[OSM-talk] GpsWeather: When conditions for mapping are good

[Michael Collinson]

At 11:16 PM 1/2/2008, Jo wrote:
>Mike Collinson schreef:
>>At 10:11 PM 2/01/2008, ivom wrote:
>>
>>>Folks!
>>>
>>> >From time to time, I am suffering from the limited reception 
>>> capabilities of my Garmin Etrex Venture Cx. I guess this is a 
>>> recognizable state of being, during a mapping session in urban 
>>> canyons, walking around with an accuracy of 17 meters or more...
>>>
>>>I am looking for some sort of indication telling me, at which 
>>>time-of-day there would be excellent conditions for creating 
>>>tracks in a dense city area. Has anybody come about such a service 
>>>on the web yet?
>>>
>>>Currently I am not planning to upgrade on the hardware side, but 
>>>do not hesitate to suggest different makes, models or add-ons, 
>>>which would suffer less from this urban canyon problem.
>>>
>>>Kind regards,
>>>IvoM
>>>
>>
>>IvoM,
>>
>>I think what you may be after is being able to predict date/times 
>>when a) there is a good number of satellites in the sky around you 
>>so that your GPS device can get as many readings as possible and 
>>choose the best, b) the satellites are well distributed over the 
>>sky to help the mathematical calculation of the GPS device and so 
>>that they are not all blocked by a tall building at the same time.
>>
>>If so, try typing into Google: GPS Satellite predictor
>>
>>I came up with
>>
>>https://stellarsupport.deere.com/stellar/SatellitePredictor?language=en&country=US
>>
>>If I remember, http://sirius.chinalake.navy.mil/satpred/, is a good 
>>one, but it is dead when I just checked it.
>>
>>Unfortunately,  even that probably won't  help that much with urban 
>>canyoning - you'll probably have to do several runs and then tie it 
>>in with Yahoo imagery if you are lucky enough to have it for your 
>>area.  One tip, I've got my best results having my GPS device 
>>mounted in a bicycle saddle-bag - it provides a much more stable 
>>platform than walking.  And if you are walking and your device 
>>loses satellite connection, put it on a metal surface - a man-hole 
>>cover,  traffic-signal controllers, even large waste-paper bins. It 
>>seems to act as a ground-plane which improves the antenna gain.
>I have an external antenna, which has a magnet. When I stick it to 
>the frame of the bus or even the toddler's stroller/buggy, reception 
>increases dramatically. Could this be the same effect, or did I 
>simply increase the size of the antenna?
>
>Polyglot


Same effect.

I am sticking my head out with this new-fangled very high frequency 
satellite stuff but at lower frequencies, UHF TV and VHF, an antenna 
typically consists of several elements whose length and spacing are 
calculated relative to the wavelength (inverse of frequency).  Only 
one element is the actual antenna. The others act as 
deflectors/reflectors which concentrate the signal on to the active 
element.  I *think* this is the sort of effect we are seeing.  Your 
external antenna would actually consist of two bits I think, the 
pointy bit being the active element and a base connected to the 
antenna cable earth lead.   I dimly recall a radio amateur who 
figured out that if he strung his horizontal antenna a tuned distance 
above a local lake, he got fantastic reception - electrical 
connection of the reflector to earth is also important.  I certainly 
subjectively feel I get my best results (and strangest stares :-) ) 
putting the GPS device with internal antenna flat onto manhole covers.

Mike