diff --git a/Dockerfile b/Dockerfile
index 9d1d3cd..cfafa5a 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -19,7 +19,7 @@ RUN set -x && \
     "${KEPT_PACKAGES[@]}" \
     "${TEMP_PACKAGES[@]}" \
     && \
-    pip install --break-system-packages icao_nnumber_converter_us && \
+    pip install --break-system-packages icao_nnumber_converter_us haversine && \
     curl --location --output /tmp/BaseStation.zip https://github.com/rikgale/VRSData/raw/main/BaseStation.zip && \
     mkdir -p /opt/basestation && \
     unzip /tmp/BaseStation.zip -d /opt/basestation/ && \
diff --git a/README.md b/README.md
index ae2fdb5..d21d125 100644
--- a/README.md
+++ b/README.md
@@ -12,9 +12,13 @@ Under active development, everything is subject to change without notice.
 
 | Variable | Description | Default |
 |----------|-------------|---------|
-| `JSON_IN` | Semi-colon separated list of `host:port` entries to connect to for JSON ingest. | acars_router:15550 |
-| `SBS_OUT` | Semi-colon separated list of `host:port` entries to connect to for SBS/Basestation output. | ultrafeeder:12000 |
-| `LOG_FILE` | Set to any value to message text, type, SBS output, and adsbexchange link to files in `/log`. | Unset |
+| `JSON_IN`   | Semi-colon separated list of `host:port` entries to connect to for JSON ingest. | acars_router:15550 |
+| `SBS_OUT`   | Semi-colon separated list of `host:port` entries to connect to for SBS/Basestation output. | ultrafeeder:12000 |
+| `LOG_FILE`  | Set to any value to message text, type, SBS output, and adsbexchange link to files in `/log`. | Unset |
+| `LAT`       | Latitude of receiver. Only required if `MAX_DISTANCE` > 0 | Unset |
+| `LON`       | Longitude of receiver. Only required if `MAX_DISTANCE` > 0 | Unset |
+| `MAX_DIST`  | Set this to a nonzero value to reject parsed positions that are too far away. Only applies to positions parsed from message text. | `0` |
+| `DIST_UNIT` | The unit of the value in `MAX_DIST`. One of `km`, `m`, `mi`, `nmi`, `ft`, `in`. | `nmi` |
 
 ## Docker Compose
 
diff --git a/rootfs/scripts/acars_decode/Decoder.py b/rootfs/scripts/acars_decode/Decoder.py
index 53e0bc0..0ccca42 100644
--- a/rootfs/scripts/acars_decode/Decoder.py
+++ b/rootfs/scripts/acars_decode/Decoder.py
@@ -1,5 +1,7 @@
 from re import compile, sub
 from colorama import Fore
+from haversine import haversine as gcdist
+from os import getenv
 
 
 dlat = r"(?P<dlat>[NS])"
@@ -110,6 +112,16 @@ def d4(n):
                 compile(dlat + sd3("latdeg") + sd2("latmin") + d1("latmin10") + dlon + sd3("londeg") + sd2("lonmin") + d1("lonmin10"))],
          }
 
+_homelat = float(getenv("LAT"))
+_homelon = float(getenv("LON"))
+_maxdist = float(getenv("MAX_DIST", 0))
+_distunit = getenv("DIST_UNIT", "nmi")
+def checkpos(lat, lon):
+  if _homelat and _homelon and _maxdist:
+    return _maxdist > gcdist((_homelat, _homelon), (lat, lon), unit=_distunit)
+  else:
+    return abs(lat) <= 90 and abs(lon) <= 180
+
 def decode(msg):
   if msg.get("vdl2"):
     dat = decodeVDLM2(msg["vdl2"])
@@ -118,8 +130,8 @@ def decode(msg):
   else:
     dat = decodeACARS(msg)
 
-  if not dat:
-    return None
+  if not dat or not dat.get("txt") or dat.get("lat"):
+    return dat
 
   dat["txt"] = dat.get("txt", "").upper().replace("\r", "").replace("\n", "")
 
@@ -153,8 +165,13 @@ def decode(msg):
         dat["lon"] += float(raw.get("lonsec", 0))/3600
         dat["lon"] *= -1 if raw.get("dlon") == "W" or raw.get("dlon") == "-" else 1
 
-      if dat.get("lat") and abs(dat["lat"]) <= 90 and abs(dat["lon"]) <= 180 :
-        return dat
+      if dat.get("lat"):
+        if checkpos(dat["lat"], dat["lon"]):
+          return dat
+        else:
+          print(Fore.RED + "failed distance check" + Fore.RESET)
+          del dat["lat"]
+          del dat["lon"]
 
   for k in msgrgx.keys():
     for rgx in msgrgx[k]:
@@ -185,8 +202,13 @@ def decode(msg):
         dat["lon"] += float(raw.get("lonsec", 0))/3600
         dat["lon"] *= -1 if raw.get("dlon") == "W" or raw.get("dlon") == "-" else 1
 
-      if dat.get("lat") and abs(dat["lat"]) <= 90 and abs(dat["lon"]) <= 180 :
-        return dat
+      if dat.get("lat"):
+        if checkpos(dat["lat"], dat["lon"]):
+          return dat
+        else:
+          print(Fore.RED + "failed distance check" + Fore.RESET)
+          del dat["lat"]
+          del dat["lon"]
 
   for i,rgx in enumerate(rgxs):
     raw = rgx.findall(dat["txt"])
@@ -211,8 +233,13 @@ def decode(msg):
         dat["lat"] = (int(raw[1]) + float(raw[2])/60) * (-1 if raw[3] == "S" else 1)
         dat["lon"] = (int(raw[4]) + float(raw[5])/60) * (-1 if raw[6] == "W" else 1)
 
-    if dat.get("lat") and abs(dat["lat"]) <= 90 and abs(dat["lon"]) <= 180 :
-      return dat
+    if dat.get("lat"):
+      if checkpos(dat["lat"], dat["lon"]):
+        return dat
+      else:
+        print(Fore.RED + "failed distance check" + Fore.RESET)
+        del dat["lat"]
+        del dat["lon"]
 
   return dat