GPS Failure - Blocking

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When a GPS Signal is blocked what is the effect?

If I block a GPS Signal will it switch to outputting the telegrams with loads of ,,,,,, comma markers in the same manner as before it finds enough Sats to output data?  Or does the data, for example time stop updating and sticks with the same value.

 

I tried to remove the GPS Aerial but it funny enough keep working.  That's to be expected, the bloody thing either doesn't work or it works too well.

 

Thanks Lads

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Last Edited: Fri. Jan 25, 2019 - 06:01 PM
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Fianawarrior wrote:
When a GPS Signal is blocked what is the effect?
Figure of Merit will decrease.

 

"Dare to be naïve." - Buckminster Fuller

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Could you elaborate more gchapman?

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A GNSS receiver's FoM is proportional to the quality of its output (complete - tracking enough satellites vs acquisition of satellites, precise - P code vs C/A code vs WAAS (augmented), correct - accuracy as circular or spherical error probability is low enough)

 

"Dare to be naïve." - Buckminster Fuller

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Completely Lost me there gchapman

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Standard NMEA-0183 sentences description

(3/4 page)

Global Positioning System Fix Data. Time, Position and fix related data for a GPS receiver

[GGA]

...

 6) GPS Quality Indicator,

...

 

"Dare to be naïve." - Buckminster Fuller

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I'm going to guess that the GPS module starts outputting NMEA telegrams without any information.  i.e. looking at the telegrams something like this would be outputed as data.  ,,,,,,,,,, 

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The gps module I’m currently using outputs a gga message with blank fields.

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Hi Kartman, that's what I'm thinking.  The blank fields are a list of commas

 

EDITED: so basically the assumption I'm going to make is that the GPS module continues to fire out telegrams but with no information.

Last Edited: Thu. Jan 24, 2019 - 09:28 PM
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Be careful what you assume!

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I try, nah, I'm just an eejit

 

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I've used 1/2 a dozen different GPS modules over the years and all of them put out null fields when the signal deteriorated to the point that it could not give good data.

That said, I don't think I've ever seen that in a formal specification.

 

Many GPS modules can also output a proprietary manufacturer's data format, and that, obviously, should be spec'd in the data sheet for data loss conditions.

 

JC 

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I think best thing to assume is that GPS data invalid until you have at least a 2D lock.

with the one exception being the time. Once that has been good it will stay more or less correct. 

Then there also is the DOP value set HDOP, VDOP.......  that will tell you how accurate the GPS module thinks its own output is.

So there are a number of ways to check if your data can be used without using garbage.....

I use a receiver that has a starting date of 2009. once it has given me a date that is in 2010 I know it is valid to use that.

then I wait until I have at least a 2D lock before I start using the data.

 

 

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Surely gchapman's answer is the "official" way to do this? I don't think you can assume that just because you get ",,,," it means signal loss. You should follow #6 and look at field 6 in the GGA as the true indicator of signal quality and make your interpretation of the rest of the sentence based on this.

 

From: https://www.gpsinformation.org/d...

                  Fix quality: 0 = invalid
                               1 = GPS fix (SPS)
                               2 = DGPS fix
                               3 = PPS fix
			       4 = Real Time Kinematic
			       5 = Float RTK
                               6 = estimated (dead reckoning) (2.3 feature)
			       7 = Manual input mode
			       8 = Simulation mode

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Ha, GPS signal failed for a few times. The system kicked in and started updating date and time by itself.  That's what I was looking to do with.

 

Thanks lads.