GPS Module

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Revision as of 06:20, 4 October 2024 by NickPGSmith (talk | contribs) (→‎GPSD)
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HW-595 Board

uBlox GPS Board
  • Based on uBlox NEO-6M-0-001
  • Receiver Description
  • 5V operation (3.3 V regulator onboard)
  • UART: 4800 - 230400 bps (9600 bps 8N1 default), TTL levels buffered by 220R resistors
  • U.FL connector supplies 3.3 V to active antenna
  • Crystal Osc, not TCXO
  • PPS LED: Invertion of PPS pin
  • Battery-backed RAM contains config data and clock and speeds first-time-to-fix
  • Data in NMEA 0183 format
  • More background here

Board pinouts:

Pin Function Description
1 PPS 1 pulse/s, synchronised at rising edge, length 100 ms (by default)
2 RXD UART input
3 TXD UART output
4 GND Ground
5 VCC 3.3 - 5 V

PPS:

  • Send PPS to UART via CTS to allow GPSD to acurately sync
  • Can be configured as a frequency reference: 0.25 Hz - 10 MHz
    • Output has high frequency accuracy
    • Suffers from jitter except at a few specific frequencies
  • Configure in u-Center here
  • Different frquencies/duty cycles can be configure for GPS synced/not synced

Software

U-Center

  • U-Center
  • Select COM port and Baud rate
  • Communicates with uBlox with binary UBX Protocol

View -> Configuration View; to activate change: Send

  • CFG (Configuration)
    • Select: Save current configuration
    • Select in Devices: 2-I2C-EEPROM
  • PRT (Ports)
    • Baud Rate : 9600 -> 115200
  • TP5 (Timepulse 5)
    • PPS Settings for no GPS lock (eg use as an "alert" signal:
      • Frequency : 1 Hz -> 10 Hz
      • Duty Cycle : 100% -> 25% (alternatively set to 0% for permanent low output)
    • PPS Settings for GPS Locked:
      • Frequency Locked : 1 Hz
      • Duty Locked : 50%
    • Rising Edge on TOS
    • Cable Delay : 50 ns -> 10 ns (This compensates for cable delay between antenna and receiver)

View -> Message View:

  • Firmware version: UBX -> MON (Monitor) -> VER (Version)
    • Currently: 7.03 (45969)

GPSD

  • gpsd
  • Packages: gpsd, gpsd-clients

Use of a serial device (eg ttyS0) is prefered over a USB/serial interface (eg ttyUSB0) due to lower latency and PPS availability.

Can be tested with something like (Debug level 4) :

gpsd -N -n -s 9600 -D 4 /dev/ttyS0 /dev/pps0

Configuration in /etc/sysconfig/gpsd, which by default enables detection of USB devices via udev events including creation of a PPS device (/dev/pps0):

USBAUTO="true"

Options can be forced, eg:

OPTIONS="-n -s 9600"
DEVICES="/dev/ttyS0 /dev/pps0"

Check data stream with gpsmon:

gpsmon /dev/ttyS0

when gpsd is not started, or connected to gpsd:

gpsmon

Show raw data:

gpscat -p -s 9600 /dev/ttyS0

Enable and start gpsd:

systemctl enable gpsd
systemctl start gpsd

Monitor with:

cgps -s
gpsmon
gpscsv
gpsplot --image gps.png
xgps

Pulse Per Second (PPS)

With PPS output connected to a serial port pin, GPSD should detect it. If it is on GPIO or other serial port, this can be used to create a PPS device:

dnf install pps-tools
modprobe pps_ldisc
setserial -v /dev/ttyS0 low_latency
ldattach PPS /dev/ttyS0

Check PPS signal:

ppstest /dev/pps0
ppscheck /dev/ttyS0

Can set at boot with udev rule (/etc/udev/rules.d/gps.rules):

KERNEL=="ttyS0", SYMLINK+="gps0"
KERNEL=="ttyS0", RUN+="/usr/bin/setserial -v /dev/%k low_latency"
KERNEL=="ttyS0", RUN+="/usr/sbin/ldattach PPS /dev/%k"