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What do you do when GPS just won't link?

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#1 astrovienna

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Posted 01 December 2012 - 10:32 PM

My GPS usually links in less than a minute, but about one night out of four it just won't link. No short, no hardware issue, it just won't link. I usually enter the time - but why won't it just keep the time on its own?! Just like with GPS, some times it has the time right, some times it's off by hours. #$%*^! This is especially annoying sine I'm permanently set up. I don't even need GPS, I just need it to remember the time and place.

Okay, rant off. I feel better now.

Kevin

#2 Digital Don

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Posted 01 December 2012 - 11:34 PM

The main purpose GPS serves for the alignment process is to tell the scope what stars are currently available from your location. A GPS fix has no effect on tracking or GOTO accuracy, or anything else during alignment.

However, since your scope is permenantly mounted, you may want to use the 'Hibernate' function which does rely on GPS when you wake the scope up.

So, you might want to give THIS PROCEDURE a try.


Don:usa:

#3 astrovienna

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Posted 02 December 2012 - 10:04 AM

Sorry, let me clarify. I do use the hibernate function, which in theory should solve all this - you start it up, it shows you the time, and you just skip the GPS link. But the time is frequently wrong. Why? Got me.

And the GPS battery isn't dead anyway. This scope is only a year old, and it's always been this way. So was my previous CPC1100, and the one before that. (long story) They just won't link sometimes, or keep the correct time sometimes, for no apparent reason. Frustrating. Am I the only one finding this erratic behavior?

Kevin

#4 telescopemullet

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Posted 02 December 2012 - 10:20 AM

Mine does it very sporadically, and I do use the Hibernate function. It takes up to 1/2-hour and it does this for a few days at a time and then it reverts back to the less than one-minute routine. When it is acting funny, I plan accordingly and start the scope up 45-minutes earlier than I usually do.

#5 dragonslayer1

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Posted 02 December 2012 - 11:32 AM

Just two cents from someone in Airline Industry, wonder if may be a satellite problem? Are you near a MOA ( military ops area). You may try contacting the someone at FAA and just see if may be problem other than your scope.

#6 Digital Don

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Posted 02 December 2012 - 10:30 PM

"... and you just skip the GPS link. But the time is frequently wrong."

Methinks therein lies the rub! You cannot ignore the GPS link when you wake the scope from Hibernation. It needs that information to calculate where it's currently pointing based on the position it was left in.

When you wake the scope up, the display should cycle through “Enter if OK”, “Undo to edit” and “GPS Linking”. Once the link is established, the display changes to "CPC Ready".

If you don't wait until the link is complete, the time/date/location information will be inaccurate.

Don:usa:

#7 AhBok

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Posted 02 December 2012 - 11:13 PM

Don,

I agree. Please correct me if I am wrong, but I've always thought the battery in the GPS for the CPC series is simply to back up ephemeris data in order to get a faster GPS fix. As you have pointed out, the clock is dependent on real-time GPS data unlike the CGE and other scopes using an external GPS which have a battery backed RTC. My CPC displays the time from the last time when it was powered down, until I get a GPS fix.

#8 Digital Don

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Posted 03 December 2012 - 12:59 AM

Hi AhBok,

That's pretty much how I understand it as well.

When you put the scope into Hibernation, say you point it due south pointing 20° above horizontal. That location will correspond to a specific Right Ascension and Declination at that particular time.

When you wake the scope, it must calculate the current R.A. and Dec. it's pointing to. I assume it uses the altitude and azimuth information stored when the scope was put into Hibernation in conjunction with the current date, time, and location provided by the GPS.

Don:usa:

#9 jerwin

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Posted 03 December 2012 - 01:17 PM

Just two cents from someone in Airline Industry, wonder if may be a satellite problem? Are you near a MOA ( military ops area). You may try contacting the someone at FAA and just see if may be problem other than your scope.


When I first read this I thought that was nuts, it's GPS, it's a satellite, how can that not just work, or how would it go unnoticed. But then I got thinking about it more and more and realized whenever my GPS acts up I'm always at one of my astronomy clubs dark sites. I was convinced the GPS was simply dying on me. But then I'd get it home and it works fine. Back to the dark site, dead, back home fine, to a different dark site fine.

I think I read once that a car GPS is always talking to 3 or 5 satellites. Your gps gets time data off of each of them and can figure out your direction speed location and whatnot. But a telescope GPS should only need to hit one satellite, so if that one GPS is having issues I could see it not linking.

Might not be the problem with the OP but not as nuts as I initially thought.

Jim

#10 astrovienna

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Posted 03 December 2012 - 11:45 PM

So when the scope is hibernated, does it store anything about its location in memory?

Kevin

#11 Digital Don

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Posted 04 December 2012 - 01:25 AM

As far as I know, Celestron doesn't say how the process works.

However, they do state that the scope must not be moved while it's hibernating. When you enter hibernation, the the software must store the altitude and azimuth the scope is pointing at.

When you wake it up, the software can use the stored position infomation plus the current time, date, and location (from a GPS fix) to calculate the current R.A. and Dec. the scope is pointed at. Once it does that it can track and locate objects.

Don:usa:

#12 AhBok

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Posted 04 December 2012 - 09:50 AM

Don,

Below is a doc from Celestron confirming what you say. If the mount has an external GPS with an RTC, the mount can keep up with tracking and time/date etc. With the CPC and scope without an RTC, the scope stores the pointing model, but needs the time/date from the GPS fix.

http://www.celestron...se&_a=viewar...

#13 astrovienna

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Posted 07 December 2012 - 12:43 PM

But does it also store the coordinates of the scope? If it does, then entering the time manually should give it everything it needs. I can't remember if it does that, though.

Kevin

#14 Digital Don

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Posted 07 December 2012 - 06:32 PM

If you mean celestial coordinates, I would say no. Right Ascension and Declination can be thought of as 'relative' since the positions change with time. Altitude and Azimuth are 'absolute' and do not change with time. 180° azimuth is always due south for example.

I assume only altitude and azimuth are stored. With that information and the time, date, and location, the current R.A. and Dec. that corresponds with the stored alt-az coordinates can be calculated.

Don:usa:

#15 astrovienna

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Posted 07 December 2012 - 10:39 PM

I meant latitude/longitude. It gets those from GPS, and if it saves them when you hibernate, you really don't need another GPS link the next night. Just tell it the time, right?

Kevin

#16 Digital Don

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Posted 08 December 2012 - 01:21 AM

"you really don't need another GPS link the next night. Just tell it the time, right?"

Celestron doesn't explain what is going on when the scope is awakened so it's hard to say for sure. However, if you were able to input the time alone, it would have to be perfectly accurate. The software will use the time information you provide to model the current sky. Unless you are absolutely accurate, the calculations will be off and the scope won't be aligned correctly.

But, why not give it a try and see what happens? Let us know the results if you do.

Don:usa:

#17 AhBok

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Posted 08 December 2012 - 09:57 AM

But does it also store the coordinates of the scope? If it does, then entering the time manually should give it everything it needs. I can't remember if it does that, though.

Kevin


Yes, the battery in the GPS stores the last lat/long coordinates along with the ephemeris information on the satellites at your location (this allows for your GPS to link up more quickly when you use the same locations night after night). When you travel with your scope more than a couple of hundred miles, you will notice the GPS linkage takes a little longer. However, your GPS will load the new ephemeris data for the new location and link up more quickly after the initial linkage for that location. You will notice that your time synchs up very quickly with your lat/long fix lagging at least a few seconds. I often hit ok after the time synchs rather than waiting for the GPS linkage when viewing from my house. When going to another site, I wait for the GPS to link in order to get the new coordinates. If for any reason the internal GPS battery runs down, either charge it by turning the scope on for 24 hours with the hand controller removed, or wait a little longer for the GPS fix when you connect the CPC to your 12V supply.

I agree once more with Digital Don. Play with your scope and let it teach you. I learned what I know about my CPC by changing parameters and noting the behavior. There really isn't anything you can do to the scope by changing the settings. Just note your changes so you can easily revert back. If you forget something, do a factory reset and start all over again. If your CPCs brains get jumbled, load the latest firmware revision and reset all the values. It really all makes sense once you exercise all the features and functions. I wish Celestron would make a separate document with all of this in it. I don't blame them, though, for not putting it in their owners manual. Most users don't dive this deep into the workings and would be intimidated by this amount of information. Those of us on Cloudy Nights are the exception and want more. We just have to figure it out for ourselves.

#18 JayTee

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Posted 22 December 2012 - 06:16 PM

I had the same problem earlier this year. This is the path I followed to fix it:
1. Reseated all the cables in the fork arm going to/from the GPS interface board and the GPS module.
2 Make sure your internal battery is charged, hook up the scope to a power source, leave the scope on for 24 hours (I still had the problem)
3. Next I decided to replace the GPS interface board. This was a $15 gamble and most of what I read said it "should" fix the problem.
4. And finally, because the 3 previous steps were unsuccessful, I replaced the GPS module ($127). Voila, I now had a signal my scope could use.

Did I need the new interface board? I decided not to troubleshoot something that was now working...

I must point out that google maps would have given me my geo coordinates with enough accuracy to satisfy the scope as would the USNO for an accurate time. These manual inputs did work well for visual work and planetary/lunar AP.

#19 snowcrow

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Posted 23 December 2012 - 06:50 AM

From my CPC1100 EdgeHD manual,

"A Few Words on GPS:The CPC uses an on-board GPS to take the guesswork out of aligning your telescope with the sky. Once an alignment method is selected, the CPC automatically initiates the internal GPS module. However, there are a few things you should be aware of in order to get full use of its many capabilities:GPS alignment will only work when the telescope is set-up outdoors with an unobstructed view of the sky. If the CPC is set-up in a location that has a limited horizon in any direction, it may take longer for the telescope to find and link with the needed satellites.When using the GPS for the first time, it may take 3-5 minutes for the CPC to link-up with its satellites. Once the telescope is successfully linked, leave the telescope powered on for at least 20 minutes. During this time the CPC will download the complete almanac of orbital elements (called the ephemeris) for the orbiting GPS satellites. Once this information is received, it will be stored for future alignments. If your CPC is transported over a long distance (say from the northern to the southern hemisphere), it may take as long as one hour to establish a satellite link from its new location. Observers wishing to travel long distances with their telescope are advised to turn on their telescope in advance to allow the GPS to acquire the necessary data."

I found the only things I had to set manually were the "time zone" and "standard or daylight savings". If these are not right and conflict with the GPS, it will give it a "brain fart"!!

#20 AhBok

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Posted 24 December 2012 - 09:19 AM

I found the only things I had to set manually were the "time zone" and "standard or daylight savings". If these are not right and conflict with the GPS, it will give it a "brain fart"!!


I've made the switch to UTC, so never have to worry about time zones or Savings/Standard time changes. I would switch back to time zones if I entered the time manually, but I always let the GPS do that.

#21 Mark72

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Posted 24 December 2012 - 10:47 AM

Kevin,

Did you get it sorted out?
I look at mine and it shows correct time but keep saying “Enter if OK”, “Undo to edit” and “GPS Linking”

Not sure if I loose orientation if I enter OK. It's waking up from deep hibernate after 2 months

Mark

#22 astrovienna

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Posted 26 December 2012 - 01:07 AM

Well, it does store the GPS coordinates. Took me five years of ownership to notice that! :) So as long as I enter the time, it's not a problem. The weird thing is that sometimes it seems to read the time from my laptop (I use Nexremote), but other times it won't. Once I actually watched it report the correct time, and as I was getting ready to press the Enter button, the time suddenly jumped about twenty minutes off. Weird!

Anyway, the bottom line is that the couple experiments I've done so far indicate that GPS linking isn't actually necessary if you're permanently setup, which should solve my problem.

Kevin

#23 Kevdog

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Posted 03 January 2013 - 12:11 PM

I think I read once that a car GPS is always talking to 3 or 5 satellites. Your gps gets time data off of each of them and can figure out your direction speed location and whatnot. But a telescope GPS should only need to hit one satellite, so if that one GPS is having issues I could see it not linking.


No, you need at least 4 satellites to determine your position in 3 dimensions. The more satellites you can see, the more exact your position.

http://airandspace.s...u/gps/work.html
http://airandspace.s...ps/spheres.html

The car devices then just use the change in position between 2 readings to determine direction and speed.

#24 AhBok

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Posted 04 January 2013 - 11:02 PM

Yep. One satellite is all you need to update the time. This is why you see the time synch up first and afterwards the GPS fix once it has linked to the required number of birds.

#25 astrovienna

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Posted 07 January 2013 - 01:06 PM

I've pretty much given up on GPS linking. Since the coordinates of my obs are remembered, it isn't worth waiting for the link. I just enter the time manually.

The time issue is really weird. When I first launch Nexremote, it shows the correct time for a second, then changes to some wildly incorrect time (and a date back in 2004). I'm guessing it first reads the time from my laptop, then for some reason dumps that time for some internally stored (but incorrect) time. I wonder why Nexremote isn't written to just stay with the laptop's clock? Very weird.

Kevin






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