GPS Project – A few simple questions

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    Hi, I’m planning on buying the BerryGPS-IMUv3 board for a GPS project (mostly as a supplement to an OpenAuto/Crankshaft head unit emulator, on the same Pi 4B), but I’m pretty new to the whole Pi thing, so I’ve got a few questions. Forgive me if they’ve been answered elsewhere; I’ve been reading for many hours and days about this.

    So, my plan is to build this as a self-contained GPS & head unit inside a box (probably SmartPi Touch 2 unless someone knows of a waterproof case?), with the Pi 4B 4GB, and maybe an X725 board for its UPS & auto-off when power gets cut. I’ll probably be using Navit as per your guide and others I’ve seen; it seems to be the most complete, easy-to-use nav app out there.


    Even though this is mostly going to be mounted on a motorcycle, I feel like I should be using an external antenna if possible – does it have to be built as a GPS antenna? Or can I connect the uFL antenna wire off, say, a laptop WiFi unit, wrap it around the inside of the case, and boost signal that way? The internal architecture of GPS antennae seems to be a tightly-held secret.

    Or, should I really be using a more external antenna? I’m sure I could find a way to bolt your 28dB active antenna either to the outside of the case, or maybe elsewhere on the bike, but I’d really like it to be self-contained inside the case, with only audio and power having to be disconnectable. I know from experience that SMA is not meant to be frequently removed.

    Or will the internal antenna be enough if it’s only obsctructed by the thin plastic box in most directions? Putting a simple uFL wire in would be no problem, but I have no idea if proper passive GPS antennae have some special noise-blocking built into them. Or perhaps it would be sufficient if I mounted the entire GPS-IMU away from the RPi board, to the inside of the outer cover of the SmartPi 2?


    Will this retain RTC info after 4 hours if the Pi is being passively powered by an X725? I don’t know enough about the power delivery/architecture of Pi here. Assuming it doesn’t keep alive, I’ve read a few of your posts saying that satellite re-acquisition should take less time, even after RTC loss, after the initial one, depending on how long you wait between power-on time up to 180 days. Will I be looking at 1-2 minute acquisiton times if I leave this unplugged for a week at a time, for example?

    Also, is there a simple config option to have compass heading displayed on Navit, or maybe even in Raspbian itself always-on-top?

    Oh, and does the physical orientation of the GPS-IMU board itself matter at all, regarding the sensors inside? Obviously it’s not going to stay 90-degrees flat and in one orientation if I have it mounted to a bike, and moving between vehicles.


    Thank you for your time. I’m already looking forward to tinkering with this.


    Some comments;

    -The berryGPS-IMU does have an internal antenna
    -To improve the signal, we do sell a ceramic  antenna which would fit inside the case and work perfectly.  You need to make sure it is facing up. And  there is no need to mount it away from the Raspberry Pi. In fact, if it is sitting right above the Pi, the Pi can help reduce multipath errors.

    -The small uFL connected isn’t meant to be frequently removed. You should be fine if you are using a SMA connector.
    -To improve the reception of any GPS antenna, you can try and avoid multipath signals.  It isn’t mandatory, but if you have issues, look into this.

    -regarding RTC, as long as you have power to the Pi (even if the Pi is shutdown) it will still have power on the Pi 3.3v pin and therefore still power the BerryGPS-IMU.

    -Acquisition, the longer it is off, the longer it will take to get a FIX. If off for one week without power, and you turn it back on while in the same position… I would expect it to take 1-5 mins.  If you are moving while it is happening, it may take longer. We do sell the BerryGPS-GSM, which can get a fix in seconds.. no matter how long it has been off.  But it does not include an IMU.

    -Heading in Navit.  Im not sure how this can be done… sorry.

    -device orientation. For GPS,   If you using the internal antenna.. then it is best that it is at least facing up, doesn’t matter at what angle as long as it can see some sky.  If you are using an external antenna.. then the external antenna would work better when it is facing up and seeing some sky. It will work upside down.. but not as well.   For IMU, it is easier to work with when it is right side up and the compass works best when the pitch or roll of the device is -/+ 40 degrees.


    I hope this helps

    Peter --


    Peter, thank you for your answers. You gave me a lot of info. A few follow-ups, if I may:

    What would I be losing without an IMU? As I understand it, IMU basically helps maintain GPS position for a while if signal is lost? Or does that even work with Navit? Would I lose constant compass heading without it?

    Doesn’t the GSM unit require a GSM carrier SIM card, and thus, an active account with a GSM cellular carrier? If true, I feel like the IMU unit would be better for me; I’m not looking to spend a lot as this will be secondary to Google Maps, GSM coverage in the US is paltry, and I feel as though IMU might maintain GPS lock a bit better in concrete jungles I visit, like Boston (ie. lots of multipath signals). What do you think?


    Sorry, I was unclear about the whole antenna thing – I wasn’t saying I’d be unplugging the uFL connector often; rather, if I used it, the uFL connected antenna would be permanent inside the case. But the real question was do I need a specific GPS antenna or will a standard WiFi antenna work? Sure, I can experiment with different ones, but I’d like to order everything I’m likely to need all at once (tonight, ideally). And, how does the ceramic passive antenna compare to the active one? (those made by OzzMaker) .. since they’re both rated at 28dB, is the only true difference being that the active one can have a longer cable? I think this is generally true based on some articles I’ve read, but a few say that active reception can be better.

    I’ll have to check specs on the X725, and see if I can keep 3.3v trickling on the GPIO rails. Roughly how much power drain will the GPS-IMU v3 board continuously take at idle?


    Thanks for all your help. This is going to be a fun project.


    +Without IMU, yes.. this is the idea. However, on the BerryGPS-IMU, these work independently and any  fusing of these two devices would need to be done by you. This is called untethered dead-reckoning and there are GPS modules that do this, however they are a bit more expensive. Untethered Dead-reckoning is what would help in your Boston scenario.

    +Heading…   lets clear this up a bit.  Heading can be used to describe which way a device is pointing or which way a device is traveling.  I think you need the later, E.g.  you want to know if you are going east bound down a highway. The IMU isn’t needed for the second use case as this can be done by the GPS module. And Navit can be configured to show a compass, I don’t know if you can configure Navit show “east”,”west”, etc..

    +For the GSM module, “yes”, you would need a SIM with an active account with a carrier.

    +The active corded antenna is the best we have.  Second would be the ceramic and third would be the internal antenna on the GPS module.   In saying this, if you have clear access to sky all the time, you can just get by with the internal antenna

    +Power consumption, when trying to get a fix.. it can peak around 71mA.  When it has a fix and is running in a continuous state it would be a constant 23mA. If you put it to sleep, it will be using 100 μA


    Peter --

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