# Sundial: Using Sunlight to Reconstruct Global Timestamps

## TL;DR

Sundial is an offline algorithm that accurately reconstructs global timestamps in environmental monitoring networks by correlating solar patterns with light sensor data, achieving high precision without synchronized clocks.

## Contribution

It introduces a novel method leveraging solar patterns and light sensors for postmortem timestamp reconstruction, robust to clock restarts and unreliable sources.

## Key findings

- Achieves timestamp accuracy up to 10 ppm.
- Works with light sensor data at 20-minute intervals.
- Effective over deployments spanning 2.5 years.

## Abstract

This paper investigates postmortem timestamp reconstruction in environmental monitoring networks. In the absence of a time-synchronization protocol, these networks use multiple pairs of (local, global) timestamps to retroactively estimate the motes' clock drift and offset and thus reconstruct the measurement time series. We present Sundial, a novel offline algorithm for reconstructing global timestamps that is robust to unreliable global clock sources. Sundial reconstructs timestamps by correlating annual solar patterns with measurements provided by the motes' inexpensive light sensors. The surprising ability to accurately estimate the length of day using light intensity measurements enables Sundial to be robust to arbitrary mote clock restarts. Experimental results, based on multiple environmental network deployments spanning a period of over 2.5 years, show that Sundial achieves accuracy as high as 10 parts per million (ppm), using solar radiation readings recorded at 20 minute intervals.