MobileGPS

General support for the Pi-Star System
LU9XRL
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Re: MobileGPS

Post by LU9XRL »

The information loaded in the file is fine, it seems to me that it is not correct, how can I correct this. Someone charitable can help me.

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Darío Lanza
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W4JEW
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Re: MobileGPS

Post by W4JEW »

I own an inexpensive USB GPS that I bought on Amazon.com.

https://www.amazon.com/gp/product/B07QRGK7ZK/

I connected it to Pi-Star (after upgrading to 4.1.2 of course), then enabled the Mobile GPS service, set it to use /dev/ttyACM0 and speed 4800 (default tries to use 38400 which is WAY too high for GPS).

I rebooted and all looks good! Now the question is...what do I do with it? :-D

The contents of /etc/mobilegps on my Pi-Star are as follows:

Code: Select all

[General]
MinDistance=400
MinTime=60
MaxTime=300
Daemon=0
Debug=0

[Log]
DisplayLevel=0
FileLevel=1
FilePath=/var/log/pi-star
FileRoot=MobileGPS

[GPS]
Port=/dev/ttyACM0
Speed=4800
Debug=0

[Network]
Address=127.0.0.1
Port=7834
Debug=0

[Enabled]
Enabled=1
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Jeff Hochberg
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Atlanta, GA
n9mxq
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Re: MobileGPS

Post by n9mxq »

KE7FNS wrote: Sat May 30, 2020 12:41 am
Looks like MobileGPS is only looking for the NMEA sentences GGA and RMC everything else is ignored.

You might need to plug that into the Ublox u-center and configure it and then confirm that yours is sending those NMEA messages.

Its been a while since I've tinkered with a Ublox but I might see if I can dig one out and plug it into a TTL adapter and see what happens.
This one is USB, I'll have to download the u-center again and see what I can do. It's no big deal for me, was just seeing if it would use it.
Gene in Belvidere IL
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n9mxq
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Re: MobileGPS

Post by n9mxq »

KE7FNS wrote: Sat May 30, 2020 10:30 pm So I had some free time and dug out a GPS/compass unit for one of my fathers autonomous drone flight controllers, and I was able to configure it to spit out the NMEA messages, I also had it spitting out at a rate of 10hz, and 115200 baud just for funzies. They use the Ublox 6, 7 and 8 chips on most of them, its just that these use i2c and serial connections.

I enabled Debug=1 and DisplayLevel=1 and I was able to see the GPS messages in the log file but I'm not sure how any of that information gets to whatever it needs to go. The documentation is really lacking information.

I swore there were two posts discussing a script a few posts ago that is mysteriously missing now. I'll bet if it was still available I would of been able to use it to do more but oh well, gotta love forum censoring.
My problem appears to be lack of lock. Not seeing sats reliably enough to start spitting out data.. It's sending NMEA, it's just empty.. It's weird, since I used to use it in the same spot and see all sorts of satellites. I originally bought the thing for keeping my time sync'd for FT-8.. Now it won't lock on any satellites..

And censorship happens on every board. Just saw a big rant over on Radio Reference this morning.. Just have to remember it's Andys house, and we're guests. (And that's the last I say on the topic)
Gene in Belvidere IL
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n9mxq
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Re: MobileGPS

Post by n9mxq »

KE7FNS wrote: Sun May 31, 2020 7:32 pm
I had an old President Lincoln 10m radio in a truck that when I turned it on it would knock out my GPS signal on a handheld unit mounted on the dash, I ended up getting a higher gain external antenna. After a while the 10m just quit transmitting completely, I never knew what was causing it but maybe you have something new in the area that is interfering.
I had one of those Presidents also!! Drove me nuts until I figured it out.

I have a Byonics GPS-2 puck right next to it, and it's locked tight. So, the signal isn't blocked by RF.. I haven't used the Ublox for a couple years, so who knows, it could have toasted when I plugged it back in. It's still trying to put out NMEA strings, and occasionally blinks a sat up on the screen, but just doesn't wanna lock..
Gene in Belvidere IL
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Re: MobileGPS

Post by n9mxq »

KE7FNS wrote: Sun May 31, 2020 9:30 pm Well this wasn't due to the radios RF this was as soon as I powered the unit on, it was very odd and I never quite figured it out.
The radio put out a birdie, I didn't have a RF counter that went high enough at the time, but yea, same thing. I even remember posting about it on a popular GPS board way back when..
KE7FNS wrote: Sun May 31, 2020 9:30 pm If its been powered off for a long time, it needs to capture the new ephemeris data which only happens once every 2 hours. I'd plug it in to USB power and set it on a windowsill and in a few hours check on it and I'll bet it will have a lock once it captures enough new data.

I saw that waveshare sells an external antenna for those boards also.

I was surprised to see my Ublox was capturing both gps and glonass satellite feeds.
Been on since before my first post in this thread.. No soap.. But like I said, I was just playing with the mobile GPS idea when I saw it posted, gotta poke new things and see if I break em ;-) I don't plan to put the GPS on either of my portable nodes for daily use.
Gene in Belvidere IL
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AF6VN
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Re: MobileGPS

Post by AF6VN »

KE7FNS wrote: Sun May 31, 2020 9:30 pm If its been powered off for a long time, it needs to capture the new ephemeris data which only happens once every 2 hours. I'd plug it in to USB power and set it on a windowsill and in a few hours check on it and I'll bet it will have a lock once it captures enough new data.
Unless NavStar has changed procedures, the general ephemeris data used to "schedule" candidate satellites takes only 15 minutes to capture, and is sent by all satellites in a continuous stream. Per "GPS Satellite Surveying 2nd Ed" (Alfred Leick, 1995 John Wiley & Sons) [page 72]{any typos are likely my fault}
Each satellite transmits a data message called the navigation message, on L1 and L2 at a rate of 50 bps. ... A complete message consists of 25 frames, each containing 1500 bits. Each frame is subdivided into five 300 bit subframes, and each subframe consists of 10 words of 30 bits each. At the the 50-bps rate it takes 6 sec to transmit a subframe, 30 sec to complete a frame, and 12.5 min for one complete transmission of the navigation message. The subframes 1, 2, and 3, are transmitted with each frame; these three frames repeat every 30 sec. Subframes 4 and 5 are each subcommutated 25 times. These 25 versions of subframes 4 and 5 are referred to as pages 1 through 25. Thus, each of these pages repeats every 750 sec or every 12.5 min.

Subframe 1 has clock correction terms.
Subframe 2 and 3 have just that satellites precise ephemeris
Subframe 4 has general messages, ionospheric terms, and GPS to UTC time coefficients, and almanac for satellites with SVN >+ 25
Subframe 5 has the almanac and health status for for up to 24 active satellites (one per major frame cycle).

Now -- it may be that ground control sends ephemeris updates every two hours, but that doesn't change the fact that every satellite is transmitting the full constellation almanac in ~15 minutes (give 2.5 min for for receiver to synchronize with whichever satellite it has manage to read.

It may even be possible to download the almanac for the constellation in less time IF the receiver has true parallel decoding logic, as the frames may not be in sync -- while one satellite is on "page 1" another could be on "page 12"...

--
AF6VN
Dennis L Bieber
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Re: MobileGPS

Post by AF6VN »

KE7FNS wrote: Mon Jun 01, 2020 7:40 pm
AF6VN wrote: Mon Jun 01, 2020 5:34 pm Now -- it may be that ground control sends ephemeris updates every two hours, but that doesn't change the fact that every satellite is transmitting the full constellation almanac in ~15 minutes (give 2.5 min for for receiver to synchronize with whichever satellite it has manage to read.
I was strictly pulling from memory from a project I had worked on in the past. However I found multiple sites that mention that ephemeris 2 hour update, so maybe I was remembering correctly. Shrug.

https://en.wikipedia.org/wiki/GPS_signals
From that article:
Wikipedia wrote:The navigation message conveys information of three types:

The GPS date and time and the satellite's status.
The ephemeris: precise orbital information for the transmitting satellite.
The almanac: status and low-resolution orbital information for every satellite.

An ephemeris is valid for only four hours; an almanac is valid with little dilution of precision for up to two weeks.[6] The receiver uses the almanac to acquire a set of satellites based on stored time and location. As each satellite is acquired, its ephemeris is decoded so the satellite can be used for navigation.
The ALMANAC is the information a ground receiver uses to locate visible satellites for subsequent ranging. The ephemeris is the precision orbit information used for said ranging.
Wikipedia wrote:The almanac serves several purposes. The first is to assist in the acquisition of satellites at power-up by allowing the receiver to generate a list of visible satellites based on stored position and time, while an ephemeris from each satellite is needed to compute position fixes using that satellite. In older hardware, lack of an almanac in a new receiver would cause long delays before providing a valid position, because the search for each satellite was a slow process. Advances in hardware have made the acquisition process much faster, so not having an almanac is no longer an issue.
Granted -- I suspect many of the small plug-in utility chips may not be quite as optimized as dedicated navigation equipment. A lot of consumer gear is probably still just using L1 (C/A), likely with WAAS, and maybe also receiving GLONASS in parallel.
KE7FNS wrote:
Satellites broadcast a new ephemeris every two hours. The ephemeris is generally valid for 4 hours, with provisions for updates every 4 hours or longer in non-nominal conditions. The time needed to acquire the ephemeris is becoming a significant element of the delay to first position fix, because as the receiver hardware becomes more capable, the time to lock onto the satellite signals shrinks; however, the ephemeris data requires 18 to 36 seconds before it is received, due to the low data transmission rate.
The part you didn't include is:
Wikipedia wrote:Satellite data is updated typically every 24 hours, with up to 60 days data loaded in case there is a disruption in the ability to make updates regularly. Typically the updates contain new ephemerides, with new almanacs uploaded less frequently. The Control Segment guarantees that during normal operations a new almanac will be uploaded at least every 6 days.
The two paragraphs together imply that the ground daily uploads 720 (24 hours / 2 hours * 60 days) discrete ephemeris sets per satellite. The "broadcast a new ephemeris every two hours" is the rate at which the satellite switches from one set of parameters to the next set, it is not the rate of broadcast. During the two hour period, ground receivers are receiving copies of the same ephemeris parameters every "18 to 36 seconds"

--
AF6VN
Dennis L Bieber
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Re: MobileGPS

Post by WA1OKB »

Would be so much better if this just used the existing hooks for gpsd...
Seems like a lot of re-inventing the wheel, and limits functionality.

For example: If Pi-star could read GPS from gpsd.... I could have one single GPS dongle, and provide GPS data via gpsd to both Direwolf (APRS) and Pi-Star and any other number of applications that might need GPS data.... all from one single GPS device, instead of having to buy a separate GPS device just to have the capability in Pi-Star.
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