Technology > Calibration > NISAR Calibration
The NISAR mission is a satellite SAR jointly developed by NASA and ISRO, India's space agency. The NASA-ISRO SAR (NISAR) mission will measure our planet's changing ecosystems, dynamic surfaces, and ice masses. This will provide scientists with valuable information about biomass, natural hazards, soil moisture, and sea-level rise.
Post-launch cal/val activities may involve imaging well established ground targets. NISAR cal/val will partially use a Corner Reflector (CR) array as these targets are commonly used for SAR systems. As many as 20 corner reflectors located in NISAR's swath over the Oklahoma and Texas areas will be deployed for NISAR calibration. Additional corner reflectors will be deployed near Fairbanks and Delta Junction, Alaska.
Alaska NISAR Calibration Array
Each NISAR CR measures 2.8 meters (9.2 ft) and is a triangular-trihedral reflector constructed with 0.125" Aluminum Alloy 5052 H32 perforated sheets with staggered round holes of size 0.188". They are retained to the ground with 1/16" guy-wires pulled down to the ground with turnbuckles attached to duckbill anchors buried 18" into the ground. The front and rear wooden platforms rest on 2" high cinder blocks attached with wedge anchors and held down by 14" metal stakes.
The University of Alaska at Fairbanks (UAF) CR measures 3.0 meters (9.8 ft) and is also a triangular-trihedral reflector. It is mounted on a ring stand and can be rotated to other azimuth directions.
Front View UAF 3.0 m CR N31A
Side View UAF 3.0 m CR N31A
Initial deployment of two 2.8 m NISAR Corner Reflectors occurred in late August 2021 near Delta Junction, Alaska. Two additional 3.0 m UAF Corner Reflectors were also deployed, one also near Delta Junction, and the other near Fairbanks. The image below is of N01A. Signage was also placed with all the deployed CRs
N01A facing mostly West
Metrology of N01A at the Panoramic Bison Range
Signage for N01A
The coordinates listed here are in IGS14 Reference Frame. See a chronology of reference frames for additional information. The coordinates are expressed as WGS-84 geodetic latitude and height above the reference ellipsoid such that:
- a = 6,378,137 meters
- f = 1/298.257222
The coordinates are referenced to epoch 01-Sep-2021 and modeled with 2.2291 cms/year of plate motion with an azimuth direction of 198.19° and 0.3274 cms/year vertical. Specifically the changes in East, North, and Vertical are modeled as:
- delta_east_in_meters = -0.006958 * ( gps_time_past_01-September-2021_in_secs) / 365.25 / 86400
- delta_north_in_meters = -0.021177 * ( gps_time_past_01-September-2021_in_secs) / 365.25 / 86400
- delta_vertical_in_meters = +0.003274 * ( gps_time_past_01-September-2021_in_secs) / 365.25 / 86400
These rates are from the multi-year time series analysis of the permanent PBO site AC71 as here.
Nearby velocity vectors from GPS multi-year GPS analysis.
Accuracy & Maintenace History
August 30th 2021 to September 1st 2021
Initial deployment of two NISAR 2.8 m CRs N01A and N02A near Delta Junction, and deployment of UAF's 3.0 m CRs N31A near Fairbanks and also N32A near Delta Junction.
UAF's 3.0 m CR N31A was deployed 14 kms ( 9 miles ) south west of Fairbanks, Alaska. Due to time constraints, precise measurement of its position was not possible. It's horizontal position below is likely good to 10s of meters and its vertical postion is likely good to a few meters. It's azimuth heading from true north is 10.5° and is mostly East facing. There are 5 CMS (2 inches) of sag in the bottom panel that will effect radiometrics. This will be corrected for in the near future, as will the metrology of its location.
Deployment of CR N31A, August 30, 2021
Some sagging of the bottom panel in CR N31A, August 30, 2021
NISAR's 2.8 m CR N01A was deployed in the Panoramic Bison Range near Delta Junction, Alaska. It's azimuth heading from true north is 169.5° and is mostly West facing. Over 16 hours of GNSS data was acquired and it is believed that the accuracy of the corner vertex is better than 3 CMS.
Deployment of CR N01A, August 30, 2021
Front edge of CR N01A, August 30, 2021
NISAR's 2.8 m CR N02A was deployed 28 kms ( 17 miles ) north east of Delta Junction, Alaska. It's azimuth heading from true north is 9.5° and is mostly East facing. Nearly 19 hours of GNSS data was acquired and it is believed that the accuracy of the corner vertex is better than 3 CMS.
Deployment of CR N02A, August 31, 2021
Signage of CR N02A, August 31, 2021
UAF's 3.0 m CR N32A was deployed 12 kms ( 8 miles ) south west of Delta Junction, Alaska at the Cold Regions Test Track. Its azimuth heading from true north is 168° and is mostly West facing. Over 20 hours of GNSS data was acquired and it is believed that the accuracy of the corner vertex is better than 3 CMS. There are a few (<2) CMS of warp in the bottom left panel near the vertex. It's unlikely this will affect radiometrics at L-band.
Deployment of CR N32A, September 1, 2021
Some misalignment of the bottom panel in CR N32A, September 1, 2021
Corner Reflector (CR) Data
KMZ file NISAR and UAF CRs in Alaska. In orange are the currently established CRs. In green are potential 3.0 m UAF CR locations and in pink are potential 2.5 m UAF CR locations.
Use the 'Get Corner Reflector Data' button to retrieve the vertices of the the NISAR corner reflectors (latitude, longitude, orientation, etc) for a particular date. Data goes back to September 01, 2021. Output is a comma-separated values (CSV) file. Except for N31A, these are tide-free coordinates; the FES2004 Ocean Tide Model was used in the computation. The entered date applies the plate motion from nearby GPS sites AC71. The precise location of N31A has not yet been aquired.