GPS Module

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 that divide into internal 48 MHz clock
• 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
    • Duty Cycle : 0% (for permanent low output)
  • PPS Settings for GPS Locked:
    • Frequency Locked : 1 Hz
    • Duty Locked : 50%
  • Falling Edge on TOS
  • Cable Delay : 50 ns -> 20 ns (This compensates for cable delay between antenna and receiver)
    • At 0.7c, 1 m <=> 4.8 ns

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

Lines showing visible/used satellites:

gpsd:PROG: SVINFO: visible=15 used=5 mask={SATELLITE|USED}
gpsd:INFO: CORE: PRN   6 az  47.0 el 20.0 ( 0.687248,  0.640869,  0.342020)
gpsd:INFO: CORE: PRN  11 az  93.0 el 22.0 ( 0.925913, -0.048525,  0.374607)
gpsd:INFO: CORE: PRN  12 az  70.0 el 58.0 ( 0.497961,  0.181243,  0.848048)
gpsd:INFO: CORE: PRN  28 az 301.0 el 35.0 (-0.702150,  0.421894,  0.573576)
gpsd:INFO: CORE: PRN  32 az 257.0 el 44.0 (-0.700903, -0.161816,  0.694658)
gpsd:INFO: CORE: Sats used (5):

Line showing time in UNIX Epoch: gpsd:PROG: TIMEGPS: time= 1728071281.000150311 mask={TIME}

Line showing PPS is asserted on control line. Here the control line is asserted with 0.5s delay:

gpsd:PROG: PPS:/dev/ttyUSB0 Assert accepted @  1728071281.503229046 offset  0.496770954

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

By default, gpsd listens on port 2947.

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"