diff --git a/pyModeS/decoder/surv.py b/pyModeS/decoder/surv.py
index 746f47f..abda10d 100644
--- a/pyModeS/decoder/surv.py
+++ b/pyModeS/decoder/surv.py
@@ -1,5 +1,120 @@
 """
 Warpper for short roll call surveillance replies DF=4/5
 
-[To be implemented]
 """
+
+from pyModeS import common
+
+def fs(msg):
+    '''tell the flight status of the aircraft, returns airborne, ground or None'''
+    if common.df(msg) in [4,5]:
+        binaryraw=common.hex2bin(msg)
+        fs=int(binaryraw[5:8],2)
+        if fs in [0,2]:
+            return "airborne"
+        if fs in [1,3]:
+            return "ground"
+        else:
+            return None
+    else:
+        raise RuntimeError("Incorrect or inconsistent message types")
+
+def alert(msg):
+    '''returns True if there is an alert, False otherwise '''
+    #The alert indicates that the TXPPR Mode A code is changed manually by the pilot. Icao Annex10, Vol IV, 3.1.2.6.10.1.1
+    if common.df(msg) in [4,5]:
+        binaryraw = common.hex2bin(msg)
+        fs = int(binaryraw[5:8], 2)
+        if fs in [2,3,4]:
+            return True
+        if fs in [0,1,5]:
+            return False
+        else:
+            return None
+    else:
+        raise RuntimeError("Incorrect or inconsistent message types")
+
+
+def SPI(msg):
+    '''returns True if there there is Special Pulse Indicator, False otherwise '''
+    if common.df(msg) in [4, 5]:
+        binaryraw = common.hex2bin(msg)
+        fs = int(binaryraw[5:8], 2)
+        if fs in [4, 5]:
+            return True
+        if fs in [0, 1, 2, 3]:
+            return False
+        else:
+            return None
+    else:
+        raise RuntimeError("Incorrect or inconsistent message types")
+
+def dr(msg):
+    '''Returns Downlink Request Status.
+     Args:
+        msg (str): 14 hexdigits string,
+    Returns:
+        str: Downlink Request Status
+    '''
+    if common.df(msg) in [4,5]:
+        binaryraw = common.hex2bin(msg)
+        dr = int(binaryraw[8:13], 2)
+        if dr in [2,3,6,7]:
+            return "ACAS"
+        if dr == 0:
+            return "no downlink request"
+        if dr == 1:
+            return "request to send Comm-B message"
+        if dr in 4:
+            return "Comm-B broadcast 1 available"
+        if dr in 5:
+            return "Comm-B broadcast 2 available"
+        if dr > 7 and dr < 16:
+            return None
+        else:
+            return str(15-dr)+" downlink ELM segments available"
+    else:
+        raise RuntimeError("Incorrect or inconsistent message types")
+
+def um(msg):
+    '''returns the Interrogator Identifier and the type of reservation
+    Args:
+        msg (str): 14 hexdigits string,
+    Returns:
+        int: II code of the identifier that triggered the reply
+        str: type of reservation made by the interrogator
+    '''
+    if common.df(msg) in [4,5]:
+        binaryraw = common.hex2bin(msg)
+        iis = int(binaryraw[13:17], 2)
+        ds = int(binaryraw[17:19], 2)
+        if ds == 0:
+            ds=None
+        if ds == 1:
+            ds='Comm-B'
+        if ds == 2:
+            ds='Comm-C'
+        if ds == 3:
+            ds='Comm-D'
+        return iis, ds
+
+    else:
+        raise RuntimeError("Incorrect or inconsistent message types")
+
+def ac(msg):
+    '''return the altitude in ft'''
+    if common.df(msg) == 4:
+        binaryraw = common.hex2bin(msg)
+        altcode = binaryraw[19:32] # Here we could also put the common.altcode(msg).
+        return common.altitude(altcode)
+    else:
+        raise RuntimeError("Incorrect or inconsistent message types")
+
+def id(msg):
+    '''return the squawk code (identifier)'''
+    if common.df(msg) == 5:
+        binaryraw = common.hex2bin(msg)
+        altcode = binaryraw[19:32]  # Here we could also put the common.idcode(msg).
+        return common.squawk(altcode)
+    else:
+        raise RuntimeError("Incorrect or inconsistent message types")