On 2012-02-28, A C <agcarver+...@acarver.net> wrote: > On 2/27/2012 15:46, unruh wrote: >> On 2012-02-27, A C<agcarver+...@acarver.net> wrote: >>> On 2/27/2012 08:49, unruh wrote: >>> >>>> Ssure tracks with a S/N of between about 18 and 54. (higher is stonger) >>>> The Garmin trackes with S/N between about 35 and 54. >>>> >>>> What happens here is that the sattelites will be strong (S/N of 45-54) >>>> then suddenly over the time frame of seconds, all loose tracking ( but >>>> the gps still sees the sattelites and still reports their location in >>>> the sky), and when they come back again they are at the 45-54 level. >>>> I have now seen one such dropout from the Garmin 18 that is located >>>> about 5 m away from the sure antenna. (The sure at that time was offline >>>> for about 15 hrs). >>>> >>>> There seems to be some problem, wither with the antenna or with the >>>> board. This kind of dropout behaviour makes the Sure much less useful >>>> for timing purposes. >>>> >>>> have written to Sure, but have had no response. >>>> >>> The GPS doesn't see the satellites if your signals hit zero. It does >>> happen to know the ephemeris data for all the satellites and simply >>> plots their position based on its internal clock. If it didn't have >>> this capability, any reboot would take many minutes until the next >>> ephemeris transmission. With the tracking capability you get teh >>> ability to have 10-12 second warm boots. >>> >>> That said, I would check your antenna cables for broken or corroded >>> connections. I had an external antenna on a Garmin 12XL receiver that >>> worked great for years until one day I got exactly the behavior you >>> describe. Eventually I discovered that the shield crimp on the >>> connector to the receiver (an MCX connector) had become loose and the >>> shield was making intermittent contact. Sometimes, the on-board >>> amplifier in the antenna was no longer receiving power if the shield >>> disconnected. The loose crimp also allowed the wire to corrode slightly >>> which creates a rectifying junction. The connection would be mostly ok >>> in that power would be flowing to the amplifier but the rectifying >>> junction makes the long coax into an antenna capable of receiving and AM >>> demodulating (via the rectification of the junction) any signal nearby. >>> A signal strong enough would flood the RF front-end and all my signals >>> would go away. >> >> This is a brand new Sure (8 months old which had only been inside until >> a week ago). So it is hard to imagine that there could be corrosion. >> Surely it does not report based on predicted orbits. I could imagine >> that the signal is too weak for timing but strong enough to get location >> info from the sattelite. I guess I should try disconnecting the antenna >> (when it starts to work agin-- it has been out for hours now). and see >> if it continues to report positions. > > Every GPS I've ever had knows where the satellites are supposed to be > given the last known ephemeris data. That's why they all have backup > batteries and how they manage to calculate the position and also > reacquire the position if the receiver loses signal for a while. It's > able to make a very reasonable guess at where the satellites are, scan > only those visible PRNs to lock onto the signal and then compute a > solution. My very old 12XL will remember the satellites and if I shut > it off for an hour then turn it back on, the sky map is different but > very close to that of a GPS that is still running with an active map. > The map even updates when I have the GPS indoors and not receiving any > signal at all (it isn't using a SiRF chipset so it has very poor indoor > performance). Reporting the sky map is trivial for the GPS whether it > has signal or not, they all do it. > > > As for age, it doesn't really matter. Copper corrodes easily, even > tinned copper. It doesn't take much to create a rectifying junction
They actually look like gold plated connectors, not copper or tinned copper. > that will swamp your RF input on the receiver and render it useless. > The receiver front end is very wide bandwidth, strong signals will > desense it (just look at the LightSquared fiasco). Even if there were > no corrosion it's entirely possible to have a bad crimp connection. Could be. Does not look like it, but then again, it probably would not look like it anyway. _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
