RTCM

2.1 About RTCM

RTCM is stand for Radio Technical Commission for Maritime Services. RTCM data format that related to

GPS corrections is called RTCM SC-104 data format. It is a standard data format for differential

pseudorange corrections took hold fairly quickly and is currently in wide use. Users of Real-Time-

Kinematic (RTK) carrier phase positioning systems also have recognized the usefulness of such systems.

But because real-time carrier phase positioning systems were developed later than pseudorange

systems, carrier phase infrastructure systems are only now coming into widespread use. In addition,

various special committees set the standards for International maritime radio navigation and radio

communication especially in GPS application and Electronic Chart.

Figure 1

The correction of RTCM that being used in DGPS system especially in Kau Yi Cau DGPS services.

Radio transmission

The corrections are formatted to RTCM 2.0 and broadcast via Radio antenna

The correction is formatted to RTCM 2.0 and broadcasted through radio antenna.

Figure 2

2.2 RTCM SC-104

RTCM SC-104 also known as RTCM Special Committee (SC) 104 is the one that concerned on GPS

correction signal. It is a compact binary data that is good for real time processing. It is also a

transmission of GPS correction from GPS reference station to GPS Rover. Mostly it is used for DGPS/RTK

surveys. There are several version of RTCM SC-104. The versions are:

RTCM-2.0 (Code correction ->DGPS)

RTCM-2.1 (Code + Phase correction -> RTK)

RTCM 2.2 ( …+ GLONASS )

RTCM 2.3 ( ….+ GPS Antenna Definition)

RTCM 3.0 (….+ Network RTK & GNSS)

The RTCM SC-104 has defined a tentative set of messages for carrier phase positioning systems in

(Version 2.1), and is now working on a revised version of these messages in RTCM SC-104

recommended standards Version 2.2. Two pairs of RTK messages are defined in the RTCM SC-104

standard, with the idea (but not the requirement) that each pair of messages would be used together.

The message types 18 and 19 contain raw carrier phase and pseudorange measurement information.

Message types 20 and 21 contain information based on the same measurements, but formatted as

corrections to the carrier phase and pseudorange measurements, in a manner similar to the RTCM Type

1 pseudorange correction message.

RTCM SC-104 V.2.0

2.3 RTCM Data Structure

Each data record contains several message types for various contents.

Message 1, Message2…. Message N

Each message comprises header and body

Header: message type, time, length of message,

Body: data for every data type

2.4 RTCM Message Type

RTCM Message Type Description

1 Differential GPS Correction

2 Delta Differential GPS Correction

3 GPS Reference Station Parameters

4 Reference Station Datum

5 GPS Constellation Health

6 GPS Null Name

7 DGPS Beacon Almanac

8 Pseudolite Almanac

9 GPS Partial Correction Set

10 P-Code Differential Correction

11 C/A Code, L1, L2 Delta Correction

12 Pseudolite Station Parameter

13 Ground Transmitter Parameter

14 GPS Time of Week

15 Ionospheric Delay Message

16 GPS Special Message

17 GPS Ephemerides

18 RTK Uncorrected Carrier Phases

19 RTK Uncorrected Pseudorange

20 RTK Carrier Phase Corrections

21 RTK Pseudorange Corrections

22 Extended Reference Station Parameters

23 Antenna Type Definition

24 Reference Station: Antenna Reference Point (ARP) Parameter

25,26 Undefined

27 Extended GPS Radio Beacon Almanac

28..30 Undefined

31 Differential GLONASS Corrections

32 Differential GLONASS Reference Station

33 GLONASS Constellation Health

34 GLONASS Partial Differential Correction Set

GLONASS Null Name (N<=1)

35 GLONASS Radio Beacon Almanac

36 GLONASS Special Message

37 GNSS System Time Offset

38..58 Undefined

59 Proprietary Message

60..63 Multipurpose Usage

2.5 RTCM Data Transmission (Real Time)

Global System for Mobile Communication (GSM)

GSM modem and SIM Card with the current services

Charge with the continuous time.

General Packet Radio Service (GPRS)

An upgrade to GSM

Connect to the mobility network to the IP network

Charge with the total data download but not in continuous time

Cheaper than GSM

Latency of DGPS and RTK correction

Figure 3

2.5 RTCM Version 3.0

It is a new data structure developed for network RTK applications. It consist new signals which are L2C

and L5. Besides that, it is also one of the GNSS system data that has a compact data structure.

RTCM 2.0 – 0.5 kbit / sec. (DGPS)

RTCM 2.1-2.3 requires 5 kbit / sec (DGPS+RTK)

RTCM 3.0.requires 2 kbit / sec

The four groups of RTCM 3.0 GNSS RTK messages (RTCM 2004) are:

Group Name Message Type Message Description

Observations 1001 L1-only GPS RTK Observables

1002 Extended L1-only GPS RTK Observables

1003 L1 & L2 GPS RTK Observables

1004 Extended L1 & L2 GPS RTK Observables

1005 L1 only GLONASS RTK Observables

1006 Extended L1-only GLONASS RTK Observables

1007 L1 & L2 GLONASS RTK Observables

1008 Extended L1 & L2 GPS RTK Observables

Station Coordinates 1009 Stationary RTK Reference Station ARP

1010 Stationary RTK Reference ARP with Antenna

Height

Antenna Description 1011 Antenna Descriptor

1012 Antenna Descriptor & Serial Number

Auxiliary Operation Information 1013 System Parameters

2.6 NovAtel RTK Rover Software

Figure 4

Basic assumptions:

Base station position is transmitted. Measurement quality similar to NovAtel narrow correlation receivers. Full wavelength L2 carrier phase. Any unclear points in the Version 2.1 RTCM standard will follow the clarification in RTCM SC-104

Version 2.2

Phase I (base and rover software):

RTCM SC-104 Version 2.1 type 20/21s used as a pair, with L1 pseudorange and carrier phase and L2 carrier phase, all transmitted at the same rate. This is the set of messages used by the German AdV system. Phase II(base and rover software): Phase I, plus

RTCM SC-104 Version 2.2 type 18/19s, used as a pair. L1- only or L1/L2 for type 18/19s Processing of the messages at the rover occurring as often as pseudoranges are transmitted. Base station co-ordinate accuracy extended with proposed new message type 22.

Phase III: Phase II, plus

The basic carrier phase positioning portion of the rover RTK system will run whenever a carrier phase message is received (without requiring an accompanying pseudorange message).

Phase I has been implemented at this point, and Phase II is in the final stages of implementation and test. Phase III will require some changes to the existing RTK positioning software, and will be implemented at a later point in time bases the estimated clock offset of the base station receiver, in seconds. It is defined here as the amount by which the local clock is ahead of GPS time. c is the speed of light in meters per second. λ is the appropriate wavelength for L1 or L2 in meters. φ is the carrier phase measurement in cycles, defined to have a sign which increases as the range to the satellite increases. A is an arbitrary constant integer number of cycles chosen at the start of tracking to keep the size of the correction small. ρ is the pseudorange measurement in meters. corrφ and corrρ are the corrections transmitted in the type 20 and 21 messages, respectively. The corrections can be applied directly to the measurements taken at the rover, rather than doing a difference between the base station and rover measurements.