Clarification of the GRACE/GRACE-FO product-level convention
established following internal discussions around 07 May 2026.
The framework distinguishes observational, SHC-domain, and
synthesized spatial products in a more rigorous and consistent way.
Level-1A/B
Level-1A/B products remain the original mission observation products.
These correspond to the fundamental observational domain and include:
KBR/LRI observations
Accelerometer measurements
GPS observations
Attitude data
Auxiliary mission measurements
These products remain unchanged and represent the original calibrated
or raw instrument observations of the GRACE/GRACE-FO missions.
Level-1C
Products derived from external processing of Level-1 observations
are classified as Level-1C. These products remain
tied to the observational domain rather than the gravity-field domain.
Externally filtered or adjusted Level-1 observables
The defining characteristic is that the products are still represented
in the measurement domain and are not yet spherical harmonic gravity-field solutions.
Level-2
Level-2 products are defined by representation in the
spherical harmonic coefficient (SHC) domain.
Any product remaining in SHC form is, by definition,
still a Level-2 product regardless of applied corrections
or post-processing steps.
This includes:
Geocenter corrections
SLR low-degree coefficient replacement
GIA corrections
Destriping or filtering
Leakage mitigation
Other SH-domain adjustments
The May 2026 clarification establishes that corrected SHC products
remain Level-2 products as long as they remain represented in SHC space.
Level-2B
Level-2B is interpreted as a corrected or enhanced
Level-2 product still fully represented in the SHC domain.
The defining criterion is not the amount of processing applied,
but whether the product continues to exist in spherical harmonic coefficient space.
Level-3
Level-3 products begin once synthesis or gridding into spatial fields is performed.
The defining characteristic of Level-3 is therefore the
gridded spatial representation, independent of whether the
product fully represents a direct geophysical quantity.
Examples include:
Gridded EWH/TWSA products
Synthesized LGD grids
Latitude-longitude spatial products
Synthesized uncertainty fields
Other gridded products derived from SHCs
The synthesis or gridding step itself is what fundamentally transitions
a product from Level-2/2B into Level-3.
Implications for TUD Product Naming (5-day v2 & Monthly Uncertainties v5)
Following this clarification, the naming convention for the upcoming
v5 release will be adjusted accordingly.
Products remaining in SHC representation retain Level-2/Level-2B classification
Gridded synthesized products are classified as Level-3 products
Example Naming Transition
Previous naming: TUD-L2B-EWH_UNC-GRACE
→
New naming: TUD-L3-EWH_UNC-GRACE
Under the updated interpretation, gridded synthesized EWH uncertainty products
are explicitly treated as Level-3 products.