• Thread starter Andre Pablo G. Fausto
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Andre Pablo G. Fausto

Hi everyone, If a GPS at a defined location can track 6-8 satellites at a time, would a different GPS track at the same location track the same 6-8 satellites considering that they are Geosynchronous? Can anyone answer this? The reason for the question is that we are seeing whether having two GPS at the same location provide a different time (up to the ms). Regards, Andre Pablo "Apaul" G. Fausto Nayon Kontrol Systems 354 (44) Quezon Avenue Quezon City, Philippines Tel. +63 2 4485074 Fax. +63 2 4485077

Peter Whalley

Hi Andre, GPS satellites are not Geosynchronous. They are low earth orbit (otherwise the signals would be to weak to receive with a hand held receiver and low gain, wide angle antenna). As a result they move around fairly quickly and the number visible at any time in a particular place varies from about 4 as a minimum up to 12 or more. My GPS (Garvin 12XL) shows all of the satellites visible from it's location, their position in the sky at that time and the relative signal strength for each of up to 12 satellites (it has a 12 channel receiver). The accuracy of the GPS can be substantially effected by obstruction of signals from particular satellites and reflection of signals from nearby objects (buildings, cars, hills etc). I suspect that to get ms accuracy would require very idealised conditions (open ocean or flat desert at night). In a multipath environment (lots of reflected signals) moving the antenna even a few cm would radically change the relative signal strength and time delays of all the signals and hence change the reading on the GPS at least a little bit. For best results you may need to use 2 hemisperical pattern antennas (facing straight up) and mount them on top of a mast or tower so that they are getting strong clear reception of multiple satellites via direct paths. Regards Peter Whalley Managing Director Magenta Communications Pty Ltd 121 King Street, Melbourne, VIC 3000 Australia. e-mail: [email protected]

Friedrich Haase

Moin Andre, I use GPS at sailing high sea. GPS usually select 3 or 4 satellites for their calculations. Should depend on best S/N ratio and best result precision, which depends on the satellite positions relative to each other. According to my experience even to same brand and same modell GSP do not display the same locations. I would also expect some variations in the time. Since DoD has switched of SA in May 2000 the positions are damned good - mostly below 10 or 20 m. Often much better than the charts. I haven't checkedthe time and I would NOT have recognised differences in msec range. BTW, some GPS get the time wrong. Please double check. GPS time differs from UTC/UT1 by some seconds (currently 12, not sure). Most GPS display UTC but some display GPS time. Best regards Friedrich Haase

Johan Bengtsson

Regardless of if they actually track the same or different satellites a 1ms error is a lot in this case (a position error of about 300km, (speed of light)). This gives that an error of 1ms can simply not be there. I think it is the output stage of the time from the two recievers that is delaying the signal differently. Btw. It should be about the same satellites anyway... geosynchronus or not it is what the receiver gets at any given moment that counts. /Johan Bengtsson ---------------------------------------- P&L, Innovation in training Box 252, S-281 23 H{ssleholm SWEDEN Tel: +46 451 49 460, Fax: +46 451 89 833 E-mail: [email protected] Internet: ----------------------------------------
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