Show HN: Red Grid Link – peer-to-peer team tracking over Bluetooth, no servers
53 points by redgridtactical 4 days ago | 35 comments
I go on a lot of backcountry trips where I barely get cell service. If my group splits, nobody knows knows where anyone is until you regroup at camp or at your destination. You can buy Garmin radios or try to set up an ATAK, but ATAK is Android-only and assumes you have a TAK Server running somewhere to make use of all of the functionality. Cool tools themselves, but expensive to set up correctly. I just wanted two iPhones to share their location directly over Bluetooth when cell coverage was lacking.

Red Grid Link does that. Start a session, and anyone nearby running the app shows up on your offline map. When they walk out of range their marker stays as a "ghost" that slowly fades.

The hard part was making sync reliable over BLE. The connections drop all the time. Someone turns a corner, walks behind a vehicle, whatever. I built a CRDT sync layer (LWW Register + G-Counter) so there's never merge conflicts. Each update is just under 200 bytes (from what I have tested so far). When a user/teammate disappears the app does exponential backoff from 2 to 30 seconds before giving up and marking them as a ghost.

Everything is encrypted (AES-256-GCM, ECDH P-256 key exchange per peer pair). Sessions can require a PIN or QR code to join. It also offers offline topo maps with MGRS grid coordinates, same system as in my other app, Red Grid MGRS.

The app is free, and I'm looking for some honest feedback from other real-world users. Let me know if you have any questions!

  • shminge 4 days ago |
    Very cool idea. What's the range of BLE connectivity? I can't imagine it gets far
    • redgridtactical 4 days ago |
      It's not long range by any stretch. The use case is more "my group split up on a trail and I want to know which fork they took" vs "track someone across a mountain." The ghost marker system helps here too. If someone walks out of range, their last known position and direction stays on your map, so you at least know where they're heading and how long ago they traveled that direction.
      • pokstad 3 days ago |
        This is exactly what I want for my family hikes. We carry walkie talkies right now, which is just one more thing. Awesome app!
        • redgridtactical 3 days ago |
          Thank you! Would love some feedback if you try it out!
  • idiotsecant 4 days ago |
    Isnt the range of Bluetooth pretty much the range of a loud yell?
    • redgridtactical 4 days ago |
      Haha fair point. In open terrain you get maybe 50-100m which I agree, not far. In practice it's more useful than it sounds though. You don't need miles of range when the point is "where did my buddy go 5 minutes ago." And if they walk out of range their last position and heading stick on your map as a ghost marker so you're not totally blind or left guessing.
      • jdyer9 4 days ago |
        This comment got me thinking that it might be worth using their second-to-last location to try and derive some vector. Obviously that's super informative as you already know the edge of the map they left, but maybe it's useful?
      • sneak 3 days ago |
        Yeah the range of BLE makes this an order of magnitude less useful or practical than a $50 LoRa transceiver.
        • redgridtactical 3 days ago |
          All true, but the tradeoff is zero additional hardware cost and zero setup. For a family hike, small team, or a hunting party staying within a few hundred meters of each other, pulling out your phones beats buying and configuring radios for everyone.

          That said, BLE Long Range (Coded PHY) pushes it to 400m-1km and is on the roadmap for the next version. I'm also planning a Meshtastic bridge so if you already own LoRa hardware, the app can route through it for multi-kilometer range. Best of both worlds: works without hardware, works better with it. Would love some feedback if you ever try it out!

    • schobi 4 days ago |
      Well... I have a hard time imagining to get location by yelling "where are you? " "by the trees!" It really sounds immensely useful to know last location (while they were still in range).

      Use case: I was out, picking wild lingonberries in the forest with a group of ~10, some kids. At a "secret" location, with everyone wandering off in a direction they see more of them. Shouting did not help much.

      • redgridtactical 3 days ago |
        If you ever do get the chance to use, let me know what you think!
  • RankingMember 4 days ago |
    Cool idea, but doesn't Meshtastic already do this but with better range because it uses radio instead of BT?
    • redgridtactical 4 days ago |
      Meshtastic is great if you're willing to carry extra hardware. Everyone in your group needs a LoRa radio (which can range from $30-50 each), and you need to pair them, flash the firmware, configure channels, etc. For a SAR team or preppers who already own the gear, it's definitely a solid choice.

      Red Grid Link was more so for those already carrying their phones and those that don't want to buy anything else. The trade-off is range for convenience. BLE gets you maybe ~50-100m in the open, ~20-60m in densely woooded areas. That's enough to keep tabs on a hunting party spread across a hillside or a hiking group. Absolutely not a replacement for a radio relay across a valley.

      Different tools for different problems. If I need a 2km mesh range I'd set up Meshtastic too.

      • RankingMember 4 days ago |
        Good points. I like the ghost marker functionality, helps with the shorter-range of BT.
        • redgridtactical 4 days ago |
          Thanks, yeah the ghost markers ended up being one of those features that came out of necessity. Once I accepted that the BLE range was never going to be amazing, the question became what happens when someone drops off? Felt wrong to just remove them from the map.
  • seriousmice 4 days ago |
    Super cool idea! This would be amazing for Airsoft games
    • redgridtactical 4 days ago |
      Definitely was one of the use cases I designed around!
  • svpk 4 days ago |
    Have you looked at Bluetooth LE Long Range? I believe more recent phones have it and it claims communication of up to 1km. In practice less in the woods I'm sure. Still a dramatic win over standard Bluetooth though.

    https://novelbits.io/bluetooth-long-range-coded-phy/

    • redgridtactical 4 days ago |
      BLE Coded PHY is on my radar. The theoretical range boost is huge, something like 4x over standard in ideal conditions. The challenge right now is that flutter_blue_plus (the BLE library I'm using) has limited support for negotiating Coded PHY, and both devices need to support it. But phone hardware has been shipping with it since around 2020 so the install base is there. Definitely something I want to add, probably as an automatic upgrade when both peers support it.
      • lxgr 4 days ago |
        It’s not available on iOS devices though, right?
        • redgridtactical 3 days ago |
          Yeah iOS supports BLE Coded PHY since the iPhone 12 / iOS 14. The tricky part is negotiating it at the library level. flutter_blue_plus doesn't fully expose Coded PHY yet so I'd need to handle it through platform channels on both sides. It's on my list though, the range improvement would be significant!
          • lxgr 3 days ago |
            Oh, I thought that support was only present in some iOS 13 beta and then disappeared again? (At the OS/driver level; I’m pretty sure the hardware supports it.)

            If it’s really available, that would be amazing!

            • redgridtactical 3 days ago |
              You're right, I was wrong about iOS 17 auto-negotiating Coded PHY. After digging deeper, Apple did support it briefly in iOS 13.4 betas but pulled it in iOS 14 and it hasn't returned. CoreBluetooth still doesn't expose PHY selection at all. The iPhone hardware supports it but the OS won't use it. So BLE Long Range is going to be Android-only for now. I've updated the roadmap to reflect that.
  • jbaber 3 days ago |
    Is this iOS only?
    • redgridtactical 3 days ago |
      For the time being yes, I am looking for Android testers (need 12 to publish and I am a solo dev), if you are interested please reach out!
  • deknos 3 days ago |
    I cannot wait until it's in an appstore for android and that it will work with android/iphone together. And i seriously hope you do not abandon it. That being said, i have questions:

    1. When i have a higher version, but others have a lowerpaid version, can i still connect to more members, or am i constrained by the lowestpaying customer in the network? 2. Do you have reproducible builds? 3. airdrop sadly only works with iphones, how will this be done with android? 4. would it be a possibility, that after some time you made decent money, you consider a even higher paid stage, where i can fully build the best /most feature version myself? just in case you ever leave the project? 5. https://github.com/RedGridMGRS/RedGridMGRS is a 404 for me? 6. Would it be possible you also use openstreetmap or better to say integrate map loading from there? 7. Would you consider adding something like https://www.fixphrase.com / what3words-alternative into the app, so coordinates could be uses with phrases?

    • redgridtactical 3 days ago |
      Thanks you! Just started working on this project based on systems I use, and have gripes with, in the military. No plans to abandon it, but I understand your concern. This is the project I actually want to use myself so it's going to keep getting built.

      1.Your tier determines your own device limit. If you have Pro+Link (8 devices), you can host a session with 8 people. The people joining don't need Pro+Link, they just need the free version with Field Link support (which covers 2 devices). So the session creator's tier sets the ceiling, not the lowest paying member. 2.Not yet, but it's open source so you can build from source and verify. Reproducible builds are something I'd like to get to but haven't prioritized over features yet. Fair ask though. 3.Good catch. AirDrop is just one option in the iOS share sheet for AAR exports. On Android it'll use the native share intent, so whatever sharing apps you have installed (email, Drive, Nearby Share, etc). The session export is just a JSON file so it works over any transfer method. I have put off building out the Android version for now because as a solo dev I need 12 testers before I can submit to the Play Store. If you are interested, or know anyone that is, please reach out! Would love to get testers for the Android version going! 4.That's an interesting idea. Something like a "source license" tier where you get build access and can self-host updates if the project goes dark. I'll think about that. The MIT + Commons Clause license already lets you build from source for personal use, but a paid tier with commercial self-build rights could make sense for teams that need that guarantee. 5.Good catch, the correct link is https://github.com/RedGridTactical/RedGridMGRS. Must be a typo somewhere, I'll fix it. 6.Yes, OpenStreetMap is actually what the offline maps use under the hood already. The tile sources are OpenTopoMap (which is built on OSM data) and USGS Topo. Adding more tile sources including vanilla OSM is on the list! 7.I hadn't seen fixphrase before, that's interesting. what3words has licensing issues that make it hard to integrate into open source projects, but an open alternative like that could work. I'll look into it. The main concern would be making sure it works fully offline since the whole point is no network dependency.

      I appreciate all the feedback!

  • redgridtactical 2 days ago |
    If anyone is interested in testing the Android versions, please use the following link:

    https://play.google.com/apps/testing/com.redgrid.redgridtact...

    I welcome all feedback. My goal with this app is to make something I myself want to use. Hopefully it becomes that for you all as well!

  • pabs3 a day ago |
    Have you considered adding WiFi when someone goes out of BLE range?
    • redgridtactical 21 hours ago |
      Yes, definitely as a fallback! Currently working on a new build. If you are interested in testing please see the following link to become a tester!

      https://play.google.com/apps/testing/com.redgrid.red_grid_li...

      • pabs3 21 hours ago |
        I currently don't have a use-case for the app, but will keep it in mind if I ever do.
  • iosifnicolae2 18 hours ago |
    Here you can find some Meshtatic devices, some of them which can be paired to your phone using BLE: https://openhardware.directory/devices/search/meshtastic-com...
    • redgridtactical 3 hours ago |
      Nice list, thanks. Meshtastic bridge is on the V2.0 roadmap so the app can route through LoRa hardware for multi-kilometer range. Being able to detect and pair with these devices over BLE is exactly how I'd want the integration to work.