GPS won’t/can’t acquire satellites

[DougPaul]

This is quite interesting! When I read Wikipedia article on GPS and more specifically the section on navigation equations I thought that each gps would have some kind of non-linear least square solver such as Gauss-Newton or Levenberg-Marquardt and it would be enough to solve for time-delay, but it didn't occur to me that the chips would be performing the search for time-delay in parallel (I don't remember anything like this mentioned in the Wikipedia article.) Can you recommend any place on the Web where I could read more about this? I think I know enough basic math that I'm not scared of reading somewhat technical Web pages or papers. Tks!

The GPS has to determine the pseudo-range (time_of_flight * speed_of_light) and doppler shifts to 4 or more satellites before it can solve the equations for PVT (position (3D), velocity (3D), time (1D)). (The fourth satellite is required to determine the time offset between the GPS internal clock and the GPS satellite atomic clocks.) Most descriptions of the solution describe an iterative method so I presume that iterative methods are used more often than the closed form solutions.

The Wikipedia article (http://en.wikipedia.org/wiki/Global_Positioning_System) is reasonable, but it misses many details. However, it looks like many of the details are contained in the linked articles.

There is a nice overview at http://www.colorado.edu/geography/gcraft/notes/gps/gps.html, a highly technical set of course notes on the principles at http://www-gpsg.mit.edu/~tah/12.540/, and a set of course notes on modern navigation at http://www-gpsg.mit.edu/~tah/12.215/ . There is also some nice info at http://www.edu-observatory.org/gps/gps.html.

Parallel vs serial search for acquisition of the satellites is "just" an implementation detail... However, its effects are visible to non-technical users. The older GPSes with a small number of correlators depend on the almanac and knowledge of the approximate location and time to reduce the acquisition time while the newer ones with large numbers of correlators can search so many time-doppler bins simultaneously that they do not need to use the almanac for a fast start.

FWIW, I learned the details of how GPSes work from a variety of sources over an extended period so I cannot give you a short list of comprehensive references. All of the details would fill a large book... (I think I qualitatively understand pretty much (or at least most...) of everything going on in a typical consumer GPS, but I certainly do not understand all of them well enough to build one from scratch. I'm also an electrical engineer so I had a head start on a number of aspects, eg direct-sequence spread spectrum signals and circular polarized radio signals.)

There is lots of info out on the web--once you learn some of the technical terms, you can search on them to learn more.

Doug

 

[Raymond]

I wrote to Garmin and was sent a link for an external antenna, but the cable was more than 8 feet long, which seemed ridiculous, and I didn’t even know for sure if using it would fix the problem. Plus I didn’t know where I could go to find one in stock around here so I could try it out first, so I went ahead and bought myself a new one. Well, not brand new — used/returned, actually; I got a 64st at the L.L. Bean outlet store in Nashua.

Now I have to unpack it and check it out. Hope it works the same as the 60CSx.

By the way, as far as the memory being overloaded, I don’t think that could be the problem. There were only two saved tracks on it. I’ve gone up to the limit of 20 a couple of times, and it has still worked. I couldn’t save any more tracks, but I could get them off of the saved track logs. Also, the unit had worked perfectly fine the day before.

 

[Colenso]

This is just an aside--but I truly enjoy using map, compass, and altimeter to navigate. I've bushwhacked more than 30 years without GPS. Don't get me wrong, I don't hate GPS, but for many reasons I prefer the above tools. OK, end of digression!

 

[iAmKrzys]

For whatever it is worth, I typed in "how to repair garmin 60csx" into Google and a number of links came up. One of them is a crude video that shows how to remove the motherboard with the antenna from the case: https://www.youtube.com/watch?v=3aecW_W90jA If you decide that you can't fix it in any other way and Garmin's repair service is too expensive then you may have not much to loose by opening your gps.

 

[iAmKrzys]

There is a nice overview at http://www.colorado.edu/geography/gcraft/notes/gps/gps.html, a highly technical set of course notes on the principles at http://www-gpsg.mit.edu/~tah/12.540/, and a set of course notes on modern navigation at http://www-gpsg.mit.edu/~tah/12.215/ . There is also some nice info at http://www.edu-observatory.org/gps/gps.html.

Thanks! I will check these links out.

 

[DougPaul]

I wrote to Garmin and was sent a link for an external antenna, but the cable was more than 8 feet long, which seemed ridiculous, and I didn’t even know for sure if using it would fix the problem. Plus I didn’t know where I could go to find one in stock around here so I could try it out first,

The antenna is most likely designed for car use where one might put the antenna on the roof.

so I went ahead and bought myself a new one. Well, not brand new — used/returned, actually; I got a 64st at the L.L. Bean outlet store in Nashua.

Now I have to unpack it and check it out. Hope it works the same as the 60CSx.

I haven't used a 64x, but I do have a 62s. (I suspect that the 64x follows the trends found in the 62x.) The user interface has a lot more toy-like features. I went through the menus and set it to be as much like a 60CSx as I could. Mapsource will not talk to the 62x--you have to use Basecamp.

FWIW, while there is a new feature or two in the 62s that I like, I generally prefer to use my 60CSx...

By the way, as far as the memory being overloaded, I don’t think that could be the problem. There were only two saved tracks on it. I’ve gone up to the limit of 20 a couple of times, and it has still worked. I couldn’t save any more tracks, but I could get them off of the saved track logs. Also, the unit had worked perfectly fine the day before.

In my experience, a corrupted memory card generally causes the boot-up procedure to fail before the GPS attempts to acquire satellites. If the card is the problem, it can be removed. If there is a problem with the internal memory, you can only access it if you can first boot the GPS. (It sounds as if you can get far enough to switch to memory access mode, but if you fool with the firmware and/or system files you could brick the unit.)

I presume your 60CSx currently hangs at the "Acquiring Satellites" display or announces that it cannot find the satellites after searching for a while. And you have tried a master reset https://support.garmin.com/support/searchSupport/case.faces?caseId={85f5ae00-14aa-11de-76c5-000000000000} . Did you leave the GPS out in a good skyview location for >20 min for it to acquire the satellites?

Doug

 

[Raymond]

Yes. First I had it sitting on a rock at the Crawford Path parking lot while I donned my boots and pack, then it rode in my pack for at least another half hour while I walked around. The picture of the satellites overhead and the signal-strength bars never appeared at all. And continue to not appear. Back at home, I tried the original 128 MB chip that came with it and no memory chip at all. No difference from the 8 GB chip I’ve had in it for a few years.

Thanks for the aside about MapSource not working with the 64ts. I can’t even get MapSource to open on this Windows 7 computer — the version of my map is supposedly a subsequent version of the program that is on the computer (?) — so I guess I won’t bother trying to figure out how to open it. But I can’t get Topo! to open, either. Windows 7 lets me navigate to Topo!, but won’t give me the opportunity to open it. Any ideas about that?

I have been opening both programs on the old Windows XP computer that I haul out of the crawl space whenever I want to transfer tracks, because the Windows 7 computer wouldn’t recognize the 60CSx. It will recognize the 64ts, so hallelujah, but now I can’t get the maps to open. Grr.

 

[DougPaul]

Yes. First I had it sitting on a rock at the Crawford Path parking lot while I donned my boots and pack, then it rode in my pack for at least another half hour while I walked around. The picture of the satellites overhead and the signal-strength bars never appeared at all. And continue to not appear. Back at home, I tried the original 128 MB chip that came with it and no memory chip at all. No difference from the 8 GB chip I’ve had in it for a few years.

Removing the card should be adequate if the card is the problem. (The GPS should work perfectly well without a card.)

It is harder for a GPS to acquire satellites if it is moving, but it sounds like you gave it an adequate chance.

While the problem could be as simple as a broken antenna connection or damaged antenna, it could also be something else. (Of course, a broken antenna connection might also prevent an external antenna from working too.)

Thanks for the aside about MapSource not working with the 64ts. I can’t even get MapSource to open on this Windows 7 computer — the version of my map is supposedly a subsequent version of the program that is on the computer (?) — so I guess I won’t bother trying to figure out how to open it. But I can’t get Topo! to open, either. Windows 7 lets me navigate to Topo!, but won’t give me the opportunity to open it. Any ideas about that?

I have been opening both programs on the old Windows XP computer that I haul out of the crawl space whenever I want to transfer tracks, because the Windows 7 computer wouldn’t recognize the 60CSx. It will recognize the 64ts, so hallelujah, but now I can’t get the maps to open. Grr.

Sorry, I can't help you with Windows software problems. (I run Linux here and have to run MapSource (or rarely BaseCamp) in a Windows emulator (www.winehq.com) or in a virtual machine (www.virtualbox.org) running XP.) I also use GPSBabel (www.gpsbabel.org) transfer track and waypoint data to/from the 60CSx. The interface to the 62x (and presumably also a 64x) is different from the one used by the 60CSx--I don't know if GPSBabel can talk to a 62x/64x.

You might consider running XP in a virtual machine on your Win 7 machine if you wish to continue using MapSource without having to visit the crawl space...

TOPO! is a totally different program from the Garmin programs, but if it only runs on XP again you might consider virtual machine running XP.

A suggestion for your 64st: if you install any additional maps, install them to the memory card. This will allow you to remove the card if there is a problem with the maps. (For instance, too many map segments will prevent the GPS from booting and if you can't boot, you can't enter computer interface mode to fix the problem thus effectively bricking the device. A removable card can be mounted on your computer and the offending files erased.)

Doug

 

[Raymond]

Thanks.

I had the National Parks East maps on the 8 GB card, so that is now in the 64, but my Waypoints list of nearly every Northeast 3000-footer I had not yet climbed, carefully entered one-by-one eight years ago, doesn’t appear anywhere anymore, not even on the 60.

I don’t know if I’ll be able to go through that again, entering all those figures.

 

[DougPaul]

Thanks.

I had the National Parks East maps on the 8 GB card, so that is now in the 64, but my Waypoints list of nearly every Northeast 3000-footer I had not yet climbed, carefully entered one-by-one eight years ago, doesn’t appear anywhere anymore, not even on the 60.

I don’t know if I’ll be able to go through that again, entering all those figures.

Try downloading the waypoints from the 60CSx with MapSource under XP*, save them in a .gpx file, copy the file to the Win7 machine, and then loading them into the 64st with BaseCamp.

* You don't need satellite reception to download waypoints or tracks. You can also turn off the GPS receiver if you wish (hit "menu" from the satellite screen).

Doug

 

[Raymond]

I will give that a shot. Thank you.