eess.IVOct 13, 2026
SaveDeficit-Driven Stratified Acquisition for Building STRATA-S12: An Open SAR-Optical Corpus for Self-Supervised Learning
Organizations: Software College, Northeastern University, Shenyang, 110819, China
Abstract
Building a co-registered SAR-optical corpus for self-supervised Earth Observation is above all an acquisition problem, yet the main multi-modal datasets are collected opportunistically, with no per-stratum quota control and no mechanism against the geographic dropout that non-uniform cloud cover leaves behind. We present Deficit-Driven Stratified Acquisition (DDSA), an algorithm that treats building a co-registered Sentinel-1/Sentinel-2 corpus as a constrained maximum-coverage problem over a hemisphere-corrected (class x season x region) stratification, enforces strict temporal pairing and a multi-stage quality gate, and closes the loop during acquisition with a human-in-the-loop deficit-repair stage. Measured on its own product, the algorithm yields three results a single-pass acquisition cannot report. Deficit-driven ordering makes a partial run a covering one: it holds every one of the 192 strata populated from the first few percent of the budget, where an abundance-first order leaves most strata empty until the end. Whether a failed attempt can be recovered depends on the temporal persistence of its cause, not the kind of defect: geometric failures such as nodata are among the most repairable (eta = 41.4%) while cloud is among the least (eta = 18.8%), because a cloudy location is cloudy on most dates of its season. The repair loop supplies 39.5% of the released pairs at no measurable quality cost, with 89.8% surviving manual review. The quality guarantee has a price: 2,928,370 acquisition attempts produced 740,746 released pairs. We release the corpus, STRATA-S12, with STAC/PDS4-aligned FAIR sidecars, and a pre-registered linear-probe check confirms it is fit for self-supervised pre-training. A metadata-level coverage comparison against SSL4EO-S12 and SEN12MS situates the corpus over a shared frame; systematic cross-corpus and cross-method comparison is left to follow-up work.
Figures & tables
Figure 1: DDSA algorithmic flow. The 10 m ESA WorldCover map and a set of parameters (target , strata , sensor latitude limits , temporal tolerance , cloud thresholds , max inter-block tile overlap ) drive a six-phase pipeline. Phases A–C build, stratify and order an acquisition plan. The block-level bulk acquisition (Phase D) is followed by an iterated human-in-the-loop tile-level recovery (Phase E) that explicitly counteracts cloud-induced geographic dropout. Phase F derives the full corpus , the reduced benchmark split , and the per-stratum statistics report .
Figure 2: Representative Phase-E manual removals, one per failure mode. In each panel the left half is the Sentinel-2 optical tile and the right half the Sentinel-1 SAR composite of the same pair. These are defects the automatic Phase-D gates cannot detect by construction; for the temporal seam (c) the discontinuity appears co-registered in both the optical and the SAR channel, which a single-sensor artefact cannot produce. Dense fire-smoke removals also occur but are not shown. The complete list, with source-scene identifiers, is the Phase-E ledger released with the corpus.
| Setting | Value | Role |
|---|---|---|
| Land-cover classes | 8 of 11 (WorldCover v200) | stratification axis |
| Continental regions | 6 | stratification axis |
| Seasons | 4 (hemisphere-corrected) | stratification axis |
| Season window | 3-month meteorological | winter = Dec –Feb |
| Pairs per season | 2 (balanced) | atomic S1–S2 pairs per location |
| Tile size | px @ 10 m | released tile |
Table 1: Parameter settings used to build STRATA-S12. Symbols match the phase descriptions above and Algorithms 1 – 2 .
Figure 3: DDSA system architecture. Three external data sources feed a Docker-containerised pipeline of four stages (pre-computation, acquisition, human-in-the-loop, archival), backed by two SQLite (WAL) state stores that drive scheduling and label-purity lookup. The released corpus is the bottom output.
| Rejected | Repair loop | ||||
| Reason | % | recovered | exhausted | (%) | |
| Tile-level quality gates | |||||
| clouds | 444,136 | 21.4 | 41,131 | 177,878 | 18.8 |
| nodata | 256,183 | 12.3 | 55,722 | 78,774 | 41.4 |
| edge_padding | 318 | 0.0 | 58 | 7 | 89.2 |
| Image-wide corruption (source scene unusable) | |||||
Table 2: Acquisition rejection counts by reason, from the rejected_tiles log. Counts are per acquisition attempt, not per unique tile: a location may fail several times before a valid pair is found, so the total exceeds the number of released tiles. The left block is the rejection; the right block is what the repair loop then did with it — how many of those attempts it refilled, how many it retried without success, and the resulting efficiency (Equation ( 4 )). Reading the two together is the point of the table: varies by a factor of five across reasons, and not in the direction the reason suggests. Pairs removed after acceptance by the manual review do not appear here; they are logged as withdrawn pairs rather than as gate rejections and are counted in the scale-and-effort summary. Attempts not yet retried are excluded from and reported there as well. Percentages are over the total number of rejected attempts, so the table is closed over its own rows and sums to 100 up to rounding; the wider count of every attempt the acquisition made (delivered, withdrawn and rejected) is reported separately in the scale-and-effort summary.
Figure 4: Recovery efficiency by rejection reason, ordered by . Bars are coloured by defect family (atmospheric/archive in one hue, geometric in another) and annotated with the number of retried attempts behind each ratio. The ordering inverts the intuitive reading of the taxonomy: the geometric failures, which look terminal, sit high, while cloud sits at the bottom — what separates them is whether the cause persists in time, not the kind of defect. Thin categories, whose rests on few retries, are read against their annotated count rather than taken as stable estimates.
| Pair attempts | Delivered pairs | ||||
|---|---|---|---|---|---|
| Scope | recovered | exhausted | (%) | from repair | share (%) |
| Global | 562,747 | 912,290 | 38.2 | 292,432 | 39.5 |
| Africa | 147,611 | 174,121 | 45.9 | 78,309 | 39.6 |
| Asia | 158,733 | 302,889 | 34.4 | 88,156 | 46.2 |
| Australia/Oceania | 29,001 | 48,679 | 37.3 | 16,863 | 33.3 |
| Europe | 122,321 | 134,892 | 47.6 | 55,541 | 39.7 |
Table 3: Phase-E repair by scope. The left block counts pair attempts and yields (Equation ( 4 )); the right block counts delivered pairs and the share of each scope’s corpus that the loop supplied. The two blocks are in different units and are not additive.
Figure 5: Per-region recovery efficiency against cloud burden, the share of a region’s rejections that were cloud-driven. Cloud burden modulates recovery but does not determine it: the cloudier regions tend lower, yet the relation is not monotone — North America breaks it, with a moderate cloud share but the lowest efficiency — so what bounds recovery is regional scene availability and orbital geometry rather than cloud alone. The dashed line is a descriptive trend, not a fit.
| Region | STRATA-S12 | SSL4EO-S12 | SEN12MS |
|---|---|---|---|
| N. America | 12.1 | 25.8 | 22.1 |
| S. America | 9.7 | 12.0 | 7.3 |
| Europe | 18.9 | 17.5 | 24.7 |
| Africa | 26.7 | 14.1 | 17.5 |
| Asia | 25.8 | 29.7 | 23.2 |
| Oceania | 6.8 | 0.9 | 5.2 |
Table 4: Regional share of each corpus (% of that corpus’s own samples) over the six continental regions of the shared frame. Columns sum to 100 up to rounding.
| Descriptor | DDSA | DDSA (no repair) | SSL4EO-S12 | SEN12MS |
|---|---|---|---|---|
| Hemisphere ratio (N:S) | 2.92 | 2.74 | 4.99 | 4.09 |
| Land-cover classes covered | 8 | 8 | 7 | 8 |
| Normalised coverage entropy | 0.904 | 0.912 | 0.871 | 0.864 |
| Tropical-band fraction (%) | 32.7 | 33.6 | 18.9 | 22.7 |
Table 5: Coverage distribution of the DDSA corpus against opportunistic corpora, computed from public scene metadata over the shared (class season region) stratification. Higher normalised entropy and more classes covered indicate a broader spread across strata. The hemisphere ratio is read with care: land area is itself Northern-skewed, so part of any imbalance is geographic, but DDSA’s ratio is driven by its per-stratum quotas rather than by scene availability, so a strong skew in an opportunistic corpus points to cloud-induced dropout beyond that baseline.
Figure 6: Coverage of STRATA-S12 against SSL4EO-S12 and SEN12MS over the shared (class season region) strata. Each top panel is one corpus; cell colour is the share of that corpus’s samples falling in a (region, season) stratum, so the panels are comparable across corpora that differ in size by orders of magnitude, and the seasonal axis is read left-to-right within a panel. The lower-left panel is the difference: its near-zero cells show that the Phase-E repair loop adds volume roughly in proportion to the existing distribution rather than redistributing it, so the stratum balance comes from the deficit-driven ordering of Phase C upstream (cf. Table 5 and Section 4.8 ).
Figure 7: Global sample density of the three corpora, binned on a grid and normalised to each corpus so the panels compare distribution rather than size. The colour scale is logarithmic and shared: densities span several orders of magnitude between an empty cell and a dense continental one, and on a linear scale everything but the hottest cells collapses into the background. The three regimes are visibly different: SSL4EO-S12 concentrates its densest cells along the northern mid-latitudes and thins below the equator, the signature of a frame that follows population; SEN12MS is sparse everywhere, being two orders of magnitude smaller; STRATA-S12 spreads across the worked continents with its densest cells in equatorial Africa, the Amazon and southern Asia. The empty regions of the bottom-left panel are countries this acquisition has not reached yet, not strata it failed to fill (Section 4.9 ).
Figure 8: Marginal share of each corpus over the three stratification axes. Bars are percentages of each corpus’s own samples, so corpora differing in size by more than an order of magnitude remain comparable. The three marginals are shown in place of the full grid, which is unreadable at this width and answers the same question less directly: which continent, which land cover and which season each acquisition regime over- or under-represents. The season panel is drawn as a profile because the axis is an ordered cycle, with the uniform share marked so departures are read against a reference rather than against a truncated axis.
| Budget | Ordering | Strata covered | Min-fill | Norm. entropy |
|---|---|---|---|---|
| 5% | deficit-greedy (Phase C) | 192 | 7 | 0.996 |
| abundance-first | 2 | 0 | 0.130 | |
| random | 192 | 0 | 0.904 | |
| 25% | deficit-greedy (Phase C) | 192 | 7 | 0.982 |
| abundance-first | 12 | 0 | 0.463 | |
| random | 192 | 2 | 0.904 |
Table 6: Coverage of the delivered pool under three acquisition orderings, at three budgets, over the shared strata. Min-fill is the count in the poorest stratum. Deficit-greedy is the Phase C rule; abundance-first emulates an availability-driven download. The orderings re-order the same delivered pairs, so they coincide at the full budget; the separation at a partial budget is what the ordering contributes.
Figure 9: Coverage versus acquisition budget for the three orderings over the delivered pool. Left: min-fill, the number of pairs in the poorest stratum — deficit-greedy holds a floor from the first few per cent of the budget, while abundance-first leaves the poorest stratum empty until the run is almost complete. Right: normalised coverage entropy. The orderings coincide at the full budget because they re-order the same pool; the gap at a partial budget is the contribution of the ordering (Section 4.9 ).
| Setting | Value |
|---|---|
| Backbone | vit_small_patch16 |
| Input channels | 6 |
| Mask ratio | 0.7 |
| Effective batch size | 256 |
| Base learning rate | 1.5e-04 |
| Weight decay | 0.05 |
Table 7: MAE pre-training configuration (frozen for all seeds).
Figure 10: MAE pre-training on the representative subset of STRATA-S12: masked-reconstruction loss (top) and learning rate (bottom) over 100 epochs. The dashed line marks the end of the learning-rate warmup.
| Metric | Pre-trained on | Random init |
|---|---|---|
| micro-mAP (primary) | 0.5552 0.0022 | 0.5158 0.0068 |
| macro-mAP (descriptor) | 0.3366 0.0004 | 0.2920 0.0046 |
Table 8: Frozen linear-probe performance on reBEN (BigEarthNet v2.0, 19-class multi-label), mean standard deviation over 3 seeds. The pre-registered criterion is met when exceeds on micro-mAP.
Figure 11: Frozen linear-probe scores on reBEN (BigEarthNet v2.0), pre-trained on versus random initialisation. Bars are the mean over 3 seeds and the black dots the individual seed values; error bars are one standard deviation. Both micro-mAP (the primary metric of the pre-registered criterion) and macro-mAP are shown so that neither is read in isolation.
Figure 12: Per-class average precision on reBEN, pre-trained on versus random initialisation, averaged over 3 seeds. Each row is one of the 19 classes; the grey and green dots are the two arms and the connecting bar is the per-class gain from pre-training. Classes are ordered by that gain. The pre-trained encoder is ahead on every class, with the widest gains on the compositionally rich land covers and the narrowest on the classes either arm already separates well or that are too rare for a linear probe to recover.