note that my GPS is a Magellan Meridian with a compass mode, not
>> just a heading.  You can also use the heading mode to a fixed landmark. 
>> If
>> you walk slowly the compass mode works like a hand held compass.  If you
>> stop it maintains the last direction.  I just went and walked up and down
>> the street in front of my house to check it out.  As long as you are 
>> moving,
>> it has an N that rotates like a compass.  As you change directions the 
>> big N
>> points north within the distance stated above.  Turn the GPS while 
>> walking
>> and N points north.
>> Ed K
>> Greenville, SC, USA
>>
>> -----Original Message-----
>> From: rhodes22-list-bounces at rhodes22.org
>> [mailto:rhodes22-list-bounces at rhodes22.org] On Behalf Of Hank
>> Sent: Thursday, April 07, 2005 9:11 AM
>> To: The Rhodes 22 mail list
>> Subject: Re: [Rhodes22-list] Slim's Compass
>>
>> Ed,
>>
>> It is true that the GPS tries to show a heading of where it is
>> pointed, but I suspect it is deriving that from forward movement.
>> Next time you go out, put the GPS perpendicular to your direction of
>> travel for a few minutes and see what it shows for a heading.  I
>> suspect it will be based upon your movement, not on where it is
>> pointed.
>>
>> Hank
>>
>> On Apr 7, 2005 9:00 AM, ed kroposki <ekroposki at charter.net> wrote:
>> >
>> >
>> > Slim:
>> >        If you are going to keep the compass on the boat, then do a 
>> > basic
>> > calibration.  If your drive is paved, then get a cheap scout compass 
>> > and a
>> > piece of chalk.  Plot N-S, E-W on the driveway, or street in front of 
>> > your
>> > house.  Adjust the compass to N-S, then E-W, then S-N, then W-E and 
>> > again
>> to
>> > fine tune.
>> >        The headings may not be perfect, but they will give you the 
>> > general
>> > direction, so that you do not do Brad's trick of going in circles.  If 
>> > you
>> > decide to do serious navigating then do more serious calibrations of 
>> > the
>> > compass.  Keep in mind it will never be perfect to a degree.
>> >        As to which way do charts use, just look at a navigation chart 
>> > in
>> > the light.  The answer is usually somewhere on the chart in degrees.
>> >        Bill said that a GPS is referenced from where you are coming 
>> > from.
>> > On my Magellan Meridian, it tries to show a heading of where it is
>> pointed.
>> > If you use a Meridian GPS, you should be able to calibrate the compass 
>> > to
>> > within a few degrees.
>> >        Are you planning a cruise across Lake Superior this summer?
>> > Are you planning a visit to your mother's anytime soon?
>> >
>> > Ed K
>> > Greenville, SC, USA
>> > Addendum:  Navigators answer:   A number of different approaches are 
>> > being
>> > tried. (We are still guessing at this point.)
>> >
>> > __________________________________________________
>> > Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>> >
>> __________________________________________________
>> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>>
>>
>> Name: !cid_010101c4b914$76242020$0a00000a at D9X6Q611.gif Type: image/gif 
>> Size: 16714 bytes Desc: not available
>> Url: 
>> http://www.rhodes22.org/pipermail/rhodes22-list/attch/200504/07/cid_010101c4b914762420200a00000aD9X6Q611.gif
>>
>>
>> __________________________________________________
>> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>>
>>
> __________________________________________________
> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list