GPS Admin screen

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

  • 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
    M0TAZ
    kiwigps.JPG
    1518 x 675 - 106K
  • 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
  • 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 28
    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.

  • 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 28 Accepted Answer
    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 28 Accepted Answer
    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 6
    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 6
    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
  • Great, thanks update applied last night.
  • Hi John,

    That seem to be a lot better - thanks.

    Martin - G8JNJ


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