[time-nuts] Crude Survey Technique

[J. Forster]

The INS has to be 'aligned' before takeoff.

Remember, the earth is spinning on an axis, and the INS's platform is
stable in Inertial space.  If you know the INS is sitting on the ground
(or even deep in a mine shaft) it's just trig. Nothing external needed,
not even stars.

-John

====================



> The simplest ones do it by difference in position from one fix to another.
> no need for a map for True North, just trig. For Magnetic North you need a
> map with variation data. More advanced units have magnetic sensors and
> accelerometers. An interesting question that I once spent some hours
> discussing in a pub (bar) about 20 years ago is how does an inertial
> navigation unit determine True North without the aircraft moving? No
> magnetic sensors, just a self contained black box. We know it did it
> because we had just finished installing one and watched it do it. We
> worked it out after a few pints ;-) Hint, it takes longer the closer to
> the pole you are. For Magnetic headings you still need a table of
> variations. At least one common aircraft inertial nav unit has magvar
> tables that are several years out of date with the OEM (well current IP
> owner, they bought out the real OEM) say it's not possible to update them!
>
> Robert G8RPI.
>
>
>
>
> ________________________________
>  From: Neville Michie <namichie at gmail.com>
> To: Discussion of precise time and frequency measurement
> <time-nuts at febo.com>
> Sent: Thursday, 21 November 2013, 21:46
> Subject: Re: [time-nuts] Crude Survey Technique
>
>
> How do the car mounted GPS guidance devices find North?
> Is it by deciding which direction you are moving on the map that they have
> stored?
> It is amazing how quickly they show the orientation of the map/your
> direction of travel.
> Do they have a sensor to register when you turn a corner?
> What they do does not seem possible from just a series of position fixes.
> cheers,
> Neville Michie
>
>
>
>
>
> On 22/11/2013, at 7:45 AM, <TMiller at skylinenet.net> wrote:
>
>> If you want true north, set up a camera that has time or bulb shutter at
>> the south end of your property. Put in a marker stake. take a time
>> exposure at night with the camera facing north. If you expose for about
>> 30 minutes, you will get a landscape with the stars rotating around the
>> north star. You can then mark the north location on your property under
>> the north star.
>>
>> A compass siting can give the mag north.
>>
>> ________________________________________
>> From: time-nuts-bounces at febo.com <time-nuts-bounces at febo.com> on behalf
>> of Brian Lloyd <brian at lloyd.com>
>> Sent: Thursday, November 21, 2013 2:04 PM
>> To: Discussion of precise time and frequency measurement
>> Subject: Re: [time-nuts] Crude Survey Technique
>>
>> On Thu, Nov 21, 2013 at 12:52 PM, <johncroos at aol.com> wrote:
>>
>>> I wish to establish a north south line on my property to an accuracy of
>>> +/- 2 degrees.
>>> Could this be done by loading a T-bolt, Antenna, Power source, and
>>> laptop
>>> into my
>>> little red wagon? The idea being to find two positions several hundred
>>> ft
>>> apart where either LH or T-bolt Mon report the same latitude? Will
>>> either
>>> of these programs report to sufficient accuracy? The base line would be
>>> 300
>>> ft, though more is possible.I realizes that the T-bolt is not a survey
>>> device, but I can spend several hours fixing each position if required.
>>>
>>
>> True north or magnetic north? In any case, a good lensatic compass and
>> knowing your magnetic variation should easily get you to within 2
>> degrees
>> and is a lot easier than doing a survey with your GPS.
>>
>>
>> --
>> Brian Lloyd, WB6RQN/J79BPL
>> 706 Flightline Drive
>> Spring Branch, TX 78070
>> brian at lloyd.com
>> +1.916.877.5067
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