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An Investigation of GNSS Techniques
- 1. ! Geodesy & GNSS! University of Glasgow GNSS Survey! ! ! ! ! SECTION 1:! ! PROJECT:!! ! ! ! UoG STATIC GNSS SURVEY! ! SURVEY LOCATION:! ! ! University of Glasgow! DATA COLLECTION DATE:!! 17/01/2015! SURVEY GROUP:! ! ! Group 1! SURVEYORS:! ! ! Peter McCready, Colin McAteer, Wen Tian, Manpreet Puri! ! 16th February 2015! ! ! EXECUTIVE SUMMARY:! ! The purpose of the project was to gain precise positioning for two Control stations located within the University of Glasgow. This involved using Leica Static Global Navigation Satellite System to establish a reference station and two Control stations (A and B). The reference station was processed against the permanent Glasgow GNSS station and the control stations were processed against the reference station using precise ephemeris data for more accurate positioning. Leica GeoOffice 8.1 was used to process the GNSS data. A full list of processing parameters are stated below. The project fieldwork took place on Monday the 17th of January 2015 between 11am and 4pm at the University of Glasgow, Glasgow.! ! Key findings of the survey included the Easting (m), Northing (m) and Orthometric Height (m) coordinates of the reference station (256902.8959, 666545.5206, 41.0871) and stations A (256941.2634, 666539.6903, 38.4212) and B (256809.9889, 666575.8877, 39.1985). Station setup errors, inconsistency of occupation times and human error in processing may explain varying positioning solutions. ! ! ! PROJECT OBJECTIVE:! ! Obtain precise GNSS Easting, Northing and Orthometric Height positioning coordinates for two control stations (A & B) within the University of Glasgow 0806514|1 12Geodesy & GNSS
- 2. STUDY AREA:! ! ! ! ! STATION, ANTENNA AND DURATION INFORMATION:! ! ! ! ! ! ! ! STATION WITNESS DIAGRAMS:! ! STATION A! ! ! ! ! ! ! ! STATION B! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! EQUIPMENT USED:! ! • Tripod x2! • Optical Plummet! • Leica Static GNSS Reference Station and Rover! STATION ANTENNA ANTENNA HEIGHT(m) DURATION GLASGOW AR25 LEIT 7h 59’ 30” REFERENCE AX1202 GG TRIPOD 5h 06’ 59” STATION-A GS08 1.45 16’ 00” STATION-B LEIGS08 PLUS 25’ 36” 0806514|2 12Geodesy & GNSS • AX1202 GG Tripod Antenna! • GS08 Tripod Antenna! • LEIGS08PLUS Antenna N N Station B Station A N N NN
- 3. FACILITIES:! ! Leica GeoOffice (LGO) 8.1 PC software to process the GNSS data.! ! ! DATA PROCESSING:! ! Default LGO processing parameters were used to calculate co-ordinates for stations A & B.! ! ! GPS-Proc! ! Antenna! ! ! GNSS LGO PROCESSING STEPS:! ! • Glasgow OS Net RINEX data were downloaded for the surveyed time and imported into LGO.! • Reference Station SmartWorx data was processed against the Glasgow RINEX data.! • Stations A & B SmartWorx data were processed against the Reference Station SmartWorx data.! • Geoid Separations were calculated to generate Orthometric height of the stations.! • Display modified to Local and Grid to display co-ordinates in Easting and Northing.! • Results were exported for analysis.! ! ! Point_ID Start End Duration Height_Reading Antenna GLAS 08:59:44 16:59:14 7h 59’ 30” 0.0000 AR25 LEIT ref 11:24:14 16:31:13 5h 06’ 59” 1.2070 AX1202 GG Tripod STATION-A 12:41:55 12:57:55 16’ 00” 1.8100 GS08 STATION-B 12:33:47 12:59:23 25’ 36” 1.4240 LEIGS08PLUS Name Vertical Offset Phase Center Offset (vertical) L1 Phase Center Offset (vertical) L2 Additional Corrections AR25 LEIT 0 0.1551 0.1631 Elevation and azimuth AX1202 GG Tripod 0.36 0.0644 0.0640 Elevation and azimuth GS08 0.36 0.0878 0.0850 Elevation and azimuth LEIGS08PLUS 0 0.0648 0.0622 Elevation and azimuth 0806514|3 12Geodesy & GNSS
- 4. DATA PROCESSING RESULTS:! ! ! ! ! ! ! ! ! Positional and height quality of station A and B lateral (Easting and Northing) and vertical (Orthometric) coordinates appear to be of sufficient quality to accept the positioning solution for the project.! The final positioning co-ordinates for each station are displayed below (E, N, H):! ! ! ! ! ! ! ! ! ! ~:~ 0806514|4 12Geodesy & GNSS POINT_ID DATE_TIME EASTING NORTHING ORTHOMETRIC_ HEIGHT POSITION_ &_HEIGHT_ QUALITY POSITION_ QUALITY HEIGHT_ QUALITY GLAS 02/02/2015 18:24:18 256340.9141 664697.2608 17.4038 0 0 0 ref 19/01/2015 11:24:44 256902.8959 666545.5206 41.0871 0.0001 0 0.0001 STATION-A 19/01/2015 12:41:55 256941.2634 666539.6903 38.4212 0.0002 0.0001 0.0002 STATION-B 19/01/2015 12:33:47 256809.9889 666575.8877 39.1985 0.0003 0.0002 0.0003 ! ! Glasgow:! ! ! (256340.9141, 664697.2608, 17.4038)! ! Reference Station:! ! (256902.8959, 666545.5206, 41.0871)! ! Station A:! ! ! (256941.2634, 666539.6903, 38.4212)! ! Station B:! ! ! (256809.9889, 666575.8877, 39.1985)
- 5. SECTION 2:! ! OCCUPATIONAL TIME INVESTIGATION! ! Purpose:! Investigate the effect of length of observation session on data quality for ! ! ! static GNSS.! ! Definitions of utilised terms:! Position Quality:! ! ! Root Mean Squared (RMS) of the Standard Deviation of the ! ! ! ! ! two position elements.! Height Quality:! ! ! Standard Deviation of the height element.! Position and Height Quality:! ! Root Mean Squared (RMS) of the Standard Deviation of the ! ! ! ! ! position and height elements.! OBSERVATION PARAMETERS:! ! The Reference (ref) station was processed against the Glasgow station as more data was available for comparison. The following observation interval times were selected to assess the effect of the duration of occupation on the quality of data output:! ! ! ! ! ! ! ! ! ! ! ! ! ! ! The precise ephemeris .SP3 file was downloaded and imported into LGO. The GPS-Proc parameter for ephemeris was changed to use precise ephemeris. This was done to improve positional output by accounting for satellite orbital fluctuations from the lower-accuracy broadcast information. However, this information is not perfect and could be improved by better force models.! ! GNSS LGO PROCESSING STEPS:! ! • Precise ephemeris SP3 file downloaded and imported into LGO - processing parameter changed to use precise ephemeris information! 0806514|5 12Geodesy & GNSS TIME FRAME DURATION 12:00-16:00 4hrs 12:00-15:00 3hrs 12:00-14:00 2hrs 12:00-13:00 1hr 12:00-12:30 30mins 12:00-12:15 15mins 12:00-12:10 10mins 12:00-12:05 5mins
- 6. • Glasgow OS Net RINEX data were downloaded for the surveyed time and imported into LGO.! • Reference Station SmartWorx data was processed against the Glasgow RINEX data.! • Station A and Station B SmartWorx data were processed against the Reference Station SmartWorx data.! • Geoid Separations were calculated to generate Orthometric height of the stations.! • Display modified to Local and Grid to display co-ordinates in Easting and Northing.! • Results were exported for analysis.! ! DATA PROCESSING RESULTS:! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! 0806514|6 12Geodesy & GNSS Time_Frame Duration Point_ID Easting Northing Ellipsoid _Height Position _Quality Height_ Quality Position_ +_Height _Quality 08:59:44-16:59:14 7hr 59' 30" ref 256902.8960 666545.5206 95.2579 0.0002 0.0003 0.0003 12:00-16:00 4hr ref 256902.8966 666545.5204 95.2560 0.0002 0.0003 0.0003 12:00-15:00 3hr ref 256902.8965 666545.5202 95.2572 0.0002 0.0003 0.0004 12:00-14:00 2hr ref 256902.8965 666545.5199 95.2573 0.0003 0.0004 0.0005 12:00-13:00 1hr ref 256902.8979 666545.5207 95.2593 0.0004 0.0006 0.0007 12:00-12:30 30’ ref 256902.8972 666545.5201 95.2577 0.0006 0.0009 0.0010 12:00-12:15 15’ ref 256902.8969 666545.5238 95.2602 0.0008 0.0012 0.0014 12:00-12:10 10’ ref 256902.8958 666545.5228 95.2605 0.0009 0.0014 0.0017 12:00-12:05 5’ ref 256902.8961 666545.5201 95.2650 0.0014 0.0021 0.0025
- 7. RESULT ANALYSIS:! Easting varied by up to ±0.0021m (2.1mm), Northing varied by up to ±0.0039m (3.9mm), and Ellipsoid height varied by up to ±0.0090m (9mm) as a result of the varying occupational time. ! ! Position Quality is of similar quality between 3-8 hours of occupation time with 0.0002 but steadily decreases in quality to 0.0014 when occupation time reduces to 5mins.! ! Similarly, Height Quality is of similar quality between 3-8 hours of occupation time with 0.0003 but steadily decreases in quality to 0.0021 when occupation time reduces to 5mins.! ! Furthermore, Position & Height Quality is of similar quality between 4-8 hours of occupation time with 0.0003 but steadily decreases in quality to 0.0025 when occupation time reduces to 5mins.! ! CONCLUSIONS:! It can be concluded that in order to achieve better lateral and vertical positional quality the observer should measure static GNSS for between 3-8 hours at each location if possible. It appears that the longer the observation time the better the quality of positional solution, therefore, it would be advisable to observe for even longer to achieve higher levels of accuracy if required. This is because more readings can be taken over a longer occupational time so can average out to calculate a truer position. However, observational time is dependent on the quality required.! ! ~:~ 0806514|7 12Geodesy & GNSS
- 8. SECTION 3:! ! GROUP RESULTS COMPARISON! ! Each group processed GNSS data in Leica GeoOffice (LGO) 8.1.! ! ! ! ! ! ! ! ! ! The table above shows the static GNSS results of groups 1-5 for Station A and Station B. The co- ordinates generated by each group appear to be within ±0.01m (±1cm) laterally for station A and approximately ±0.02m (±2cm) for station B. Orthometric heights showed more significant differences for station A, approximately ±0.3m (±30cm), and around ±0.02m (±2cm) height difference for station B. ! ! ! ! The orthometric height difference for station A of ±0.3m (±30cm) is large and is an indication that errors may be present. Possibly errors in setup or processing may be present. These errors could result from a range of possibilities including:! ! ! • Measured to top of the peg! ! ! -! Incorrect height reading values may impact processing! ! • Poor centring and levelling when setting up stations! ! • Varying occupational time (as shown in the comparison in section 2 above)! ! • Incorrect selection of vertical datum! ! ! -! The use of a different ellipsoid (e.g. WGS84 instead of OSGB36) that is ! ! ! ! selected in project setup in LGO.! ! • Antenna not in LGO 8.1! ! ! -! If the antenna is not in LGO then the user must create a new antenna ! ! ! ! and input the vertical offset and the L1 and L2 vertical phase centre offsets ! ! ! values. The user must also select the Corrections type (e.g. Elevation and 0806514|8 12Geodesy & GNSS
- 9. ! ! ! Azimuth). Zenith Angle values may also be required. If a height hook was ! ! ! ! used then the user must account of this.! ! ! -! For example the GS08 antenna had to be altered to include the 0.36m ! ! ! ! (36cm) vertical offset. A new GS08 Tripod antenna was generated to ! ! ! ! account for this.! ! • Not processing correctly! ! ! -! Using Navigated (Point class) instead of Measured, i.e. did not process ref ! ! ! against GLAS before processing STATION-A and STATION-B (did not ! ! ! ! process the data relative to the base station)! ! ! -! If point class indicates ‘Navigated’ then the data has not been fully ! ! ! ! processed, in! this case the system may have processed the pseudo-ranges ! ! ! instead, giving only an approximate position! ! • Human error! ! ! -! Incorrect height or other manual inputs being entered incorrectly when post- ! ! ! processing, i.e. typos! ! ! It should be noted that it can’t be said for sure exactly why the coordinates produced by each group are different unless the exact cause of error is known.! ! ~:~ 0806514|9 12Geodesy & GNSS
- 10. SECTION 4:! ! GNSS ERROR MODELLING:! ! Although Leica GeoOffice (LGO) 8.1 is a very useful, relatively easy-to-use tool to process GNSS data it is not entirely flawless or without limitations. Some GNSS errors not modelled by LGO include the following:! ! Satellite! ! -! Clock Drift, Channel Biases! Propagation! ! -! Multipath! Receiver (Internal)! -! Measurement Noise, Channel Biases, Clock Error! Receiver (Position)! -! Ocean Tide Loading, Subsidence, Human Error! ! Small satellite clock drift occurs over time (3x10-9s ~ 1m pseudo range error). This can affect the positional accuracy as time is a key component of positional calculation. A time standard is maintained by ground control. It is common for manufacturers to apply leap-second offsets to correct drift but these corrections are not a one-time fix and must be continually applied as drift breaches a set threshold. A ‘steered’ oscillation may be applied instead to correct over time but is less commonly used.! ! Carrier phase have very low noise level and can be very precise but may suffer channel bias that can lead to thousands of kilometre error (navipedia.net). Differential channel biases associated with pseudo-range is due to the delay of codes in hardware. L1 and L2 channel differences are the main concern. Effect can be prevented by using a constant bias over a certain period of time. Can also be solved for as additional parameters.! ! Multipath error relates to satellite-to-receiver geometry and involves the reflection of satellite signals prior to being picked up by the GNSS receiver. It is dependent on the environment in which the receiver is positioned and can be reduced by positioning the receiver in an open or non- reflective environment. For example; buildings, vehicles, snow, rain water and vegetation can cause multipath. Multipath can be reduced by using cut-offs, choke rings or radomes with the receiver itself. Low pass, matched and Kalman filters may also be used. Time averaging over a certain time-frame may be used as a means of averaging out the multipath effect.! ! 0806514|10 12Geodesy & GNSS
- 11. Very low quality of receiver clock can result in a loss of satellite lock meaning the satellite must re- establish lock in order to re-calculate the integer ambiguity (number of wavelengths) using the phase reading at receiver, i.e. re-calculate receiver position. Hardware improvement, using a higher quality external clock or using differential GPS can prevent this issue.! ! Ocean Tide Loading (OTL) can cause considerable periodic lateral and vertical displacement of the Earth surface. Lateral displacement is generally one third of the magnitude of vertical displacement (Clarke & Penna, 2010). Clarke and Penna (2010) estimate displacement error in the United Kingdom of several mm in plan and more than 10mm in vertical (Clarke & Penna, 2010). Network relative positioning can be used to mitigate the error associated with OTL but it is recommended that OTL-modelling should be incorporated into GNSS processing softwares packages to eliminate OTL-related error and obtain more precise point positioning (Clarke & Penna, 2010).! ! Receiver subsidence and human error when measuring and inputting receiver antenna height can result in incorrect setting of receiver antenna height. This can affect the positioning accuracy as incorrect antenna height can be used as part of the positioning solution. Error can be minimised by careful set-up on more stable ground or averaging antenna height over time to minimise the effect of subsidence. Taking extra care when measuring antenna height and input when processing can help to eliminate or minimise human error.! ! Human error can occur at any stage of the positioning data capture and positioning solution. Set- up error when centring and levelling the receiver or incorrect input when setting processing parameters can lead to an inaccurate positioning solution. Extra care must be taken by the surveyor in set-up and post processing to prevent error affecting the positioning solution.! ! ~:~ 0806514|11 12Geodesy & GNSS
- 12. REFERENCES! ! Clarke, P. J., Penna, N. T., (2010) Ocean Tide Loading and Relative GNSS in the British Isles, Survey Review. [Online] 42 (317). p.212-228. Available From: https://www.staff.ncl.ac.uk/ peter.clarke/offprints/Clarke_Penna-2010-pp.pdf (Accessed: 5th February 2015)! ! N a v i p e d i a ( 2 0 1 2 ) [ O n l i n e ] h t t p : / / w w w . n a v i p e d i a . n e t / i n d e x . p h p / GNSS_Measurement_features_and_noise (Accessed: 5th February 2015)! ! Dr. Elizabeth Petrie - Lecture PowerPoint Slides 0806514|12 12Geodesy & GNSS