[time-nuts] Re: GPS failed

[Carsten Andrich]

The u-blox SAW filtering is great. We've carried out various RF 
measurements with +40 dBm EIRP at 2.53 and 3.75 GHz with some u-blox 
ANN-MB within <2m of the Tx antennas. While we haven't conducted 
in-depth comparisons with a superior ground-truth, my current conclusion 
is that the u-blox RTK performance is not (noticeably) affected by 
strong out-of-band emissions. Without extensive filtering the Tx power 
would likely steamroll any LNA/receiver. Of course, as John pointed out, 
this won't help against in-band interference.

Best regards,
Carsten

On 11.07.22 15:16, John Ackermann via time-nuts wrote:
> Hi Skipp -- there is a lot of info about interference mitigation in the u-blox integration manual for the ZED--F9T (available under the docs at https://www.u-blox.com/en/product/zed-f9t-module).  It might give you some clues, and I think might also point to another u-blox app note on the topic.
>
> Most of the antennas I've seen that have an LNA also include a SAW filter.  I also once found on either Amazon or eBay so e new-product, relatively inexpensive, high pass filters with cutoff around 1 GHz.  Those would help knock down broadcast, trunking, etc. stuff.  (But of course nothing will help with on-frequency crud coming from outside the GPS system.)
>
> John
>
> On Jul 11, 2022, 8:49 AM, at 8:49 AM, skipp Isaham via time-nuts <time-nuts at lists.febo.com> wrote:
>> Hello to the Group,
>>
>> I'd like to get some opinions and war stories regarding GPS reliability
>> at
>> high RF level and elevation locations.
>>
>> Background:  Three different hill-top GPS receivers, all different
>> types, using
>> different antennas mounted on an outside fixiture, plain view of the
>> open sky,
>> all stopped working.
>>
>> Test antennas were brought in and placed on a fixture well away from
>> the
>> original antennas, the recevers went back in to capture and lock.
>>
> >From what I understand, the original antennas are what I would call
>> straight
>> preamp with no pre-selection / filtering.
>>
>> The ordered and now inbound replacements are said to contain a SAW
>> filter
>> system. It is the intent of the client to just place these "improved
>> antennas" in
>> to service and get on with life.
>>
>> I would suspect a GPS antenna (and receiver) could be subject to RF
>> overload
>> or blocking, however, we're assuming nothing major has changed at the
>> site, nor
>> any nearby location.  One might think there are more GPS receivers
>> being pushed
>> out of reliable operation by the world around them, I'm just not
>> hearing those stories
> >from a lot of people using them (GPS receivers). 
>> Any new install GPS receiver antenna ordered will/should contain some
>> pre-selection
>> to potentially avoid a problem, even some years down the road? Seems
>> like that's
>> where things are going... no more off the shelf, wide band, (hot)
>> preamplified GPS antennas
>> in busy locations?
>>
>> Thank you in advance for any related comments and/or opions ...
>>
>> cheers,
>>
>> skipp
>>
>> skipp025 at jah who dot calm
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