diff --git a/pyModeS/decoder/acas.py b/pyModeS/decoder/acas.py
index 47a2dd8..ace13bd 100644
--- a/pyModeS/decoder/acas.py
+++ b/pyModeS/decoder/acas.py
@@ -2,7 +2,9 @@
 Decoding Air-Air Surveillance (ACAS) DF=0/16
 """
 
-from pyModeS import common
+from __future__ import annotations
+
+from .. import common
 import warnings
 
 warnings.simplefilter("always", UserWarning)
@@ -23,7 +25,7 @@ def isACAS(msg: str) -> bool:
         return False
 
 
-def rac(msg: str) -> str:
+def rac(msg: str) -> None | str:
     """Resolution Advisory Complement.
 
     :param msg: 28 hexdigits string
@@ -52,7 +54,7 @@ def rac(msg: str) -> str:
     return "; ".join(RAC)
 
 
-def rat(msg: str) -> bool:
+def rat(msg: str) -> None | int:
     """RA terminated indicator
 
     Mode S transponder is still required to report RA 18 seconds after
@@ -70,7 +72,7 @@ def rat(msg: str) -> bool:
     return mte
 
 
-def mte(msg: str) -> bool:
+def mte(msg: str) -> None | int:
     """Multiple threat encounter.
 
     :param msg: 28 hexdigits string
@@ -85,7 +87,7 @@ def mte(msg: str) -> bool:
     return mte
 
 
-def ara(msg: str) -> str:
+def ara(msg: str) -> None | str:
     """Decode active resolution advisory.
 
     :param msg: 28 bytes hexadecimal message string
diff --git a/pyModeS/decoder/bds/bds61_st2.py b/pyModeS/decoder/bds/bds61_st2.py
index 2726873..90bc061 100644
--- a/pyModeS/decoder/bds/bds61_st2.py
+++ b/pyModeS/decoder/bds/bds61_st2.py
@@ -5,10 +5,10 @@
 #   (Subtype 1)
 # ------------------------------------------
 
-from typing import Tuple
-from pyModeS import common
+from __future__ import annotations
 
-from pyModeS.decoder import acas
+from ... import common
+from .. import acas
 
 
 def threat_type(msg: str) -> int:
@@ -40,7 +40,7 @@ def threat_identity(msg: str) -> str:
         raise RuntimeError("%s: Missing threat identity (ICAO)")
 
 
-def threat_location(msg: str) -> Tuple:
+def threat_location(msg: str) -> None | tuple[int, int, int]:
     """Get the altitude, range, and bearing of the threat.
 
     Altitude is the Mode C altitude
@@ -58,6 +58,8 @@ def threat_location(msg: str) -> Tuple:
         bearing = common.bin2int(mb[51:57])
         return altitude, distance, bearing
 
+    return None
+
 
 def has_multiple_threats(msg: str) -> bool:
     """Indicate if the ACAS is processing multiple threats simultaneously.
@@ -68,7 +70,7 @@ def has_multiple_threats(msg: str) -> bool:
     return acas.mte(msg) == 1
 
 
-def active_resolution_advisories(msg: str) -> str:
+def active_resolution_advisories(msg: str) -> None | str:
     """Decode active resolution advisory.
 
     Uses ARA decoding function from ACAS module.
@@ -91,7 +93,7 @@ def is_ra_terminated(msg: str) -> bool:
     return acas.rat(msg) == 1
 
 
-def ra_complement(msg: str) -> str:
+def ra_complement(msg: str) -> None | str:
     """Resolution Advisory Complement.
 
     :param msg: 28 hexdigits string