GPS Data Automatic Computation Services of Hong Kong SatRef System JAMES WONG, LS/G(NT)

GPS Data Automatic Computation Services Purposes To provide simple GPS static survey computation services to general public. To provide simple GPS static survey computation services to general public. Target Users Non survey professional users who require to perform GPS simple data computation for positioning. Non survey professional users who require to perform GPS simple data computation for positioning.

GPS Data Automatic Computation Services Description of Service Users can upload their RINEX data into the SatRef System via Internet for performing GPS static survey computations. Users can upload their RINEX data into the SatRef System via Internet for performing GPS static survey computations. The system shall automatically select 3 nearest SatRef stations or user-defined SatRef stations to compute the position of the rover station. The system shall automatically select 3 nearest SatRef stations or user-defined SatRef stations to compute the position of the rover station. The system shall send the computed rover position to the users ’ e- mail address via Internet within 5 minutes. The system shall send the computed rover position to the users ’ e- mail address via Internet within 5 minutes.

GPS Data Automatic Computation Services

Requirements of the RINEX Data Files 1. Maximum number of RINEX data files to be uploaded is 3 pairs. 2. Each pair of RINEX files (n and o files) contains only one unknown point's GPS data. 3. Each pair of RINEX files (n and o files) contains only one observation session data. 4. All uploaded RINEX data must not be older than 3 months. 5. All uploaded RINEX data must be surveyed in the same GPS day. 6. Total file size of RINEX data to be uploaded shall not be greater than 10 MB.

Web Demonstration

GPS Data Automatic Computation Services Normally, two s shall be sent to the user. They are listed as follows: Acknowledgement Acknowledgement Acknowledgement Acknowledgement Reply with Processing Result Reply with Processing Result Reply with Processing Result Reply with Processing Result

This automatic computation process includes : 1. Automatic computation of baselines 2. Comparison of the repeated baselines 3. Preliminary quality checking 4. Rejection of blunders 5. Automatic least squares adjustment 6. Detection of outliers 7. Elimination of outliers 8. Iteration of the outlier detection process until completion of data cleaning 9. Computation of final adjustment results 10. Computation of residuals and standard errors

Testing of the Automatic GPS Data Computation

Limitations of the Automatic GPS Data Computation Service 1. No checking on GPS antenna model 2. No checking of number of satellites observed 3. No checking of length of observation session 4. No checking of the time separation between sessions 5. No rejection of bad satellites 6. No rejection of GPS data with many cycle slips/ large DOP values 7. No result output of origin checking 8. No checking by terrestrial observations, if necessary 9. Simple GPS network computation only 10. Different organizations and jobs have different survey specifications and computation approaches. 11. Waste of GPS observations because of no data cleaning, no data windowing and GPS baseline quality checking in 3D

Recommendations This automatic service provides a convenient tool to the non- survey professional users for processing their GPS static data and computing the position of their unknown points in a more systematic manner. This automatic service provides a convenient tool to the non- survey professional users for processing their GPS static data and computing the position of their unknown points in a more systematic manner. For the more reliable and precise GPS computations, the users must consult the professional land surveyors or professional GPS specialists. For the more reliable and precise GPS computations, the users must consult the professional land surveyors or professional GPS specialists.

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