More than that, as I learned in designing mobile and portable
commercial equipment, cars and airplanes have pretty much infinite
power available, while the battery-powered iPhone does not. Radios
(especially GPS devices) require lots of power, draining the battery
fairly quickly. You can manage a modern cell phone radio to limit
power drain (thus extending battery life), but a GPS receiver needs to
be on all the time to maintain its status. The hall effect sensors
provide an excellent alternative to this, keeping the power drain low
so you can go several days without recharging.
By the way, TPA went through a runway renumbering because of mag north
change this past year. The real story appears to be that they actually
misnumbered at least one of the runways, but at least now they seem to
have it right.
Jack B, W6FB
On Jan 4, 2012, at 9:45 AM, Kevin Normoyle wrote:
> Your plane is moving, so differential measurements using GPS works.
> Human standing still, ain't.
> On 1/4/2012 9:37 AM, K8RI wrote:
>> On 1/4/2012 12:03 PM, Kevin Normoyle wrote:
>>> Iphone uses AKM AK8975 chip to sense the earth's magnetic field
>>> (four Hall
>>> effect sensors on a chip)
>>> so GPS isn't providing compass. I would think it's pretty
>>> accurate. Sure there's
>>> software and calibration, and the possibility for error, but I
>>> would imagine
>>> it's really good. I have a Garmin gps/compass that senses magnetic
>>> field for
>>> it's compass similarly.
>> If they have an app and GPS why on earth would they want to sense
>> magnetic North?
>> The nice thing about my GPS in the airplane is I don't have to
>> to get true North. With the magnetic compass you need to know the
>> magnetic variation or declination and add or subtract. IIRC When I
>> started flying it was + 5 degrees and now it's approaching - 7 in
>> Central Michigan. At one time it was zero. So for 50 years our
>> have all kept their same numbers. However they are fast approaching a
>> one digit change. This far North it can vary over 20 degrees in some
>> areas. In Northern Michigan it's even more than that, but due to the
>> iron deposits.
>> As for hams, it would seem to me that unless using a dish for EME or
>> very high gain array, "That looks about right" or "North is that
>> way" is
>> close enough for tribanders and even pretty big monobanders. Those
>> the only methods I've used for over 50 years on many antennas.
>> Roger (K8RI)
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