GPS Admin screen

M0TAZM0TAZ
in KiwiSDR Discussion
I dont really understand what the screen is telling me ? I will enclose a screen grab, but my questions are..

The antenna here isnt in a great location, but I would expect a fix here,... as I dont know what to expect and haven't seen anything in the user guide any help appreciated.

Enable GPS set to YES
Ch 0 to 11 mostly blank except Ch0

ACQ (assume acquired GPS) YES
tracking blank
Good blank
Fixes blank
run 59:41:28
TTFF blank
GPS time blank
ADC clock 66.666070 (0)
Lat blank
Lon blank
Alt blank
Map blank.

Is this fixed ?
What should it display when fixed?
Timing seems OK for WSPR having checked earlier.

Thanks Dave M0TAZ
image

Comments

  • BjarneBjarne
    It may seem like a silly question to ask, but is the GPS antenna facing the sky? Like being positioned outside? For comparison, mine hangs in the antenna wire just outside the window, probably far from ideal, but the attachment shows what it looks like here.

    Bjarne Mjelde
    arcticdx.blogspot.com


    Attachments:
    https://forum.kiwisdr.com/uploads/Uploader/8b/a375f7026e72f3e183a8d1b6f0c4a4.jpg
    M0TAZ
  • GaryGGaryG
    Hi

    I see the same as Dave with the exception that very occasionally I do see a RSSI bar for a short period (seconds).

    The antenna is facing toward the sky with approximately a 10 degree by 70 degree opening  although through double glazed glass.

    For a comparison, my phone, placed beside the patch antenna shows seven satellites (varies) with S/N readings between 18 and 28 (or so). I'm using the Android GPS Test app (Ver. 1.5.4) in the phone (a Moto G).

    GaryG

    M0TAZ
  • G8JNJG8JNJ
    Hi,

    The patch antenna supplied with the KiWi doesn't seem to be as good as some other GPS antennas.

    A lot of modern double glazing uses a metalised film to provide additional heat retention, but this also has the effect of greatly reducing the strength of GPS signals.

    I rarely used to get two satellites at any time with the supplied patch, but this week I ran in some more cables so that I could split the feed from an existing Trimble GPS antenna and now I'm typically seeing six at any moment with four or more at good strength most of the time.

    Regards,

    Martin - G8JNJ
    M0TAZ
  • jksjks
    edited February 2017
    I need to add a description of the GPS page fields to the Kiwi FAQ. But that's a separate issue.

    Are you using the GPS "puck" antenna supplied with the Kiwi? Have you extended the cable in any way? The attenuation through the glass might be an issue. If possible try it with the antenna outdoors temporarily for comparison.

    The Kiwi GPS is very sensitive to input signal strength. During development I was using the supplied active antenna with its 3m rg174 cable. But I extended it maybe another 3m so I could place the antenna in a location that had a better view of the sky. The signals were always marginal. When I moved to a new location I was able to remove the cable extension and this made a huge improvement in the number of sats received and their RSSI (like double). rg174 has large losses at 1.5 GHz. Your cellphone doesn't suffer from the cable loss and may also have more antenna amplification or a better noise figure.

    Bjarne: this (http://www.abracon.com/Support/PatchAntenna-Application-Note.pdf) says the best gain is when the patch antenna is placed horizontally on a surface ("base down"). But it looks like yours is working fine whatever position it happens to be in.

  • M0TAZM0TAZ
    Thanks to all who contributed. I think the GPS antenna / receiver doesn't seem to be as sensitive as others I have. Now I understand the screen I have isn't showing a GPS lock, that wasn't very clear from the information presented.

    I've moved the Antenna outside and now see 1 to 3 sats and have a GPS location showing with an associated map.

    Still don't fully understand how the GPS has been implemented, the time for instance is stopped at 19.23.13 so I guess it's now lost its fix?

    Indoors I would expect 3 to 5 fixes as I get on the QRP Labs WSPR txm using the internal antenna. I understood these units have the same GPS chip, so should really perform in a similar way.

    I will try another active antenna, see if that changes anything but don't really understand why it's not that sensitive, most modern GPS receivers are amazingly sensitive these days?

    Regards Dave M0TAZ
  • jksjks
    edited February 2017
    The GPS time displayed is the time computed when the last position solution was calculated. This can be arbitrarily delayed for a bunch of reasons. Maybe I should call it something else.

    First of all GPS time is offset from UTC by, what, 17 to 20 seconds now because of UTC leap second adjustments? So keep that in mind.

    Second, the GPS code I use, which is from Andrew Holme's wonderful Homemade GPS Receiver project (http://www.holmea.demon.co.uk/GPS/Main.htm), while excellent is not as sophisticated as what you'll find in a cellphone or modern GPS receiver. The maximum rate at which solutions are calculated is every 4 seconds under the best conditions. Not 10 times a second +/- like most GPSs. Modern GPSs also have better software & firmware algorithms that do better tracking and thus have better sensitivity. That's why they perform better even though we use the same front-end chip (and if you factor out the antenna differences).

    The GPS process of searching (acquiring) new satellites is stopped when there are active SDR connections on the Kiwi. Existing sats will continue to be tracked (and timing solutions calculated) until they eventually drift out-of-range. This is because the acquisition process uses an expensive FFT which cannot run simultaneously when there are multiple SDR connections. The Beagle doesn't have enough CPU to do all this work and still meet the realtime deadlines.

    So all these reasons are why the displayed GPS time will lag UTC, sometimes significantly. The GPS isn't used for anything except calibrating the ADC clock. And for that the timeliness of the GPS time measurement doesn't matter (only the difference between the last two GPS time calculations is used). Things like WSPR time presume accurate time is being received over the network (Internet) via NTP.

     
    M0TAZ
  • jksjks
    edited February 2017
    Quick comment about the GPS page.

     Bottom row:
    "acq" Acquisition state, "yes" is acquiring new sats, "paused" if active SDR connections prevent acquisition of new sats.
    "tracking" Number of sats being tracked and appearing in one of the 12 GPS channel slots.
    "good" Of the sats tracked, then number of those that are considered good enough to compute a solution from (min 4 for a solution).
    "fixes" Total number of GPS position and time fixes computed.
    "run" How long the GPS has been running (H:M:S)
    "TTFF" Time-To-First-Fix, how long it took to get the very first solution after startup.
    "ADC clock" The GPS-measured true frequency of the ADC clock (nom 66.666600 MHz) In parens after are the number of computed corrections.

    Channel display: (this is a 12-channel receiver currently)
    "acq" Shows which channel the acquisition process is working with.
    "PRN" Sat identifier.
    "SNR" Signal-to-Noise-Ratio at time of acquisition. Min 16 to begin tracking.
    "gain" Goes negative if RSSI > about 1500 and software attenuation needs to be applied.
    "hold" How many bits in the current frame, out of 300, have been decoded.
    "wdog" Watchdog that will timeout and free tracking channel if bits are not being decoded. Different timeout values depending on channel state.
    "err" U = tracking loop unlocked. P = parity error. Momentary errors are not necessarily a bad thing.
    "subframe" Colored when GPS subframe correctly received. Blinks grey when updated. Subframes 1-3 required for position solution.
    "novfl" Number of holding register overflows (a debugging aid).
    "RSSI" Relative Signal Strength Indicator. Above 250-350 means sat actually present. Above 1000 is an excellent signal.

    M0TAZ
  • jksjks
    edited March 2017
    The GPS admin page no longer displays the GPS time. In its place is the GPS/UTC offset. This is the number of seconds GPS time is ahead (or behind) UTC. It changes as leap seconds are applied to UTC and is a parameter extracted from the satellite transmitted navigation data.

    This is important now because starting with the v1.59 release the Beagle time-of-day clock will be set to the UTC-adjusted GPS time if it differs by more than 2 seconds (value picked because WSPR needs to be within roughly 2 seconds of UTC). The Beagle clock could be incorrect because NTP is no longer running, there is no Internet connection, etc.

  • jksjks
    edited March 2017
    v1.60, to be released tomorrow, will fix a GPS bug that I finally understood today. Apparently, since v1.38 the GPS acquisition process has not been restarting after active users disconnect like it's supposed to. That will lead to the situation where the admin GPS page says "acq yes" (instead of "acq paused"), but there is no refresh icon shown in the "acq" column next to an incrementing PRN as is typically the case. The satellites that are already acquired and being tracked will be be fine until they drift out-of-range and are dropped. So eventually the GPS page will show nothing even though new satellites should be getting acquired.

    M0TAZWA2ZKDKA7U
  • M0TAZM0TAZ
    Great, thanks update applied last night.
  • G8JNJG8JNJ
    Hi John,

    That seem to be a lot better - thanks.

    Martin - G8JNJ


  • UlliUlli
    Hi everybody,
    I have still problems getting GPS running ...
    Can somebody suggest a better GPS receiving OUTDOOR antenna, as I hardly can copy more than two GPS sattelites at a time with the supplied active GPS antenna...
    I understand as the GPS-functionality in software is not as clever as in GPS (mine is 6years old and captures easily constant 10 sattelites at a time)....
    If one sattelite is lost, it needs about 2-3minutes until a new one is found... with RSSI hardly reaching more than 350 (SNR just 16 or 17)
    So not to wait so long, I need a really good GPS antenna outside not to loose satellites, found earlier, because it takes so long to find a new one...

    Unfortunately there is no spec of the supplied antenna, so I really don't know what to search for, for considerable improvement, which is really necessary to get GPS running.

    I have the shack in the basement - there is no GPS at all...;-(
    But even on the first floor (under the roof)  - no luck with sufficient GPS reception... so outdoor is a must.

    No trees - flat terrein... my very old phone sees 10...12 satellites at once outside... so there is headroom for improvement with a good outdoor GPS antenna....

    Any suggestion? And where to buy ? Ebay?

    Ulli, ON5KQ
  • jksjks
    edited September 2017
    Ulli,

    To answer your question from the Github issue you opened:
    Do you have spec's of the little GPS antenna , which was supplied with the kiwiSDR ?

    Yes, see here

    In your new location, is it possible to locate the Kiwi closer to the point in the building where the GPS cable goes outside? So the antenna can be positioned in a better place without having to extend the cable? You can run Ethernet up to 100m maximum. And hopefully power can be supplied at this point.

    Or maybe get one of these outdoor active antennas with an N connector so you can use a long run of low-loss coax without having to splice the cable:

    See here for cabling recommendations: http://freqelec.com/gps_gnss/gps_ant_issues_r1_5-07.pdf
    Cable loss of rg174 at GPS L1 freq is -1.5 dB/m (-4.5 dB/3m)

    I've related this story before someplace, but it is worth telling again. It shows just how sensitive the Kiwi is to any reduction in GPS signal strength due to cable extension. At the location where I was first developing I needed to extend the 3m cable to about 6m to get the GPS antenna up to the roof with a clear view of the whole sky. I did this by cutting the SMA connector off the supplied puck antenna and splicing in 3m of RG174 coax, and then crimping my own SMA on the end of that cable. I did not have the proper crimping tools for this, so it was a very crude connector termination. The cable splice was even worse. Imagine the impedance bump at that point! I knew that at 1.5 GHz the RG174 loss was going to be terrible.

    Sure enough, I had very poor performance compared to a little handheld GPS receiver. The handheld receiver even worked okay indoors. When I moved to a new location I was able to compare that antenna with the one supplied with the Kiwi (3m cable). The unmodified Kiwi antenna received the sats with twice the RSSI as my extended cable antenna! So the difference is very significant.

  • UlliUlli
    Many thank John, for very good advice...
    One possibility is indeed to move the Kiwi completely outside in my Gardenhouse. Then GPS antenna can be installed easyly outside...
    But it needs some more work of installation necessary powerlines

    The outdoor GPS antenna seem to be also a good deal ...

    What I also wonder is, whether I might have additional interference sometimes from Wifi routers arround. Sometimes I get strange SNR-values in the admin panel, like 82 in the SNR colomn... that seem to be a spike coming from some noise  ?!
    Certainly I need to test more to find a good solution.

    Ulli, ON5KQ
  • jksjks
    Interference to the GPS L1 frequency (1575.42 MHz) from a clock harmonic or other source is very possible. Especially given the extremely weak GPS signals at the SMA connector even if using an active antenna.

    My original prototype used a different ADC clock frequency with a harmonic that fell within the 2 MHz GPS passband at L1. I could not run the GPS at all unless the SDR ADC clock was disabled!

  • WA2ZKDWA2ZKD
    I have been trying various alternative GPS antennas, including a "big" one which I'll report on it a few days. However, if you look at the data sheet for the supplied patch unit, they specify a 70X70 mm groundplane for their gain spec. I can confirm that mounting it on a 4X4" piece of steel improved things here. 

    I can also confirm that adding more coax hurts. 
  • WA2ZKDWA2ZKD
    an extension that allows the GPS data to be viewed by others would be handy!
  • jksjks
    That's not a bad idea. And it could be done in a way that doesn't leak the Kiwis precise location (i.e. with any greater resolution than the current maps)

    WA2ZKD
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