update README for filters

README.md

@@ -145,10 +145,27 @@ heavies, _ := c.Aircraft(ctx,
 )
 ```
 
-Available filters include `WithPosition`, `WithHex`, `WithCallsign`,
+| Filter Name | Description | Sample Usage |
-`WithSquawk`, `WithCategory`, `WithType`, `InEmergency`, `IsMLAT`,
+|---|---|---|
-`MinAltitude`, `MaxAltitude`, `OnGround`, `WithinNM`, `SeenWithin`, `MinRSSI`,
+| `WithPosition` | Keeps only aircraft that currently report a lat/lon. | `readsb.WithPosition()` |
-plus the `Not` and `Any` combinators.
+| `WithHex` | Keeps aircraft whose ICAO address matches any of the given values. | `readsb.WithHex("A1B2C3", "D4E5F6")` |
+| `WithCallsign` | Keeps aircraft whose (trimmed) callsign matches any value. | `readsb.WithCallsign("UAL2350")` |
+| `WithSquawk` | Keeps aircraft transmitting any of the given Mode A squawk codes. | `readsb.WithSquawk("7700")` |
+| `WithCategory` | Keeps aircraft of any of the given emitter categories. | `readsb.WithCategory(readsb.CatLarge)` |
+| `WithType` | Keeps aircraft of any of the given source types. | `readsb.WithType(readsb.TypeMLAT)` |
+| `InEmergency` | Keeps only aircraft squawking a non-routine emergency/priority code. | `readsb.InEmergency()` |
+| `IsMLAT` | Keeps only aircraft whose position was derived by multilateration. | `readsb.IsMLAT()` |
+| `MinAltitude` | Keeps airborne aircraft at or above the given barometric altitude (feet). | `readsb.MinAltitude(30000)` |
+| `MaxAltitude` | Keeps aircraft at or below the given barometric altitude (feet); on-ground aircraft always pass. | `readsb.MaxAltitude(10000)` |
+| `OnGround` | Keeps only aircraft reporting an on-ground barometric altitude. | `readsb.OnGround()` |
+| `MinSpeed` | Keeps aircraft at or above knots, measured by src (GroundSpeed or TrueAirspeed). | `readsb.MinSpeed(200, readsb.GroundSpeed)` |
+| `MaxSpeed` | Keeps aircraft with a positive reading at or below knots, measured by src; aircraft with no reading are dropped. | `readsb.MaxSpeed(500, readsb.TrueAirspeed)` |
+| `WithinNM` | Keeps aircraft within the given range (nautical miles) of the receiver. | `readsb.WithinNM(30)` |
+| `WithinNMOf` | Keeps aircraft whose current position is within nm nautical miles of the given point (decimal degrees). | `readsb.WithinNMOf(40.7, -74.0, 50)` |
+| `SeenWithin` | Keeps aircraft heard from within the given duration. | `readsb.SeenWithin(5*time.Minute)` |
+| `MinRSSI` | Keeps aircraft whose average signal strength is at or above dbfs (e.g. -24). | `readsb.MinRSSI(-24)` |
+| `Not` | Inverts a filter. | `readsb.Not(readsb.OnGround())` |
+| `Any` | Keeps aircraft matching at least one of the supplied filters (logical OR). | `readsb.Any(f1, f2)` |
 
 ### Streaming