prop/updates.md

Algorithm Updates — April 2026 (Full Dataset Analysis)

Analysis of 58,367 contacts matched against 41M HRRR profiles in the local database. This extends the earlier Part 2b correlation analysis with binned distance analysis across all atmospheric variables.

Finding 1: Shallow Boundary Layer Bonus Not Supported

The algorithm previously awarded a score of 82 when HPBL < 300m (shallow BL), on the theory that shallow BL indicates a surface inversion trapping refractivity. The full dataset contradicts this:

BL Depth	n	Avg Distance (km)	Median	Avg Refrac Gradient
1000-2000m	6,021	222.1	210.0	-98.7
500-1000m	10,834	214.2	188.4	-103.2
< 200m (shallow)	7,020	209.6	178.8	-114.0
200-500m	10,018	204.1	178.7	-112.3
2000m+ (deep)	803	196.2	167.1	-101.3

Medium-depth BL (1000-2000m) produces the longest average contacts, not shallow. Shallow BL does correlate with stronger refractivity gradients (-114 N/km avg), but this doesn't translate to longer contacts — likely because shallow BL often means fog/low stratus that attenuates the signal.

Code change: Removed the @shallow_bl_threshold_m 300 / @shallow_bl_score 82 refractivity fallback. Refractivity scoring now falls through to the default score when gradient is weak, regardless of BL depth.

Finding 2: Td Depression at 24 GHz — Moist Range is Worst

At 24 GHz, the moist range (2-5C Td depression) produces the shortest contacts, while both dry and near-saturated conditions are better:

Td Depression	n	Avg Distance (km)	Median	P90
15+ (very dry)	311	109.1	110.6	193.5
0-2 (saturated)	447	104.9	108.2	178.2
10-15 (dry)	278	99.5	97.8	179.7
5-10 (moderate)	841	94.9	87.1	177.2
2-5 (moist)	727	72.1	57.7	146.7

Near-saturation (0-2C) likely produces ducting that overcomes the absorption penalty, while the 2-5C range gives enough moisture for absorption without the refractivity benefit. Very dry conditions minimize absorption entirely.

No code change — the current harmful-band Td scoring already penalizes low depression. The existing curve is directionally correct; the moist-range penalty emerges naturally from the humidity and PWAT factors working together.

Finding 3: PWAT Sweet Spot Confirmed at 20-30mm

The PWAT scoring curve already peaks at 20-30mm for beneficial bands:

PWAT Bin	n	Avg Distance (km)	Median
20-30mm	13,376	219.7	211.5
< 10mm	463	211.8	195.5
10-20mm	8,500	208.3	183.5
40-50mm	3,118	207.7	177.3
30-40mm	8,947	203.5	171.8
50mm+	292	190.5	173.7

The existing code's PWAT beneficial curve (<10→55, <20→75, <30→90, <40→70, else→50) is well-calibrated. No change needed.

Finding 4: Pressure Relationship Has Fine Structure

Finer pressure binning reveals a clear monotonic relationship for the bulk of the data:

Pressure Bin	n	Avg Distance (km)	Median
< 970 mb	6,010	242.7	232.9
970-980	5,254	229.8	225.5
980-990	7,234	207.3	184.2
990-1000	9,336	184.1	160.4
1000-1010	3,505	200.7	161.0
1010+	3,357	221.3	191.1

The <970 to 990-1000 range shows a clean 30% gradient. The uptick at 1010+ likely reflects high-altitude western US stations with inherently longer radio horizons, not a genuine pressure effect.

Code change: Refined pressure scoring bins to add a <980 tier (score 88) and increase the <1005 score from 80 to 85, better reflecting the strong low-pressure signal.

Finding 5: Mode Distance Advantage Quantified

Mode	n	Avg Distance (km)	Median	Max
CW	18,001	247.2	227.4	1,606
PH (SSB)	35,089	190.9	176.2	999.2
FM	180	141.2	116.5	664.1

CW achieves 29% longer average distance than SSB at 10 GHz, consistent with the ~7 dB bandwidth advantage (24% theoretical range increase). FM is 26% shorter than SSB. These ratios are useful for range estimate calibration.

Finding 6: Refractivity Gradient is Flat Within the Bulk Distribution

Gradient Bin	n	Avg Distance (km)	Median	Ducting %
< -200 (strong)	659	202.2	189.0	100%
-200 to -150	2,804	205.0	179.9	52.8%
-150 to -100	14,113	211.8	186.6	0%
-100 to -75	12,138	215.8	192.3	0%
-75 to -55	4,390	207.5	180.0	0%
>= -55 (weak)	592	176.3	151.9	0%

The -150 to -75 range (the bulk of HRRR gradients) shows remarkably flat distance distributions. Only the weakest gradients (>= -55) show meaningfully shorter contacts. The current 8% weight is appropriate given this weak discriminatory power.

Finding 7: Dataset is 99.5% Aug-Sep

Month	Contacts (tropo)	Avg Distance
Aug	29,725	202.4
Sep	28,468	198.7
All other months	174	varies

All atmospheric correlations in this analysis are effectively summer-only findings. Seasonal scoring curves cannot be validated from this dataset. The physics-based seasonal tables remain the best available approach for non-summer months.

Updated Dataset Statistics

• Contacts: 58,367 total, 58,367 tropo (<3000 km)
• HRRR profiles: 41,071,398 (37,925 matched to contacts, 65% coverage)
• Terrain profiles: 58,361 (56,735 BLOCKED 97.1%, 1,284 CLEAR 2.2%, 342 FRESNEL_PARTIAL 0.6%)
• IEMRE observations: 15,092
• Weather stations: 3,268
• Commercial samples: 0 locally (available on production only)