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Cellular forums Home > Archive > GPS > April 2007 > to Jim+andy, GPS and solar flares
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to Jim+andy, GPS and solar flares
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> Assuming your antenna has an unobstructed view of the horizon in all
> directions,
> it will be able to see way more than 4 satellites at any given time. Many
> modern GPS units can track 12 or more satellites.
The GPS has a limited view of the horizon as it is placed in between large
buildings, the odd thing is that you can have fix for several months and
then suddenly the 1 pps is lost for hours, there's no obvious
periodicity/pattern in this. That's why I began suspecting sun activity,
especially if this activity coincides with only few sattelites being
received and considering this GPS being an older type with a lot less
receiver sensitivity than newer types!
The receiver/antenna comb. is an older Motorola one, I don't have any
further ID regarding model etc. The antenna and receiver is built into a
cone housing.
> These satellites orbit the earth and move across the sky. They are not
> fixed.
> The number of satellites in the sky varies with the time of day. Also,
> satellites
> can be taken offline periodically.
>
> So the 4 satellites you see can be many different satellites throughout
> the day.
> If your antenna's view of the sky is obstructed in part, then it's not
> uncommon to
> lose reception now and then as a satellite moves behind the obstruction.
>
> As far as flares go.. You need an unusually big one to knock out GPS
> reception.
> I know there has been a bit of press lately, but complete GPS signal loss
> due to
> flares is a pretty rare event.
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| nitespark 2007-04-15, 7:33 am |
| RK wrote:
>
>
> The GPS has a limited view of the horizon as it is placed in between large
> buildings, the odd thing is that you can have fix for several months and
This is most likely your problem. I would say if you can somehow get
the antenna to where it has a clear view of the horizon, you will start
seeing more satellites. The GPS constellation is in constant movement
and my best guess is, due to the cyclical nature of the satellite orbits
and their relative position on the horizon, there are going to be some
times when the majority of available satellites is lower on the horizon
or may be blocked by the two buildings.
> then suddenly the 1 pps is lost for hours, there's no obvious
> periodicity/pattern in this. That's why I began suspecting sun activity,
> especially if this activity coincides with only few sattelites being
> received and considering this GPS being an older type with a lot less
> receiver sensitivity than newer types!
I am seriously doubting sunspot activity.
>
> The receiver/antenna comb. is an older Motorola one, I don't have any
> further ID regarding model etc. The antenna and receiver is built into a
> cone housing.
>
The receiver and the antenna are all in one unit outside? I have never
seen an arrangement like this. I have a "cone" shaped antenna likewise
on the roof of my house but the main receiver is on my repair bench.
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| > The receiver and the antenna are all in one unit outside? I have never
> seen an arrangement like this. I have a "cone" shaped antenna likewise on
> the roof of my house but the main receiver is on my repair bench.
There's a lot of sense in this, you avoid the cable attenuation!
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| nitespark 2007-04-15, 3:33 pm |
| RK wrote:
>
>
> There's a lot of sense in this, you avoid the cable attenuation!
>
>
Depends on the cable.
According to the user guide for the Z3801, using LMR400 cable, a cable
length of 115 meters (377 ft), no in-line preamplifier is needed.
Using RG213, anything less than 53 meters (177ft), no in line
preamplifier is needed.
Unless you are using extremely small cable or you have an extremely long
cable run, or both, cable attenuation is not a major concern. In your
application (timing), you will need to factor in the velocity factor of
the cable and the length of the cable to obtain an accurate 1pps output
from the receiver, but that is not a major problem.
In my situation, I am not concerned with the 1pps output. Just
frequency output which I get. My frequency stability is just a little
bit worse than a rubidium standard, which for establishing frequency
accuracy on my test bench, is way more than adequate.
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