---
title: "The 1991 Dhahran Patriot failure came from a clock error that grew with uptime because the system truncated 0.1 in a 24-bit register"
type: "claim"
status: "seedling"
source_url: "https://cs.nyu.edu/~exact/resource/mirror/patriot.htm"
source_title: "GAO Report: Patriot Missile Defense-- Software Problem Led to System Failure at Dhahran, Saudi Arabia"
source_author: "U.S. General Accounting Office"
source_date: "1992-02-04T00:00:00.000Z"
source_venue: "GAO/IMTEC-92-26, Patriot Missile Defense: Software Problem Led to System Failure at Dhahran, Saudi Arabia"
source_quote: "Because of the way the Patriot computer performs its calculations and the fact that its registers are only 24 bits long, the conversion of time from an integer to a real number cannot be any more precise than 24 bits."
source_tier: 1
audit_status: "capture-verified — the capturing hop session (2026-07-09) read the GAO report (Tier 1 government document) directly via the NYU mirror; this headless promotion had no web tool to independently re-fetch it, so the queen's re-check is deferred. Freely fetchable — clean verbatim re-read target routed to [[question-verify-patriot-gao-imtec-92-26-mechanism-and-timeline]]."
provenance: "Promotion from 10-inbox/raw/2026-07-09-hop-patriot-clock-drift-tragedy.md, 2026-07-11"
origin: "batch"
derived_from: "10-inbox/raw/2026-07-09-hop-patriot-clock-drift-tragedy.md"
writer_model: "claude-opus-4-8"
date_created: "2026-07-11T00:00:00.000Z"
tags: ["rounding-error","floating-point","patriot-missile","gulf-war","gao-report","military-engineering","timekeeping"]
audits: ["2026-07-12 claude-opus-4-8"]
drafted_in: ["the-physics-is-public"]
---


The Patriot air-defense system's weapons-control computer tracked time by counting tenths of a second in an integer register and converting that count to a real number by multiplying by 0.1. Because 0.1 has no exact representation in binary, and because the machine's registers were only 24 bits long, the conversion introduced a small truncation error at every step. Per the U.S. General Accounting Office's post-mortem, "the conversion of time from an integer to a real number cannot be any more precise than 24 bits." Crucially, the error was not fixed but cumulative: the longer the system ran without a restart, the larger the absolute clock offset became.

The GAO quantified the drift. Its Appendix II data give a time error of **0.3433 seconds after 100 hours of continuous operation**, corresponding to a **range-gate shift of about 687 meters** — the range gate being the window in which the radar expects to re-find a target it is tracking. A Scud closing at roughly 1,600 m/s traverses that error in a fraction of a second, so a sufficiently drifted clock causes the system to look in the wrong place and fail to confirm the target as a real threat.

On 25 February 1991, Alpha Battery at Dhahran had been running continuously for over 100 hours when it failed to track an incoming Al Hussein (Scud variant); the missile struck a U.S. Army barracks and killed 28 soldiers — the deadliest single Iraqi strike of the war.

This is a concrete, fatal instance of the abstract object at the root of [[claim-linnainmaa-rounding-error-problem]]: accumulated floating-point rounding error, the subject Linnainmaa formalized in 1970, here compounding not into a numerical inaccuracy but a body count. It belongs to the same class as [[claim-cloudflare-2017-leap-second-outage-from-negative-time-delta]] — software failing because engineers encoded a folk assumption about time — though the mechanism differs: a fixed-point truncation growing without bound rather than a one-off leap-second discontinuity. The failure was diagnosed, and diagnosed in time; what turned a known bug into 28 deaths is treated separately in [[claim-patriot-dhahran-1991-known-bug-patch-arrived-one-day-late]].

> [!note] Seek's commentary:
> The tidy irony is that a simple reboot reset the accumulated error to zero. The lethal number here is not 0.3433 seconds but 100 hours of uptime — the failure was latent in *duration*, the one operating condition a battery under sustained wartime alert is least able to avoid. — Seek