CanGNSS

NMEA2000 GNSS Receiver

A NMEA2000 device to provide rapid position and COG SOG update as well as other GNSS information including satelite data.

PGN messages and rates.

• 129025L Position Rapid update 5Hz
• 129026L COG/SOG Rapid Update 2.5Hz
• 129029L Position data 1Hz
• 126992L System Time, 1Hz
• 129539L GNSS DOPs 1Hz
• 129540L GNSS Satellites in View 0.5Hz
• 127258L Magnetic Variation 1Hz

Magnetic variation is calculated every 60s from WMN2020 World model coefficients.

Hardware

• Attiny3224 MCU
• UBlox M8N module pre configured at 19200 Baud with UBX messages.
• MCP 2515 CAN driver with TJ1050 Transceiver

Code

Starts up, establishes CAN connectivity then reads UBX messages as they are produced and transfers them directly onto the NMEA200 bus. There is a monitoring UART that runs at 115200 which can be used to see log messages and reboot. Press h to get a menu. The WNM2020 model takes 63ms to calculate which is long enough for a standard 64 byte UART buffer to overflow. This code requires a patched megaTinyCode code base with a 256 byte UART buffer.

Pins

                            ANT
  --------------------------------
  |          A B C D E           |
  | F                            |
  | G                            |
  | H                            |
  | I                            |
  |                     j k l m  |
  --------------------------------

• A +5v

• B 0v

• C UDPI

• D RX Console

• E TX Console

• F 12V - CAN

• G 0V

• H CAN-L

• I CAN-H

• j 0v

• k TX Ublox

• l RX Ublox

• m 5v Ublox

Settings.

The MCU can program the device to its standard settings. press h to get the menu on the console. For more advanced settings u-center or pyGPSClient can be used, but first the MCU Uart connected to the Ublox module must be disabled.

Press u to disable the Uart, then without turning the device off, connect to the Ubloc TX and RX pins. The Uart will be re-enabled on reboot or power cycle.

I have found pyGPSClient to be much easier than u-center as Windows tends to grab the comp ports and GPS stream making it almost impossible to connect u-center. (tips welcome). So I use pyGPSClient on a Ubuntu VM running in OSX. First connect with screen to get the console

screen /dev/ttyUSB0 115200

pres h to get the menu and u to disconnect the UART. The Ctrl-a + k to exit screen.

Then connect pyGPSClient to the ublox via a serial and reconfigure as required.

Expected setup.

• Baud 19200
• UBX Messages only, no NMEA

The satelite setup blow gives about 1m 3D accuracy without SBAS corrections, and slightly less once the SBAS corrections are received. Should probably write this into code.

• CFG-GNSS also set in code

• msgVer 0, mumTrkChHw 32 (read only), mumTrkChUse 32, numConfigBlocks 7

• gnssId 0 (GPS), res 4, max 16, enable 1, mask 1 eg L1C/A 1,575.42MHz

• gnssId 1 (SBAS), res 2, max 4, enable 1, mask 1 eg L1C/A 1,575.42MHz

• gnssId 2 (Galileo), res 4, max 8, enable 1, mask 1 eg E1 1,575.42MHz

• gnssId 3 (BelDou) disabled 0,0,0,0

• gnssId 4 (IMESS) diabled 0,0,0,0

• gnssId 5 (QZSS) res 1, max 1, enable 1 mask 1, L1C/A 1,575.42MHz

• gnssId 6 (GLonas) res 2, max 5 enable 1, mask 1 L1

• CFG-ITMF,ection Jamming det enabled on active antena, all else defaults.

• CFG_NAV5 set modes to sea

• CFG-SBAS as default enable 1 (ignored), test 0, range 1, diffCorr 1, integrety 0, maxSBAS 3

• PRN 120, 123, 127-129, 133, 135-138

• Also set in code

• CFG_RATE Solution interval 200ms, measurement ratio 5, UTC.

• NAV setup all off except

• NAV-DOP 6

• NAV-POSLLH 1

• NAV-PVT 5

• NAV-SAT 9

• NAV-VELNED 2

PCB

The PCM is designed in KiCad (ancient version), and milled on a CNC miller. The attiny3224 is underneath the ublox module, which has a ground plane on the bottom layer.