6. Global Positioning System

6.1 Objective

The Global Positioning System (GPS) module within the ground station is essential to provide accurate real-time position and heading information before we can begin satellite tracking operations. We need to know the exact location of the ground station and where it is facing as we must establish a ground position relative to the satellite that we are tracking. This step is required as the ground station is portable and the location of it will vary.

The objective of this global positioning system is to implement and calibrate a GPS-based positioning subsystem that interfaces and works with the gimbal control software. This integration ensures that the ground station can maintain consistent tracking accuracy, making the entire system work even when deployed in different locations.

6.2 Design Specifications

The Global Positioning System (GPS) module shall fulfil the following design requirements to ensure accurate and reliable position and heading determination for satellite tracking operations:

GPS Module: AS-STARTKIT-HEAD-L1L2-NH-00
Heading Accuracy: < 0.2°
Position Accuracy: Within ±2.5 m
Update Rate: Minimum 10 Hz to support real-time gimbal motion updates
Communication Interface: Serial (UART) connection to the main control unit
Power Supply: 5 V DC regulated input
Integration Requirement: Must interface seamlessly with the TLE parsing and gimbal control subsystems for continuous azimuth and elevation computation.

The selected GPS module meets the above specifications, providing dual-frequency L1/L2 capability for improved heading precision and resistance to multipath errors. These specifications collectively ensure stable and accurate satellite tracking performance in mobile ground station configurations.

6.3 Code Required

The GPS data acquisition and validation were implemented in Python.
The serial port verification script (check_ports.py) and the GPS parsing and heading computation module (gps_reader.py) are provided in Gimbal Control Software Folder for reference. Gimbal Control Software Folder -(This will bring you to a github folder containing all the code)

A. check_ports.py - Serial Port Discovery
Purpose:

Provides a quick way to list all available serial ports on the host machine and verify that the target GPS device/port can be opened at the expected settings (baud rate, timeout).

This is used before running the live GPS reader to avoid silent connection failures.

Key Components

Port enumeration: Uses serial.tools.list_ports.comports() to print detected ports and human-readable descriptions.
Connection probe: Attempts to open a candidate list of device names for Windows ("COM6" and the escaped "\\.\COM6") at 38400 baud with a 2 second timeout.
Result reporting: Prints “Opened OK” / “Open failed” with the exception text to aid troubleshooting.

Inputs / Parameters

target = "COM6" - change to match the actual GPS port.
baud = 38400 (fixed in code) - must match the GPS module’s configured baud rate.

Outputs

Console log listing ports and the success/failure of opening each candidate device.

B. gps_reader.py - UBX-First GPS Fix & Heading Utility
Purpose

Acquire a single high-quality GPS fix (time, latitude, longitude, altitude) using a dual-antenna setup to find the heading. Prioritises u-blox UBX messages for robustness and falls back to National Marine Electronics Association (NMEA )if needed.

External Dependencies

pyserial (serial I/O)
pyubx2 (parsing u-blox UBX protocol)

Module Structure

1) Helper Converters

_auto_deg(v) and _auto_alt_m(h)
- Auto-scale raw UBX integer fields (e.g., 1e-7 deg for lat/lon, millimetres to metres for height).
_auto_heading_deg(rh, ah)
- Scales UBX relative heading and heading accuracy (from 1e-5 deg), then normalises heading with a –90° frame adjustment to align with the project’s heading convention.
_dm_to_deg(dm, hemi)
- Converts NMEA “degrees+minutes” strings into signed decimal degrees using hemisphere flags.

2) Public API

read_once(port="COM6", baud=38400, max_wait_s=8.0, want_heading=True, extra_heading_wait_s=2.0)
- Goal: Return one consolidated fix as a Python dict:
  {time_utc, lat, lon, alt_m, heading_deg, heading_acc_deg, source}.
- Acquisition strategy:
  1. Prefer UBX NAV-PVT for position & time (requires fixType ≥ 2).
  2. Listen opportunistically for UBX NAV-RELPOSNED to obtain dual-antenna heading and its accuracy.
  3. Fallback to NMEA (GGA or GLL) for position if UBX is not yet available within the wait window.
  4. If position is obtained but heading is missing, keep listening up to extra_heading_wait_s for a late RELPOSNED.
- Timeout behaviour: Raises a TimeoutError if no position is acquired before max_wait_s.
- Source flag: Sets source = "UBX", "NMEA-GGA", or "NMEA-GLL" to document where the fix came from.
stream_status(port="COM6", baud=38400, timeout=1.0)
- Live console monitor for continuous UBX telemetry.
- Prints fix type (e.g., “2D/3D”), satellite count, position, altitude, and (when available) heading with ±accuracy.
- Useful for antenna placement tests, sky-view diagnostics, and verifying heading accuracy.

3) Command-Line Entry Point

When executed directly:

Lists available ports; 2) Calls read_once(...); 3) Prints a formatted “GPS FIX” summary; 4) (Optionally) enables stream_status(...) for continuous monitoring.

Data & Accuracy Notes

Heading source: Relies on UBX NAV-RELPOSNED (dual-antenna/RTK-capable receivers) to obtain true heading and a manufacturer-reported heading accuracy (heading_acc_deg).
Time handling: Prefers UBX-validated time; on NMEA fallback, constructs a UTC timestamp from sentence fields for logging consistency.
Units & normalisation: All lat/lon in decimal degrees, altitude in metres, heading in degrees 0–360.

Typical Use in the System

Bring-up: Use check_ports.py to confirm COM assignment.
Single-fix acquisition: Call read_once(...) from the tracking stack to seed ground-station location/time (and heading if available).
Live QA: Run stream_status(...) to evaluate antenna placement and confirm that heading ± accuracy meets operational thresholds before a satellite pass.