`instrupy.synthetic_aperture_radar_model` — Synthetic Aperture Radar Model

Description

This module provides the SyntheticApertureRadarModel class which can be used to represent instruments imaging at optical/near-optical wavelengths. The SyntheticApertureRadarModel object can be obtained from json/ dict representations using the from_json or from_dict functions. Please refer to synthetic aperture radar model for description of the instrument model and the expected key/value pairs.

Refer to the base module and OrbitPy coverage calculator module (orbitpy.coveragecalculator) to know how to use the SyntheticApertureRadarModel object for coverage calculations.

The calc_data_metrics function can be used to calculate the data-metrics given the viewing-geometry. There are two ways in which the viewing-geometry can be specified:

1. With spacecraft-state and target location:

Here the spacecraft-state and observed-location (target) are specified. The expected key/value pairs for the spacecraft-state input argument (of type dict) are:

Column

Data type

Units

Description

time [JDUT1]

float

Julian Day UT1

Time in Julian Day UT1. Corresponds to the time of observation.

x [km], y [km], z [km]

float

km

Cartesian coordinates of satellite in EARTH_CENTERED_INERTIAL frame at the time of observation.

vx [km/s], vy [km/s], vz [km/s]

float

km/s

Velocity of spacecraft in EARTH_CENTERED_INERTIAL frame at the time of observation.

The expected key/value pairs for the observed-location input argument (of type dict) are:

Column

Data type

Units

Description

lat [deg], lon [deg]

float

degrees

Ground-point accessed (geocentric latitude, geocentric longitude).

2. With incidence-angle specification:

Here the following input arguments is expected to the calc_data_metrics function:

Column

Data type

Units

Description

alt_km

float

km

Spacecraft altitude.

sc_speed_kmps

float

km/s

Spacecraft speed in orbit.

sc_gnd_speed_kmps

float

km/s

Spacecraft ground-speed/ antenna-footprint speed.

inc_angle_deg

float

degrees

Incidence angle at the observed location.

`instru_look_angle_from_target_inc_angle` flag

In addition to the above mentioned input-arguments to the calc_data_metrics function, there is an additional optional flag which can be specified to instruct how to obtain the instrument look-angle. If the instru_look_angle_from_target_inc_angle flag is set to False (also default value), the instrument look-angle is obtained from the instrument orientation specification. If the flag is set to True, the instrument look-angle is obtained from the target location incidence-angle.

The instrument look-angle influences the calculation of the swath-width, (valid) PRF and hence the NESZ.

Examples

Example model parameters derived from real-world instruments can be found in the examples folder. Note that the models do not emulate the real-world instrument entirely, but are meant to be approximate versions.

Example of MicroXSAR (X-band SAR). The default polarization (SINGLE), swath (FULL) and scan-technique (Stripmap) is considered. The view geometry is specified with the spacecraft state and ground-point geo-coordinates. The instrument look angle considered is from the specified instrument orientation.

import numpy as np
from instrupy.synthetic_aperture_radar_model import SyntheticApertureRadarModel

RE = 6378.137
def orbital_speed(alt_km):
return np.sqrt(398600.5/(RE + alt_km))

microxsar_dict = {"@type": "Synthetic Aperture Radar",
                  "name": "MiroXSAR",
                  "mass": 130,
                  "volume": 0.343,
                  "power": 1100,
                  "orientation": {
                        "referenceFrame": "SC_BODY_FIXED",
                        "convention": "SIDE_LOOK",
                        "sideLookAngle": 30
                  },
                  "dataRate": 2000,
                  "bitsPerPixel": 16,
                  "pulseWidth": 31e-6,
                  "antenna":{"shape": "RECTANGULAR", "height": 4.9, "width": 0.7,
                        "apertureEfficiency": 0.5,
                        "apertureExcitationProfile": "UNIFORM"},
                  "operatingFrequency": 9.65e9,
                  "peakTransmitPower": 1e3,
                  "chirpBandwidth": 75e6,
                  "minimumPRF": 3000,
                  "maximumPRF": 8000,
                  "radarLoss": 3.5,
                  "sceneNoiseTemp": 290,
                  "systemNoiseFigure": 4.3
                  }
microxsar = SyntheticApertureRadarModel.from_dict(microxsar_dict)
epoch_JDUT1 = 2451623.999630# 2000 3 20 11 59 28.000
# lat = 0, lon = 0 corresponds to [6378, 0, 0] km in ECI for observer position, check using Matlab function: eci2lla([6378, 0, 0] ,[2000 3 20 11 59 28.000])
sc_orbit_state = {'time [JDUT1]':epoch_JDUT1, 'x [km]': 6378.137 + 600, 'y [km]': 0, 'z [km]': 0, 'vx [km/s]': 0, 'vy [km/s]': orbital_speed(600), 'vz [km/s]': 0} # equatorial orbit, altitude 600 km
target_coords = {'lat [deg]': 2, 'lon [deg]': 0} # incidence angle to the target is 28.9855 deg
obsv_metrics = microxsar.calc_data_metrics(sc_orbit_state, target_coords, instru_look_angle_from_target_inc_angle=False)
print(obsv_metrics)

>> {'ground pixel along-track resolution [m]': 2.09, 'ground pixel cross-track resolution [m]': 4.95,
'NESZ [dB]': -12.89, 'incidence angle [deg]': 28.99, 'swath-width [km]': 37.3, 'PRF [Hz]': 3751}

In the below snippet, the instrument look angle is considered from the incidence angle at the target ground-point (28.99 deg). Note that the calculated swath-width is smaller. A different PRF is evaluated (higher PRF) which results in a different (improved) NESZ.

obsv_metrics = microxsar.calc_data_metrics(sc_orbit_state, target_coords, instru_look_angle_from_target_inc_angle=True)
print(obsv_metrics)

>> {'ground pixel along-track resolution [m]': 2.09, 'ground pixel cross-track resolution [m]': 4.95,
    'NESZ [dB]': -14.12, 'incidence angle [deg]': 28.99, 'swath-width [km]': 34.4, 'PRF [Hz]': 4976}

Example of dual-polarization SAR and FIXED swath configuration. View geometry is specified using the altitude, spacecraft speed, footprint speed and incidence angle specifications. The instru_look_angle_from_target_inc_angle flag is set to True.

import numpy as np
from instrupy.synthetic_aperture_radar_model import SyntheticApertureRadarModel

RE = 6378.137
def orbital_speed(alt_km):
return np.sqrt(398600.5/(RE + alt_km))

alt_km = 500
orb_speed = orbital_speed(alt_km)

test_sar_dict = { "@type": "Synthetic Aperture Radar",
                  "name": "DSHIELD L-Band SAR",
                  "orientation": {
                        "referenceFrame": "SC_BODY_FIXED",
                        "convention": "SIDE_LOOK",
                        "sideLookAngle": 45
                  },
                  "pulseWidth": 14.16e-6,
                  "antenna":{"shape": "RECTANGULAR", "height": 14.38, "width": 1.48,
                        "apertureEfficiency": 0.6, "apertureExcitationProfile": "UNIFORM"},
                  "operatingFrequency": 1280e6,
                  "peakTransmitPower": 1000,
                  "chirpBandwidth": 0.86e6,
                  "minimumPRF": 1,
                  "maximumPRF": 20000,
                  "radarLoss": 2,
                  "systemNoiseFigure": 2,
                  "swathConfig": {
                        "@type": "fixed",
                        "fixedSwathSize": 25
                  },
                  "polarization": {
                        "@type": "dual",
                        "pulseConfig": {
                        "@type": "SMAP"
                  }
                  }
            }
test_sar = SyntheticApertureRadarModel.from_dict(test_sar_dict)

inc_deg = 35
obsv_metrics = test_sar.calc_data_metrics(alt_km = alt_km, sc_speed_kmps = orb_speed, sc_gnd_speed_kmps = orb_speed*(RE/(RE+alt_km)), inc_angle_deg = inc_deg,
                                          instru_look_angle_from_target_inc_angle = True)

print(obsv_metrics)
>> {'ground pixel along-track resolution [m]': 6.67, 'ground pixel cross-track resolution [m]': 364.66,
      'NESZ [dB]': -40.69, 'incidence angle [deg]': 35.0, 'swath-width [km]': 25.0, 'PRF [Hz]': 2666}


inc_deg = 45
obsv_metrics = test_sar.calc_data_metrics(alt_km = alt_km, sc_speed_kmps = orb_speed, sc_gnd_speed_kmps = orb_speed*(RE/(RE+alt_km)), inc_angle_deg = inc_deg,
                                          instru_look_angle_from_target_inc_angle = True)

print(obsv_metrics)
>> {'ground pixel along-track resolution [m]': 6.67, 'ground pixel cross-track resolution [m]': 295.79,
      'NESZ [dB]': -37.41, 'incidence angle [deg]': 45.0, 'swath-width [km]': 25.0, 'PRF [Hz]': 2279}

inc_deg = 55
obsv_metrics = test_sar.calc_data_metrics(alt_km = alt_km, sc_speed_kmps = orb_speed, sc_gnd_speed_kmps = orb_speed*(RE/(RE+alt_km)), inc_angle_deg = inc_deg,
                                          instru_look_angle_from_target_inc_angle = True)

print(obsv_metrics)
>> {'ground pixel along-track resolution [m]': 6.67, 'ground pixel cross-track resolution [m]': 255.33,
      'NESZ [dB]': -32.87, 'incidence angle [deg]': 55.0, 'swath-width [km]': 25.0, 'PRF [Hz]': 1578}

API

Classes

instrupy.synthetic_aperture_radar_model — Synthetic Aperture Radar Model

Description

instru_look_angle_from_target_inc_angle flag

Examples

API

`instrupy.synthetic_aperture_radar_model` — Synthetic Aperture Radar Model

`instru_look_angle_from_target_inc_angle` flag