Hello,
To determine if the coordinate is negative, convert it to binary format and check the very first bit. If it is 0, coordinate is positive, if it is 1, coordinate is negative.
Example:
Received value: 20 9C CA 80 converted to BIN: 00100000 10011100 11001010 10000000 first bit is 0, which means coordinate is positive converted to DEC: 547146368. For more information see two‘s complement arithmetic.
3F 3F 78 63 gives 00111111 00111111 01111000 01111000 which is 1061124195 in decimal.