Running GNATSS

Running GNATSS#

Overview#

GNATSS has two processing modes, a “posfilter” mode and a “solver” mode, which are used at different phases of the processing chain. It is possible to run both processing modes in sequence or individually. The posfilter module outputs GNSS-Acoustic travel times and transducer positions in the community standard GNSS-Acoustic data format, so if you have obtained data already in that format you may skip the posfilter mode and immediately run the solver.

The basic step-by-step guide to position a GNSS-Acoustic array using GNATSS is:

Step 1: Gather the required input files

Step 2: Create working directory and configuration file

Step 3 (optional): Run the posfilter module to generate transducer positions at the interrogation/reply times of the GNSS-Acoustic survey

gnatss run --posfilter config.yaml

Step 4: Run the solver to calculate an array position

gnatss run --solver --distance-limit 150 --residual-limit 10000 config.yaml

Step 5: Run the solver again to with the –remove-outliers option to remove flagged residuals.

gnatss run --solver --distance-limit 150 --remove-outliers config.yaml

Step 6: Repeat Steps 4-5, reducing the residual limit as desired in each successive iteration in order to remove erroneous residuals.

Step 7: Array positions, offsets, and statistics are stored in the process_dataset.nc file.

Alternative: GNATSS supports end-to-end processing, so you may execute Steps 3 and 4 simultaneously. This will generate an array position as well as input to run the solver again without having to repeat the posfilter analysis. For repeated calls to the solver module during Step 5, add the –from-cache flag to skip the posfilter and immediately load gps solutions from your output directory.

gnatss run --distance-limit 150 config.yaml
gnatss run --from-cache --distance-limit 150 --residual-limit 10000 config.yaml
gnatss run --from-cache --distance-limit 150 --remove-outliers config.yaml

Required Input Data#

GNATSS requires the following data in order to generate an array position:

  • Acoustic two-way travel times

  • Surface platform positions

  • A sound velocity profile

  • (Optional) Velocities and Roll, Pitch, Heading values for the surface platform

These data may ingested in a Level-1 (L-1) or Level-2 (L-2) data format, with L-1 data required for the posfilter mode and L-2 data required for the solver.

L-1 Data#

Acoustic Two-Way Travel Times

Acoustic two-way travel times are defined as the duration between when the transducer on the surface platform sends an acoustic interrogation and when it receives a reply from a seafloor transponder. Multiple transponders may respond to the same interrogation if they each receive the ping.

GNATSS supports multiple formats for the two-way travel time input files, which may be designated by defining a format key in the configuration file. Supported formats are:

  • DFOP: DFOP00.raw files generated by the Sonardyne GPS-A payload

  • legacy, no value: Custom text-column format with the following columns:

26-JUN-23 00:00:07.00   2241985   2337422   2469996
26-JUN-23 00:00:22.00   2240506   2346387   2463020
26-JUN-23 00:00:37.00   2240627   2353454   2454958
26-JUN-23 00:00:52.00   2243723   2359242   2445547

Each row in this file corresponds to a ping, containing UTC timestamps of the ping interrogation and two-way travel times in microseconds. Each of the two-way travel time columns corresponds to the replies from a specific transponder as defined in the config.yaml file.

In order to prevent interference between the replies, each transponder may be assigned an internal delay time that will elapse before sending a reply to an interrogation ping. These delay times must be defined in the config.yaml file and will be removed during positioning.

There may also be a delay time associated with the surface platform corresponding to the time between when the computer sends an interrogation command and when the transducer emits an acoustic pulse. For instance, the Sonardyne GNSS-A payload has a 0.13 second delay time that is recorded in its raw two-way travel times. This delay must be removed prior to GNATSS positioning because there is not an acoustic pulse physically traveling through the water column during this time. GNATSS will perform this operation automatically if two-way travel time files are provided in the DFOP format.

Surface Platform Positions

There are two positions on a GNSS-Acoustic surface platform relevant for array positioning, the positioning of the GNSS antenna and the position of the transducer. While GNATSS requires positions for the transducer for array positioning, raw positioning data in the form of RINEX files track the GNSS antenna. Thus, the user must provide GNATSS with a time series of antenna positions from which GNATSS will derive the transducer positions during the posfilter operation.

The antenna positions may be calculated with a GNSS processing software of the user’s preference, which may be designated by setting a format key in the configuration file. Supported formats are:

  • CSRS: .pos file generated by the CSRS-PPP online tool offered by the Canadian Geodetic Survey

  • PRIDE: kin file generated by PRIDE PPP-AR

  • legacy, no value: Custom text-column format with the following columns:

   712497600.0000000    GPSPOS       -2605855.7990    -3707733.0210     4472994.7430    0.0100    0.0100    0.0100
   712497601.0000000    GPSPOS       -2605855.2850    -3707733.4330     4472995.0110    0.0100    0.0100    0.0100
   712497602.0000000    GPSPOS       -2605855.1770    -3707734.0660     4472995.1420    0.0100    0.0100    0.0100
   712497603.0000000    GPSPOS       -2605854.7550    -3707734.4340     4472994.8080    0.0100    0.0100    0.0100

The columns of the legacy file format are as follows:

  • The first column is time in J2000 seconds, relative to 2000-01-01 12:00:00.

  • The second column is a text flag.

  • Columns 3-5 are the antenna positions in ECEF XYZ coordinates with units of meters.

  • Columns 6-8 are the standard deviations of the antenna positions, also in meters.

Velocities and Roll, Pitch, Heading values

GNATSS is currently equipped to ingest raw orientation data collected by the Sonardyne GNSS-A payload used by the Liquid Robotics model SV-3 Wave Gliders currently deployed in the Near Trench Community Geodetic Experiment. These payloads include a Novatel dual-antenna navigation system that reports the orientation in NMEA text strings:

  • #INSPVA includes the platform positions, velocities, and orientations

  • #INSSTDEV includes the uncertainties for the velocities and orientations

Pin Files

The LRI model SV-3 Wave Glider telemeters small extracts of data to shore subsampled at a 2-minute sampling rate for quallity control purposes. These data are stored in zipped tarballs, each of which contains a set of .pin files. Each .pin file contains the data recorded during one ping stored in a JSON structure, which includes:

  • The real-time position and orientation at ping interrogation

  • The real-time position and orientation recorded reasonably close (within <0.2s) to each of the ping replies

  • The times at ping interrogate and reply

  • The ranges to each transponder

  • The NovAtel #RANGEA record for the interrogation epoch

Because these files only contain a subsample of the data set, they are only used for an alternative pre-processing mode using the --parsed flag instead of the default --posfilter flag. (See “Real-time” Processing.)

L-2 Data#

L-2 data is generated by the posfilter module or may be acquired independently of GNATSS. These data should be in the Community Standard GNSS-Acoustics Data Format. These data are logged in a csv format and include the following data and header:

T_receive,MT_ID,TravelTime,T_transmit,ant_cov_XX1,ant_cov_XY1,ant_cov_XZ1,ant_cov_YX1,ant_cov_YY1,ant_cov_YZ1,ant_cov_ZX1,ant_cov_ZY1,ant_cov_ZZ1,X_receive,Y_receive,Z_receive,ant_cov_XX0,ant_cov_XY0,ant_cov_XZ0,ant_cov_YX0,ant_cov_YY0,ant_cov_YZ0,ant_cov_ZX0,ant_cov_ZY0,ant_cov_ZZ0,X_transmit,Y_transmit,Z_transmit
712483771.569176,NDP1-2,3.569176,712483768.0,0.0002271,9.8e-07,-2.2e-07,9.8e-07,0.0002278,-3.1e-07,-2.2e-07,-3.1e-07,0.00022849,-2606685.75482955,-3705817.00172181,4474091.26651063,7.841e-05,1.1e-07,-4e-08,1.1e-07,7.849e-05,-6e-08,-4e-08,-6e-08,7.857e-05,-2606687.12007168,-3705817.05582391,4474089.56415817

By default, L-2 Data generated by GNATSS will report time in J2000 seconds relative to 12:00:00 on Jan 1, 2000. GNATSS automatically corrects for leap seconds.

Sound Velocity Profile#

The sound velocity profile is a simple two-column text file with the first column being the depth (m, negative down) and the second column being the sound velocity in m/s. This may be derived from a CTD or XBT during instrument deployment. For example:

-1.9840000000000000 1502.5000000000000
-2.9760000000000000 1502.5400000000000
-3.9670000000000001 1502.5699999999999
-4.9589999999999996 1502.5100000000000
-5.9509999999999996 1502.4900000000000
-6.9429999999999996 1502.3599999999999
-7.9349999999999996 1502.3000000000000
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