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NISAR Mission

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. See here for additional information.

NISAR Calibration

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.

Oklahoma and Texas NISAR Calibration Array

Construction

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.

Deployment

Trial deployment of two NISAR CRs occurred in June, 2021 near Clinton, Oklahoma. The images below are from left to right, N01K and N03K.

NISAR N01K elevated at 15 degrees, East facing
East facing CR N01K in the morning
 
NISAR N03K elevated at 15 degrees, West facing
West facing CR N03K in the evening

Coordinates

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:

The coordinates are referenced to epoch 4-June-2021 and modeled with 1.5364 cms/year of plate motion with an azimuth direction of 254°. Specifically the changes in East, North, and Vertical are modeled as:

These rates are the average of a multi-year time series analysis of the permanent CORS sites OKSY and OKCL. The PBO site WMOK for example here also shows simliar horizontal plate motion.

Plate-motion for SWOT and NISAR Oklahoma and Texas Corner Arrays
Nearby velocity vectors from GPS multi-year GPS analysis

Accuracy & Maintenance History


June 2021

Initial deployment of N01K and N03K occurred June 2nd and June 3rd, 2021. Sites locations were computed with PPP for absolute positioning. Nearly 16 hours GNSS data was collected for N01K and over 14 hours of GNSS data was collected for N03K. It is believed that the accuracy of the NISAR corner vertices is better than 3 CMs 1-sigma.

NISAR N01K elevated at 15 degrees, East facing
Deployment of CR N01K, June 3, 2021
 
NISAR N03K elevated at 15 degrees, West facing
Deployment of CR N03K, June 2, 2021

July 28, 2021

N01K

The turnbuckles were tightened to re-tension the guy-wires. Removed tall grasses and reinforced fence.

N03K

Fence down, and metal posts bent. Stakes were partially out of the groud, and two guy-wires were not attached to CR. All remaining guy-wires were loose. Removed tall grasses, restaked, tightened turnbuckles after removing some S-hooks and reinforced the fence.

NISAR N01K elevated at 15 degrees, East facing
Maintenance of CR N01K, July 28, 2021
 
NISAR N03K elevated at 15 degrees, West facing
Maintenance of CR N03K, July 28, 2021

December 14 & 15, 2021

N01K and N03K

Removed all prior fencing and tall grasses. Reconstructed N01K and N03K with cap blocks as bases which elevated the CRs 1 5/8" in height. Erected 8.6 kV electric fences with two lines of polywire each and energized with solar chargers. Restaked aft guy-wires and replaced all side stakes with duck-bill anchors. Did not redo metrology but rather adjusted prior coordinates in vertical due to the offsets introduced by the cap blocks. There is non-trivial sagging of both bottom panels as this was necessary at this time to reduce oscillations in strong wind conditions. Future maintenance will reinforce the bottom panel and move the forward platform stands out toward the edges. This should reduce the sagging of the bottom panel. When this maintenance is performed, we will redo the full metrology of these CRs.

Reconstructed NISAR N01K elevated at 14.26 degrees, East facing
Reconstructed East Facing CR N01K, December 15, 2021
 
Reconstructed NISAR N03K elevated at 14.33 degrees, West facing
Reconstructed West Facing CR N03K, December 14, 2021

December 16, 2021

N02K and N04K

Installed new NISAR corners N02K and N04K. Both these corners have a reinforced bottom panel and due to placement of the front stands, sagging of the bottom panels is greatly reduced. Erected 8.6 kV electric fences with two lines of polywire each and energized with solar chargers. Completed full metrology on these reflectors. Used SWOT Eastern Reference site (SEOT) to perform RTK to N04K with 4 hours of GPS data, and RTK to N02K with 1.5 hours of GPS data.

NISAR N02K elevated at 12.55 degrees, West facing
Newly Constructed West Facing CR N02K, December 16, 2021
 
NISAR N04K elevated at 16.60 degrees, East facing
Newly Constructed East Facing CR N04K, December 16, 2021

September, 19 to September 27, 2022

N03K, N01K, N02K, N04K

Re-monumented N03K and re-secured N01K. N03K was overturned and placed near its original location. N01K was mounted on 20x20x2" capstones for better retention in the wind. Both CRs were cinced down and remeasured. Nearly 17 hours of GNSS data was acquired for both of these CRs. N02K was also overturned and was relocated 8 meters East and 1.4 meters South of it's original location. Over 19 hours of GNSS data was aquired for N02K. Complete metrology on these 3 reflectors was obtained. N04K was not relocated or re-monumented and has survived well for over 1 year. For N04K the azimuth was measured as 0.5 degrees (was 359.0) and the tilt angle was measured as 17.9 degrees (was 16.6).

NISAR N03K, West facing
Re-Constructed West Facing 2.8 m CR N03K, September 20, 2022
 
NISAR N01K, East facing
Re-Monumented East Facing 2.8 m CR N01K, September 21, 2022
NISAR N02K, West facing
Re-Constructed West Facing 2.8 m CR N02K, September 26, 2022
 
NISAR N04K, East facing
East Facing 2.8 m CR N04K, September 26, 2022

Ronald Muellerschoen
Jet Propulsion Laboratory
Mail Stop 300-238
4800 Oak Grove Drive
Pasadena, California, 91109-8099     

Tel: (818) 354-0717
Email: rjm@jpl.nasa.gov

Corner Reflector (CR) Data

KMZ file of all deployed NISAR CRs in Oklahoma and Texas

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 June 4, 2021. Output is a comma-separated values (CSV) file. These are tide-free coordinates; the FES2004 Ocean Tide Model was used in the computation. The entered date applies the average plate motion from nearby GPS sites OKSY and OKCL.

Invalid date. Please enter a date in YYYY-MM-DD HH:MM format between 2021-06-04 00:00 and 00:00.

Get Corner Reflector Data