[time-nuts] OT - GPS and North

[Magnus Danielson]

mike cook wrote:
> 
> ----- Original Message ----- From: "Magnus Danielson" 
> <magnus at rubidium.dyndns.org>
> To: "Discussion of precise time and frequency measurement" 
> <time-nuts at febo.com>
> Sent: Saturday, November 21, 2009 4:05 PM
> Subject: Re: [time-nuts] OT - GPS and North
> 
> 
>>
>> Robert Darlington wrote:
>>> It depends on the GPS receiver.  The GPS chipset won't know (it knows 
>>> where
>>> it is and can remember where it was to know what direction it's 
>>> moving), but
>>> some consumer GPS receivers (Garmin, Magellan) have electronic compasses
>>> built in.  My Garmin eTrex Legend does NOT have a compass built in, but
>>> taking about 2 steps in any direction will tell me which way I'm 
>>> moving. It
>>> can't do this when stationary.  My buddy's eTrex can point north without
>>> moving, but that feature drains the battery faster.
>>
>> The actual GPS receiver and the navigator is two different things. A 
>> GPS navigator may use additional sensors such as accelerometers and 
>> magnetic sensor to aid in addition to the GPS receiver built into them.
>>
> 
> Agreed Magnus, but I dont think any gizmos are required. If the the 
> positions of the satellites are known, as they must be to enable the 
> antennas position to be calculated, I think just an extra set of 
> calculations is necessary to indicate the direction to anywhere else on 
> the planet (or elsewhere) including the geographic poles. Getting the 
> magnetic pole directions would need something else I suppose.

Yes, but you don't know in what direction from the unidirectional 
antenna. The antenna has no sense of direction, but the receiver knows 
very well in what direction relative north each sat is. The NMEA stream 
even include the coarse directions.

If you has a steerable directional antenna, several antennas or moveable 
antenna, then you can convert the internal directions of the antenna 
into a north heading or heading towards anything else.

But a normal unidirectional GPS antenna and associated receiver will be 
able to know where it is and from it provide very high precision 
direction relative north, but unless you move it or make any other form 
of aid, it will not be able to show the heading... but when you move 
around in with the car, then it can show it relative to the current 
heading of the car... as that forms a vector for which you can relate an 
angle... to north or to your intended destination.

Cheers,
Magnus