Connect with us

GPS Satellites Visible But No Fix

Discussion in 'Radio and Wireless' started by Nishanth, Nov 21, 2018.

  1. Nishanth

    Nishanth

    3
    0
    Nov 21, 2018
    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
     
  2. davenn

    davenn Moderator

    12,998
    1,661
    Sep 5, 2009

    I don't know if you understand you are not using the correct terminology ? you CANNOT get a FIXED location with a single unit
    so what do you REALLY mean ?

    how different are the LAT and LONG ? .... be specific
     
  3. Nishanth

    Nishanth

    3
    0
    Nov 21, 2018
    LAT and LONG -15 to 20 meters difference
    In NMEA PROTOCOL
    GSV:- SATELLITE IN VIEW:-15 TO 20
    GGA:- POSITION FIX:- INVALID
     
  4. davenn

    davenn Moderator

    12,998
    1,661
    Sep 5, 2009
    around 10 - 15 is the best you would ever get, anywhere between 10 and 20m would be typical for a single unit


    well that is correct as I stated previously

    you CANNOT get a fixed location with only one unit



    I do hi precision Trimble GPS for my job, to get less that 10 metres accuracy and to get a FIXED location requires much better
    technology than a simple car or handheld receiver
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    24,860
    2,571
    Jan 21, 2010
    We'll... You can also average over time. That's what my GPS disciplined frequency standard does. The accuracy of the reference frequency is partially based on it knowing where it is with respect to the satellites. While it can get an approximate value quickly, it improves over time (up to a week allegedly) as it fine tunes it's actual location.
     
  6. kellys_eye

    kellys_eye

    4,289
    1,143
    Jun 25, 2010
    What positional accuracy are you seeking? GPS accuracy is deliberately degraded - it's a military system after all .

    Precision position accuracy (centimetric) is expensive and uses differential correction systems.

    Select ONE system and see if that makes a difference.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    24,860
    2,571
    Jan 21, 2010
    I believe selective availability is turned off these days (since May 2000 by a decision on the Clinton white house).

    And future satellites may not even have the capability included.
     
    davenn likes this.
  8. kellys_eye

    kellys_eye

    4,289
    1,143
    Jun 25, 2010
    Wow - that long ago! How time flies!
     
  9. davenn

    davenn Moderator

    12,998
    1,661
    Sep 5, 2009

    yes, that is true

    there are just a speed and height restrictions in place

    As I said up the page, I do high precision GPS for my main job
    our precision accuracy is down to 3mm horiz and 7mm vertical
    vertical accuracy is always at least twice as "poor" as horizontal.... it's just the way GPS works
     
  10. davenn

    davenn Moderator

    12,998
    1,661
    Sep 5, 2009
    here's an example of a good quality FIXED position I am used to getting .........

    DSCF5563sm.jpg
     
  11. Ylli

    Ylli

    103
    22
    Jun 19, 2018
    3mm, that translates to a time accuracy of 10 ns or better. That deserves a WOW.
     
    davenn likes this.
  12. kellys_eye

    kellys_eye

    4,289
    1,143
    Jun 25, 2010
    I recently bought some extra land adjacent to my property and during the negotiation process the sellers wanted me to pay for centimetric GPS surveying to establish the boundaries.

    I got a tape measure out and did it manually and saved a few 100££. I had to - when I told them I wasn't paying for GPS surveying they sent a couple of goons to do a manual measurement but they didn't know HOW.

    What's the education system coming to these days?????

    Cool accuracy on those GPS results though Dave!
     
  13. davenn

    davenn Moderator

    12,998
    1,661
    Sep 5, 2009
    indeed ... seriously impresses me every day :)




    yeah ... totally reliant on computer systems !




    thanks ......

    without hijacking the thread too much, here's an overview of the system above for obtaining high accuracy ......

    That above screen show is one of these ... a CB460 Machine control, control box,

    CB460.jpg

    these sit in a dozer, grader, excavator etc and is the control hub for all the sensors receivers etc

    Grader Kit.jpg

    the guts of the system is a GPS base station in the construction site hut that broadcasts GPS corrections out on 450-460 MHz.
    The SNRx10 you see on the right receives those signals and sends them to the CB460 (450) along with GPS data from one or two GPS dome receivers
    as seen in the upper centre (MS992)

    when those 2 sets of GPS data are combined in the control box, a high accuracy FIXED location can be derived.

    And this is what I was trying to impress upon the OP @Nishanth ...... good or high accuracy cannot be obtained by a single budget GPS receiver.
    The best you can get is around 10m ( 10 - 20m avg)

    I don't install this gear on machines, the company has installation and support guys for doing that. Rather those items come into my workshop for repair

    Dave
     
  14. kellys_eye

    kellys_eye

    4,289
    1,143
    Jun 25, 2010
    I recall installing DGPS systems on ships a few decades ago. These relied on separate receivers to pick up correction signals sent from fixed base station on land. Such systems were fitted a part of DSP (dynamic ship positioning) systems such as mobile rigs and tasks like dropping sea anchors accurately.

    Can't recall what frequency the Differential part worked on - ISTR it was 425kHz ??? Or thereabouts. To long ago for this old brain to recall....
     
    davenn likes this.
Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.