diff --git a/README.md b/README.md
index ad5a219..193ce9c 100644
--- a/README.md
+++ b/README.md
@@ -58,11 +58,12 @@ For best performance you should use [TimescaleDB](https://www.timescale.com), wh
     
 8. Get world elevation data (needed for AGL calculation)
 	Sources: There are many sources for DEM data. It is important that the spatial reference system (SRID) is the same as the database which is 4326.
-	The [GMTED2010 Viewer](https://topotools.cr.usgs.gov/gmted_viewer/viewer.htm) provides data for the world. Just download the data you need.
+	The [GMTED2010 Viewer](https://topotools.cr.usgs.gov/gmted_viewer/viewer.htm) provides data for the world with SRID 4326. Just download the data you need.
+	
 	For Europe we can get the DEM as GeoTIFF files from the [European Environment Agency](https://land.copernicus.eu/imagery-in-situ/eu-dem/eu-dem-v1.1).
     Because the SRID of these files is 3035 and we want 4326 we have to convert them (next step)
     
-9. Convert the elevation data into correct SRID
+9. Optional: Convert the elevation data into correct SRID
 
 	We convert elevation from one SRID (here: 3035) to target SRID (4326):
     
diff --git a/ogn_python/collect/celery.py b/ogn_python/collect/celery.py
index 2a14eb7..516af20 100644
--- a/ogn_python/collect/celery.py
+++ b/ogn_python/collect/celery.py
@@ -18,12 +18,11 @@ logger = get_task_logger(__name__)
 
 
 @celery.task(name='update_takeoff_landings')
-def update_takeoff_landings():
+def update_takeoff_landings(delta):
     """Compute takeoffs and landings."""
 
-    now = datetime.datetime.now()
-    yesterday = now - datetime.timedelta(days=1)
-    takeoff_update_entries(session=db.session, start=yesterday, end=now, logger=logger)
+    now = datetime.datetime.utcnow()
+    takeoff_update_entries(session=db.session, start=now-delta, end=now, logger=logger)
 
 
 @celery.task(name='update_logbook_entries')
@@ -53,3 +52,18 @@ def update_receivers_country_code():
     """Update country code in receivers table if None."""
 
     receivers_update_country_code(session=db.session, logger=logger)
+
+
+@celery.task(name='purge_old_data')
+def purge_old_data(age):
+    """Delete AircraftBeacons and ReceiverBeacons older than given 'age'."""
+
+    from ogn_python.model import AircraftBeacon, ReceiverBeacon
+    now = datetime.datetime.utcnow()
+    db.session(AircraftBeacon) \
+        .filter(AircraftBeacon.timestamp < now - age) \
+        .delete()
+
+    db.session(ReceiverBeacon) \
+        .filter(ReceiverBeacon.timestamp < now - age) \
+        .delete()
diff --git a/ogn_python/config/default.py b/ogn_python/config/default.py
index facf942..150d624 100644
--- a/ogn_python/config/default.py
+++ b/ogn_python/config/default.py
@@ -11,7 +11,7 @@ from datetime import timedelta
 CELERYBEAT_SCHEDULE = {
     'update-ddb': {
         'task': 'import_ddb',
-        'schedule': timedelta(minutes=1),
+        'schedule': timedelta(hours=1),
     },
     'update-country-codes': {
         'task': 'update_receivers_country_code',
@@ -20,6 +20,7 @@ CELERYBEAT_SCHEDULE = {
     'update-takeoff-and-landing': {
         'task': 'update_takeoff_landings',
         'schedule': timedelta(minutes=1),
+        'kwargs': {'delta': timedelta(minutes=10)}
     },
     'update-logbook': {
         'task': 'update_logbook_entries',
@@ -29,4 +30,9 @@ CELERYBEAT_SCHEDULE = {
         'task': 'update_logbook_max_altitude',
         'schedule': timedelta(minutes=1),
     },
+    #'purge_old_data': {
+    #    'task': 'purge_old_data',
+    #    'schedule': timedelta(minutes=10),
+    #    'kwargs': {'age': timedelta(days=2)}
+    #},
 }