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GPS Satellites Visible But No Fix

Thread starter #1
Hi,
I am using Quectel L86 GPS Module, I am facing following problems:-
1. GPS Satellite Fix takes more time or sometimes not at all fixing. (Even using external chip Antenna problem exists)
2. Which of these following satellites, I need to enable:- (All I need to enable or according to my location I need to turn ON/OFF)
* GALILEO_FULL_Enable
* GALILEO_Enable
* GLONASS_Enable
* GPS_Enable
3. Even in 12-15 Satellites View also.... The GPS not giving same location repeatedly..It is giving different lat&log every 2-3 minutes....
4. When wanted to use HOT,WARM,COLD Starts.
5. After these Starts, Even though I didn't get satellite connectivity. How long, I need to wait to resend these Start commands.

Thanks
 

rjenkinsgb

Active Member
#2
Do you have a _permanent_ supply to the v_bckup pin? Without that, it may take a very long time to initialise when powered up.

I don't know with that exact unit, but some modules can take 15 mins or more to become operational after losing all backup data; they have to download the updated satellite almanac and try to get a rough location before they can even work out what sats they should be trying to use.
That also assumes they have a full 180' view of the sky, an "open field" outdoor location. If the signal from the sats is obstructed in any way, that will further slow down the startup.

A "cold start" is probably what you need if the location has significantly changed from when the device was last working properly.
A warm or hot start means all the stored data and current position are valid (or near enough so).


From having no backup supply connected, just do a cold start then leave it for 30 mins or so with a clear view of the sky. It will probably have locked up OK by then. Reacquisition with working backup should then be quite quick.

The backup supply keeps its internal clock running, as well as storing data. Accurate time is needed to calculate the expected satellite positions before it can start to get a "fix".
 

unclejed613

Well-Known Member
Most Helpful Member
#3
one thing that can make it difficult to get a fix, is being indoors where there may be metallic objects blocking, reflecting, or refracting the signals. most nav units i've worked on have a diagnostic menu option that allows you to see where the satellites are, as well as the signal strengths. you need good reception from 3 satellites for a fix. if the diagnostic screen shows all of the satellites grouped together in the middle of the "sky map", or satellites identified as "255" instead of numbers between 1 and 32, you probably aren't getting good reception. the worst-case of using a GPS indoors would be an industrial building with a steel frame, and a steel roof. if you are outdoors, and still not able to get a fix, then the diagnostic display is still a good tool to use if you have any doubts about the antenna. be aware that nav units with a remote antenna (such as an in-dash GPS with the little square antennas) usually provide DC power for the antenna through the RF cable. if the GPS unit came with an active antenna, there should be 3.3V on the center conductor of the coax. if it's not there, then the reason for bad or no reception is the lack of power to the antenna's preamp. if a passive antenna has been connected that has a DC short across it (like a "j-pole", or the typical "F" shaped antenna etched on a circuit board) then the 3.3V for the antenna preamp is no longer present, and there's an open fuse or resistor on the GPS receiver board. also, check the setup menu to see if there's an "active/passive" configuration setting for the antenna. make sure it's off (passive) for passive antennas, or on(active) for antennas with a built-in preamp.
 

MikeMl

Well-Known Member
Most Helpful Member
#4
My electronics lab is inside my metal aircraft hangar. I cannot get a fix with the airplanes inside the hangar, they have to pulled out. Working on various GPS modules on my workbench, I have to use a remote, outside antenna to get a fix. If I wait long enough, the workbench GPS might eventually get a lock through a skylight that is within about 2m of the workbench, but it only gets two or three sats, and the lock disappears from time to time...
 

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