Japan: Augmenting navigation

Akio Yasuda

MSAS (MTSAT Satellite-based Augmentation System is the Japanese satellite based augmentation system which has been prepared by the Civil Aviation Bureau of Ministry of Land, Infrastructure, Transport and Tourism (MILT) and started on 27 September 2007, to serve for improving accuracy, integrity, and availability of GPS positioning on the civil aircrafts. It has two geostationary satellites, called MTSAT-1R and MTSAT-2 located at 140 and 145 degrees in the east longitude, on the orbit of 3600km above the Equator. The satellites were launched in February of 2005 and 2006 respectively, long after the launch failure on November 1999. MTSAT, which stands for Multi-functional Transport SATellite, also has a function of geostationary metrological satellite.

The satellites transmit the augmentation data signal for GPS positioning using the same frequency L1 of GPS, which specification is the same as the WAAS in USA, EGNOS in Europe and GAGAN in India. It also works as an extra GPS satellite by sending the ranging signal.Though the data are transmitted by the global beam, the ionosphere grid data for L1 frequency, which are estimated from the L1 and L2 measurements, are in the area between 105 and 170 degrees in the east longitude and between 5 and 65 degrees in the north latitude at every 5 degrees.

The preliminary evaluation of the accuracy in Tokyo area at about 140 degrees in east longitude and 36 degrees in north latitude in rather ideal circumstance shows that the 2drms is reduced to 2 m from 3 m without MSAS correction. The improvement is not remarkable presumably due to the recent calm ionosphere condition.

The SBAS is originally designed to serve civil aircrafts flying long range by geostationary satellites with a wide service area However, the users in land and ocean can equally access freely without extra device by just reforming the firmware of GPS receiver.

There are two types of DGPS services in Japan. One is the Maritime Differential GPS (DGPS) service, which transmits the correction data followed by RTCM SC-104 for DGPS positioning with the radio beacon waves of around 300 kHz. It has been operated by Japan Coast Guard, also managed by MILT since 1999 with no direct charge. Twenty seven stations are covering whole coastal sea around Japanese archipelago and serving the steaming ships. Each station has the service area of 200 km in radius including the coastal land. Radio beacon itself stopped the function of direction finding in the August 2006 and is just transmitting the correction data for DGPS. Almost all of the GPS receivers can accept the correction data to improve the accuracy, but it requires the extra antennas and receivers for the reception. The other DGPS service is for the car navigation systems. The cumulative shipping number of car navigation system in Japan got to 27 million on June 2007 in the last decade. In the 1990th, the GPS signal was degraded by so-called selective availability (SA). Then the announced accuracy of 100 m did not satisfy the demands of the drivers and hence the manufacturers. Thus a new private company was established in 1997 named G-PEX to supply the correction data which were multiplexed on the FM broadcasting waves. Seven reference stations are located covering all over the Japanese islands and forty FM radio stations are disseminating the correction data. About 4.5 million sets of the car navigation devices were furnished with the data reception apparatus by the end of 2002. It is about 40 % of the cumulative shipping number by that time. There is no direct charge but the manufactures pay some amount of money to the data supplying company for each device when shipping. After the suspension of SA on May 1st 2000, the accuracy of GPS without DGPS was dramatically improved to less than 10 m. The accuracy is enough for the car navigation systems, especially with the aids of the odometer, direction sensor and the technique of map matching. Then the manufactures became to conceive the less importance of DGPS. Nevertheless, the system exists, probably because the main capital of G-PEX comes from the manufactures group. But as the service by the MSAS is certificated, they decided to quit service on the end of March 2008.

It is very convenient to access the MSAS, i.e. free of charge, no extra antenna and no extra receiver. The accuracy is high enough for cars and coastal vessels, even though it is a little bit less than that by marine DGPS service according to our evaluations. We must consider some time continuation of the service by maritime beacon system in considering the maintenance expense of twenty seven stations. The accuracy of standalone GPS positioning is already enough for coastal vessels. Quasi Zenith Satellite System (QZSS) is a Japanese Satellite Positioning System to augment the performance of GPS positioning. The first satellite shall be launched in 2010 being prepared by JAXA (Japan Aerospace Exploration Agency). And after proving the effectiveness by the first satellite, the following satellites will be launched and the full system will be furnished: i.e. the users will be able to receive the signals from the zenith continuously for 24 hours. The higher accurate SBAS with one-degree- pitch denser ionospheric data suitable to Japanese islands and surrounding seas is under examination by Electronic Navigation Research Institute (ENRI) to transmit by the L1- SAIF (L1- Submeter-class Augmentation with Integrity Function). However, the simulated experiment showed recently that the accuracy was not so much improved as was expected. The further examination will be continued. Precise tropospheric correction data service is also considered.

QZSS will also provide the data through the LEX (experimental signal with higher data rate of 2 kbps of message), compatibility with Galileo E6 signal at 1279 MHz. This channel will be devoted to the transmission of the carrier phase measurements data with charge for RTK-GPS positioning.

As for the cm-order positioning data services, there are several private companies which transmit the network RTK-GPS data via mobile phone line in Japan. They are using the carrier phase data at about 1200 electronic reference stations called GEONET (Gps Earth Observation NETwork system), being operated by Geographic Survey Institute, covering all over the Japan to observe crustal deformations.

Maritime GPS Positioning Solutions, non profit organization, is serving RTKGPS data for the marine construction activities via VHF radio link. They establish a reference station by themselves at each operational area in Japan.

The council for High Accuracy GPS Experiment has developed the data transmission system for DGPS and RTKGPS by utilizing the data space in the terrestrial digital TV broadcasting, which has been employed experimentally in Japan since December 2003 and will replace the analogue TV broadcasting on 26 July 2011. The system is going to be certified by Association of Radio Industries and Broadcast (ARIB).

There are still many to be developed to serve the effective data via low data capacity of 2 kbps via LEX on QZS, making the best use of the advantage of the wide service area of the satellite.