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Forum Index : Microcontroller and PC projects : GPS problem
| Author | Message | ||||
| PeterB Guru  Joined: 05/02/2015 Location: AustraliaPosts: 651 |
G'Day All. I have been using GPS to get speed of agricultural things for years but now I do not understand what is happening. We are seeing 11 sats. with a lot of numbers but no speed. GSA seems to indicate that only 1 sat is being used so I dunno. Can you young blokes help this old bloke out? Peter $GPRMC,115327.00,V,,,,,,,090923,,,N*7F $GPVTG,,,,,,,,,N*30 $GPGGA,115327.00,,,,,0,00,99.99,,,,,,*65 $GPGSA,A,1,,,,,,,,,,,,,99.99,99.99,99.99*30 $GPGSV,3,1,11,08,26,225,,10,72,206,,15,09,139,,16,18,301,*78 $GPGSV,3,2,11,18,35,088,,21,02,240,,23,45,139,,24,08,107,*73 $GPGSV,3,3,11,26,04,328,,27,59,250,,32,40,002,*49 $GPGLL,,,,,115327.00,V,N*49 |
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| oh3gdo Regular Member  Joined: 18/11/2021 Location: FinlandPosts: 47 |
PeterB Can old antenna being broke or get something wrong? Please look carefully the antenna and cable. Or test with other antenna. Regards Pekka OH3GDO |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 651 |
I have the thing mounted in my study window so the signal strength is not good so that is the probable reason but I'm curious as to why it can get all that data but not the stuff it needs. I'm off to bed now.  Peter |
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phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2135 |
That there are so many commas with no numbers between suggests missing data. Either a weak signal or interference corrupting the signal perhaps. As noted above if the antenna is separate from the receiver check the cable and connections. The GPS module I have can take quite a long time to output useful data if it has been switched off for any length of time. The orbital data of the satellites is often kept in battery backed RAM. It's a tiny rechargeable cell and when it goes flat the receiver must wait for the next Almanac transmission to renew the data. |
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| Andrew_G Guru Joined: 18/10/2016 Location: AustraliaPosts: 847 |
Peter, Tin roof? Try moving it more into the open sky? If is is only reporting one satellite it wont get speed. What is your nearest city - coverage across Aus. can still vary. A new day might get better results. Cheers, Andrew |
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6100 |
Lots of satellites (should be) in view, but none with any reported signal strength. Either antenna not functioning or stored data out of date and it needs a long time with a very good sky view to update. Some GPSs have a date roll-over problem which needs a firmware update to fix but that should only effect dates, not positions. Give it 30 minutes in the middle of the lawn. If that doesn't fix it, run over it with the lawn mower. It won't fix the GPS but might feel good... Jim VK7JH MMedit MMBasic Help |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 651 |
Good morning All. Yes, out on the lawn and away from the mower it will probably be O.K. but I don't understand how it can collect that rock solid data but not the stuff that matters. I would have thought that it would be all or nothing. I never did understand RF or women or........ Peter |
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| phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2135 |
It may be that the receiver and antenna are ok. Despite getting a lot of information it still doesn't have enough to perform the calculations. First it must "see" at least 3 satellites to do any thing at all and at least 4 to calculate Lat, Long and Alt. Next it must synchronize its clock to that of the satellites, compensating for the Doppler effect and Relativistic effects of gravity and acceleration on time for the satellites. Then it must acquire an error free copy of the Almanac, which is quite large so only transmitted a couple of time per hour. This provides the parameters for the equations that describe the orbits of all the satellites so their exact 3D positions and velocities can be calculated at any given instant. It also contains predictions of atmospheric propagation delay, which affects the time signals take to get from them to the receiver. Only then can it calculate your position from the minute differences in the time signals received from each satellite. All that for less than ten bucks! Edited 2023-09-10 12:00 by phil99 |
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| TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6100 |
Most of the data you supplied is a list of the satellites the GPS thinks it should be seeing. All are listed as null for the signal strength which indicates that it is NOT receiving and data from them. It needs time to update its internal data. Or it's stuffed. Jim VK7JH MMedit MMBasic Help |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 651 |
Thanks Jim. The difference between what it can see and what it could see is interesting. This unit has probably been fired up in the past so it would know where it lives and what to look for. Very interesting. Actually I don't have a lawn. I gave up because we don't have water in S.A. but I do have a large area of shrubs, trees, weeds, etc. Peter |
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Chopperp Guru Joined: 03/01/2018 Location: AustraliaPosts: 1057 |
My external GPS stopped the other day, but I found it swimming in water in it's enclosure. Got moved to a more sheltered position, but that wouldn't be your problem being inside. I've run a number of GPS modules near windows with no problems (after warm-ups), but I do have one that doesn't work at all. I've mainly been operating F4's using the GPS Functions. I assume the supply voltage at the unit is OK. Are you able to check the DOP? It should be < 2 (I think)? Brian ChopperP |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 651 |
Brian et al. I have had these working inside the window in the past so when I got lots of numbers I assumed that it was receiving data but not all the data. Jim has explained that, rather than receiving data, it was just using its memory. When moved 300mm outside the window and after waiting for 30 minutes I got all the data. I am a bit gun shy about GPS receivers after buying 20 dud ones from a well known company. The one I am using came from CORE and I am confident it is good. For getting the speed of agricultural things they beat a magnet and a reed relay. Peter |
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