/gnss-attitude

GNSS single antenna attitude determination

Primary LanguageCGNU General Public License v3.0GPL-3.0

gnss-attitude

Introduction

gnss-attitude is a GNSS-based single antenna attitude determination software implementing the SNR-based algorithm first proposed by Axelrad and Behre (1999). As part of my undergraduate capstone design project, gnss-attitude is being developed for future use in VIOLET, a nanosatellite by CubeSat NB, to support its camera function. Acting as an extension for RTKLIB, this software accepts processing results from RTKLIB and outputs the determined antenna boresight vectors in local coordinates (ENU). It can be used for both space and ground vehicles.

In single-antenna GNSS/INS fusion, GNSS usually provides position and time, not attitude; and relative attitude is provided by IMU. This program enables absolute attitude by GNSS directly. With several new techniques employed, preliminary results show that the accuracy is a few degrees RMS, in contrast to 15° RMS reported by Wang et al. (2005) and within 10° degrees from a combination of two opposite-pointing antennas reported by Eagleson et al. (2018).

For potential testers:

  1. This software is under initial development. Anything is subject to substantial change and considered unstable. Initial development release is expected Feburary 2022.
  2. The missing part is the calibration for the specific antenna-receiver pair you're using. For this, you have to do it yourself.
  3. The complete methodologies will be made publicly available in a technical report, expected April 2022.

Algorithm

  1. Before determining the attitude using observation data, a calibration data set is collected. A multiparameter nonlinear regression obtains SNR adjustment parameters for each satellite group and the SNR mapping function.
  2. For a given epoch, the line-of-sight (LOS) vectors from satellite to receiver are calculated from satellite and receiver locations. The satellite and receiver locations are derived from the GNSS navigation and observation files.
  3. SNR values from the observation file are adjusted according to the adjustment terms developed in Step 1.
  4. The off-boresight angles can be found by the SNR mapping function developed in Step 1 and the adjusted SNR values from Step 3.
  5. The LOS vectors from Step 2 and off-boresight angles from Step 4 are put into a multiple linear regression to determine the antenna boresight vector.

Performance

Using a general SNR mapping function, the system delivers an accuracy of about 5° - 20° (RMS). Better performance can be achieved if calibration is performed (under evaluation).

Limitations

  1. Accuracy is subject to the number of signals received and satellite geometry, particularly when the antenna points down, in the woods, outside of GNSS service volume, etc.

  2. Determines the boresight vector only. Rotation around the boresight axis is undetectable.

  3. Does not work with antennas not following the typical gain pattern of a GNSS antenna, such as chip antenna.

  4. Requires one-time calibration.

Prerequisites

  1. Linux machine (update package list)

     sudo apt update

  2. RTKLIB

  3. GNU Scientific Library

     sudo apt-get install libgsl-dev

  4. Build-essential (gcc & make)

     sudo apt install build-essential

Usage

  1. Save AZ/EL/SNR/MP from RTKLIB as input.txt

  2. Edit snr.c with parameters obtained from calibration

  3. Edit config.c

  4. Compile and run

    Option A: custom input file & multiple frequencies

     make antenna
 ./antenna [input_1] [input_2] [input_3]

    Option B: default input file path & single frequency

     make run

License

gnss-attitude is licensed under the GNU General Public License v3.0. See LICENSE.txt for more information.