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. See here for additional information.
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
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.
Trial deployment of two NISAR CRs occurred in June, 2021 near Clinton, Oklahoma. The images below are from left to right, N01K and N03K.
East facing CR N01K in the morning
West facing CR N03K in the evening
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 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:
- delta_east_in_meters = -0.01486 * ( gps_time_past_4-June-2011_in_secs) / 365.25 / 86400
- delta_north_in_meters = -0.00428 * ( gps_time_past_4-June-2021_in_secs) / 365.25 / 86400
- delta_vertical_in_meters = 0.0 * ( gps_time_past_4-June-2021_in_secs) / 365.25 / 86400
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.
Nearby velocity vectors from GPS multi-year GPS analysis
Accuracy & Maintenance History
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.
Deployment of CR N01K, June 3, 2021
Deployment of CR N03K, June 2, 2021
July 28, 2021
The turnbuckles were tightened to re-tension the guy-wires. Removed tall grasses and reinforced fence.
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.
Maintenance of CR N01K, July 28, 2021
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 East Facing CR N01K, December 15, 2021
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.
Newly Constructed West Facing CR N02K, December 16, 2021
Newly Constructed East Facing CR N04K, December 16, 2021
Jet Propulsion Laboratory
Mail Stop 300-238
4800 Oak Grove Drive
Pasadena, California, 91109-8099
Tel: (818) 354-0717
Corner Reflector (CR) Data
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.