Lock on Satellite Still or Moving

 

-- Since the 21 GPS satellites are not in synchronous orbit 11,000 miles above the earth (they don't stay in relatively one position all the time),

-- and since they are not even synchronous with each other,

-- and a spot on the earth is constantly moving (probably at a high velocity) as the earth spins and travels through space,

---Why would it matter greatly if one activates their GPSr while being stationary or while moving?

Some suggest that it is more difficult for the GPSr to lock-on if it is moving. Is that really true, or is it a guess.

Based upon my conjecture, the GPSr is already moving (even if it is standing still) -- perhaps greatly, and most probably a extra 35, 45, 55 mph, while driving, would not have any appreciable effect at all in initially accessing the satellites as compared to if it were standing still.

And, when operating, the GPS is moving and reflecting almost instantaneous information be in vehicle, boat or air.

I'm asking, just to better understand. A real technical answer might be lost on me.

I've owned 5 GPSrs over the past 9.5 years starting with the Garmin GPS III Plus in 1999, and have not found any difference, at least that I've noticed, for access time based upon whether I am moving or not. Have I just not noticed?

If anyone has some 'non-guessing' answers, I would appreciate seeing them posted.

[I am not talking about initial start-up if unit has been off and moved a considerable distance. That, I know, takes longer.]

Gary Hayman
Garmin Tricks
arrow http://snipurl.com/garmin_gps_tricks

--
Garmin DriveSmart 61 LMT-S, Prev.GPSs: Drive61 LM, nuvi 3790LMT, 755T & 650, GPSIII+, SP 2610, 250W; Magellan 2200T; Originator of GARMIN NUVI TRICKS, TIPS, WORKAROUNDS, HINTS, SECRETS & IDEAS http://bit.ly/GARMIN-TNT

When a GPS receiver is

When a GPS receiver is moving in a car, it becomes much more likely that obstructions will briefly interrupt the reception of the satellites in view.

Before a GPS receiver begins nav, it needs almanac (general system health and rough orbits of all GPS satellites) and ephermis (precise orbital data for each satellite being used for the position solution). The data is broadcast at a slow rate; it takes 30 secs for a full transmission.

The almanac data is good for longer periods but the ephermis (precise orbit) data for satellites goes stale much faster and may need to be updated before a satellite in view can be used. The ephemeris is updated every 2 hours and is generally valid for 4 hours, with provisions for updates every 6 hours or longer in non-nominal conditions. When a GPS receiver is off for a while, the satellites it last used may go out of view and new ones are in view that it hasn't "seen"/used for a longer period.

GPS receivers can receive the ephermis data in chunks and put the full message back together but frequent interruptions can result in this taking a long time.

If you live in a very open area, it's probably less of an issue but in a dense urban environment, signal interruptions from buildings, poles, trees, etc can mean it takes a long time to get a full ephermis transmission so a satellite can be used.

Hope that helps.

about the moving thing, When

about the moving thing, When i was in a plane flying at about 500 miles per hour i was able to get a satellite lock from the small window which is only facing one direction, and it worked fine. so I don't think moving should affect it much.

??

Moving wasn't the issue, the obstructions WHILE moving were. At least, that was what I read.

When you were in the plane and held the unit to the window, there shouldn't have been any obstructions.

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Glenn - Southern MD; SP C330 / Nuvi 750 / Nuvi 265WT

There is constant movement, isn't there?

I would assume that to a satellite that is not in a synchronous orbit with the earth has to do some real quick math right off. If you are located at the equator you are already moving at over 1000 miles per hour due to the rotation of the planet. It's probably closer to 700-800mph at 45 degrees latitude.

Could this be some of the math taken care of during the ephemeris transmission?

--
-damdefoe (Nüvi 200W, GPS12) "Everything that can be invented has already been invented." -- Charles H. Duell, director of the U.S. Patent Office, 1899

GPS Relativity

Thank you.. Makes sense and clear. Much appreciated.

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Tom

Lock on Satellite Still or Moving

Thank you all for this quick and clear explanation on GPS satellites. I really appreciate this kind of information. smile

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Clem - Garmin 2495LMT, 1450T & C530