5 ms·

How do these things do GPS tracking from under a car seat? Wouldn't the seat frame and car body basically block the device from acquiring satellites?

by yborg 5y ago

livueta 5y ago
I'm not really an expert (corrections welcome!) but having spent some time recently playing with positioning systems: Most of GPS's perceived suck (in cold boot (TTFF) time, general receptivity) is actually just bad antennas/chips. I bet this thing has a large, possibly active, antenna and a modern, possibly multi-service, gnss module. As an example, I have some very cheap ~10mm passive antennas that never pull a signal indoors, and some very cheap modules that take >20 minutes to TTFF even with a clear view of the sky. I also have some big chonker 30mms that, with a modern module, have a <2min TTFF inside/in a car/whatever.
- SAI_Peregrinus 5y ago
  The antenna in the picture looks a lot like a Taoglas L1 active patch antenna. So an antenna (possibly several, hard to tell if it's a stacked-patch or not), low-noise amplifier, and possibly a Surface Acoustic Wave (SAW) filter.
kube-system 5y ago
Apple Maps keeps working fine when I invariably drop my phone under the seat in my car. GPS receivers are pretty good these days and are capable of picking up very weak signals.
- guenthert 5y ago
  Is the Maps application getting the position data from GPS alone or rather from accelerometer plus earlier good reading (using GPS or cell tower known position) or some fancy sensor data merging (Kalman filter)?
  - Bedon292 5y ago
    Location data can come from a lot of things that are being pieced together into a really good guess even when parts are missing. GPS and the other satellites like GLONASS. Nearby WiFi hotspots since they are all pretty much known locations. What cell towers you are connected to, which are also known locations, and the signal strength getting you general ranges. Can even be done with bluetooth beacons within building like malls and conference centers I believe. And accelerometer and compass data to fill in gaps too.
    - blamazon 5y ago
      In the famous "big dig" highway tunnels in Boston your phone may be connecting to the 850 Bluetooth beacons stuck to the tunnel walls by Waze: https://blog.mass.gov/transportation/massdot-highway/highway-waze-app-tunnel-service-improvements/ https://blog.mass.gov/transportation/massdot-highway/highway...
  - jfoutz 5y ago
    I've only fiddled with this in a hobby way, but it's tricky to get accurate position information by integration of acceleration vectors. you could use the compass to get device orientation relative to motion, you could look for available wifi names to narrow down localization. It's possible to make good guesses, but it's a pain, and they're still guesses. (Well, they were at my level anyway)
    - userbinator 5y ago
      They can be quite accurate if well-designed: https://en.wikipedia.org/wiki/Inertial_Navigation_System https://en.wikipedia.org/wiki/Inertial_Navigation_System
    - AlotOfReading 5y ago
      The guesses can get scarily good. Typically you start with the GNSS signal as your initial position/velocity estimate and inertial measurements to estimate travel from that point. With decent sensors and no other constraints, you can get lane level accuracy for upwards of a minute at highway speeds without a satellite fix. However, people (and vehicles) don't have arbitrary paths and the places where they do tend to be easy to get a satellite fix. The kinds of places where you have difficult GNSS environments also tend to have networks of paths and streets that can be used to refine your estimate., a technique called "map matching". In good conditions, you can localize position to within a block inside an entire urban area on that alone.

How do these things do GPS tracking from under a car seat? Wouldn't the seat frame and car body basically block the device from acquiring satellites?

by yborg 5y ago

How do these things do GPS tracking from under a car seat? Wouldn't the seat frame and car body basically block the device from acquiring satellites?

livueta 5y ago

I'm not really an expert (corrections welcome!) but having spent some time recently playing with positioning systems: Most of GPS's perceived suck (in cold boot (TTFF) time, general receptivity) is actually just bad antennas/chips. I bet this thing has a large, possibly active, antenna and a modern, possibly multi-service, gnss module. As an example, I have some very cheap ~10mm passive antennas that never pull a signal indoors, and some very cheap modules that take >20 minutes to TTFF even with a clear view of the sky. I also have some big chonker 30mms that, with a modern module, have a <2min TTFF inside/in a car/whatever.

SAI_Peregrinus 5y ago
The antenna in the picture looks a lot like a Taoglas L1 active patch antenna. So an antenna (possibly several, hard to tell if it's a stacked-patch or not), low-noise amplifier, and possibly a Surface Acoustic Wave (SAW) filter.

kube-system 5y ago

Apple Maps keeps working fine when I invariably drop my phone under the seat in my car. GPS receivers are pretty good these days and are capable of picking up very weak signals.

guenthert 5y ago
Is the Maps application getting the position data from GPS alone or rather from accelerometer plus earlier good reading (using GPS or cell tower known position) or some fancy sensor data merging (Kalman filter)?
- Bedon292 5y ago
  Location data can come from a lot of things that are being pieced together into a really good guess even when parts are missing. GPS and the other satellites like GLONASS. Nearby WiFi hotspots since they are all pretty much known locations. What cell towers you are connected to, which are also known locations, and the signal strength getting you general ranges. Can even be done with bluetooth beacons within building like malls and conference centers I believe. And accelerometer and compass data to fill in gaps too.
  - blamazon 5y ago
    In the famous "big dig" highway tunnels in Boston your phone may be connecting to the 850 Bluetooth beacons stuck to the tunnel walls by Waze: https://blog.mass.gov/transportation/massdot-highway/highway-waze-app-tunnel-service-improvements/ https://blog.mass.gov/transportation/massdot-highway/highway...
- jfoutz 5y ago
  I've only fiddled with this in a hobby way, but it's tricky to get accurate position information by integration of acceleration vectors. you could use the compass to get device orientation relative to motion, you could look for available wifi names to narrow down localization. It's possible to make good guesses, but it's a pain, and they're still guesses. (Well, they were at my level anyway)
  - userbinator 5y ago
    They can be quite accurate if well-designed: https://en.wikipedia.org/wiki/Inertial_Navigation_System https://en.wikipedia.org/wiki/Inertial_Navigation_System
  - AlotOfReading 5y ago
    The guesses can get scarily good. Typically you start with the GNSS signal as your initial position/velocity estimate and inertial measurements to estimate travel from that point. With decent sensors and no other constraints, you can get lane level accuracy for upwards of a minute at highway speeds without a satellite fix. However, people (and vehicles) don't have arbitrary paths and the places where they do tend to be easy to get a satellite fix. The kinds of places where you have difficult GNSS environments also tend to have networks of paths and streets that can be used to refine your estimate., a technique called "map matching". In good conditions, you can localize position to within a block inside an entire urban area on that alone.