Concepts

Granules and frames

A DISP granule is one NetCDF file holding the displacement between two dates, a reference date and a secondary date, on a fixed grid called a frame. A DISP-S1 frame is about 7,000 x 9,000 pixels at 30 m in a UTM zone. A frame's history is a few hundred granules.

Ministacks and reference dates

The processor (dolphin) works on ministacks: blocks of about 15 acquisitions that share one reference date. Every granule in a ministack measures displacement from that ministack's reference date, so the raw series restarts at zero at each new ministack.

The next ministack's reference date is the last secondary date of the previous one. That shared epoch, the crossover, links them: chain adds to each ministack the displacement accumulated at its crossover, so the whole series becomes relative to the first reference date. A missing crossover granule breaks the chain, and chain raises MissingCrossoverError. A NaN at a crossover makes later epochs of that pixel NaN (nan_policy="propagate", the default).

open_disp returns the series as stored, relative to each ministack reference; reference_time says which one. chain returns it chained, with attrs["reference_date"] set and a zero epoch prepended at the first reference date.

Spatial reference

DISP displacement is relative to a reference point chosen by the processor. rereference subtracts, at every epoch, the value of a pixel you choose or the median of an area, so the series is relative to ground you trust.

Units and sign

Products store meters, positive toward the satellite (uplift or motion along the look direction toward it). to_units converts length layers to cm or mm; plots use mm by default.

Quality layers

Each granule carries quality layers alongside displacement:

Layer	Meaning
recommended_mask	1 where the processor recommends the pixel
temporal_coherence, phase_similarity	0 to 1; higher is better
connected_component_labels	unwrapping regions; 0 is unreliable
water_mask	0 over water
persistent_scatterer_mask	1 for persistent scatterers
shp_counts, estimated_phase_quality, timeseries_inversion_residuals	diagnostics

mask sets displacement to NaN where pixels fail: the "recommended" preset uses recommended_mask, "strict" adds connected components, water and a temporal-coherence threshold. quality_summary maps how often each pixel passes.

Corrections

The /corrections group holds the ionospheric delay and solid-earth tide that the processor estimated but did not subtract. correct subtracts them once, before chaining, and records what it applied; it refuses a correction the product already applied (DISP-NISAR ionosphere).

Chunks

Data is opened lazily in the layout of the files: one epoch and one block per dask chunk (chunks="native"). Each operation then reads what it needs:

Question	Reads
a map of one epoch	that epoch, plus one crossover epoch per earlier ministack
a pixel's time series	one block from every granule
a per-pixel fit	rechunks to whole time series itself

rechunk switches layouts when one kind of access repeats. Stores written with to_zarr choose their layout with preset=: "timeseries" for per-pixel work, "maps" for maps.