Real-Time Grid-ETA System Design

You are tasked with designing a real-time system that maintains the remaining ETA for every driver currently located within each cell of a city grid. You must use only the following data sources:

• GPS pings from driver devices
• Trip events (e.g., trip_start, trip_end, destination provided at trip start)

No external maps, POI data, or manual features are allowed. Assume:

• The city is pre-partitioned into a fixed grid (e.g., S2/H3 or a rectangular grid) with a known cell_id function latlon → cell.
• For an active trip, the destination (lat, lon) is known at trip_start.
• GPS pings arrive at ~1 Hz with fields including timestamp and optional accuracy.

Design the system and describe, step-by-step, how you will:

1. Ingest and partition GPS pings and trip events.
2. Denoise GPS signals and correct drift to robustly localize drivers within grid cells.
3. Compute and continuously update each driver's remaining ETA.
4. Select and train a model suitable for online serving.
5. Perform online feature computation strictly from the approved data.
6. Meet low-latency, high-throughput SLAs.
7. Monitor, alert, and rollback models.
8. Evaluate offline and online.

Be specific about:

• Algorithms for GPS error reduction (e.g., filtering, map matching)
• Data schemas
• State stores and keys
• Windowing strategy and update frequency
• Failure handling
• Validation methodology