The Hidden 3x Inside the E911 Tiers

Ada re-derives this chapter’s own numbers step by step, at full precision

ADA · CALCULATION AUDIT

The Hidden 3x Inside the E911 Tiers

When you dial 911 from a mobile phone, FCC E911 rules force the carrier to report where you are — and the chapter tabulates how accurately. Handset-based A-GPS must reach 50 m for 67% of calls and 150 m for 95%; network-based positioning, 100 m and 300 m. The chapter’s design rule is to spec to the 95% number and expect roughly 3x worse than the brochure figure, so this audit asks whether that 3x is a loose rule of thumb or something the regulation actually encodes.

Companion to the chapter Location Privacy and Regulations — every number here comes from that chapter.

Ada: The E911 table gives two accuracy tiers for each technology, and the chapter’s design rule is to spec to the 95% number and expect roughly 3x worse than the advertised figure. Let me check whether that 3x is a loose rule of thumb or something the regulation actually encodes.

Take the ratio of the 95%-of-calls bound to the 67%-of-calls bound for each technology:

• Handset-based (A-GPS): 150 m / 50 m = 3.0
• Network-based: 300 m / 100 m = 3.0

They are not merely close, they are both exactly 3.0. The FCC set the worst-case (95%) requirement at precisely three times the typical-case (67%) requirement, and it did so identically for two physically different positioning methods. So the chapter’s heuristic is not folklore; it is reading a constant straight out of the mandate. That is why the “advertised 5 m -> expect 15-20 m in hard environments” guidance (a 3x to 4x spread) is well founded: a location system’s typical accuracy and its dependable accuracy differ by a roughly fixed multiple, and safety-critical design should budget for the 3x-worse number rather than the brochure number.

Every number above is taken from the chapter’s own material and re-derived step by step.