# FastFix GPS snapshot positioning algorithm #
The FastFix algorithm uses a least-squares optimization to estimate receiver position and GPS time from measurements of the relative codephase, and Doppler-shift of GNSS satellite signals. A practical application of this algorithm is demonstrated in a small, lightweight, low-power tracking tag that periodically wakes-up, records and stores 4 ms of GPS L1-band signal and returns to a low-power state—reducing power requirements by a factor of ∼10,000 compared to typical GPS devices. Stationary device testing shows a median error of 27.7 m with a small patch antenna.
## Usage ##
The process for use is a two-step process. Step 1 is acquisition of the satellite signals. There is a command-line tool for that.
### Acquisition ###
usage: acquire [-h] [--tag-dir TAG_DIR] [--binfile BINFILE] --outfile OUTFILE [--decodedfile DECODEDFILE] --fc0 FC0 [--resolution RESOLUTION] [--epochs EPOCHS] [--iq] [--spectrum] [--corr-plot] [--start-date START_DATE]
Acquire SV signals from Max2769B GPS data.
optional arguments:
-h, --help show this help message and exit
--tag-dir TAG_DIR The FastFix tag directory.
--binfile BINFILE The binary data file.
--outfile OUTFILE The output data file.
--decodedfile DECODEDFILE
The decoded file.
--fc0 FC0 Center Frequency.
--resolution RESOLUTION
Number of bits per sample.
--epochs EPOCHS Number of code epochs.
--iq Use I/Q complex baseband.
--spectrum Plot the spectrum of the data.
--corr-plot Plot the correlations of each SV.
--start-date START_DATE
Date and time for the start of the clock.
Example:
acquire --binfile test_data/FIX00033.BIN --fc0 4.092e6 --outfile test_data/FIX00033.json
### FastFix algorith ###
This processes a json file that contains one or more acquisition outputs.
usage: fastfix [-h] --json-file JSON_FILE --output-file OUTPUT_FILE [--mcmc] [--plot] [--n N] [--clock-offset-std CLOCK_OFFSET_STD]
FastFix algorithm.
optional arguments:
-h, --help show this help message and exit
--json-file JSON_FILE
The JSON acquisition file.
--output-file OUTPUT_FILE
The JSON output file.
--mcmc Use an MCMC.
--plot Make plots of the posteriors.
--n N How many fixes to process.
--clock-offset-std CLOCK_OFFSET_STD
Estimated error in the local clock (seconds).
Example:
fastfix --json-file test_data/FIX00033.json --output-file test_data.json
## References ##
The FastFix algorithm is described in Molteno, Timothy CA. "Estimating Position from Millisecond Samples of GPS Signals (the “FastFix” Algorithm)." Sensors 20.22 (2020): 6480.
Citing this work:
@article{molteno2020estimating,
title={Estimating Position from Millisecond Samples of GPS Signals (the “FastFix” Algorithm)},
author={Molteno, Timothy CA},
journal={Sensors (Basel, Switzerland)},
volume={20},
number={22},
year={2020},
publisher={Multidisciplinary Digital Publishing Institute (MDPI)}
}
## Test Data ##
A full dataset including test data is available at doi:10.5281/zenodo.4266994.
