SSP Leave Report of Chris Rizos

Objectives of the Special Studies Program (SSP)

Almost the entire period of the 6 month SSP leave, from 1st August 1999 to 31st January 2000, was undertaken at the Surveying & Mapping Laboratory, at the School of Civil and Structural Engineering, Nanyang Technological University (NTU), Singapore, working with colleagues A/Prof. Goh Pong Chai and A/Prof. Tor Yam Khoon. NTU are partners with SNAP-UNSW in a National Science & Technology Board (NSTB) grant project, "Development of an integrated multiple base station infrastructure to support concurrent high precision differential GPS positioning applications", which funds the establishment of a GPS test facility in Singapore. This 'open air' laboratory comprises four high precision permanent GPS reference stations located in the east, west, north and south of the Republic of Singapore, and linked by high speed data lines to the NTU computer/server. Several real-time GPS system designs will be tested over the next few years. An ARC large grant "Development & testing of innovative high precision GPS techniques with the aid of the Singaporean multi-base station network" assists in the funding of SNAP projects related to this Singaporean project. Activities during the SSP can be grouped under the following headings:

 

Details of SSP Activities

Host at NTU was A/Prof. Goh Pong Chai, who over the last two years has worked closely with SNAP to promote a joint UNSW/NTU project to establish an operational multi-reference station network in Singapore (referred to here as the SGRSN Ð Singapore GPS Reference Station Network). SNAP has developed algorithms by which the data from a network of GPS reference receivers scattered about a city (separated by 30-50km) is optimally processed, and correction data generated that may be subsequently used by a variety of GPS user applications to improve their positioning performance. (This 'correction data' may be used for post-processing of the combined user and reference receiver data, or somehow transmitted to users by a communications link, in order that positioning results are provided in 'real-time'.) Tests to date have shown that centimetre-level accuracy positioning is possible, without the constraints of current off-the-shelf GPS systems. However, to carry out further research into the optimal real-time data processing algorithms and to improve user application software requires access to an 'open air' laboratory such as the SGRSN.

 

Singapore GPS Reference Station Network, showing locations of GPS receivers and NTU server (users will access data via the NTU server through appropriate wireless communication links).

 

The SGRSN was funded after a successful application to Singapore's NSTB by the UNSW and NTU investigators. This SSP was therefore undertaken at an opportune time to permit real progress to be made on this joint project.

 

Activity 1: Development of the SGRSN

GPS is an all-weather, global, satellite-based, round-the-clock positioning system developed by the U.S. Department of Defense that became available to the civilian surveying and navigation community in the early 1980s. The standard mode of high accuracy differential positioning requires simultaneously observing GPS receivers, one (reference) GPS receiver needs to be located at a 'reference' or 'base' station whose coordinates are known (in an analogous manner to the base stations within a cellular phone system), while the second (user) GPS receiver simultaneously tracks the same satellite signals. Depending on the data processing technique used, accuracies ranging from a few metres down to the centimetre-level can be achieved in this Differential GPS (DGPS) mode of relative positioning.

The technique of DGPS is essential for high precision applications. There are, however, a number of ways in which DGPS can be implemented. If the positioning results are required immediately (i.e. in 'real-time'), then there must exist a communications link from the reference GPS receiver (or receivers) to the user(s). Otherwise the data processing must take place offline, in the so-called 'post-processing' mode. The DGPS user has the choice of either operating their own reference station (and possibly the communications channel between reference and user receivers as well), or relying on others to do this for them. Obviously the latter implies a service, generating and transmitting the necessary information to the users. The provision of a unique service, based on not one, but a network of reference receivers is one of the objectives of the Singapore GPS Reference Station Network (SGRSN).

High precision DGPS implies that the carrier phase data from the reference and user receivers be combined (in real-time or offline) and processed. But the use of carrier phase data comes at a cost in terms of overall system complexity because the measurements are ambiguous, requiring the incorporation of an Òambiguity resolutionÓ (AR) algorithm within the data processing software. Carrier phase-based positioning is an engineering challenge that is yet to reach its full potential. If GPS satellite signals were continuously tracked and loss-of-signal-lock never occurred, the integer ambiguities determined at the beginning of signal lock-on would be valid for the whole period that GPS was being used. However, the GPS satellite signals are occasionally shaded (for example, due to buildings in 'urban canyon' environments), or momentarily blocked (for example, when the receiver passes under a bridge or near a tall building), in which case the ambiguity values are ÒlostÓ and must be redetermined. This process can take from several tens of seconds up to a few minutes with present GPS systems. During this Òre-initialisationÓ period the GPS carrier-range data cannot be obtained, centimetre accuracy positioning is not possible, and hence there is ÒdeadÓ time until sufficient data has been collected to Òresolve the ambiguitiesÓ. If interruptions to the GPS signals occur repeatedly, then ambiguity re-initialisation is at the very least an irritation, and at worse a significant weakness of carrier phase-based GPS positioning systems.

Research at SNAP has demonstrated the feasibility of reliable, single-epoch, carrier phase-based positioning, opening up the way for robust, instantaneous sub-decimetre-level positioning with a minimum of operational constraints (apart from the need to receive multiple satellite signals for an instant). Furthermore, the use of a network of reference receivers (rather than the single reference station approach used hitherto) has a number of advantages, one of which is that the distance between reference and user receivers may be lengthened to 3-4 times current limits without sacrificing accuracy. Hence, the combined use of multiple reference stations and state-of-the-art ambiguity resolution/validation algorithms means it is possible to obtain centimetre accuracy positioning faster and with less operational constraints than current systems. (Current commercial off-the-shelf high precision DGPS products do not have the same level of performance, typically requiring tens of seconds AR time and recommending user-reference receiver separations be kept to below 10km.)

The SSP at NTU coincided with the period of early deployment of the SGRSN infrastructure, first testing in the offline mode of network-based GPS positioning in Singapore, and planning for the development of the reference station data processing 'engine' that will be at the heart of the SGRSN. It is envisaged that the operational system will be ready by the end of 2000, or early 2001. The following activities and events took place within the context of this SSP activity:

 

Activity 2: Development and Testing of GPS User Applications

The SGRSN is an invaluable resource for the development of new and innovative DGPS-based products and services. In effect, this 'open air laboratory' can be used for testing data processing algorithms, hardware, software, communication links, new integrated products and services. The SGRSN will have several innovative features that will deliver benefits to certain users beyond what is possible with present DGPS systems. Firstly, the communications link options will allow for more cost effective, better targeted services, which are likely to also be more economical and convenient for certain classes of users. Secondly, users requiring high precision, real-time, carrier phase-based positioning will have access to a unique service that is optimised for their applications.

SNAP research has shown that a network-based approach for positioning (via, for example, the SGRSN) can lead to significant advantages in terms of improved accuracy, faster and more robust carrier phase-based positioning (i.e. faster 'ambiguity resolution' -- AR), less operational constraints (maximum distance of user receiver from reference receiver), and lower-cost (through the use of single-frequency receivers rather than the more expensive dual-frequency receivers used for high precision applications). In particular, we have identified three user applications that benefit from the SGRSN: (a) centimetre-accuracy kinematic positioning with instantaneous AR, (b) efficient, low-cost static positioning for surveying, and (c) mixed mode (single- and dual-frequency) receiver arrays for such applications as building or ground deformation monitoring.

During November 1999 five SNAP group members (Dr. S. Han, Mr. H.Y. Chen, Mr. L. Dai, Mr. C. Satirapod, Ms. X. Han) visited NTU for a two week period to perform experiments in which various user application scenarios were simulated. Data was collected, and then processed offline with the SGRSN data. Their results were presented at the "GPS/GIS Showcase", 19-20 November 1999. Other activities associated with user applications undertaken during the SSP include:

 

Activity 3: Visit to Japan

The period covered by this visit was 14-30 October 1999. The academic activities can be considered in two parts: (a) part one, 18-22 October 1999, in Tsukuba Science City; and (b) part two, the remaining period while a guest of the Earthquake Research Institute at the University of Tokyo.

The visit to Japan by Chris Rizos as a Fellow of the Japan Society for the Promotion of Science (JSPS) coincided with the "International Symposium on GPS -- Application to Earth Sciences and Interaction with Other Space Geodetic Techniques", held during 18-22 October 1999 at the Tsukuba International Convention Center. This afforded an invaluable opportunity to meet with many Japanese and foreign earth-scientists, as well as to visit a number of Japanese research institutes in the Tsukuba area. This symposium was the largest "GPS geodesy" conference of 1999, and one of the biggest such events ever held in the Asian region. Over 370 participants learned of the tremendous progress being made in ultra-high precision GPS geodesy techniques for a range of applications: from the "traditional" measurement of crustal motion to the new applications in meteorology. Over 300 oral and poster presentations were delivered. The symposium was truly international, with a total of almost 200 people attending from 40 countries apart from the host nation Japan. Nevertheless, the conference was an ideal opportunity for Japanese geodesists to present GPS results pertinent to the North West Pacific region, and in particular to highlight the value of the GEONET facility for geodetic research. (GEONET is the nationwide network of over 900 permanent GPS receivers operated by the Geographical Survey Institute of Japan.)

Three members of the SNAP group attended the symposium: Dr. Chris Rizos, Dr. Shaowei Han and Mr. Linlin Ge (PhD student). Six papers were presented. This symposium also presented an opportunity to visit the following research institutes: (a) Geographical Survey Institute (GSI), (b) Meteorological Research Institute (MRI), (c) National Aeronautics & Space Development Agency (NASDA), and (d) the Communications Research Laboratory at Kashima. In particular, the discussions with colleagues at GSI and MRI were extremely useful for proposing further collaboration with SNAP:

The host in Japan was A/Prof. Teruyuki Kato, of the Earthquake Prediction Research Center, Earthquake Research Institute (ERI), University of Tokyo. A/Prof. Kato is one of Japan's foremost researchers in the application of GPS technology for regional crustal dynamic studies. For many years he has been establishing and maintaining permanent GPS instruments in a number of East and South East Asia countries. This WING network (West Pacific Integrated Network for Geodesy) has been a valuable source of data for many investigators around the world working in the field of regional and global geodynamics. Fruitful discussions were also held with his colleague Dr. Shigeru Nakao. Dr. Nakao has established, and been operating, a network of single-frequency GPS receivers in an area approximately 10x10km extent around Ito City, on the Izu Peninsula, for about two years. The network was operating during the April 1998 earthquake swarm referred to above. The combined use of low-cost GPS instruments and permanent, dual-frequency receivers (such as those deployed in the GEONET network) has been a SNAP research topic for several years. It was suggested to Dr. Nakao that his raw GPS measurements could be processed in this way.

 

Activity 4: Supervision of Undergraduate Students on Exchange Programme

The UMAP (University Mobility in Asia and the Pacific) is a programme to facilitate the establishment of undergraduate student exchange programs with universities in the Asia-Pacific region. The programme is funded by the Australian Federal Government's Department of Education, Training & Youth Affairs (DETYA).

In 1998 a UMAP proposal was submitted to DETYA, "Globalising UNSW Training for Future Surveyors", and was granted funding of $15,000. In June 1999 four final year UNSW geomatic engineering students travelled to the Dept. of Geomatics, University of Technology Malaysia (UTM), Johor Bahru, Malaysia, to undertake one semester's study of several geomatics subjects. In addition, the students were able to undertake practical work, in order to gain professional practice experience. Following this UTM-based program, the students spent one month at the Surveying & Mapping Laboratory, Nanyang Technological University (NTU) in Singapore, completing their final year thesis project. In addition, a faculty-to-faculty Memorandum of Understanding between UTM and UNSW to facilitate future student exchanges and to promote further research collaboration was signed.

 

 

More Photos ...