Product Specification Find Data
Orbit Ephemeris Overview¶
NISAR Orbit Ephemeris (OE) products describe the position, velocity, and acceleration of the NISAR satellite through time. Accurate ephemeris measurements are essential for focusing, mapping, and geolocating SAR imagery, and are often used as input to SAR processing workflows.
Orbit Ephemeris Datasets¶
Satellite ephemeris can be reasonably predicted in advance. Measurements at the time of acquisition are more accurate, and further accuracy improvements are possible after acquisition by incorporating additional GNSS data.
Four types of Orbit Ephemeris products are provided for NISAR reflecting this trade-off between latency and quality:
Forecast Orbit Ephemeris (FOE)
Near real-time Orbit Ephemeris (NOE)
Medium precision Orbit Ephemeris (MOE)
Precise Orbit Ephemeris (POE)
FOE products are the earliest available and lowest data quality, while POE products are the latest available and highest data quality. The characteristics of these datasets are described in Table 1.
Table 1:Orbit Ephemeris Dataset Characteristics
| Product Type | Latency | Delivery frequency | Time coverage | Sampling |
|---|---|---|---|---|
| Forecast (FOE) | Forecast | Daily | One week into the future | Every 30 sec |
| Near real-time (NOE) | Hours | Hourly | One product file released every hour containing up to 30 hours of reconstructed orbits | Every 10 sec |
| Medium precision (MOE) | Days | Daily | One product file per day, including 3 hours of orbits from previous and subsequent day | Every 10 sec |
| Precise (POE) | Weeks | Daily | One product file per day, including 3 hours of orbits from previous and subsequent day | Every 10 sec |
The NISAR mission uses Medium precision (MOE) data when generating new NISAR data products, as MOE offers a reasonable compromise between data quality and prompt delivery.
Orbit Ephemeris File Contents¶
Each OE product is an XML file containing a list of position, velocity, and acceleration vectors at regular time intervals. A full description of the XML structure and data values is available in Fattahi & Buckley (2024).
Orbit Ephemeris Naming Convention¶
Orbit Ephemeris data files use the following naming scheme:
NISAR_ANC_J_PR_{ProductType}_{CreationTime}_{ValidityStartTime}_{ValidityEndTime}.xml
ProductType: FOE, NOE, MOE, or POECreationTime: UTC time of product creation in YYYYMMDDThhmmss formatValidityStartTime: UTC time of the first orbit state vector in the product in YYYYMMDDThhmmss formatValidityEndTime: UTC time of the last orbit state vector in the product in YYYYMMDDThhmmss format
For example, a file named NISAR_ANC_J_PR_POE_20260107T213450_20251224T205942_20251226T025942.xml is a Precise Orbit Ephemeris product created on Jan 7, 2026 that contains orbit state vectors from Dec 24, 2025 20:59:42 through Dec 26, 2025 02:59:42 in UTC time.
Finding and Downloading Orbit Ephemeris Files¶
You can find and download NISAR OE products using the NISAR_OE short name in Earthdata Search or via the Earthaccess Python package.
You can also find and download NISAR OE products via Direct AWS S3 Access. OE files are organized in a separate prefix for each product type (FOE/, NOE/, MOE/, POE/), as described in Prefix Structure.
- Fattahi, H., & Buckley, S. (2024). NASA SDS Orbit Ephemeris Product Software Interface Specification (NASA SDS Product Specification JPL D-102253 (Revision B)). NASA Jet Propulsion Laboratory. https://nisar.asf.earthdatacloud.nasa.gov/NISAR-SAMPLE-DATA/DOCS/NISAR_D-102253B_NASA_SDS_Orbit_Ephemeris_SIS_revB__20240624_URS_v2_w-sigs.pdf