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sci.geo.satellite-nav (Global Satellite Navigation) (sci.geo.satellite-nav) Discussion of global navigation satellite systems (GNSS). Topics include the technical aspects of GNSS operation, user experiences in the use of GNSS, information regarding GNSS products and discussion of GNSS policy (such as GPS selective availability).

Too much gain?



 
 
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  #1  
Old March 24th 14, 06:42 AM posted to sci.geo.satellite-nav
claudegps
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Posts: 217
Default Too much gain?

Hi all,
what to extect on the GPS receiver in case there is the antenna/LNA gain that is too high?

I case of gain too high, I'd expect to have difficulties in the reception of satellites with higher elevation (generally the ones with stronger signal).
This would may also prevent the reception of the lower elevation ones, because the AGC would be set to lowest possible (because too much signal in input) making not possible for the receiver to raise the gain where needed to low signal satellites.

We have a receiver that has often very high CN0 values, but sometimes we experience problems in receiving all satellites available.
(I'm investigating to understand if there a releationship between the satellites not received and their elevatio/expected signal)

(I understand the question maybe too generic... but any help would be appreciated)

Thanks
  #2  
Old March 24th 14, 07:25 AM posted to sci.geo.satellite-nav
Joop[_3_]
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Posts: 7
Default Too much gain?

On 24-3-2014 8:42, claudegps wrote:

what to extect on the GPS receiver in case there is the antenna/LNA gain that is too high?

I case of gain too high, I'd expect to have difficulties in the reception of satellites with higher elevation (generally the ones with stronger signal).
This would may also prevent the reception of the lower elevation ones, because the AGC would be set to lowest possible (because too much signal in input) making not possible for the receiver to raise the gain where needed to low signal satellites.

We have a receiver that has often very high CN0 values, but sometimes we experience problems in receiving all satellites available.
(I'm investigating to understand if there a releationship between the satellites not received and their elevatio/expected signal)

(I understand the question maybe too generic... but any help would be appreciated)


I'm not sure if this could be an issue. Signal level of GPS is way
below the noise floor, conventional radio-thinking does not apply fully.

Could you have an issue with other radio activity on nearby frequency's?

I always wonder how GPS reception on 1575MHz can work at all while at
the same time having transmitter activity on GSM 1800MHz or UMTS 2100MHz
within a few cm's distance (on smartphone's).

Your GPS preamp could be overloaded if there is insufficient filtering
on the input and other (transmitter) signals close by (in physical
distance and/or frequency distance).

  #3  
Old March 24th 14, 11:30 AM posted to sci.geo.satellite-nav
claudegps
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Posts: 217
Default Too much gain?

Il giorno luned́ 24 marzo 2014 09:25:40 UTC+1, Joop ha scritto:
On 24-3-2014 8:42, claudegps wrote:



what to extect on the GPS receiver in case there is the antenna/LNA gain that is too high?




I case of gain too high, I'd expect to have difficulties in the reception of satellites with higher elevation (generally the ones with stronger signal).


This would may also prevent the reception of the lower elevation ones, because the AGC would be set to lowest possible (because too much signal in input) making not possible for the receiver to raise the gain where needed to low signal satellites.




We have a receiver that has often very high CN0 values, but sometimes we experience problems in receiving all satellites available.


(I'm investigating to understand if there a releationship between the satellites not received and their elevatio/expected signal)




(I understand the question maybe too generic... but any help would be appreciated)




I'm not sure if this could be an issue. Signal level of GPS is way

below the noise floor, conventional radio-thinking does not apply fully.


Yes, but I think that in any case an excess of gain may degrade the reception performance.
Not sure about the effect in any case.
having too much amplification may also amplify the noise, leading to signal (useful signal+noise) overload.


Could you have an issue with other radio activity on nearby frequency's?



I always wonder how GPS reception on 1575MHz can work at all while at

the same time having transmitter activity on GSM 1800MHz or UMTS 2100MHz

within a few cm's distance (on smartphone's).


Generally it works.
There are many devices that are GPS+GSM/UMTS and they work. Of course the design must take into account the transmitter activity.


Your GPS preamp could be overloaded if there is insufficient filtering

on the input and other (transmitter) signals close by (in physical

distance and/or frequency distance).



Of course this could be an issue I'm investigating.
Thanks
  #4  
Old March 24th 14, 11:07 PM posted to sci.geo.satellite-nav
Alan Browne
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Posts: 1,339
Default Too much gain?

On 2014.03.24, 03:42 , claudegps wrote:
Hi all,
what to extect on the GPS receiver in case there is the antenna/LNA gain that is too high?


If the front end of the receiver does not need a powered (amplified)
antenna, and the cable is short, then if a powered antenna is used (with
a T to inject the DC bias for the antenna power, or separate power
feed), then yes, the gain will be too high and will saturate the front end.

This will cause a lack of accuracy because the correlators (each
channel) will not drive to a sharp peak but rather to a flat top.

Desired:
^
/ \
/ \
/ \
/ \
error range: |



Too much gain:
-----
/ \
/ \
/ \
error range: | |

That error will be largest (as you say) on those sats at highest
elevation and lowest (and perhaps no longer present) towards the horizon.

So, your horizontal error might not be an issue, but your vertical error
will be higher.

--
... it may be that "in the cloud" really isn't the best term
for the services these companies offer. What they really
want is to have us "on the leash."
-David Pogue, Scientific American, 2014.02
  #5  
Old March 26th 14, 12:21 PM posted to sci.geo.satellite-nav
claudegps
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Posts: 217
Default Too much gain?

Il giorno marted́ 25 marzo 2014 01:07:21 UTC+1, Alan Browne ha scritto:
On 2014.03.24, 03:42 , claudegps wrote:

Hi all,


what to extect on the GPS receiver in case there is the antenna/LNA gain that is too high?




If the front end of the receiver does not need a powered (amplified)

antenna, and the cable is short, then if a powered antenna is used (with

a T to inject the DC bias for the antenna power, or separate power

feed), then yes, the gain will be too high and will saturate the front end.



This will cause a lack of accuracy because the correlators (each

channel) will not drive to a sharp peak but rather to a flat top.


Thanks for your answer.
I have some problems in understanding how the excess of gain to the RF input may generate the problem post-correlation.
Maybe I just need to take some time to review all the process
What makes me think is that in case of your example, probably I'd have a sharp peak with the replica-signal not perfectly in phase with the input signal (lowering the correlation gain)
The effect would be inaccuracy anyway(because range/doppler measurements would be wrong - even generating the sharp peak)
  #6  
Old March 26th 14, 07:19 PM posted to sci.geo.satellite-nav
Alan Browne
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Posts: 1,339
Default Too much gain?

On 2014.03.26, 09:21 , claudegps wrote:
Il giorno marted́ 25 marzo 2014 01:07:21 UTC+1, Alan Browne ha
scritto:
On 2014.03.24, 03:42 , claudegps wrote:

Hi all,


what to extect on the GPS receiver in case there is the
antenna/LNA gain that is too high?




If the front end of the receiver does not need a powered
(amplified)

antenna, and the cable is short, then if a powered antenna is used
(with

a T to inject the DC bias for the antenna power, or separate power

feed), then yes, the gain will be too high and will saturate the
front end.



This will cause a lack of accuracy because the correlators (each

channel) will not drive to a sharp peak but rather to a flat top.


Thanks for your answer. I have some problems in understanding how the
excess of gain to the RF input may generate the problem
post-correlation.


Getting back to your original question:

The gain on the antenna is fixed - even for active antennas.

At the receiver input: gain would be minimized because the highest
powers observed are saturating the receiver. This _could_ mean the
lower elevation satellites are not being seen as a saturated signal -
they could be "just right" (since there is still the over sensitive
antenna gain you believe to be present).

Do you have actual results (channels, elevation, SNR's) you can share?

What is the antenna gain? Is it a powered antenna (active)?

Maybe I just need to take some time to review all the process What
makes me think is that in case of your example, probably I'd have a
sharp peak with the replica-signal not perfectly in phase with the
input signal (lowering the correlation gain) The effect would be
inaccuracy anyway(because range/doppler measurements would be wrong -
even generating the sharp peak)


If the input is saturated then there is less to correlate to the
"replica" (I assume you mean the gold-code for that PRN) so the
correlator peaks will be flat (or otherwise not "peaky").

Doppler is used to drive the correlator position and feeds off the
result - so it will remain close to the peak when tracking - but will be
noisy for the same reason (somewhere in the "flat"). Any "products" of
the Doppler number will be noisy as a result (velocity).

--
Those who have reduced our privacy, whether they are state
or commercial actors, prefer that we do not reduce theirs.
- Jaron Lanier, Scientific American, 2013.11.


 




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