commit 8d89cf024bc93f22c8cfb2513d0ec4b176ab6de2
Author: David Honess <david.honess+github@gmail.com>
Date:   Fri Jan 25 09:14:59 2019 +0100

    Python script for Doppler correction

diff --git a/doppler.py b/doppler.py
new file mode 100644
index 0000000..6202e37
--- /dev/null
+++ b/doppler.py
@@ -0,0 +1,128 @@
+#!/usr/bin/python3
+import urllib.request
+import math
+import socket
+import ephem
+import time
+import sys
+
+
+C = 300000000.0
+F0 = 145800000.0
+
+LATITUDE = "52.219308"
+LONGITUDE = "4.419926"
+ALTITUDE = 20
+
+
+class tle_reader(object):
+    """
+    For keeping ephem two line element sets up to date
+    """
+    def __init__(self,
+                 tle_name="ISS (ZARYA)",
+                 tle_file="https://celestrak.com/NORAD/elements/stations.txt",
+                 tle_max_age=3600):
+        self._tle_name = tle_name
+        self._tle_file = tle_file
+        self._tle_max_age = tle_max_age
+        self._tle = None
+        self.reload()
+
+    def build_index(self, tle_lines):
+        index = {}
+        for i in range(0, len(tle_lines), 3):
+            index[tle_lines[i].strip()] = (tle_lines[i + 1], tle_lines[i + 2])
+        return index
+
+    def reload(self):
+        print("Loading: %s" % self._tle_file)
+
+        try:
+            with urllib.request.urlopen(self._tle_file) as response:
+                tle_lines = response.read().decode("utf-8").splitlines()
+            index = self.build_index(tle_lines)
+            tle_data = index[self._tle_name]
+            self._tle = ephem.readtle(self._tle_name, tle_data[0], tle_data[1])
+        except Exception as e:
+            print(e)
+
+        self._tle_age = time.time()
+
+    @property
+    def tle(self):
+        return self._tle
+
+    @property
+    def tle_expired(self):
+        return time.time() - self._tle_age > self._tle_max_age
+
+
+class rtl_fm_remote(object):
+    """
+    For remote control of rtl_fm command line program
+    """
+    def __init__(self,
+                 host="localhost",
+                 port=6020):
+        self._host = host
+        self._port = port
+        self._s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+        self._s.connect((self._host, self._port))
+
+    def set_freq(self, freq):
+        self.send_cmd(0, freq)
+
+    def set_mode(self, mode):
+        self.send_cmd(1, mode)
+
+    def set_squelch(self, squelch):
+        self.send_cmd(2, squelch)
+
+    def set_gain(self, gain):
+        self.send_cmd(3, -100 if gain == "auto" else gain)
+
+    def send_cmd(self, cmd, param):
+        cmd_bytes = (cmd).to_bytes(1, "little")
+        param_bytes = (param).to_bytes(32, "little")
+        self._s.send(cmd_bytes + param_bytes)
+
+    def __del__(self):
+        self._s.close()
+
+
+rtl = rtl_fm_remote()
+iss = tle_reader(tle_name="ISS (ZARYA)", tle_max_age=5520)  # 92 minutes
+
+if iss.tle is None:
+    sys.exit(0)
+
+myloc = ephem.Observer()
+myloc.lon = LONGITUDE
+myloc.lat = LATITUDE
+myloc.elevation = ALTITUDE
+
+freq = F0
+running = True
+
+try:
+    while running:
+        myloc.date = time.strftime('%Y/%m/%d %H:%M:%S', time.gmtime())
+        iss.tle.compute(myloc)
+        alt = math.degrees(iss.tle.alt)
+
+        if alt > 6:  # iss is flying over our location
+            new_freq = int(F0 - iss.tle.range_velocity * F0 / C)  # doppler
+            if new_freq != freq:
+                print(new_freq, round(alt, 2), myloc.date)
+                rtl.set_freq(new_freq)  # set new frequency in rtl_fm
+            freq = new_freq
+        elif iss.tle_expired:
+            iss.reload()  # we could be running for days / weeks
+        else:
+            time.sleep(10)  # do nothing, wait for iss to arrive
+            freq = F0
+except KeyboardInterrupt:
+    running = False
+
+print("Bye")