ZED F9P

ZED-F9P RTK GPS Receiver

Board for an autonomous cart that follows the edge of a lawn and sprays weed killer along the border. Goal is centimeter level positioning using RTK corrections so the cart can hold a precise line along a mapped boundary. Prototyping this board is the only part of the project built so far.

Status

Not currently in use on the cart. Prototyping this board cost way more than expected, and the rest of the cart, drive base, sprayer, controller, was never built as a result. Right now this is just the GNSS board sitting on its own.

Overview

The ZED-F9P decodes both L1 and L2 bands at the same time, which speeds up how fast it locks a fixed RTK solution and helps under partial sky obstruction, useful near fences, trees, or the edge of a house. With a clear view of sky and a valid RTCM3 correction stream, this board is capable of about 10 mm 3D accuracy.

Two things it needs to actually hit that accuracy:

• Clear sky view. RTK does not work well indoors or under heavy tree cover.
• A correction stream, either from a second ZED-F9P set up as a base station, or from an NTRIP correction service.

Features

• u-blox ZED-F9P module, L1/L2 dual band, multi constellation
• Rover mode for the cart, with base station mode available if running your own fixed reference
• About 10 mm 3D accuracy with valid RTK corrections
• Update rates up to 20 Hz in high precision RTK mode
• Five simultaneous communication interfaces, see below
• U.FL antenna connector for a low profile L1/L2 active antenna
• Rechargeable backup cell that keeps module config and almanac data through power off, so warm starts are faster. Holds state for roughly two weeks unpowered
• Status LEDs for power, PPS, and fix state

Communication Interfaces

Interface select jumper: I2C and SPI share pins, so only one is active at a time.

• Jumper open, default: I2C/DDC enabled, UART1 and I2C both active
• Jumper closed: SPI enabled, this disables UART1 and I2C

Set this before wiring your host MCU. An unexpected jumper state is the most common reason I2C or SPI fails to come up.

Hardware

Pinout

Getting Started

1. Connect an active L1/L2 GNSS antenna to the U.FL connector.
2. Set the I2C/SPI select jumper to match your intended interface. Default is open, I2C.
3. Power the board from 5V or 3.3V.
4. Connect to the module with a UBX capable configuration tool such as u-center over USB or UART1 to check satellite lock and fix status.
5. Once a valid 3D fix shows up, move to RTK setup below.

RTK Setup

As a rover on the cart:

• Feed RTCM3 correction messages into UART2, or via I2C/SPI if you'd rather.
• Once corrections are flowing and satellite geometry cooperates, fix state moves from no fix to 3D to RTK float to RTK fixed.
• Fixed RTK is what you want before trusting the position for holding a line along the lawn edge.

If running your own base station instead of NTRIP:

• Configure survey in mode with a target accuracy and minimum observation time, for example 60 seconds and 2 meters.
• Once survey in completes, the module holds a fixed reference position and starts outputting RTCM3 corrections on UART2.
• Typical RTCM3 messages to enable: 1005 for station coordinates, 1077, 1087, 1097, 1127 for per constellation observations, 1230 for GLONASS bias.

Correction delivery options:

• A radio telemetry link, LoRa or 900 MHz, between a yard mounted base and the cart
• An NTRIP client and caster over the internet for networked corrections