Decode latitude/longitude data from CPR-encoded hex strings

decode.py

import socket
import math

def nround(n):
    if n % 0.5 == 0:
        return math.ceil(n)
    else:
        return round(n)

def hex2bin(hex_str):
    bin_str = bin(int(hex_str, 16))[2:]
    return(bin_str.zfill(math.ceil(len(bin_str)/2)*2))

def NL(lat):
    # this is a mess, dammit faa
    return(nround(2*math.pi*(math.acos(1 - (1-math.cos(math.pi/(2*15)))/(math.cos((math.pi*abs(lat))/180))))**(-1)))

def extract_latlon_b(bin_str):
    latlon = {
        "lat": None,
        "lon": None
    }
    
    latlon["lat"] = bin_str[22:39]
    latlon["lon"] = bin_str[39:56]
    
    return(latlon)

def calc_latlon(cpr1, cpr2):
    frame_1 = cpr1
    frame_2 = cpr2

    frame_1_latlon_b = extract_latlon_b(hex2bin(frame_1))
    frame_2_latlon_b = extract_latlon_b(hex2bin(frame_2))

    lat0 = int(frame_1_latlon_b["lat"], 2)
    lat1 = int(frame_2_latlon_b["lat"], 2)
    lon0 = int(frame_1_latlon_b["lon"], 2)
    lon1 = int(frame_2_latlon_b["lon"], 2)

    j = int(((59 * lat0 - 60 * lat1) / 131072) + 0.5)

    rlat0 = 6 * (j % 60 + lat0 / 131072)
    rlat1 = (360 / 59) * (j % 59 + lat1 / 131072)

    nl0 = NL(rlat0)
    nl1 = NL(rlat1)

    ni = max(1, nl1-1)

    dlon1 = 360 / ni

    M = nround((((lon0 * (nl1 - 1)) - (lon1 * nl1)) / 131072) + 0.5)

    lon = dlon1 * (M % ni + lon1 / 131072)

    return([rlat1, lon])

def analyse_stream(stream):
    for frame in stream:
        frame = frame[1:]
        first_byte = frame[0:2]

        if len(frame) == 28 and hex2bin(first_byte)[0:5] == "10001": # check that it has correct length and downlink format (DF) = 17
            icao = frame[2:8]

            if not icao in craft_frames:
                craft_frames[icao] = [None] * 2 # empty list of length 2

            # substring the useful part
            cpr_data = frame[8:-6]

            # extract type code (TC)
            tc_hex = cpr_data[0:2]
            tc_bin = hex2bin(tc_hex)[0:5]

            if tc_bin == "01011": # check that TC is 11
                # extract flag
                flag = int(hex2bin(cpr_data)[21])
                craft_frames[icao][flag] = cpr_data

    for icao in craft_frames:
        if craft_frames[icao][0] and craft_frames[icao][1]:
            print(icao + ": " + str(calc_latlon(craft_frames[icao][0], craft_frames[icao][1])))

craft_frames = {}

ip = "INSERT_IP_HERE"
port = INSERT_PORT_HERE

client_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
client_socket.connect((ip, port))

while True:
    data = client_socket.recv(512).decode("utf-8")
    analyse_stream(data.split(";\r\n"))