院士论坛| 杨元喜：我国海洋大地测量基准与海洋导航技术研究进展与展望（英文版）

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构建与学术的桥梁        拉近与权威的距离

Progresses and Prospects of Marine Geodetic Datum and Marine Navigation in China

YANG Yuanxi^１,2  XU Tianhe³  XUE Shuqiang⁴

1.State KeyLaboratory of Geo-information Engineering, Xi’an , 710054, Shaanxi, China

2. Xi’an Research Institute ofSurveying and Mapping, Xi’an, 710054, Shaanxi, China

3. Institute ofSpace Science, Shandong University, Weihai, 264209, Shandong, China

4. ChineseAcademy of Surveying and Mapping, Beijing, 100830, China

Abstract: Territorial water is a significant part of national sovereignty ofChina, thus the infrastructures of national space datum and location servicesshould cover the sea areas except for the land areas. China has establishedrelatively complete geodetic coordinate frame in land areas over the pastdecades, including the lastest developed China Geodetic Coordinate System 2000(CGCS 2000) with its reference frame and the national gravity datum 2000.However, the currently used geodetic infrastructures have not well covered thesea areas of China. The marine geodetic datum and marine navigationtechnologies need to be further developed and extended to satisfy the nationaldemands of marine environment and resource detection, scientific investigation aswell as marine economy development in new era of China. This paper mainlyreviews the development and the progress of Chinese marine geodetic datum andmarine navigation, analyses related key technologies in establishing ournational marine geodetic datum. Some current trends and future directions forindependently developing our national marine geodetic datum and marinenavigation technologies are discussed.

Keywords: marine surveying; geodesy; datum; positioning; underwater navigation

Foundation support: Key R&D Program‘Marine Geodetic Datum and Navigation Techniques’ (2016YFB0501700)

1. Introduction

The ocean has become an important field for human sustainabledevelopment and resource exploitation.China is a big marine country with more than 300km² seaareas and rich marine resources.To utilize the ocean, to enhance the marine economy development, to safeguardthe maritime rights and interests and to ensure maritime security within the nationalwaters, we need at first to provide supports to the maritime transport. Inaddition we need to strengthen the scientific research on the marine and therelevant technology to support the environment monitoring and marine resourceexploring and so on. These aims and tasks have a wide range of demands indeveloping the marine geodetic infrastructures and marine navigation of highprecision. It will be a base for schematizing and performing all nationalmarine strategies.

After entering the 21stcentury, the US, Canada, Japan and other developed countries developed and improvedtheir marine geodetic infrastructures by constructing seafloor geodeticnetworks with advanced technologies ^[1-3].These efforts have greatly promoted the Earth science researches and themonitoring ability of marine geological hazards^[4-7]. Meanwhile, marine navigation technologies weregradually innovated to support the security for various ocean activities.Especially in recent years, the US proposed a project to investigate the feasibilityof developing a global underwater positioning system similar to the GPS (GlobalPosition System), by constructing a set of acoustic signal sources on theseafloor to realize the underwater positioning of high precision for variousunderwater vehicles without surfacing ^[8].

Techniques of establishingthe marine geodetic datum were developed relatively later compared to theground-based datum, but this technique was rapidly developed over the lastdecades. As early as the end of 1980s, scholars had ever considered andproposed the conception of constructing the seafloor geodetic control network^{[9, 10]}, but nowadays, only a fewdeveloped countries have formed the ability and mature techniques inestablishing and maintaining the seafloor geodetic stations. As is well known, themarine underwater location is generally based on the GNSS/acoustic technique^{[7, 11-14]}; thus one can establish themarine geodetic datum and perform the over/underwater seamless navigation byacoustic connection measurements between the sea surface GNSS buoys or ship’scontrol lines with the seafloor geodetic stations. This has recently become ahot topic and frontier research direction in the field of marine surveying,positioning and navigation.

Since the 1970s, thesatellite altimetry has been utilized to determine the mean sea surface leveland marine geoid, and to calculate the marine gravity field by the inversionmethods^{[15, 16]}. The newtrends in geodesy are embodied in establishing the ocean tide model with high precisionand high resolution, developing the vertical datum model and realizing theseamless vertical datum transformation of high precision^[17]. By assimilating the observationsfrom tide gauge stations and the satellite altimetry data to the tide wave mode,the ocean tide model of high precision was realized^[18]. Besides, scholars also developed high-precision water-levelcalculation methods by combining the ocean tide model with the surrounding tidestation observations^[19]. Currently,the US, Canada and other developed countries have made great achievements inthe marine vertical datum establishment, unification and transformation betweendifferent vertical datum types, and this seems meaningful for reference in developingChinese marine geodetic datum architecture.

Underwater positioning/navigationand marine LBS (location based services) has gradually become more and moresignificant in implementing the national oceanic developing strategy includingvarious ocean activities, security, rescue, law-enforcement, exploring oceanicresources and protecting the oceanic environment as well as disaster mentoring,etc. As to the underwater positioning and navigation, nowadays it still seriouslyrelies on the acoustic signals for easily penetrating the water. This is verydifferent to that of ground-based positioning and navigation based on electromagneticsignals. This difference can lead to a number of new challenges for theshortages in the acoustic communication and data transmission. Besides, thereis still a lack of high-precision models that can be applied in the underwaterpositioning and navigation with high precision. Just in recent years, marine navigationinstruments have been innovated and hardware instrument series were proposedand updated rapidly in the past. New trends in navigation instrument innovationmainly are of the machinery combination, multi-functional integration, wide-bandsignal; thus, multi-sensor integration has become a hot direction in seamlessover/underwater positioning and navigation^[20,21]. Nowadays, widely applied underwater positioning systems are mainlydivided into three groups, the LBL system, Short baseline system and Ultra-Shortbaseline system ^{[22, 23]}, andthere are a series of mature products in abroad as well as in domestic.

Differential underwaterpositioning model is helpful to reduce the positioning influences from varioussystematic errors and spatial correlated errors. This technique uses the single/doubledifferential observations of acoustic distances from the sea surface acousticranging equipment to the seafloor geodetic station to precisely calculate thethree-dimensional coordinates of the seabed transponders. The differentialtechnique has been rapidly concerned and developed since it was proposed^[24].

During the 1990s, BellAerospace Textron successfully developed the gravity gradiometer for submarine vehiclenavigation, of which the accuracy for both in-line and cross gradient measurementswas on the order of 1 E (where E denotes an Eotvos unit of 10^-9m/s²),and the navigation precision 62m/8h were achieved. Lockheed Martin Inc. developeda navigation system add-on called the Universal Gravity Module (UGM) containingtwo types of gravity sensors (a gravimeter and three gradiometers) satisfying the14-day sub-surface navigation requirement^[25,26]. With the rapid developments in acoustic positioning, the INS(Inertial Navigation System) and gravity matching navigation, and themulti-sensor integrated underwater navigation are gradually mature and become anew trend in underwater navigation system development.

2. Research progresses of China’s marine geodetic datum

As early as in the 1990s, Chinese scholars initiated to discuss theseafloor datum transfer method by combining the shipbone GNSS positioning withthe acoustic ranging technique to realize the location and orientation of aseafloor geodetic network^[27].Subsequently, China made a series of satisfactory achievements and progressesin developing the SBL system and LBL system^[28,29]. Latest studies further discussed the seafloor datum transformationmethod and the seafloor geodetic network data processing issues for improvingthe precision of datum definition and realization, by improving the arrayconfiguration of GNSS buoys or the ship’s control lines, and by using the free/constrainednetwork adjustment^{[28, 30]}. Technicalarchitecture for establishing and maintaining the seafloor geodetic datum andnew models and algorithms for precisely processing the marine geodetic network aredesired to be deeply and systematically studied in the future.

Over the last twodecades, the precision of marine geoid and ocean tide model covering the ChinaSea has been greatly improved. The gravity anomaly digital model with resolution2'x2' was established over the China's offshore and Chinese territorial waters,and the model precision reaches 3-5mGal. A series of global mean surface heightmodels with resolution 2'x2' were realized and the precision was better than 4cm.The trend characters of the global sea level change over the past sixty yearswere analyzed to quantify the main influence factors. Global seafloor digitalterrain model was obtained by applying the ocean inversion approaches; Globalocean tide model of 15'x15' resolution was also established ^[31]. During the 12th National Five-YearPlan, the national geoid model was established by the multi-source gravity datafusion^[32].

In recent years,seamless vertical datum construction technique becomes a hot topic in marine geodesyand the relevant issues are gradually concerned by scholars of China ^[33-40]. The systematic bias in the meansurface level of China and the datum difference between the 1985 NationalHeight Datum and the Global Quasigeoid were preliminarily studied^[41]. Both the Global height datum and localheight datum unification issues were highly recognized and widely discussed ^{[42, 43]}. Under the support of National863 Project “Key technologies of island (reef) surveying and mapping demonstrationapplications”, technologies for establishing the seamless marine vertical datumwere developed to realize the marine vertical datum transformation^{[44, 45]}. According to an uncompletedstatistic, there are at least 70 coastal long-term tidal stations in China,they have played an important role in determining the multi-year mean sea leveland the seamless chart datum as well as in revealing the tidal processessurrounding the China's offshore. Precise ocean tide model of China was alsoestablished by combining multi-altimeter data with the coastal long-term tidal observationdata^[18]. Beside, by comprehensivelyutilizing the local tie between the GNSS CORS stations and the long-term tidal stations,the precise transformation model between the national height datum and thechart datum was established in China^[46,47]. However, the precision of the national vertical datum overthe sea area needs to be improved in the future, and the technology for completelyrealizing and dynamically maintaining the vertical datum need to be further developed.

3. Progresses in underwater positioning and navigation technology inChina

Over the past two decades, China made great achievements in developingthe equipments used in underwater positioning and navigation. In the aspect ofdeveloping underwater acoustic devices, the technical gap between China and developedcountries was gradually reduced. During the 12th Five-Year Plan of China, the longrange and ultra short baseline positioning system was successfully developed.This system can realize the underwater beacon location of 3800m water depth. Bycombining the GPS technique with acoustic technique, the long baseline systemand the underwater DGPS system were also developed ^{[22, 48, 49]}. GRAT LBL is the first set of underwaterpositioning system using the radio-controlled GNSS buoys array^[50]. The underwater DGPS system adoptsthe advanced differential technique to reduce the acoustic signal accumulatedand systematic errors, but the positioning performance may be limited to thenumber of the used GNSS buoys; meanwhile, the vertical positioning precisionneeds to be further improved. Although in ideal situations, these systems couldbe used to achieve underwater there-dimensional positioning and navigation of meter-levelprecision, the overall positioning performance, engineering applications andtheir practicality need to be further enhanced. As to the aspect of LBL dataprocessing, there are great number of simulation studies and engineeringexperiences that can be referenced in the future studies, such as the array measuringand calibration technique for the long baseline array^[51]. However, there are still a numberof advanced technologies developed in the last years that need to be further developedby absorbing and digesting the new achievements in abroad.

Many advancements andexperiences in applying the ocean acoustic positioning has been accumulated to supportthe detection of resource over the deep-sea and shallow water, such as the applicationsin the oil exploration^[52]. Forthe OBC (Ocean Bottom Cable) exploration, deep-sea towed-streamer explorationand submarine earthquake data collections, it shows that the acoustic positioningtechnique can be applied to realize the high precision positioning and navigationfor the ships and underwater vessels, but the positioning precision in these applicationsneed to be further enhanced in many situations.

Recently, real-time underwater gravimetryand gravity matching navigation techniques have been widely concerned for supportingthe gravity-aided navigation system^[53-58].The gravity-aided navigation system was preliminarily developed and its performancehad been passed the marine test^[59].However, most of studies were based on the simulations to validate the system. Thereare also explorative studies in the underwater magnetic matching navigation^[60-62], but because of the lack ofhigh-resolution geomagnetic data, the rapid time-variation of the terrestrialmagnetic field and the easily-influenced features, there is still a great gap forpractical applications.

To summarize, in thepast decades great achievements have been made not only in developing a seriesof underwater navigation equipments but also in developing underwaterpositioning techniques. However, these self-developed underwater positioning equipmentsneed to be further improved and developed in the directions of the product serialization,integration miniaturization and standardization. The comprehensive marine navigationsystem integrated with acoustic/gravimetric/inertial sensors needs to be developedin the future to realize the seamless underwater navigation.

4. Key techniques for developing the marine geodetic datum and underwaterpositioning and navigation

As the GNSS/acoustic technique is applied to a large-scaled geodeticnetwork for establishing and maintaining the marine geodetic datum, or forrealizing the underwater navigation over a large area, there are still technologicalbottlenecks that need to be further solved^[63],such as the accurate corrections for the acoustic ranging with or without thesound profile data^[64-67]. Animproved seafloor datum realization was proposed by optimally designing theGNSS buoy array or the dynamic ship’s controlled lines to enhance thepositioning precision and to simply the seafloor data processing models and algorithms^{[29, 68, 69]}, but the verticalpositioning precision needs to be further improved since the remainingsystematic errors in acoustic ranging cannot be removed by only optimizing the positioningconfiguration only distributed on the sea level^[70]. Therefore, we need to systematically study and revolutethe three-dimensional seafloor geodetic datum realization methods, the seafloormark construction scheme, the facilities maintenance ways, and marine seamlessnavigation technologies.

(1) The theories and methodologies forestablishing the marine geodetic datum

Firstly, we need to breakthrough the technical bottlenecks in establishing a dynamic seafloor geodeticdatum and to make it to be consistent with the terrestrial datum. Thecorresponding theories, models and methodologies for realizing the marine LBS (LocationBased Services) need to be studied. The primary taskis to study the necessary density and distribution of seafloor geodeticstations to optimally allocate the sea surface GNSS buoys and seafloorreference stations^[71]; Theeconomic and feasible observational technique and scheme for the marinegeodetic network need to be further developed and innovated. Secondly, the precisionoceanic models and techniques for real-time correcting the acoustic signal propagationerrors need to be developed by utilizing the global or regional oceantemperature  and salinity information; Somenew models and algorithms for marine geodetic data processing need also to beestablished for the seafloor geodetic network adjustment. At last, the globaland regional marine geodetic observatory network and marine LBS prototype needto be explored to develop a technical architecture that serves to marinegeodetic datum and marine LBS.

(2) Techniques and methods for realizing the land/sea unified andseamless datum

We need to tacklethe problems in precisely establishing the three-dimensional sea 44 35036 44 15535 0 0 2813 0 0:00:12 0:00:05 0:00:07 3107floor datum, indetermining the coastal geoid using multi-source gravity data fusion, and in realizingthe marine seamless vertical datum. The nonlinearity of short-distanceequations will be taken into account to reduce the nonlinearity influences^{[72, 73]}, in order to reduce the uncorrectedacoustic ranging errors, and perform prior constrains especially for stabilizingthe vertical coordinate component. At last, the CORS stations, long-term tidal stations,GNSS buoys and satellite altimetry data need to be integrated to completely realizeand dynamically maintain the marine vertical datum.

(3) Techniques for constructing and maintaining the seafloorgeodetic stations

The geodeticreference network optimization, reference station facilities shelter and maintenance,as well as location calibration, needs to be carefully considered, some technicalobstacles in the pressure protection, antiseptic treatment, installation and retrievefor submarine geodetic facilities need also to be studied. Besides, the way foroperating the seafloor geodetic facilities, energy supply and system status monitoringneed to be explored to give a practical and feasible solution.

(4) Techniques for marine gravimetric and magnetic matchingnavigation

There are still key technologiesthat need to be solved in developing gravimetric and magnetic matchingnavigation, such as the determination of the proper resolution of referencegravity anomaly grids, and fast algorithms to match the real-time gravimetricmeasurements with the reference gravity anomaly ^{[61, 74]}. As we know that the temporal and spatial magneticmodel is the foundation to realize the magnetic matching navigation^[74], and the precision and reliabilityof the reference magnetic field model seriously affect and restrict thedevelopment of magnetic navigation, we need to take the rapid time-variationcharacters of the terrestrial magnetic field and its easily-influenced featuresinto account. Therefore, we need to develop some location correlation matchingtechnique based on the gravimetric, magnetic and topographic information, andthe gravimetric and magnetic matching navigation equipments as well. Theintegration navigation theory and methodologies of the gravimetric and magneticsensors with the INS navigation should also be considered for improving the availabilityand continuity of underwater navigation performance, based on optimalmulti-sensor data fusion.

(5) Techniques for multi-sensor integration navigation andmulti-source data fusion

Integrating themulti sensors and the multi-source data fusion are the keys in realizing the marineseamless navigation of high precision. The multi-sensor integration navigation equipmentshould be smart and tightly integrated^[75];Multi sensors should be meet the compatibility and interoperability^[76], while multi-source navigation data fusionshould be realized adaptively to provide users intelligent services. Besidethis, we need to develop calibration platform and techniques for independently developingthe underwater positioning and navigation equipments and multi-source dataprocessing software.

(6) Development of polar navigation techniques

In the polar regionof the Earth, the GNSS positioning geometry is very poor and the GNSS signal maybe greatly affected by the ionosphere, thus the positioning performance may becomeworse relative to that in mid and low-latitude areas; meanwhile; the INSnavigation may be invalidated and easy to lose the direction more occasionallythan other areas^[77]; the magneticnavigation is also difficult to be performed in the polar areas. Therefore, facedto the polar navigation, the multi-source data fusion and the positioningperformance analysis are also worthy of study.

5. Conclusions and prospects

In the past decades, China established the complete terrestrialgeodetic datum with advanced technologies and made great achievements in theaspects of the geodetic data processing theory, models and algorithms^[78-82], but there still exists a large gapin aspects of marine datum construction and marine location relative to theinternational advanced level. This is mainly reflected by the lack of technologiesand approaches of underwater positioning and navigation. Moreover, there isstill no complete and specific architecture for unifying the land and seaspatial geographic information. Therefore, we recommend to speed up China’smarine geodetic network design, demonstration and construction, developfeasible marine signal sources for underwater positioning, and realize theland/sea geodetic datum unification. To break through key techniques inconstructing and maintaining the seafloor reference facilities and those in themarine multi-sensor positioning and navigation, we need independently developthe national marine geodetic datum of high precision and the technicalarchitecture for developing the national marine location based services. Forsupporting the national Maritime Silk Road, marine resource exploitation,national ocean right and interests, submarinevehicle navigation and maritime navigation safety, we recommend to independentlydevelop the national marine positioning and navigation equipments and the advancedcalibration platform, and then we need to develop the global/regional locationbased service frame, technical architecture and standards.

Besides, thetechnique advancement in the marine geodetic datum and submarine navigation cangreatly promote developments in marine science, and it has a wide application prospectsin monitoring the changes of seafloor sediments in specific sea area, obtainingthe seafloor spreading and plate movement, preventing and reducing the oceanic disasters.

Author: YANG Yuanxi (1956-), male, PhD,professor, acdemician of Chinese Academy of Sciences, majors on dynamicgeodetic data and satellite navigation data processing.

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