Add idea 015: Bus Stop ETA Beacon

ideas/015-bus-stop-eta-beacon.md

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+# 015 · Bus Stop ETA Beacon
+
+**title:** Bus Stop ETA Beacon — an offline, solar "next bus in…" sign that uses real GPS, not a schedule
+**status:** idea / MVP ready
+**license:** MIT
+
+## why
+
+GTFS schedules lie when buses run late, reroute, or get stuck. Meanwhile riders wait in the rain refreshing an app that needs signal. A solar + LoRa beacon at the stop can show real, crowd-sourced ETAs: volunteer "rider pings" from onboard phones (BT/BLE) + a tiny GPS tracker on a route vehicle relay the actual position to each stop's e-ink sign. No cellular network, no app install, no data plan — just a #digital-clock that tells the truth for once.
+
+## what
+
+- **Stop sign**: e-ink display (1–7") + ESP32 + LoRa (or Wi-Fi where available), solar panel + LiFePO4 battery.
+- **Vehicle puck**: $25 GPS (NEO-6M/gNSS) + ESP32 riding a school/community shuttle; broadcasts position every 15s over LoRa.
+- **Routing on the sign**: each sign only displays the bus(es) on its own route — distance/speed → ETA.
+- **Fail-degraded**: if no ping for 5 min, sign flips to schedule-based ETA (GTFS) and shows a ⚠️ "live signal lost".
+- **Hop-by-hop**: LoRa range 1–3 km, can chain across a route; no towers, no fees.
+
+## plan
+
+1. Build a one-vehicle proof: GPS puck + one e-ink sign at the terminal stop.
+2. LoRa frame: route_id, vehicle_id, lat, lon, speed, heading, ts — tiny binary, signed with a CRC.
+3. Sign firmware: parse frames, compute distance along route vs. last-known, display "12 min".
+4. Add schedule fallback + stale-signal warning state.
+5. Multi-vehicle + multi-stop: mesh relay so stops beyond direct range still hear the bus.
+6. Enclosure + solar sizing; pilot on a school route or community shuttle for a term.
+
+## code
+
+LoRa packet + sign-side ETA logic (~60 lines):
+
+```python
+# beacon.py — ESP32 LoRa vehicle puck (MicroPython, SX1276)
+import time, ubinascii, struct
+from machine import Pin, UART
+# LoRa via SPI (RA-02/SX1276) — see your board's wiring; use a lib like sx127x
+
+def gps_fix():   # returns (lat, lon, speed_kph) — from ATGM336H/NEO-6M over UART
+    u = UART(1, 9600)
+    for _ in range(40):
+        line = u.readline()
+        if line and line.startswith(b"$GPRMC"):
+            parts = line.decode(errors="ignore").split(",")
+            if parts[2] == "A":   # valid fix
+                lat = float(parts[3][:2]) + float(parts[3][2:]) / 60.0
+                lon = float(parts[5][:3]) + float(parts[5][3:]) / 60.0
+                if parts[4] == "S": lat = -lat
+                if parts[6] == "W": lon = -lon
+                return lat, lon, float(parts[7]) if parts[7] else 0.0
+    return None
+
+def packet(route, veh, lat, lon, spd):
+    # binary: route(1B) veh(1B) lat(4B, 1e6) lon(4B) spd(2B, 0.1kph) crc(2B)
+    payload = struct.pack("<BBiiH", route, veh, int(lat*1e6), int(lon*1e6), int(spd*10))
+    crc = ubinascii.crc32(payload) & 0xFFFF
+    return payload + struct.pack("<H", crc)
+
+while True:
+    fix = gps_fix()
+    if fix:
+        lora_send(packet(ROUTE_ID, VEH_ID, *fix))   # your lora_send() from sx127x lib
+    time.sleep(15)
+```
+
+```python
+# eta.py — …and on the sign: distance-but-no-schedule math
+def eta_minutes(route_m, bus_lat, bus_lon, stop_lat, stop_lon, speed_kph):
+    from math import radians, cos, sin, asin, sqrt
+    R = 6371.0
+    dlat = radians(stop_lat - bus_lat); dlon = radians(stop_lon - bus_lon)
+    a = sin(dlat/2)**2 + cos(radians(bus_lat))*cos(radians(stop_lat))*sin(dlon/2)**2
+    km = 2 * R * asin(sqrt(a))
+    if speed_kph < 2: return "~unknown (idle)"
+    return max(1, int(km / speed_kph * 60))
+```
+
+## next
+
+- Route-arc projection: use stop order + last two fixes to estimate ETA along the actual path.
+- Store-and-forward repeaters for stops out of direct range.
+- Sign sleep schedule: wake 2s before expected bus window, else deep-sleep (e-ink = zero idle power).
+- Pedestrian/cyclist "live" mode (no GPS needed — bits of route from the puck).
+- Accessibility: audible ETA button + high-contrast large digits (see #013 for reading ease).
+- Fleet dashboard on the hub for maintenance, and open LoRa frame spec for other tools.