IGS Real Time Working Group Message Protocol

RT-IGS Message Protocol

All real-time IGS messages are transmitted using the User Datagram Protocol (UDP). The UDP protocol was adopted following the IGS "Towards Real-time" workshop and has proven to be a very reliable means of transmitting GNSS data in real-time over the open internet.

The RT-IGS protocol is designed to allow for any message type to be defined both in terms of the message content and the frequency at which the message type is sent out. The following message types have been defined.

Message	Frequency	Comment
Station Description	Twice per hour	IGS station name and link to essential processing information
GNSS Observations	Once per second - recommended	Data pertaining to a GNSS constellation
Ephemeris	On issue by receiver
Meteorological data	At least once per 5-minute

RT-IGS Message Structure

Each RT-IGS message contains a common message header followed by message specific content including the data.

Message Header

Every RT-IGS message has a common 11 byte header. The header contains the following information:

Bytes	Name	Type	Description
2	rec_id	unsigned short	Message Type ID
2	sta_id	unsigned short	Unique RT-IGS number assigned to each station.
4	GPSTime	unsigned long	GPS time message issued, GPS time is seconds from the beginning of GPS (6 Jan. 1980)
2	num_bytes	unsigned short	Total number of bytes in the message including the header
1	IODS	unsigned char	Issue of Data Station. This number will change in order to alert real-time users of a service affecting change at a station.

Message Types

Each RT-IGS message type is assigned an identification number. The following message type IDs have been reserved.

ID Number	Message Type
100	Station
200-219	GPS Observations
220-239	Glonass Observations
240-259	Galileo Observations
300	GPS Ephemeris
400	Surface meteorological data

RT-IGS Message Content

The following message types have been defined and are present in today's RT-IGS data streams.

100: RT-IGS Station Message

Content: Station information that enables real-time users to link data to a specific station configuration. This information is to be valid for the real-time data stream content and may be more current than site log information immediatley following a configuration change at a station.

Bytes	Name	Type	Description
2	rec_id	unsigned short	Station type message issued at least twice per hour
2	sta_id	unsigned short	Unique RT-IGS number assigned to each station.
4	GPSTime	unsigned long	GPS time message issued, GPS time is seconds from the beginning of GPS (6 Jan. 1980)
2	num_bytes	unsigned short	Total number of bytes in the message including the header
1	IODS	unsigned char	Issue of data station. This number will change in order to alert real-time users of a service affecting change at a station.
1	sta_rec_type	unsigned char	Various station data formats are possible: 0 indicates that there is no additional station data
1	IGS_UniqueID[8]	char	IGS ID eg. "NRC1", this variable will use the IGS 4 character standard but c requires a \0 to terminate the string
	*data	unsigned char	Default sta_rec_type 0 does not contain data. The pointer is shown only to illustrate options

200: RT-IGS Observation Message

The GPS Observation Message can be in any format. Message Type 200 consists of GPS observations in the JPL SOC format. SOC is a highly compressed format and was chosen for the prototype network in order to test the advantages of compressed data formats for real-time applications.

SOC Compression

SOC compresses a complete GPS observation to 21 bytes/epoch. Each observation set is fully idependent making it ideally suited for real time data transmission from remote sites that may only have low bandwidth access to the Internet.

Message Type 200 Content:

Bytes	Name	Type	Description
2	rec_id	unsigned short	200 indicates RT-IGS GPS observation. Data is in SOC format.
2	sta_id	unsigned short	Unique RT-IGS number assigned to each station.
4	GPSTime	unsigned long	GPS time message issued, GPS time is seconds from the beginning of GPS (6 Jan. 1980)
2	num_bytes	unsigned short	Total number of bytes in the message including the header
1	IODS	unsigned char	Issue of data station. This number will change in order to alert real-time users of a service affecting change at a station.
1	num_obs	unsigned char	Number of GPS Observations in the data block
21/sat	*data	unsigned char	The data starts here. The pointer is not in the data stream.

300: RT-IGS Ephemeris Message

Content: SV Broadcast Ephemeris (Subframes 1-3, parity removed)

Bytes	Name	Type	Description
2	rec_id	unsigned short	300 indicates RT-IGS ephemeris message.
2	sta_id	unsigned short	Unique RT-IGS number assigned to each station.
4	CollectedGPSTime	unsigned long	Time ephemeris was received at a station. GPS time is seconds from the beginning of GPS (6 Jan. 1980)
2	num_bytes	unsigned short	Total number of bytes in the message, including the header, but not the size of the data pointer
1	IODS	unsigned char	Issue of data station. This number will change in order to alert real-time users of a service affecting change at a station.
1	prn	unsigned char	PRN number for the data in the message.
72	*data	unsigned char	The data starts here. The ephemeris data format consists of the 3 broadcast sub frames with the parity bits removed so 3 subframes = 72 bytes per eph.

400: RT-IGS Meteorological Message

Content: Station Meteorological Observations

Bytes	Name	Type	Description
2	rec_id	unsigned short	400 indicates RT-IGS meteorological message.
2	sta_id	unsigned short	Unique number assigned to each station.
4	GPSTime	unsigned long	GPS time message issued, GPS time is seconds from the beginning of GPS (6 Jan. 1980)
2	num_bytes	unsigned short	Total number of bytes in the message, including the header, but not the size of the data pointer
1	IODS	unsigned char	A flag indicating the current station configuration that derived the met. observations, this flag will change every time the station configuration changes
1	Numobs	unsigned char	If only temperature, pressure, and relative humidity then: 3
	*mets	long	Temp. (deg C), press (mb), rel. hum. (%), zenith wet (m), zenith dry (m), zenith total (m), and each scaled by 1000: so 1000.123mb = 1000123. If the zenith observations are not present only enter 3 for the number of observations.