[time-nuts] Re: GPS failed
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 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 ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Realtime comparing PPS of 3 GPS
On 31.05.22 01:10, glen english LIST via time-nuts wrote: Be aware not to confuse the antenna ground plane (the patch will always have its own plane because the top metalization must be fed against a plane or counterpoise - and a ground plane behind the antenna. I can see the usefulness of the larger ground plane for any purchased patch antenna to reduce the likelihood of interference underneath (if the feed coax has a good RF contact with the plane), and if the plane is coupled well, it may improve the low angle response . The supplementary ground plane doesnt have to have a galvanic connection if the gap between the underside of the patch is low- IE use purely a capacitive coupling to tie the patch antenna ground to the large ground sheet- [...] That means reducing the gap to about 0.05mm OR increasing the area- probably means using a bigger patch. Hi Glen, thank you for the insight. I was referring to a ground plane behind the antenna. Gaps below 1~2 mm between a magnetic "puck"-type patch antenna with IP67 housing and an external ground plane seem practically challenging to me. When it comes to stacked patch multi-band antennas like u-blox' ANN-MB [1], the gap between the top patch and the external ground plane is probably significantly higher. Yet, u-blox generally recommends the use of a symmetric ground plane for the RTK applications [1,2]. From my experience, the M8P and F9P RTK fix barely works without a ground plane under the u-blox antennas. While it's just an empirically educated guess, I'd assume that what is required for RTK will not hurt for timing. Could you share your expert opinion on this? My antenna expertise is admittedly limited to reading data sheets and picking the right one for the particular RF measurement requirements. Thanks and best regards, Carsten [1] https://content.u-blox.com/sites/default/files/ZED-F9P_IntegrationManual_UBX-18010802.pdf#page=114 [2] https://content.u-blox.com/sites/default/files/ZED-F9P-MovingBase_AppNote_(UBX-19009093).pdf#page=8 ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
[time-nuts] Re: Realtime comparing PPS of 3 GPS
Hi Erik, have you tried running all receivers off the same antenna via a power splitter (make sure to dc block all but one receiver)? That should remove the uncertainty due to antenna differences (location, RF characteristics, etc.). Also, are you using ground planes for your puck antennas? These types of antennas typically require a ground plane for optimal performance [1]. Best regards, Carsten [1] https://content.u-blox.com/sites/default/files/products/documents/GNSS-Antennas_AppNote_(UBX-15030289).pdf#page=16 On 30.05.22 13:00, Erik Kaashoek via time-nuts wrote: Further evaluation did shown the time differences between the 3 GPS modules was due to difference in the trigger level setting of the timer/counter and difference in length of GPS antenna cables. After removal of the phase drift due to Rb frequency offset the attached image shows the phase differences of the 3 modules versus a Rb reference. The two ATGM modules are very consistent over a 2.8 hours period. The NEO-7M varies wildly with phase errors above 100 ns. Possibly due to a somewhat less optimal antenna position. It seems phase variations over time in the order of 10-20 ns are indeed unavoidable, even with a good antenna. Erik. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
