International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 5

Number 1

March 2011

1 INTRODUCTION

Differential GPS (DGPS) has been an established

technique that provides meter level positioning in

real time over wide area typically using single fre-

quency Global Navigation Satellite System (GNSS)

receivers. DGPS systems may utilize single refer-

ence station based corrections, such as IALA mari-

time stations, or orbit and clock corrections derived

from wide area network, such as US WAAS and Eu-

ropean EGNOS. The accuracy of DGPS technique is

limited to about one meter level because of inherent

pseudorange noise. Also, single frequency based

DGPS accuracy may deteriorate during ionospheric

storms.

Real Time Kinematic (RTK) positioning tech-

nique can potentially provide centimeter-level posi-

tioning accuracy. However, RTK requires the dis-

tance to the closest reference station to be within

several tens of kilometers. Because of its fundamen-

tal distance limitations RTK is not well suited for

wide area and offshore positioning.

To address the needs of higher accuracy users

Fugro has pioneered over the last ten years several

decimeter level wide area real time positioning sys-

tems, called HP, XP, and the most recent G2. The

XP and HP systems are based on the use of GPS sat-

ellites only, while G2 is augmented with GLONASS

satellites. G2 is the first real-time positioning system

combining observations from GPS and GLONASS

that has wide area coverage. The addition of

GLONASS satellites in G2 positioning solutions re-

sults in improved availability and robustness of high

precision positioning compared to HP and XP, GPS

only systems.

These real-time systems developed by Fugro

achieve decimeter accuracy by using dual frequency

carrier phase observations to eliminate ionospheric

effects and provide high accuracy positioning. By

offering decimeter level accuracy over wide area,

Fugro high accuracy systems have bridged the accu-

racy and coverage gaps between a confined area

centimeter level RTK and a wide area meter level

DGPS.

Fugro high precision services use wide footprint

area geostationary satellites to transmit corrections

determined from global reference networks to the

GNSS users, conceptually shown in Figure 1. Fugro

developed HP, XP and G2 solutions are embedded

in virtually thousands of GNSS receivers. These re-

ceivers may be enabled to receive the corrections

broadcast via geostationary satellites. The receivers

with Fugro enabled HP, XP and G2 high precision

solutions are used in various applications such as

Dynamic Positioning of drill ships/supply vessels,

airborne lidar surveys, hydrographic surveys, tidal

control and precision agriculture.

Recent Advances in Wide Area Real-Time

Precise Positioning

D. Lapucha

Fugro Chance, Lafayette, USA

K. de Jong & X. Liu

Fugro Intersite, Leidschendam, The Netherlands

T. Melgard, O. Oerpen & E. Vigen

Fugro Seastar, Oslo, Norway

ABSTRACT: This paper describes briefly new high precision wide area real time positioning systems, dis-

cusses their evolution, implementation and presents recent results. In the latter part the paper is primarily fo-

cused on discussing the benefits of GLONASS augmented high precision positioning.

Figure 1. Geostationary Satellite Correction Broadcast

This paper describes briefly Fugro high precision

systems, discusses their evolution, implementation

and presents recent results. In the latter part the pa-

per is primarily focused on discussing the benefits of

combined GPS and GLONASS positioning as of-

fered by G2 system.

2 CARRIER PHASE POSITIONING

Unlike standard DGPS systems that use lower preci-

sion pseudorange observations Fugro high accuracy

systems use higher precision dual frequency carrier

phase data as primary observations in a positioning

solution. The use of dual frequency carrier phase ob-

servations enables virtual eliminating of ionospheric

delays, one of the major error sources of GNSS posi-

tioning. The carrier phase observations are adjusted

with the respective HP, XP or G2 correction infor-

mation before entering the user positioning filter.

Carrier phase observations are also corrected for var-

ious disturbing effects using high fidelity error mod-

eling.

The satellite ambiguities, inherent in carrier phase

observations, are recomputed for the rising satellites

that are brought in a solution. The carrier phase am-

biguities and residual atmospheric effects are then

estimated, along with a position, primary parameter

of interest in a user solution.

Achieving high accuracy after the cold start re-

quires some initial convergence time, typically 10 to

30 minutes. The convergence time is a function of

satellite geometry and tracking environment. Gener-

ally, shorter convergence time is achieved with bet-

ter satellite geometry.

3 MULTI REFERENCE HP

Fugro introduced the first commercial decimeter

level wide area real time positioning system in 2001.

This service named HP, is based on the application

of virtual station corrections optimized for the user

location (Lapucha et al, 2001). The virtual correc-

tions are computed using the corrections from the

region of the multiple reference stations typically

close to or surrounding the user. The virtual base

station corrections are applied to the rover observa-

tions to mitigate satellite clock and orbit errors.

Fugro operates a worldwide network of about a

hundred HP reference stations providing high preci-

sion coverage for major land masses and coastal are-

as. The HP solution approaches RTK accuracy if the

user is within the reference network and the closest

station is generally less than 1000 km. However, HP

positioning accuracy gradually deteriorates if the us-

er is outside of the network and the distance to the

closest station is more than 1000 km. Thus the HP

system is not suitable to provide high accuracy posi-

tioning in truly remote areas from reference stations

such as in the middle of the oceans.

Typical 24 hour monitoring results from the HP

system operating in dynamic mode at Lafayette,

USA, on the Gulf of Mexico coast, using four refer-

ence stations with the distances ranging from 350

km to more than 1000 km, observed on January 23,

2011 are shown in Figure 2. The position accuracy

given in terms of standard deviation is 3, 3 and 5 cm,

for longitude, latitude and height, respectively.

Figure 2. HP Position Results, Lafayette, January 23, 2011

HP solution was also extensively tested at various

locations and over an extended time. The results of

these tests showed that HP solution provides 10 cm

horizontal, 15 cm vertical accuracies 95% at dis-

tances up to 500 km and 15 cm horizontal, 30 cm

vertical accuracies 95% at distances up to 1000 km

from the closest reference station.

4 PRECISE POINT POSITIONING XP

The XP system, based on the Precise Point Position-

ing (PPP) method, was introduced by Fugro in 2003.

The precise orbit and clock corrections used in the

XP system are determined in cooperation with Na-

tional Aeronautics and Space Administration

(NASA) Jet Propulsion Laboratory (JPL) based on

NASA’s worldwide network of reference stations.

The PPP method used in the XP system involves

using the satellite specific precise orbit and clock

corrections instead of virtual reference station range

corrections. These corrections represent the most ac-

curate estimate of the errors of the GNSS satellites

broadcast orbit and clocks. Unlike the corrections

used in the multiple reference station HP method,

which are reference station and satellite specific, the

orbit and clock corrections used in the PPP XP

method are satellite specific only and not location

dependent. The positioning accuracy is no longer

limited by the distance from the reference stations.

Therefore, application of these corrections leads to

virtually homogeneous high positioning accuracy

worldwide.

Typical 24 hour monitoring results from the XP

system operating in dynamic mode at Lafayette,

USA, on the Gulf of Mexico coast, as observed on

January 23, 2011 are shown in Figure 3. The posi-

tion accuracy given in terms of standard deviation is

4, 3, and 8 cm, for longitude, latitude and height, re-

spectively.

Figure 3. XP Position Results, Lafayette, January 23, 2011

XP solution was also extensively tested at various

locations and over an extended time. The results of

these tests showed that the XP solution provides 10

cm horizontal, 20 cm vertical accuracies in terms of

95% statistics. Unlike HP, the accuracy of XP is not

dependent on location and distance from the refer-

ence stations.

5 GLONASS AUGMENTED PRECISE POINT

POSITIONING G2

Fugro introduced in 2009 truly the next generation

multi constellation real-time PPP system, based on

the use of precise GPS and GLONASS orbit and

clock corrections, called G2 (Melgard et al, 2009).

The development has benefited from the close coop-

eration between Fugro and the European Space Op-

eration Centre (ESOC), an establishment of the Eu-

ropean Space Agency (ESA). ESOC has contributed

with their expertise on precise orbit and clock pro-

cessing techniques while Fugro built an operational

real time system.

G2 position solution uses the PPP method with fi-

ne tuned statistical models to process GPS and

GLONASS satellite observations and precise orbits

and clock corrections determined from the global G2

network. These corrections are satellite specific only

and not location dependent, similarly to XP. There-

fore, the application of these corrections in a G2 user

solution leads to virtually homogeneous high posi-

tioning accuracy worldwide

The G2 service utilizes Fugro’s network of dual

system GNSS reference stations to calculate precise

orbits and clocks on a satellite by satellite basis for

all 50 plus satellites of the two global navigation sat-

ellite systems. The system comprises about 40 dual-

frequency GPS and GLONASS reference stations,

operated independently of HP and JPL networks,

evenly distributed around the world.

Successful integration of GLONASS carrier

phase observations in G2 solution required account-

ing for incompatibilities between GPS and

GLONASS systems. GLONASS satellites, unlike

GPS, use different satellite specific frequencies. Al-

so, GLONASS observations refer to different time

system than GPS. However, after accounting for the-

se differences GLONASS satellites act like addi-

tional GPS satellites in G2 solution.

Including GLONASS together with GPS satel-

lites improves redundancy, geometry and availabil-

ity of a positioning solution. Because of the greater

number of satellites and improved geometry, inte-

grated GPS and GLONASS G2 solution offers faster

convergence than GPS only solution (Melgard et.al,

2009). Additional GLONASS satellites offer the po-

tential to enable a positioning solution that may not

be possible with GPS only, especially in challenging

tracking environments with line of sight obstructions

such as depicted in Figure 4.

Figure 4. Challenging GNSS Tracking Environment

6 G2 POSITION ACCURACY ANALYSIS

In the following, G2 positioning results are present-

ed from different locations and times to assess the

representative G2 accuracy figures. These results

were achieved with the systems operating in dynam-

ic mode. It should be noted, the daily positioning ac-

curacy can vary from day to day and with location

depending on GNSS receiver, antenna and antenna

cable and local environment. It is therefore im-

portant to note the following results represent exam-

ples observed at the monitor stations.

Example, 24 hour monitoring results time series

from G2 system operating at Gulf of Mexico, Lafa-

yette location are shown in Figure 5. The position

accuracy given in terms of standard deviation is 2, 2

and 6 cm, for longitude, latitude and height, respec-

tively.

Figure 5. G2 Position Results, Lafayette, January 23, 2011

Example, monitoring results from the G2 system

operating in Oslo, Norway are shown in Figure 6.

The position accuracy given in terms of standard de-

viation is 2, 2 and 4 cm, for longitude, latitude and

height, respectively.

Figure 6. G2 Position Results, Oslo, January 2, 2011

Similar data were also collected in different loca-

tions around the world. Composite G2 accuracy fig-

ures given in terms of 95% statistics from some lo-

cations on January 23, 2011 are summarized in

Figure 7. These results demonstrate that the G2 solu-

tion provides consistently, even with incomplete

GLONASS constellation, 10 cm horizontal and 15

cm vertical accuracies in terms of 95% statistics.

Figure 7. G2 Position Accuracy Summary, January 23, 2011

7 BLOCKAGE SIMULATION

Combining GPS and GLONASS observations in the

integrated G2 solution offers potential to expand

availability of high precision solution when single

satellite system solution is not possible, as shown

earlier at Figure 4. Generally, at least four satellites

are necessary for single satellite system three dimen-

sional user position determination.

To assess G2 performance under blockage condi-

tions G2 data was reprocessed with simulated virtual

wall to the south blocking GNSS satellite observa-

tions, as shown in Figure 8.

Figure 8. Satellite Blockage Skyplot

During the simulated blockage times, when a

number of GPS satellites drops below the required

four, GPS only solution fails to provide position, as

shown in Figure 9. Moreover, positioning accuracy

deteriorates after outage because of reconvergence

in poor satellite geometry conditions.

Figure 9. GPS only PPP Results Under Blockage Conditions

However, during time when GPS solution was

not available, G2 solution provided seamlessly dec-

imeter level positioning with only slightly degraded

accuracy during blockage time, as can be in Figure

10. These results demonstrate improved resiliency of

G2 solution in challenging tracking environment.

Figure 10. G2 Results Under Blockage Conditions

8 GLONASS ONLY PPP RESULTS

Expanding GLONASS constellation offers potential

for PPP solution independent on GPS. As of begin-

ning of 2011, GLONASS constellation does not of-

fer 24 hour worldwide coverage. However it pro-

vides virtually 24 hour coverage of Northern

Europe.

Figure 11 presents recent position results from the

PPP system in Oslo utilizing only GLONASS obser-

vations and precise corrections in a user solution.

Moreover, GLONASS precise orbit and clock cor-

rections used in a solution were determined in the

process that also used GLONASS observations only,

completely independent on GPS.

The GLONASS PPP position accuracy from this

test given in terms of standard deviation is 5, 5 and 8

cm, for longitude, latitude and height, respectively.

Even with incomplete GLONASS constellation, the-

se results demonstrate that GLONASS PPP method

can potentially provide decimeter accuracy position-

ing independent on GPS.

Figure 11. GLONASS PPP Results, Oslo, January 26, 2011

9 OPERATIONAL SERVICES

Fugro HP, XP and G2 services and networks are op-

erated independently and redundantly to provide the

highest level of positioning integrity. All Fugro po-

sitioning services use geostationary satellites trans-

mitting data within L band, the same frequency band

as used by GPS and GLONASS satellites. Use of L

band has the advantage that user receivers can utilize

the same antenna for reception of GNSS and geosta-

tionary satellite signals. The geostationary satellites

used by Fugro provide virtually worldwide cover-

age, with the exception of polar regions, as can be

seen in Figure 12.

Figure 12. Geostationary Satellite Worldwide Coverage

Fugro developed HP, XP and G2 solutions are

embedded in major brand GNSS receivers. The re-

spective solutions can be activated with a subscrip-

tion. These receivers employ the Fugro software li-

brary that decodes subscription and correction data

and carries out high precision positioning computa-

tions. HP, XP and G2 enabled GNSS receivers are

used for numerous applications requiring high preci-

sion positioning on land, sea and in the air.

10 CONCLUSIONS

The real time positioning systems developed by

Fugro provide decimeter accuracy by using dual fre-

quency carrier phase observations. These systems

and services provide independently high accuracy

worldwide positioning using geostationary satellites

for correction broadcast.

Recently introduced G2 system is the first system

offering combined GPS and GLONASS PPP posi-

tioning. Including GLONASS together with GPS

satellites improves redundancy, geometry and avail-

ability of a positioning solution. It even opens for

the possibility to use GLONASS as a positioning

system completely independent of GPS also for

higher precision positioning adding a new dimension

to the redundancy.

REFERENCES

Lapucha D., Barker R., Ott L., Melgard T., Oerpen O. &

Zwaan H., 2001. Decimeter-Level Real-Time Carrier Phase

Positioning Using Satellite Link. In Proceedings of The In-

stitute of Navigation GPS 2001 International Technical

Meeting, September 11-14, 2001, Salt Lake City, USA

Melgard T., Vigen E., De Jong K., Lapucha D., Visser H., Or-

pen O. 2009. G2- The First Real-Time GPS and Glonass

Precise Orbit and Clock Service in Proceedings of The In-

stitute of Navigation GNSS 2009 International Technical

Meeting, September 22-25, 2009, Savannah, USA.