See More of NMEA data

The data captured by the GPS receiver is NMEA data. Now we will try to see more familiar with NMEA data. NMEA (National Marine Electronics Association) standard was developed specifically for the industry as an inter-face of a variety of marine equipment introduced since 1983. The standard is awarded to marine equipment that sends information to a computer or other device. Examples of equipment issued is the GPS NMEA data.

NMEA-0183 contains information relating to the geography as about time, longitude, latitude, altitude, speed and many more. To display the information more understandable by users NMEA-0183 data needs to be further processed.
 
NMEA-0183 standard uses a simple ASCII format, each line defines the contents of each type of message that can be sorted out. Five of the first character after the $ sign is called address field. The first two characters in the address referred Talker-ID. After Talker-ID followed behind the 3 characters that describes the type of sentence. While each data separated by a comma, if there are empty fields will not fill in anything between two commas and terminated by a Carriage Return + Line Feed (CR + LF). NMEA-0183 has a variety of sentence types, one of which is the RMC ( Recommended Minimum Navigation Information). Examples of types of RMC NMEA data is as follows:

Name	Sample	Unit	Information
Message ID	$GPRMC		RMC protokol header
UTC Position	065102		hhmmss.ss
Status	A		A=data valid or V=data tidak valid
Latitude	0745.6301		ddmm.mmmm
N/S Indicator	S		N=north or S=south
Longitude	11024.5308		dddmm.mmmm
E/W Indicator	E
Speed Over Ground	000.0	knots
Course Over Ground	066.2	degrees	E=east or W=west
Date	030306		ddmmyy
Magnetic Variation	E	degrees	E=east or W=west
Checksum	*65
CR LF			End of message termination

  more info can be found here

Various kinds of GPS Satellite Signal

There are 3 kinds of signals which is owned by GPS :
Carriers
GPS satellites send signals on two frequencies. L1 1575.42 MHz by bringing the two status messages and pseudo-random code for the purpose of calculating the time. 1227.60 MHz L2 brings with menggunakaan precession more accurate due to military purposes. Power radio signals emitted only ranged from 20-50 Watts. This is classified as very low considering the distance between the GPS and satellite to 12,000 miles. The signal emitted by line of sight (LOS), can pass through clouds, glass but can not be solid objects like buildings, mountains.

Pseudo-Random Codes
GPS is used for the public to monitor the frequency of L1 on the UHF (Ultra High Frequency) 1575.42 MHz. L1 signal that is sent will have the patterns of a particular digital code known as pseudorandom. Transmit signal consists of two parts: the code Protected (P) and the Coarse / Acquisition (C / A). Code that was sent was also unique among the satellites, allowing each receiver to distinguish signals sent by one satellite with another satellite. Some code Protected (P) also exists a randomized, so as not to be accepted by ordinary GPS. The random signal is known as the Anti-spoofing, which is commonly used by GPS specifically for certain purposes such as military.

Navigation Message
There is a weak power frequency signal added to the L1 codes that gives information about satellite orbits, clock corectionnya and other system status.