【正文】 dmaccuracy trends and developments in precise positioningRelative positioning with GPS and Differential GPS (DGPS) both involve the positioning of a second receiver with respect to a reference station. As both stations similarly experience — depending on their interdistance —the effects of satellite orbits/clocks and atmospheric delays, the relative position is largely insensitive to mismodelling of these effects and their errors. The concepts of relative positioning with GPS and Differential GPS have existed for some twenty years. Until recently, these two fields have developed relatively independently from each other. Two new trends in both DGPSpositioning and GPS RealTime Kinematic (RTK) surveying include moving from scalar corrections (from one reference station) to (state) vector’corrections’, based on a network of reference stations。 and the processing of the data, also for the global high precision IGStype (International GPS Service) of applications, is moving towards real time execution. As a result the traditional distinction between precise relative positioning with GPS and DGPS diminishes。 instead, one consistent family of applications emerges, sharing a mon concept and mon algorithms, that could be termed Networkbased Differential GPS (NDG). NetworkInitially, systems for DGPS started with one reference station, and one or more mobile receivers (rovers) in a local area. Later, the service area of Differential GPS was extended from local to regional and national, and eventually to the continental scale with Wide Area DGPS (WADGPS)systems such as WAAS (Wide Area Augmentation System) in the US and EGNOS (European Geostationary Navigation Overlay Service) in Europe. Logically, the last step is Global DGPS, as introduced by JPL (Mllersch246。n et al.,2001a). Thus making seamless DGPS positioning available across the world. The advantage is that costly infrastructure is no longer needed, however, the user has to rely on the US Department of Defence (DoD) for GPS data, on a global infrastructure of active GPS reference stations,and on NASA’s JPL for the corrective information. Realtime productsThe Internetbased Global Differential GPS (IGDG) system aims at realtime precise position determination of a single receiver either stationary or mobile, anywhere and anytime. The concept of Precise Point Positioning (PPP) was introduced in the early 1970s, for more details refer to the key article by Zumberge et al. (1997). Precise Point Positioning utilizes fixed precise satellite clock and orbit solutions for single receiver positioning. This is a key to standalone precise geodetic point positioning with cm level precision.Over the past several years the quality of the Rapid IGS satellite clock and orbit products has improved to the cm level. Today the IGS Rapid service provides the satellite clock/orbit solutions within one day, with almost the same precision as the precise final IGS solutions (IGS, 2004).A good agreement between satellite clock error estimates produced by 7 Analysis Centers (AC) contributing to the IGS is reached. These estimates agree within – ns or 3 – 6cm. Currently IGS orbits with a few decimeter precision, can be made available in (near) realtime. Ultrarapid/predicted ephemerides are available twice each day(at 03:00 and 15:00 UT), and cover 48 hours. The first 27 hours are based on observations, the second part gives a predicted orbit. It allows one to obtain high precision positioning results in the field using the IGS products. of corrective informationTraditionally, DGPScorrections are broadcast over a radiolink from reference receiver to rover. With IGDG, corrections are disseminated over the open Internet. The user can access the very modest correction data stream using a (direct and) permanent network connection, or over the public switched telephone network (PSTN), possibly using an Asynchrone Digital Subscriber Line (ADSL). For a moving user access is possible using mobile (data) munication by cellular phone (possibly General Packet Radio Service (GPRS) or the Universal Mobile Telemunication System (UMTS) in future) or satellite phone. For mercial use three Inmarsat geosynchronous munication satellites are utilized to relay the correction messages on their Lband global beams. The three satellites (at 100?W (Americas), 25?E (Africa), 100?E (Asia Pacific)) provide global coverage from latitude ?75?to +75?.2 InternetBased Global Differential GPSIn Spring 2001, the Jet Propulsion Laboratory (JPL) of the National Aeronautics Space Administration (NASA) launched Internetbased Global Differential GPS (IGDG). Compared with traditional Differential GPS (DGPS) services, the position accuracy improves by almost one order of magnitude. An accuracy of 10 cm horizontal and 20 cm vertical is claimed for kinematic applications, anywhere on the globe, and at any time. This level of position accuracy is very promising for precise navigation of vehicles on land, sea vessels and aircraft, and for Geographic Information System (GIS) data collection, for instance with construction works and maintenance. A subset of some 40 reference stations of NASA’s Global GPS Network (GGN) allows for realtime streaming of data to a processing center, that determines and subsequently disseminates over the open Internet, in realtime, precise satellite orbits and clocks errors, as global differential corrections to the GPS broadcast ephemerides (as contained in the GPS navigation message). An introduction to IGDG can be found in Mullerschonetal. (2001a) and on IGDG (2004). Technical details are given in BarSever et al. (2001) and Mullerschonetal. (2001b).Internetbased users can simply download the lowband with correction data stream into a puter, where it will be bined with raw data from the user’s GPS receiver. The user’s GPS receiver must be a dual frequency engine and be of geodetic quality in order to extract maximum benefit from the accurate correc