cellidresolver: synthesize a cell tower if no exact match is found in the database

ols/resolver/cellid/cellidresolver.py

@@ -56,13 +56,58 @@ class CellIdResolver(ResolverBase):
         else:
             return None
 
-    def _resolve_cell(self, radiotype, opc, lac, cid) -> Optional[dict]:
+    def search_nearby(self, radiotype, mcc, net, area, cell) -> Optional[dict]:
+        """
+        synthesize a cell tower from towers whose `cell` id is close to the id of interest,
+        under the assumption that towers with similar cell ids have similar location.
+        this assumption is crudely validated, by aborting if the distance between sampled
+        towers is absurdly large.
+        """
+        M_PER_DEG = 111319
+        NUM_SAMPLES = 4  #< take this many nearest towers
+        MAX_RANGE = 20000  #< if the uncertainty is > this many meters, just return `None`
+        cur = self.con.cursor()
+        # radiotype is not required in observations
+        radio = radiotype.upper() if radiotype is not None else 'GSM'
+
+        # TODO: replace this "fetchone" with a sampling
+        nets = cur.execute("""
+            SELECT * FROM cell WHERE mcc = ? AND net = ? AND area = ?
+            ORDER BY abs(cell - ?);
+            """, (mcc, net, area, cell)).fetchmany(NUM_SAMPLES)
+
+        if not nets or len(nets) != NUM_SAMPLES:
+            return None
+
+        synth = {}
+        for field in 'lat', 'lon', 'range':
+            # float fields
+            synth[field] = sum(n[field] for n in nets) / NUM_SAMPLES
+        for field in 'updated', 'created', 'samples':
+            # int fields
+            synth[field] = sum(n[field] for n in nets) // NUM_SAMPLES
+
+        # guess a range, based on the standard deviation of sampled tower locations.
+        # each degree of latitude is about 100km, and the range field is in meters.
+        sum_of_squares = 0
+        for n in nets:
+            sum_of_squares += ((n['lat'] - synth['lat']) * M_PER_DEG) ** 2 + ((n['lon'] - synth['lon']) * M_PER_DEG) ** 2
+        rms = (sum_of_squares / NUM_SAMPLES)**0.5
+        log.debug(f'synthesized cell tower from {NUM_SAMPLES} real towers with {rms:.3}m std. dev')
+        synth['range'] += rms
+
+        if synth['range'] > MAX_RANGE:
+            return None
+
+        return synth
+
+    def _resolve_cell(self, radiotype, opc, lac, cid, search_fn) -> Optional[dict]:
         radio = radiotype.upper()
         mcc, mnc = opc_to_mcc_mnc(opc)
         if mcc is None:
             log.debug(f'opc to mcc,mnc conversion failed for {radio} {opc}_{lac}_{cid}')
             return None
-        netd = self.search_network(radio, mcc, mnc, lac, cid)
+        netd = search_fn(radio, mcc, mnc, lac, cid)
         if netd is None:
             log.debug(f'Resolving failed for {radio} {opc}_{lac}_{cid}')
             return None
@@ -77,7 +122,10 @@ class CellIdResolver(ResolverBase):
         }
 
     async def resolve_cell(self, radio, opc, lac, cid, **kwargs) -> Optional[dict]:
-        return self._resolve_cell(radio, opc, lac, cid)
+        best_match = self._resolve_cell(radio, opc, lac, cid, self.search_network)
+        if best_match is None:
+            best_match = self._resolve_cell(radio, opc, lac, cid, self.search_nearby)
+        return best_match
 
     async def resolve_wifi(self, radio, mac, **kwargs) -> Optional[dict]:
         return None