MyRTKnet: Get set and go!

From 1997 to 2000, JUPEM has set up a permanent GPS network known
as Malaysia Active GPS System (MASS) which consists of eighteen (18) stations located at a spacing of 200 – 300 km from each other. Each station of the network is equipped with a high precision dual frequency GPS receiver that is operational 24 hours daily. The acquired GPS data is transferred on a daily basis to the Central Processing Centre at JUPEM’s Headquarters in Kuala Lumpur via modem/internet. This permanent GPS stations network is an integrated geodynamic network and the zero order network for the realization of the National Coordinate Reference Frame, which is known as the Geocentric Datum of Malaysia 2000 (GDM2000).

Presently, MASS provides an excellent frame of reference for geodetic positioning in Malaysia, and several kinematic applications have been carried out by post-processing thedata. However, the possibility of having realtime access to the data observed by the GPS network would dramatically and clearly increase its synergy with land surveying activities. For instance, it would allow the use of GPS for centimeter positioning over the entire territory of Malaysia through the broadcasting of RTK corrections.

Existing Scenario

Real-Time Kinematic (RTK) is now widely used for surveying and other precise positioning applications. The classical RTK technique requires that GPS data be transmitted from a single base receiver to one or more roving units.

RTK can provide centimeter position accuracy, but the accuracy and reliability of the standard RTK solution decreases with increasing distance from the Base Station. This limitation on the distance between the roving GPS receiver and the RTK Base Station is due to the systematic effects of ephemeris, tropospheric and ionospheric errors. These systematic errors result in reduced accuracy and increasing initialization time as the distance between base and rover increases.

Besides the aforementioned constraint, the limitation is also due to the range of available radio telemetry solutions. In practice, this means that a temporary RTK Base Station must be established close to the work area, often at a location that does not provide any physical security or continuous power supply. Each time such a temporary reference station is established, there is a likelihood of introducing an error in the reference station coordinates that will be transferred into the position calculated by the rover RTK receiver. Such an error can easily go undetectedwhen using a single base station.

Objectives

In order to take full advantage of the real-time capabilities of the RTK network, MyRTKnet has been designed with the following objectives:

i. To establish a network of permanently running GPS base stations, at a spacing of 30 to 150 km, feeding GPS data to a processing centre via frame-relay IPVPN communication network.

ii. To establish a central facility that will model the spatial errors which limit GPS accuracy through a network solution and generate corrections for roving receivers positioned anywhere inside the network with an accuracy better than a few centimeters (dense network) to a few decimeters (sparse network) in real time.

iii. To establish a web site that will make available near real time (1 – 3
hours) reference station data to the users for post-processing differential GPS throughout the coverage area.

Network design

Objectives

The primary goals in designing the system were for efficient and easy operations, facilitate user understanding of the network and for expandability. Some empirical numerical limits have been suggested for IP addressing and efficient operation of the system. The networking protocol used is the Internet Protocol (IP).

Though contained in a single private network, the system was split into 2 subnets which is not representative of classical IP subnet class. As such this network is considered as ‘classless’. Each subnet consists of all reference stations and the Control Center (also referred to as central control or CC).

Malaysia Real Time Kinematic GPS Network (MyRTKnet)

The RTK Network Solution concept is based on having a network of twenty-seven (27) GPS reference stations continuously connected via IP VPN to a Control Centre (Figure 1). The computer processor at the Control Centre continuously gathers the information from all GPS receivers, and creates a living database of Regional Area Corrections.

Physical Communications

The physical communications infrastructure is provided by the IPVPN over COINS frame relay system. This system works by connecting any respective site into the Telekoms Malaysia frame relay network. The service is available throughout Malaysia except for some isolated locations. The advantage of using IPVPN verses point to point leased lines is that IPVPN will require only a single high speed connection and modem at the Central Control rather than a connection comprising of a modem and separate leased line for each station. A schematic representation of the system communications is shown in Figure 2

Reference stations

Physical Enclosure

The physical enclosure for the equipment at each site is a small structure or allocated room of approximately 3m x 4m. This room is being cooled by 2 air conditioners that alternate during the day such that the total time that any one unit is on over the entire day is 12 hours.

System Schematic

The system schematic for the reference stations are shown in Figure 3.
The Reference Station consists of the following:
• Cisco 1721 router
• Dlink 5 port 100/10M switch
• Trimble 5700 CORS with Zephyr antenna
• Moxa 5410 terminal server
• Advantech Adam 6017 A/D module
• Micromate UPS for minimum 2 days backup power
• Satelit lightning protection

The system is designed for autonomous operation and is able to run for 48 hours without power.