CC BY
95
УДК 629.783:528.48
Ir. Hans Visser, Ing. Cor de Kuijper
И. Ганс Виссер, Корнелис Куйипер
Нидерланды
OMNISTAR HP WORLDWIDE POSITIONING SERVICE
OmniSTAR HP - Служба высокоточного глобального позиционирования в реальном масштабе времени
1 ABSTRACT
OmniSTAR has a worldwide network of 100 GPS reference stations, which together with broadcasted precise orbit and clock info of GPS satellites allow decimetre level absolute position accuracy. OmniSTAR broadcasts these corrections over a network of 6 geostationary reference Satellites. In this article, the application of this technology to GIS mapping and the operating parameters will be explained. Concepts such as “Kick-start” (i.e) starting at a known point) and “static convergence” will be discussed.
2 INTRODUCTION
OmniSTAR BV is located in the Netherlands and is part of the Fugro group of companies. Together with sister companies in Houston (USA), Perth (Australia), Cape Town (South Africa), Abu Dhabi (Middle East) and Singapore, OmniSTAR sells GPS correction signals for the land market. These correction signals are sent by geostationary satellites.
These correction signals allow sub-meter and decimetre level accuracy around the world without the need for having own reference stations.
OmniSTAR broadcasts over six high power geostationary satellites, located at 36,000 km anywhere above the equator, measurements to the Mobile GPS receiver. The frequencies are in the range of 1,525-1,559 MHz, (GPS L1=1, 575.42 MHz, GPS L2=1227.60 MHz)
Table 1 OmniSTAR Geostationary L-band satellites
Nr. Satellite пале Area of Coverage І I Ш Data rate Centre Longitude East Border West Border South Border North Border
1 EA-SAT Europe. Russia. Middle Africa 1 53:- 152 500 1200 bp і 25° East 56: W 106= E SD°S S0= N
2 AF-SAT Africa. Middle Ea7,t 153; ISO 000 1200 bpi 25° East 56: W 106= E SD°S S0= N
3 AP-SAT A*.ia. India. Australia. New Zealand 1 535 137 500 1200 bp:; 109° East 25= E 16S= w SD°S S0=N
4 AM-SAT Noitli and Scutli America 1 535 137 500 1200 bp:; 9S° West 179= W 17= W SD°S S0=N
5 OPTUS Australia and New Zealand 1 558 510 000 1200 bp7. 145° East 95= E 173= E 66° S 7=N
6 MSV-W USA-West 1 536 7S2 000 1200 bp7. 120° West 25“ N 67“ N so°s SO' N
MSV-C USA-Central 1 534 741 O'OO 1200 bp7. 94° West 25= N 67= N SD°S S0=N
MSV-E USA-Eau 1 530 359 000 1200 bpi SD° Wetf 25= N 67z N SD°S S0=N
7 OC-SAT Pacific Ocean. Australia. New Zealand 1 535 1 £5 000 1200 bpi 143.5" East 62= E \w- W SD°S S0=N
In order to receive the OmniSTAR signal one needs a special Combined L1/L-band for DGPS or L1/L2/L-band Antenna for HP.
OmniSTAR DGPS Virtual Base Station (VBS)
The VBS solution uses GPS pseudo range measurements from all reference stations in the network. These pseudo-range measurements are sent to the GPS receiver. In the receiver is an algorithm, which is weighting the pseudo-range measurements based on distance, elevation mask and quality. From these measurements an optimized RTCM for that specific location (VBS) is calculated which is sent internally to the GPS receiver.
Figure 2 HP principle overview
OmniSTAR High Performance (HP)
OmniSTAR HP needs a of dual frequency receiver. In the receiver Ionospheric-free phase and code measurements from all reference stations are received. In the receiver the phase and code measurements of the network are combined with the measurements from the mobile GPS receiver. The total number of wavelengths to the satellites is then estimated.
The position, receiver clock and troposphere are estimated. After a receiver has been switched on the position solution converges from meter level in the first minute to cm level in 24 hours.
3 POSITION CONVERGENCE SPEED
When the sum of the standard deviations is 30 cm a flag is raised and the solution is considered converged. The NMEA GGA quality indicator will indicate code 5, which stands for RTK float ambiguity fixing or code 4, which stands for RTK accuracy level.
The convergence speed depends on
a) The number of GPS satellites tracked
b) The constellation change of these satellites over time
c) Distance to reference stations
The convergence process can be visualised as trying to fit the high accurate relative measured distance change to the satellite with the actual orbit. In order to make this fit well the orbit needs curvature. Curvature needs time.
Figure 3 OmniSTAR HP working overview
Kick-start
When the receiver starts at a point with known coordinates the position can be injected directly in the HP algorithm. This is called Kick-start or seeding as the start position is the “Seed” of the computation process. With Kick-Start the solution is already “converged” within a minute after switching on the receiver.
Static convergence
If the receiver is static the HP algorithm takes advantage of this fact giving average convergence in 10 minutes and maximum in 20 minutes.
Dynamic convergence
When the receiver is moving in relative open areas, like on tractors or aircraft it takes an average of 25 minutes and maximum 55 minutes to converge.
Calculations in post-processing mode for one day with a processing interval of 5 minutes in the Netherlands gave the following result
Table 2 HP kickstart, static and dynamic convergence time in minutes
HP Convergence in minute SEP=30 cm
Kickstart average 00! 36”
minimum time 00! 30,?
Maximum time 02! ir
Static average 09!31"
minimum time 06! or
Maximum time ii‘ or
Dynamic average time IV 46,?
Minimum time 10! 43
Maximum time 54-: 36,?
+SEP=Spherical Error Probability
These results will vary for other locations in the world based on satellite geometry and number of satellites. However the results will be in the same order of magnitude.
Figure 4 HP convergence time till Spherical Error Position (SEP) =30 cm for
Kickstart, Static and Moving.
Convergence depending on number of satellites
As it is a heavy calculation task we have looked at the number of Satellites available at the moment of convergence as possible indicator for convergence time.
Table 3 Convergence time in minutes versus number of satellites
Convergence time HP Static HP Dynamic
NSAT average mill max average min max
6 12' OS5’ 10’ 12” 17’04” 32: 53” 25:37” 48'20”
7 12'15” 07’34” 19’58” 28!44” 12! 34” 54'10”
8 10' 56:’ 06’56” 18’15” 23: 27” 11: 59” 54' 36”
9 09'36” 06’33” 17’29” 20! 29” 10! 45” 46'25”
10 09' 15:’ 07’32” 10’48” 19! 05” 10! 43” 28' 59”
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Figure 5 HP convergence speed depending on number of satellites
These values can be used to create a rule of thumb for the time it will take before the solution is converged.
4 GPS SATELLITE INTERRUPTIONS
The HP solution be can restored after satellites phase measurements were blocked by buildings or trees as long as there are a minimum of five common satellites available before and after the interruption.
When less than 5 satellites are available, because the solution goes in a tunnel or under heavy trees HP can regain the same level accuracy up to a period of 3 minutes.
The success rate of regaining the solution is dependent on:
a) Time without GPS measurements
b) Phase and code multipath before and after the interruption
c) The number of available satellites before and after the gap.
5 OMNISTAR L-BAND SATELLITE INTERRUPTIONS
Sometimes the Geostationary satellite is blocked by buildings or trees. In VBS mode the solution is valid up to 4 minutes without correction data. In HP mode the solution remains valid for a maximum of 5 minutes. Once a correction message becomes available the solution can be again of the same quality as before the outage.
6 USING GPS REFERENCE STATIONS
HP uses measurements of the nearest surrounding reference stations together with measurements of the mobile GPS receiver. Up to a distance of 1000 km there are enough common satellites to guarantee good position accuracy. Beyond 1000 km the position solution slowly degrades.
7 USING PRECISE ORBITS AND CLOCK
In addition to the HP solution based on Reference stations the correction satellites also broadcasts precise satellite orbits every minute and precise clock information every ten seconds. It is then possible without the use of any reference stations to estimate positions up to the decimetre level. When the position of the satellites is known within 20-30 cm and also the satellite clock error is known within a nanosecond (30 cm). This technique is also called “Precise Point Positioning” (PPP).
8 COMBINED REFERENCE STATIONS WITH PRECISE ORBITS
OmniSTAR Europe combines the measurement of the reference network (HP)
and orbits and clocks into an integrated solution.
This improves the robustness of the solution considerable.
a) Far away from reference stations the solution relies on precise orbits and clocks.
b) Close to reference stations the solution is improved using a network solution
c) Rare outages of individual reference stations will then not influence the solution.
9 GPS MANUFACTURERS WITH OMNISTAR
The following GPS manufacturers have implemented OmniSTAR algorithms in their GPS hardware.
Table 4 Overview of OmniSTAR capable receivers and firmware releases
Markets version date
Hemisphere DGPSMAX Maiine GIS
(CSl.'Satloc/ SLX 1,2,3 Ар/Avionics. 4.3 Dec 2005 VBS2
Outback) Outback Ae
Corneipost AG'GES
Leica GS5DGS20 +R.TS GIS 1.37 Jan. 2005 Nearest Station
Novate! Piopak LB Propak LB Span OEM Vehicle 2.312 Nov. 2005 VBS2 HP
OmniSTAR 3000 and 3100LR series GIS 3.00.22 Sept. 2004 VBS
3200LR12 Vehicle/GIS/Ag 3.0 Sept. 2005 VBS2
35O0LR L-band Demodulator 02.06.05 June. 2005 VBS2
3510LR L-band Demodulator 02.06.05 Juue. 2005 VBS2
7114LR12 Ag/GIS 3.0 Sept. 2005 VBS2
330OHP GIS.'Vehicle 2.310 Nov. 2005 VBS2 HP
S400HP GIS.'VehicLe 1 .Op2b2 June 2005 VBS,'HP
Raven Invicta 310 GIS.Vehic le.-Maiine Q1 2005 VBS2
RPR 410 Ae Q1 2006 HP VBS2
Scldiia GSR2650 LB GIS 2.310 Nov. 2005 VBS2 HP
Axis3 GIS 4.S De; 2005 VBS2
Thales MKTV Ag/GIS 1.37 Jan. 2005 Nearest Station
Topccn MAP-RT GIS 3.0a? Feb-2006 VBS
MAP-HP GIS.-С onstmcticn 1.0p2b2 June 2005 VBS HP
Trimble AsGPS 110 Ag/GIS 3.0 Oct. 2005 VBS2
AgGPS 114 Ag'GIS 3.0 Sept. 2005 VBS2
AsGPS 132 3.0 Sept. 2005 YBS2
AgGPS 2 52 Ag Autobteer
AsGPS332 Ag
DSM 332 Maiine 3.21 Mar 2006 VBS 2,'HP
SPS-550 Maiine Conitiuction
SPS-750 С onstructicn
SPS-B50 С onstructicn
Pathfinder p ower GIS 1.70 March 2005 VBS
PRO-XRS GIS 1.70 Jan. 2003 VBS
10 PDA GIS SOFTWARE
All receivers are supporting the NMEA output control. However to set the HP process in Kickstart or Static mode a command has to be sent to the receiver. The following PDA software packages do support HP Kickstart
- PocketGIS Pocket, Positioning Resources
- Arcpad6.01 ESRI (using Script file)
- Solo Software TDSWay
Imap
Topsurv
FieldGenius
Omnilogger
Viewall
Sokkia
Topcon
MicroSurvey
OmniSTAR South Africa (Psion) OmniSTAR BV
- Others...
11 WHERE IS OMNISTAR USED
OmniSTAR is used by thousands of customers around the world in many market segments and for niche applications. The most important markets are Agriculture, GIS, Construction and machine control.
12 AGRICULTURE
One of the main markets is Agriculture. In Agriculture OmniSTAR VBS is used for manual steering aids, mapping the contours of the area and the soil quality and measuring the quantity of production as function of position. It is used for spreading fertilizer, chemicals over the fields. This leads to significant improvements in efficiency and cost savings.
OmniSTAR HP is used for automatic steering of the Tractor. To create parallel lines for ploughing, seeding and spraying.
In GIS OmniSTAR VBS is used for data collection, map road marks and electricity poles. OmniSTAR HP is for instance used to map pipeline trajectories, cadastral applications, etc
14 MACHINE CONTROL
In Agriculture OmniSTAR HP is used to steer the Tractor to the correct lane and keep the Tractor on the lane with decimetre accuracy. In Construction HP is being used for road construction.
15 AVIONICS
In aviation the system is used in photogrammetry, flight trajectory control and flight inspection.
16 CONCLUSIONS
- With OmniSTAR the Customer does not need own reference stations
- OmniSTAR works worldwide
- OmniSTAR Reference stations increase accuracy and robustness
- Kickstart allows immediate decimetre production
- Static convergence takes 10 minutes (average)
- Dynamic convergence takes 25 minutes (average) and 50 minutes max.
- Convergence time is depended on the number visible satellites
- The converged HP solution has a 4-5 cm standard deviation
- A majority of GPS manufacturers support the OmniSTAR Signal in their hardware.
- OmniSTAR HP can be used for GIS measurements in open terrain to get fast decimetre level accuracy.
13 GIS
17 REFERENCES
1. Wide Area Carrier Phase Positioning Comparison of the Two Alternate Methods ENC-GNSS Rotterdam May 2004 ,Dariusz Lapucha and Richard Barker, Fugro Chance Inc., USA , Henk Zwaan, Fugro Intersite B.V., The Netherlands
2. A note on frame transformations with applications to geodetic datums,Tomas Soler&Al, NGS, NOS, NOAA,N/NGS22, #8825
3. The ITRF2000 Claude Boucher et al IERS ITRS Centre
© Ir. Hans Visser, Ing. Cor de Kuijper, 2006
