【正文】 measurement update. Meaningful settings for the noise of the observables can easily be found from an assessment of the measurement precision. In our case, the carrier phase observables have been assigned a measurement noise of 2 cm.The process noise of the state vector elements is in general more difficult to determine. For simplicity, it is assumed to result from an integrated white noise process,which means that the process noise increases linear in time. It is denoted qi for the filter state element i and is characterized with the standard deviation σ and τ time constant s. The process noise matrix has diagonal structure and the elements of the main diagonal are found from qi =σi2Δt/τi. The time difference Δt denotes the time between the consecutive epochs.For the process noise settings of the satellite clock states, no distinction is made between the individual clock types. Instead, the process noise settings are the same for all GPS satellites. The clock offsets have a process noise with a standard deviation of 3 cm and a timeconstant of 600 s. The clock drift process noise has a standard deviation of m/s (amp。GPS Solut (2009) 13:173–182DOI Kalmanfilterbased GPS clock estimation for near realtime positioningAndre180。 Hauschild . Oliver MontenbruckAbstract In this article, an algorithm for clock offset estimation of the GPS satellites is presented. The algorithm is based on a Kalmanfilter and processes undifferenced code and carrierphase measurements of a global tracking network. The clock offset and drift of the satellite clocks are estimated along with tracking station clock offsets, troposphericzenith path delay and carrierphase ambiguities. The article provides a brief overview of already existing nearrealtime and realtime clock products. The filter algorithm and data processing scheme is presented. Finally, theaccuracy of the orbit and clock product is assessed with aprecise orbit determination of the MetOp satellite andpared to results gained with other realtime products.Keyword Clock estimation Precise orbit determination Realtime Kalman filterIntroduction A growing number of near realtime precise point positioning (PPP) applications raise the need for precise GPS orbit and clock products with short latency. One of these applications is the precise orbit determination (POD) of remotesensing satellites, which is to be performed shortly after a ground station pass. The observations of the satellite’s GPS receiver are available immediately after the download to the ground station. For processing these data,the user requires precise orbit and clock data for theplete GPS constellation. The rubidium and cesium atomic standards of the GPS satellites are subject to clock noise and frequency variations, which can originate from a variety of effects and are hard to forecast. Predictions of clock offset and drift, which are provided for example in the predicted part of the ultrarapid orbits provided by IGS or the broadcast ephemerides, will deviate quickly from the true values by several decimeters or even meters. Thus,these orbit/clockproducts bee unusable for PPP applications, where a carrierphase based positioning accuracy down to centimeter level is desired. The solution to this problem is the use of clock offsets, which have been estimated from GPS measurements originating from a network of sensor stations. Currently, only a limited set of providers for precise (near) realtime orbit/clockproducts is available. Among them are three of the IGS Analysis Centers: JPL (BarSever et al. 2003), NRCan and ESA (Pe180。1012 s/s) over 900 s. Though these simplified assumptions do not strictly reflect the selected twostate clockmodel, they are favored for a realtime capable process pared to more elaborated clock models with characteristic process noise settings depending on the satellite block type (Senior et ) or even on the individual satellite clocks (Hutsell1996) adds more plexity, since changes of the used onboard frequency standard or untypical clock behavior must be detected to adjust the process noise , the benefits of the model would not be fully exploited. Adapting the settings in realtime from recent data significantly increases the putational load and has therefore not been attempted. However, a rigor assessment of the benefits different clock models deems helpful to identify possible enhancements. The differential zenith path delay of the ground stations are assumed to vary only marginally over time. Consequently,only a small amount of process noise with a standard deviation of 2 mm over 1 h is assigned. On the contrary, the ground station clock offset will