II IN T E R N A T IO N A L C O N FER EN C E___________
T R A N SPO R T SY ST E M S T E L E M A T IC S T ST '02
PO LITEC H N IK I ŚLĄSKIEJ 2002 T R A N SP O R T z.45, nr kol. 1570
GPS, DCPS, 1GDGPS, accuracy requirements, data transfer, internet Jacek JA N U S Z E W S K I1
D ATA T R A N SFE R FO R H IG H PR E C ISIO N U SERS IN T H E SEA, R O A D AN D R A IL TR A N SPO R T
T elem atics can be defined as the branch o f inform ation technology, w hich deals w ith the long
distance transm ission o f com puterized inform ation. T he accuracy o f position obtained from satellite navigation system s depends on th e m ode o f data transfer also. In the new solution, IG D G PS (In tern et- based G lobal D ifferential G lobal P ositioning System ) the data and corrections to the G PS broadcast ephem cris to authorized users are transm itted over the internet.
T R A N SFE R PR EC Y ZY JNY C H D A N Y C H DO U ŻY TK O W N IK Ó W W TR A N SPO R C IE M O R SK IM , LĄ D O W Y M I K O L EJO W Y M
T clem atyka m oże być zdefiniow ana ja k o część technologii inform acyjnych, które z ajm u ją się tra n sm isją kom puterow ych danych. D okładność m iejsca otrzym anego z system u satelitarnej naw igacji zależy rów nież od sposobu przesyłu danych. W now ym rozw iązaniu, IG D G PS (system oparty na internetow ym globalnym różnicow ym pozycjonow aniu) dane i popraw ki do transm isji G PS są przekazyw ane do upraw nionych użytkow ników przez internet.
1. IN TR O D U C TIO N
T he continuous inform ation o f u se r’s position is one o f the m ost im portant elem ents, which determ ines the safety o f the user in the transport. Safety o f sea transport consists of safety o f navigation, people, cargo and environm ent. T he inform ation about position is obtained the m ost frequently from specialised electronic p o sitio n -fix in g system s, in particular satellite navigation system s - A m erican GPS (Global P ositioning System - N avstar) and Russian G lonass. T he new system GNSS (G lobal N avigation Satellite System ), the system of the future will be used in aviation, sea transport and rail and road transport also [2].
A ccuracy requirem ents o f the u ser’s position depend upon various factors which include three different levels o f coverage (global, regional and local) as well as safety perform ance:
- essential use - safety o f life, - essential use - other applications, - non essential use.
T he user applications sum m ary is presented in the table 1, the sum m ary o f accuracy requirem ents in the table 2. It is recognized that the categorisation are som e w hat subjectively
1 E lectronic N avigation D epartm ent, G dynia M aritim e U niversity, Z jednoczenia 3, 8 1 -3 4 5 G dynia, Poland w nknt@ vega.w sm .gdynia.p!
48 Jacek JA N U SZEW SK I
based on current capabilities and may change, particularly if dependence on GNSS increases, the applications may m ove from N o n-essential to Essential. Also the distinction betw een local and regional is not always clear [7].
2. D A TA T R A N SFER IN D IFFER EN TIA L M O D E OF GPS
D ifferential GPS (DG PS) services have been developed in response to inherent and previously im posed lim itations o f GPS (horizontal circular error with 95% confidence level - 22.5 m eters). In the sea transport for the civil users (receiver L I, code C/A only) it is not sufficiently accurate to m eet the stringent requirem ents o f navigation in harbours and their approaches. T he fundam ental principle o f DGPS technology is the com parison o f the position o f a surveyed point, referred to as the reference position, w ith absolute positions obtained from GPS receiver at that point.
T able 1 U ser applications sum m ary
S a f e t y c r i t i c a l i t y
C o v e r a g e E s s e n t ia l N o n
S a f e t y o f life O t h e r e s s e n t i a l
G lo b a l
A v ia tio n M a rin e : O c e a n ic
S A R
T im in g a n d fre q u e n c y
F is h e rie s - d e e p se a R e c re a tio n a l
R e g io n a l
A v ia tio n
M a rin e : C o a s ta l p h a s e R o a d : S a fe ty a n d s e c u rity
C o llis io n a v o id a n c e R a il: T ra in lo c a tio n an d
co n tro l
R a il: m a n a g e m e n t in fo r
m a tio n
R o a d : F le e t m a n a g e m e n t L a n d su rv e y
M a rin e su rv e y
R o a d : In fo rm a tio n s e rv ic e s N a v ig a tio n
D e m a n d m a n a g e m e n t R a il: P a s s e n g e r in fo rm a tio n
L o c a l
A v ia tio n
M a rin e : H a rb o u rs I n la n d w a te rw a y s R a il: T ra in lo c a tio n a n d
c o n tro l
M a rin e : D r e d g in g H y d ro g ra p h y
T ra c k in g p c rso n e l a n d c o n ta in e rs
R o a d : T ra f f ic c o n tro l
T able 2 Sum m ary o f accuracy requirem ents [m]
C o v e r a g e
E s s e n t ia l
N o n e s s e n t i a l S a f e ty o f life O t h e r
G lo b a l 1 0 - 1 0 0 1 0 - 1 0 0 1 0 - 1 0 0
R e g io n a l 1 - 10 1 - 1 0 1 - 1 0
L o c a l 0.1 - 10 0 .01 - 10 0 .0 3
49
T he pseuodorange technique used in DGPS receivers for the sea users involves com puting a unique correction (Pseudo R ange C orrection - PR C ) to the range o f each satellite observed at the reference station but does not necessarily require all o f the sam e satellites to be ones used by the m obile observer. Investigations o f transm itting the pseudoranges corrections to the user, identified m edium frequency m arine radiobeacons operating in the 283.5 - 325 kH z (M F band) as the m ost suitable terrestrial m ethod for use w ithin the coastal zone. An A m erican correction m essage form at know n as R TC M SC 104 has becom e the internationally standard for encoding DGPS corrections [1],
T he results o f m easurem ents o f several hundred thousands DGPS positions fixed by several different types o f DGPS receivers w ith different reference stations perform ed in G dynia (on B altic Sea) in different ports in Europe and A m erica are dem onstrated in [4).
DGPS secures the continues positions w ith accuracy o f few m etres for the m ariner users. T he results o f using o f DGPS with or w ithout S.A w ere practically the sam e in m aritim e navigation. By chance, the m easurem ents w ere realised during the last days o f April 2000 and during the first days o f M ay 2000, i.e. w ith and w ithout Selective A vailability (S.A.
was sw itched o f f 2 M ay 2000). As a result w e can say univocally, that the accuracy o f DGPS positions obtained during all this period was the same.
A ccuracy o f DGPS position depends on the param eters o f the receiver (its antenna and cable), the distance from the reference station, the “quality” o f this station, the DGPS correction type num ber (1 or 9), the age o f corrections and the suppleness o f the user’s receiver to electrom agnetic interference and radio noise. In each port the user m ust take the probability o f disturbances in the reception o f signals from satellites and reference station into consideration.
As the DGPS position accuracy decreases w ith the distance from the reference station, the effective range o f these stations is 100 -r 200 nautical m iles only. T h a t’s why the new DGPS system s using geostationary satellites have also been developed. A netw ork of reference stations provides the pseudorange corrections, w hich are transm itted to the satellites for broadcasting to users. T he coverage o f the satellite broadcast is very m uch greater than M F radiobeacons and the “range” o f the system is only constrained by the distance from the reference station over w hich the derived corrections rem ain valid.
F or land navigation and transport (cars, railway etc.) the satellite navigation system s as GPS, G lonass o r G alileo are (at present) o r w ill be (in the future) an additional positioning system only. T he position o f trucks and trains can be obtained from few other m ethod;
uninterrupted inform ation about latitude and longitude o f these vehicles is not one o f the most im portant elem ents o f the safety, on the contrary to the safety o f navigation in the sea transport [3]. T he inform ation about actual position o f the vehicle obtained from satellite system can be im m ediately transm itted to the center, to the m aster control station etc.
3. D A TA T R A N SFER O V ER IN TER N ET - IGD G PS
M any com m ercial, civilian, scientific and m ilitary applications require precise (decim etre accuracy o r better) real-tim e, onboard know ledge o f receiver’s position and velocity as a critical com ponent. R e al-tim e kinem atic (RTK ) techniques have enabled decim etre-lev el (and better) rea l-tim e G P S -based positioning, but w ith a few kilom etres local range scale only. For regional applications satellite-b ased augm entations system s as W ide A rea D ifferential GPS (W A D GPS) and various com m ercial differential system s can be
50 Jacek JA N U SZEW SK I used. B ut global (ground and airborne) rea l-tim e positioning at the decim etre level has never been attem pted nor achieved due to the perceived technical and cost challenges.
T he solution o f this problem - two param eters at the sam e tim e - very high position accuracy and global coverage, is new system - IGD G PS (Internet-based G lobal D ifferential GPS). This is a uniquely pow erful and flexible C -lan g u ag e softw are package that provides a com plete e n d -to -e n d system capability for G P S -based rea l-tim e positioning and orbit determ ination. T he features o f IGD G PS are [8]:
10 cm horizontal and 20 cm vertical real tim e positioning accuracy w ith dual frequency GPS receivers,
- 5 cm - level real tim e orbit determ ination for low Earth satellites carrying dual frequency GPS receivers,
the m ost precise and com plete set o f geodetic, satellite dynam ics, and G PS m easurem ents m odels for uncom prom ising accuracy,
dual m ode application: dedicated softw are used to com pute GPS o rbit and clock corrections and, em bedded, for user positioning/orbit determ ination,
valid corrections anyw here in the w orld o r in space, - In tern et-b a sed correction dissem ination,
- softw are proven operationally in a num ber o f critical.
T he fundam ental tenet o f the G lobal D ifferential GPS (GD G PS) is a “sta te -sp a c c ” approach in w hich the orbits o f the GPS satellites are precisely m odeled and the satellite epoch states, thus guaranteeing that the corrections will be global and uniform ly valid. In contrast, at present m ost differential system s em ploy a “m easurem ent-space” approach, w here the estim ated param eters are the user local range errors. T hese errors depend on the user’s position local relatively to the reference network.
IG D G PS is geared toward users carrying d u al-frequency receivers. T hese h ig h -en d users typically require h igh-accuracy positioning. H aving elim inated the ionosphere as an error source using d u al-b a n d receivers, these users are still susceptible to errors in the GPS ephem eris and clocks. G ro u n d -b ased users (m arine, rail and road transport) and aircraft (aviation) m u st also deal w ith errors due to the troposphere. A t present (July 2002) there is one civil GPS frequency (LI = 1575.42 M H z) only. T he situation w ill im prove in 2003 with launch o f the first IIR -M satellite with the new civil signal on L2 (1227.6 M H z). W hen the first satellite IIF satellite is launched in 2005, the num ber o f civil signals w ill increase to three by addition the L5 (1176.45 M Hz) [5].
3.1. D A T A PR O C ESSIN G
A ccurate correction for the GPS ephem eris and clock errors requires a netw ork o f GPS reference sites. W ith G lobal D ifferential G PD architecture, a w ell distributed global netw ork o f about a dozen sites is sufficient for continuously providing GPS ephem eris and clock corrections for GPS satellites. F or this purpose N A SA G lobal GPS N etw ork (GGN), w hich is operated and m aintained by Jet P ropulsion L aboratory (JPL), was used. T he GGN consists o f approxim ately 60 d u al-freq u en cy GPS geodetic reference stations, operates as a com ponent o f the International GPS S ervice’s GPS netw ork. D ata from the G G N are norm ally dow nloaded in batch m ode, b u t to support a variety o f N A SA m issions w ith low data latency requirem ents, the G G N is being upgraded to provide GPS data in real tim e over the open internet. T h a t’s w hy a subset o f the GGN is equipped w ith com puters and internet connection.
51 In tern et-b ased data com m unications w ere chosen over the m ore conventional telep h o n y - based com m unications prim arily for reason o f its significant econom ical advantage.
IG D G PS has been designed to return all GPS data in real tim e from rem ove receivers.
IGD G PS collects, edits and com presses the raw GPS observable at the rem ote site. It then transm its the packetized data over the open internet (227 bytes/second) to the processing center - the central data daem on. A daem on is a com puter program that runs continuously and exists for the purpose o f handling periodic service requests. A t this center the global data is analysed by IG D G PS to produce precise GPS orbits and clocks. T hese are form atted as global differential corrections to the GPS broadcast ephem eris, encoded and are provided over the internet (560 bit/sec m essage) to authorized users.
In addition to produce the differential corrections, IGD G supports the end user w ith a m odule for onboard, autonom ous user positioning and orbit determ ination. In this m ode IGDG com bines the correction m essage and the user GPS observables to provide precise estim ates o f the user position, tropospheric delay and the receiver’s clock estim ates. For users with know n dynam ics (spacecraft) IGD G perform s precise orbit determ ination constrained by the platform ’s dynam ics through a form o f K alm an filtering. For the users with unknown dynam ics (vehicles, airplanes) IGDG provides unconstrained kinem atic positioning [6].
T he GD G PS system has been operated at JP L since N ovem ber 1999 w ith autom ated distribution to approved users via the Internet.
T he com parison o f capability o f GPS and its two m odes - W A D G PS and IGD GPS is presented in the table 3.
Table 3 C om parison o f G PS, W A D G PS and IG D G PS
C a p a b ility G PS W A D G P S IG D G P S
C overage
Global + - +
Seam less + - +
Space + - +
A ccuracy
K inem atic
applications 5 m > 1 m 0.2 m (dem onstrated
0.1 m (projected) O rbit
determ ination 1 m - 0.01 - 0 .0 5 m
(projected)
D issem ination m ethod B roadcast B roadcast Internet/broadcast
T argeted users Dual frequency Single frequency D ual/frequency
4. CO N CLU SIO N S
T he new generation o f GPS satellites (IIR -M and IIF) with the second and third civil frequency perm it the users to profit from d u al-frequency receivers. T h e global differential corrections will be transm itted over open internet, to provide very high position accuracy and global coverage, w hich (IG D G PS) will be accessible in aviation, sea transport and rail or road applications.
52 Jacek JA N U SZEW SK I
BIB LIO G R A PH Y
[1] A dm iralty List o f R adio Signals, vol. 8 Satellite N avigation System s, N P. 2 88, T h e U nited K ingdom H ydrographic O ffice, 2002/2003
[2] JA N U S Z E W S K I J., S afety o f N avigation in th e Sea T ransport, Proceedings o f Scientific C onference T ran sp o rt System s E ngineering, Section 2 - T raffic C ontrol in T ransport, W arsaw , S eptem ber 1995 [3] JA N U S Z E W S K I J „ W hy does m aritim e navigation need G N SS?, Ill E uropean Sym posium G N S S ’99
(G lobal S atellite N avigation System s), vol. II, p. 5 2 5 -5 3 0 , G enoa, O ctober 1999
[4] JA N U S Z E W S K I J„ U ser’s T ests o f D G PS A vailability and A ccuracy, V GN SS International Sym posium (G N S S 2001), Sevilla, M ay 2001
[5] JA N U S Z E W S K I J„ System y satelitarne w naw igacji m orskiej, Fundacja R ozw oju W yższej Szkoły M orskiej, G dynia, ISB N 8 3 -8 7 4 3 8 -5 2 -9 , G dynia 2002
[6] M U E L L E R S C H O E N R „ N A S A ’s Globa! D G PS for H igh P recision U sers, G PS W orld, vol.12 N u m b er 1, January 2001
[7] SP1LLER J„ T A P S E L L T ., PE C K H A M R., P lanning o f the fu tu re S atellite N avigation System s, T he Journal o f N avigation, vol.52, N um ber 1, January 1999
[8] w w w .gipsy.jpl.nasa.gov
R eviewer: Prof. A ndrzej Lew iński
