GPS Ra dio Occultation: Re sults from CHAMP, GRACE andFORMOSAT-3/COS MIC

Jens Wickert1, *, Grzegorz Michalak1, Torsten Schmidt1, Georg Beyerle1, Chio-Zong Cheng 2, Sean B. Healy 3, Stefan Heise1, Cheng-Yung Huang 4, Norbert Jakowski 5, Wolfgang Köhler1,

Christoph Mayer 5, Dave Offiler 6, Eiji Ozawa 7, Al ex an der G. Pavelyev 8,Markus Rothacher1, By ron Tapley 9, and Chris tina Ar ras1

1 Helmholtz Cen tre Potsdam, GFZ Ger man Re search Cen tre for Geosciences, Potsdam, Ger many2 Na tional Cheng Kung Uni ver sity (NCKU), Tainan, Tai wan, ROC

3 Eu ro pean Cen tre for Me dium Range Fore casts (ECMWF), Read ing, UK4 Na tional Space Or ga ni za tion (NSPO), Hsinchu, Tai wan, ROC

5 Deutsches Zentrum für Luft- und Raumfahrt (DLR), Neustrelitz, Ger many6 Met Of fice, Exeter, UK

7 Ja pan Me te o ro log i cal Agency (JMA), Tsukuba, Ja pan8 In sti tute for Radioengineering and Elec tron ics (IRE), Mos cow, Rus sia

9 Uni ver sity of Texas, Aus tin, US

Re ceived 27 June 2007, ac cepted 26 December 2007

AB STRACT

The Tai wan/US FORMOSAT-3/COS MIC (FORMOsa SAT el lite mis sion - 3/Con stel la tion Ob serv ing Sys tem for

Me te o rol ogy, Ion o sphere and Cli mate) sat el lite con stel la tion was suc cess fully launched on 14 April 2006. It is ex pected to

le ver age the use of the GPS (Global Po si tion ing Sys tem) ra dio occultation data for at mo spheric and ion o spheric re search to

im prove global weather fore casts and aid cli mate change re lated stud ies. FORMOSAT-3/COS MIC, to gether with the Eu ro pean

MetOp, Ger man CHAMP and US/Ger man GRACE-A sat el lites, form a 9 sat el lite con stel la tion for pre cise at mo spheric

sound ing on a global scale. This sat el lite con stel la tion is expected to provide about 3500 occultation measurements daily.

Re cent re sults and the sta tus of the CHAMP and GRACE-A or bit and occultation data anal y sis are re viewed and

com ple mented with a re view of ini tial re sults from FORMOSAT-3/COS MIC at GFZ. The sig nif i cantly in creased po ten tial of

the CHAMP, GRACE-A and FORMOSAT-3/COS MIC con stel la tion for at mo spheric stud ies, com pared to sin gle sat el lite

mis sions, is dem on strated for se lected ap pli ca tions such as global mon i tor ing of wa ter va por dis tri bu tions, tropo pause

parameters and ionospheric irregularities.

Key words: Ra dio occultation, Weather fore cast, Tropo pause, Ion o sphere, Wa ter va por, Spo radic E-layer

Ci ta tion: Wickert, J., G. Michalak, T. Schmidt, G. Beyerle, C. Z. Cheng, S. B. Healy, S. Heise, C. Y. Huang, N. Jakowski, W. Köhler, C. Mayer, D. Offiler, E.Ozawa, A. G. Pavelyev, M. Rothacher, B. Tapley, and C. Ar ras, 2009: GPS ra dio occultation: Re sults from CHAMP, GRACE and FORMOSAT-3/COS MIC. Terr. Atmos. Ocean. Sci., 20, 35-50, doi: 10.3319/TAO.2007.12.26.01(F3C)

1. IN TRO DUC TION

The year 2006 marks the be gin ning of an era in which a

multi-sat el lite con stel la tion is used for pre cise at mo spheric

sound ing on a global scale us ing GPS ra dio occultation (RO,

e.g., Yunck et al. 2000). In ad di tion to the Ger man CHAMP

(CHAl leng ing Minisatellite Pay load) sat el lite (e.g., Wickert

et al. 2001a; Reigber et al. 2005), which since its launch in

2001 has pro vided oc cultation data al most con tin u ously

(e.g., Wickert et al. 2006a, b), mea sure ments from sev eral

ad di tional mis sions be came avail able in 2006. The US/

Ger man GRACE-A sa t el lite (e.g., Dunn et al. 2003; Tapley

and Reigber 2004) has pro vided occultation data con tin u -

ously since 22 May 2006. The six sat el lites of the Tai wan/

US FORMOSAT-3/COS MIC mis sion were suc cess fully

launched on 14 April 2006 (Cheng et al. 2006; Anthes et al.

2008) and once, in the fi nal or bital con fig u ra tion more than

Terr. Atmos. Ocean. Sci., Vol. 20, No. 1, 35-50, February 2009 doi: 10.3319/TAO.2007.12.26.01(F3C)

* Cor re spond ing au thorE-mail: wickert@gfz-potsdam.de

2500 pro files per day are ex pected. The Eu ro pean op er a -

tional weather sat el lite MetOp was launched on 19 Oc to ber

2006 and ini tial results of the GRAS (GNSS

Re ceiver for At mo spheric Sound ing; see,

e.g., Montenbruck et al. 2008) on 27 Oc to ber,

in di cated the po ten tial for pro vid ing more than

600 mea sure ments per day (www.eumetsat.int;

von Engeln et al. 2007). This multi-sat el lite

con fig u ra tion was fur ther extended by the

Ger man TerraSAR-X with an In te grated GPS

Oc cul tation Re ceiver (IGOR) re ceiver on board

when it was launched on 15 June 2007 via a

Rus sian Dnepr-1 rocket from Baikonur (Rus -

sia). The mea sure ments from all these sat el -

lites en hance the po ten tial of the GPS oc cul -

tation tech nique for sev eral ap pli ca tions in

at mo spheric re search, weather fore cast and cli -

mate change re lated stud ies.

We briefly re view the sta tus and the op er a -

tional data an al y sis of GPS RO aboard CHAMP

and GRACE-A and pre sent ini tial GFZ re sults

from the FORMOSAT-3/COS MIC.

2. HIS TORY AND STA TUS OF RA DIOOCCULTATION ABOARD CHAMPAND GRACE

CHAMP reached its 7th an ni ver sary in or bit

on 15 July 2007. A suc cess ful or bit up lift of

19 km was per formed at the end of March 2006

to ex tend CHAMP’s life time, which is cur -

rently ex pected to end in 2009. The sat el lite and

in strument are in ex cel lent con di tion and the

op er a tion is cur rently funded un til 2009. GPS

ra dio occultation aboard CHAMP was ac ti -

vated on 11 Feb ru ary 2001, more than 300000

pro files have been de rived as of mid Sep -

tember 2007 (Fig. 1). Ion o spheric pro files from

CHAMP are avail able since 11 April 2001 (see,

e.g., Jakowski et al. 2002).

Ini tial occultation mea sure ments from

GRACE were re corded dur ing a 25 h pe riod on

28/29 July 2004 aboard the GRACE-B sat el lite

(Wickert et al. 2005; Beyerle et al. 2006). A

lon ger pe riod of GRACE-A mea sure ments (41

days) was ob tained be tween 12 Jan u ary and 20

Feb ru ary 2006 be fore con tin u ous ac ti va tion be -

gan on 22 May 2006 (Fig. 1). Ion o spheric pro -

files from GRACE-A are con tin u ously avail -

able since 28 Feb ru ary 2007. The mis sion du ra -

tion of GRACE is cur rently ex pected to last un -

til 2012 - 2015.

A new level as far as the num ber of re -

corded oc cultations is con cerned, was reached

af ter the launch of the FORMOSAT-3/COS MIC sat el lites on

15 April 2007 (Fig. 2). Be gin ning in the au tumn of 2006, up

36 Wickert et al.

Fig. 1. Num ber of daily ver ti cal at mo spheric pro files, de rived from (a) CHAMP and

(b) GRACE-A GPS occultation mea sure ments (GFZ pro cess ing). For the 2252 days

of CHAMP RO ac ti va tion 315290 pro files have been col lected (on av er age about

140 per day) as of 17 Sep tem ber 2007. The num ber of avail able pro files from

GRACE-A for 526 days of RO ac ti va tion is 63064 as of 30 Sep tem ber 2007 (on av er -

age about 120 per day). The pro files from CHAMP form the first long-term RO data

set which is ex pected to last un til the end of 2009.

Fig. 2. Num ber of ver ti cal at mo spheric pro files from FORMOSAT-3/COS MIC as of

7 Sep tem ber 2007 (504 days, on av er age about 1270 per day). The data are pro vided

by UCAR (Uni ver sity Cor po ra tion for At mo spheric Re search) via www.cosmic

.ucar.edu.

(a)

(b)

to 2000 high qual ity pro files per day have been pro vided to

the sci en tific user com mu nity. More than 2500 daily mea -

sure ments are ex pected af ter reach ing the fi nal or bital con -

fig u ra tion in 2007 (Schreiner et al. 2007; Anthes et al. 2008).

3. OCCULTATION DATA ANAL Y SIS

CHAMP and GRACE-A or bit and occultation data are

op er a tion ally an a lyzed by GFZ be gin ning with raw data

(GPS ob ser va tions of the ze nith and occultation an tenna) up

to higher level data prod ucts, the glob ally dis trib uted ver ti -

cal pro files of at mo spheric pa ram e ters (see, e.g., Wickert et

al. 2004a). Ion o spheric occultation data are an a lyzed at the

Ger man Aero space Cen ter (DLR) Neustrelitz, Ger many

(e.g., Jakowski 2005). FORMOSAT-3/COS MIC data will be

pro cessed at GFZ for se lected time pe ri ods.

3.1 Ground In fra struc ture

The op er a tional occultation ground in fra -

struc ture from GFZ is used for re ceiv ing and an -

a lyz ing the CHAMP and GRACE-A data. It con -

sists of a po lar and mid-lat i tude re ceiv ing sta tion

(Ny Ålesund, Spitsbergen, 79.0°N, 11.5°E and

Neustrelitz, Ger many, 53.1°N, 13.1°E, DLR), a

high rate and low la tency fiducial net work of

glob ally dis trib uted GPS ground sta tions (jointly

op er ated with Jet Pro pul sion Lab o ra tory, JPL),

the au to mated sat el lite or bit and oc cultation

pro cess ing sys tems and GFZ’s data ar chive. The

op er a tional ca pa bil ity and in ter play of these

com po nents al low for the dem on stra tion of a

near-real time (NRT) oc cultation data anal y sis,

which is al most con tin u ously dem on strated with

CHAMP and GRACE-A (see also sec tion 3.4). A

more de tailed over view on the occultation infra -

structure and rel e vant ref er ences are given by

Wickert et al. (2004a).

3.2 Pre cise Sat el lite Or bit De ter mi na tion

De tails of the GFZ pre cise de ter mi na tion of

GPS and CHAMP sat el lite po si tions, ve loc i ties

and clocks are given, e.g., by König et al. (2005,

2006). This pro cess ing sys tem is also used for

the GRACE-A data anal y sis. The or bit prod uct

for the reg u lar GFZ occultation anal y sis is de -

noted as Rapid Sci ence Or bit (RSO). Fig. 3

shows a com par i son of CHAMP and GRACE-A

RSO data and Sat el lite La ser Rang ing (SLR)

mea sure ments. The mean de vi a tion in 2007 is

5.9 cm for CHAMP and 4.7 cm for GRACE-A.

Michalak et al. (2007a) pro vided a de tailed

study of FORMOSAT-3/COS MIC or bits by GFZ

for the pe riod be tween 4 and 7 Au gust 2006, when com plete

GPS data cov er age for the FORMOSAT-3/COS MIC sat el -

lites was avail able. Or bits were de rived for all 6 sat el lites us -

ing the op er a tional or bit pro cess ing sys tem for CHAMP and

GRACE-A. This sys tem was ex tended to use FOR MO -

SAT-3/COS MIC SST (sat el lite-sat el lite track ing) and at ti -

tude data. Since SLR data for FORMOSAT-3/COS MIC are

not avail able for ex ter nal val i da tion, the or bits were com -

pared with Uni ver sity Cor po ra tion for At mo spheric Re -

search (UCAR) op er a tional or bits. An ex am ple for these

com par i sons is pro vided in Fig. 4. The mean ex ter nal 3-d po -

si tion ac cu racy re sult ing from these com par i sons for all 6

sat el lites is 23 cm, for the ve loc ity the ac cu racy amounts to

0.24 mm s-1. The in ter nal po si tion ac cu racy of GFZ or bits

from the over laps is better and amounts to 13.1 cm. This

GPS Ra dio Occultation with CHAMP, GRACE and FORMOSAT-3/COS MIC 37

Fig. 3. Ac cu racy of LEO (Low Earth Or biter) RSOs ob tained from com par i son

with SLR mea sure ments. (a) CHAMP (24 April 2001 - 11 April 2007). (b)

GRACE-A (6 Oc to ber 2004 - 11 April 2007). The mean RMS for CHAMP (5.9 cm)

and GRACE-A (4.7 cm) is cal cu lated for time pe riod since Jan u ary 2007.

(a)

(b)

value is com pa ra ble to the UCAR in ter nal po si tion pre ci sion

of 15.2 cm. For the in ter nal ve loc ity ac cu racy we have ob -

tained 0.19 mm s-1. This is nearly twice the UCAR value,

0.11 mm s-1. The large dif fer ence be tween in ter nal and ex -

ternal or bit ac cu racy sug gests prob lems of sys tem atic na -

ture which will be in ves ti gated in the fu ture. Cur rently, GFZ

is also pre pared to op er a tion ally gen er ate FORMOSAT-3/

COS MIC or bits in a near-real time mode with less strin gent

ac cu racy re quire ments. We note that in for ma tion on the re la -

tion ship be tween or bit ac cu racy and at mo spheric pa ram e ters

can be found in Kursinski et al. 1997; Rocken et al. 1997;

Wickert et al. 2001b; or Wickert 2002.

3.3 Der i va tion of Ver ti cal At mo spheric Pro files

The dou ble-dif fer ence tech nique (e.g., Wickert et al.

2001b) is cur rently ap plied for the CHAMP stan dard pro -

cess ing. Near-real time data (see sec tion 3.4) are an a lyzed

us ing the space-based sin gle dif fer ence tech nique (Wickert

et al. 2002). The mea sure ments from GRACE-A were an -

alyzed ap ply ing the zero-dif fer ence tech nique (Beyerle et

al. 2006) which is im ple mented in the op er a tional oc cul -

tation anal y sis sys tem from GFZ (e.g., Wickert et al. 2005).

The zero-dif fer ence tech nique does not re quire in clu sion of

ref er ence links from space- or ground-based re ceiv ers. Data

from both GRACE sat el lites were pro cessed us ing iden ti cal

al go rithms. We note that GRACE-B occultations (ex cept the

ini tial mea sure ments, see sec tion 2) were ac ti vated only for a

short pe riod be tween 1400 UTC 23 and 1200 UTC 30 Sep -

tem ber 2005. Dur ing this time GRACE-B trailed GRACE-A

and its occultation an tenna pointed to anti- velocity di rec -

tion. On 10 De cem ber 2005 the or der of the satellites was

re versed. From 1231 GRACE-B occultations

1113 pro files (90.4%) were de rived and com -

pared with ECMWF. The val i da tion re sults

(Wickert et al. 2006b) were found to be nearly

iden ti cal with those from GRACE-A. The ma -

jor dif fer ence be tween the GRACE sa t el lites

is the better value for the long-term stability

of the Ul tra Sta ble Os cil la tor (USO) aboard

GRACE-B (about 30 ns s-1) com pared to

GRACE-A (about 230 ns s-1). At mo spheric

bend ing an gles are de rived from the time de -

rivative of the ex cess phase af ter ap pro pri ate

fil ter ing. The ion o spheric cor rec tion is per -

formed by lin ear com bi na tion of the sep a rately

de rived L1 and L2 bend ing an gle pro files

(Vorob’ev and Krasnil’nikova 2004). To cor -

rect for the ef fect of tro po spheric multipath, be -

low 15 km, the Full Spec trum In ver sion (FSI)

tech nique, a wave op tics based anal y sis met -

hod, is ap plied (Jensen et al. 2003).

Ver ti cal pro files of at mo spheric refractivity are de rived

from the ion o sphere cor rected bend ing an gle pro files by

use of the Abel in ver sion tech nique. For dry air, the den sity

pro files are ob tained from the re la tion ship be tween den sity

and refractivity. Pres sure and dry tem per a ture are ob tained

ap ply ing the hy dro static equa tion and the equa tion of state

for an ideal gas. Ba sics of the GPS ra dio occultation tech -

nique and the der i va tion of at mo spheric pa ram e ters are de -

scribed by Kursinski et al. (1997). The sci en tific soft ware

al go rithms for the cal cu la tion of the at mo spheric pro files

are em bed ded into a soft ware sys tem (CAP, CHAMP At -

mo sphere Pro ces sor), which im ple ments the au to ma tion

(Schmidt et al. 2005a) of the stan dard but also NRT data

anal y sis.

An im proved (in re la tion to GFZ soft ware 005, see e.g.,

Wickert et al. 2005) pro cess ing soft ware has been uti lized

for the NRT data anal y sis from CHAMP and GRACE-A.

Hereby, the ap pli ca tion of the sin gle dif fer ence tech nique

(CHAMP) al most com pletely elim i nates the in flu ence of ir -

reg u lar i ties which are in tro duced to the CHAMP data by

periodic clock ad just ments to achieve a 1 ms max i mum de -

viation from co or di nate time. How ever, a slight in flu ence

from these ir reg u lar i ties re mains in the form of ad di tional

noise (in tro duced by the ion o sphere cor rec tion of the ref er -

ence link) and re duces the data qual ity at strato spheric al ti -

tudes. GRACE-A pro cess ing needs no in clu sion of a ref er -

ence link (zero dif fer enc ing, use of an ul tra-sta ble os cil la tor

as sat el lite clock), there fore the ad di tional noise, ob served in

the CHAMP data, is avoided. As a re sult, al ready the anal y -

sis of the first larger data sets from CHAMP and GRACE-A

(Fig. 5) in di cate re duced bias and also RMS of the GRACE-

A refractivity pro files to the op er a tional model from the Met

Of fice above 25 km in re la tion to the CHAMP data. This is

re garded as in di ca tion of a higher qual ity of the GRACE-A

38 Wickert et al.

Fig. 4. An ex am ple from the ini tial re sults of the pre cise or bit de ter mi na tion for

FORMOSAT-3/COS MIC at GFZ. The plot shows the dif fer ence in ra dial com po -

nent be tween one 14 h GFZ arc and cor re spond ing op er a tional UCAR or bits for

FORMOSAT-3/COS MIC sat el lite 2 (4/5 Au gust 2006). The mean de vi a tion is 3.8 cm

and cor re spond ing RMS is 9.4 cm.

data com pared to CHAMP at higher al ti tudes. The lower tro -

po sphere bias of the im proved pro files (NRT, Fig. 5) is re -

duced in re la tion to ver sion 005 to val ues be low 0.5% by im -

proved im ple men ta tion of the wave op tics based data an -

alysis and more strict qual ity con trol. This slightly de -

creases the lower tro po sphere data yield in re la tion to the

GPS Ra dio Occultation with CHAMP, GRACE and FORMOSAT-3/COS MIC 39

Fig. 5. Sta tis ti cal com par i son of ver ti cal refractivity pro files (NRT data anal y sis) from (a) CHAMP (3186 pro files) and (b) GRACE-A (2387

pro files) be tween 11 March and 11 April 2007 with op er a tional anal y ses from the Met Of fice [orig i nal qual ity mon i tor ing plots from

GRAS-SAF (GRAS Sat el lite Ap pli ca tion Fa cil ity); black: mean de vi a tion, grey: stan dard de vi a tion]. Note that the S-shaped bias at up per lev els

is thought to be model-re lated, and not a fea ture of the RO data. (b) shows the num ber of com pared data points as a func tion of al ti tude.

(a)

(b)

005 prod uct ver sion (for val i da tion re sults of 005 see, e.g.,

Wickert et al. 2005). We note how ever, that the ob served

deviations also can be due to weak nesses of the me te o ro -

log i cal anal y ses.

Ion o spheric pro files from CHAMP and GRACE-A

mea sure ments are gen er ated by an au to mated pro cess ing

sys tem at DLR in Neustrelitz. The re trieval al go rithm is

based on a tomographic ap proach di vid ing the ion o sphere

and plasmasphere into spher i cal shells with up ward grow ing

thick ness and con stant elec tron den sity in side. Due to the

low or bit height of CHAMP be low 450 km (of ten within the

F-layer of the ver ti cal elec tron den sity dis tri bu tion), the

upper bound ary prob lem is more chal leng ing com pared to

LEOs at higher or bit. A spe cific model as sisted tech nique

has been de vel oped us ing an adap tive model for the top side

ion o sphere and plasmasphere above the CHAMP or bit

height. The model is based on a Chap man layer func tion

com pleted by a plasmaspheric term with ex po nen tially de -

creas ing elec tron den sity vs. al ti tude. Model pa ram e ters

such as the plasma scale height at the up per bound ary are

determined in a few it er a tions in or der to en sure a smooth

tran si tion be tween model val ues and mea sure ments (Jako -

wski et al. 2002). Due to the tomographic ap proach no

spher i cal sym me try as sump tion is needed and ad di tional in -

for ma tion from ground based GPS mea sure ments, ion o -

spheric mod els and/or ad di tional ver ti cal sound ers can im -

prove the re trieval. These data, e.g., can pro vide in for ma tion

on hor i zon tal gra di ents of the elec tron den sity dis tri bu tion

and im prove the re trieval. The anal y sis soft ware is em bed -

ded in a cor re spond ing dy nam i cally configurable au to mated

data pro cess ing sys tem (Wehrenpfennig et al. 2001). Fig. 6

shows two typ i cal pro files, which were de rived from the first

GRACE-A ion o spheric occultation mea sure ments.

A ma jor ad van tage of FORMOSAT-3/COS MIC data

com pared to CHAMP and GRACE-A is the im proved data

qual ity, com bined with higher yield in the lower tro po -

sphere. This is achieved by the ap pli ca tion of the OpenLoop

(OL) sig nal track ing tech nique by the Black Jack GPS re -

ceiver (e.g., Beyerle et al. 2006; Sokolovskiy et al. 2006a,

b). OL also al lows for the track ing of ris ing occultations.

OL data pro cess ing needs spe cific mod i fi ca tion and

exten sion of the in ver sion soft ware, which is used for the

proces sing of the Closed Loop (CL) data from CHAMP and

GRACE-A. Fig ure 7 shows the first re sult for a ris ing

occultation from FORMOSAT-3/COS MIC gen er ated with

the op er a tional GFZ pro cess ing soft ware. The mea sure -

ments were an a lyzed us ing the at mo spheric ex cess phase

data pro vided by UCAR. The re con struc tion of the fi nal

phase from the model and the dif fer en tial phase was per -

formed us ing the nav i ga tion bit in for ma tion from a sup port -

ing GPS ground net work, also pro vided by UCAR (us ing the

al go rithm ac cord ing to Sokolovskiy et al. 2006a). The GFZ

anal y sis re sults (refractivity and dry tem per a ture pro files)

agree very well with UCAR. Dif fer ences above 30 km in

refractivity and dry tem per a ture can be ex plained by the ap -

pli ca tion of dif fer ent al go rithms for the op ti mi za tion of the

bend ing an gle by UCAR and GFZ (see, e.g., Ao et al. 2003).

Be low the tropo pause the pres ence of wa ter va por leads to a

cold bias of the dry tem per a ture pro files com pared to

ECMWF wet tem per a tures [see, e.g., Kursinski et al. (1997)

for the def i ni tion of dry tem per a ture]. This dif fer ence can be

re garded as a mea sure for the wa ter va por it self and is large

for the trop i cal pro file shown in Fig. 7. We note that this cold

bias in the dry tem per a ture com pared to anal y sis tem per a -

ture (wet) is dif fer ent from the phe nom e non of the neg a tive

refractivity bias, ob served in RO data (e.g., Beyerle et al.

2006). De vi a tions in the lower tro po sphere are prob a bly re -

lated to dif fer ences of the ap pli ca tion of the FSI tech nique

by both cen ters and will be in ves ti gated in more de tail in fu -

ture stud ies uti liz ing more ex tended data sets.

3.4 Near-Real-Time Data Pro vi sion for Nu mer i calWeather Fore cast

The year 2006 marks a mile stone for the use of GPS as a

plat form for at mo spheric re mote sens ing, eleven years af ter

the US Air Force an nounced on 17 July 1995 the full op er a -

tional ca pa bil ity of the GPS. Both ground and also space-

based (RO) data have started to be used con tin u ously to

improve re gional and global weather fore casts. Pi o neer ing

work with re spect to the RO data use for global nu mer i cal

weather pre dic tion was per formed at the Met Of fice, ECMWF

and NCEP us ing op er a tion ally avail able CHAMP data (e.g.,

Healy et al. 2005; Healy and Thepaut 2006; Cucurull et al.

2007). The large vol ume of data, avail able from FOR -

MOSAT-3/COS MIC since 2006, is a fur ther ma jor im pe tus

for these ac tiv i ties. These data cur rently form the ma jor

share of the op er a tion ally avail able GPS RO data. Con -

tinuous as sim i la tion of GPS RO started on 26 Sep tem ber

2006 with CHAMP and GRACE-A at the Met Of fice (FOR -

MOSAT-3/COS MIC since 15 May 2007), with FORMO -

SAT-3/COS MIC, CHAMP and GRACE-A on 12 De cem ber

2006 at ECMWF and on 22 March 2007 at JMA (Ja pan

Meteorological Agency) with CHAMP. NOAA/NCEP (Na -

tional Oce anic and At mo spheric Ad min is tra tion/Na tional

Cen ter for En vi ron men tal Pre dic tion) at US ac ti vated the op -

er a tional as sim i la tion of GPS RO data on 1 May 2007,

Meteo France on 5 Sep tem ber 2007. A fur ther boost for

these RO re lated ac tiv i ties to im prove global weather fore -

casts is ex pected from the GRAS data, re corded aboard the

MetOp sat el lite, how ever these data are sched uled for avail -

abil ity in 2008.

The cur rent near-real time ac tiv i ties for CHAMP and

GRACE-A are funded within an in ter na tional re search pro -

ject (NRT-RO, Near-Real Time Ra dio Occultation) by the

Ger man Min is try for Ed u ca tion and Re search within the

GEOTECHNOLOGIEN re search programme (www.geo -

tech nologien.de). The pri mary goals of the pro ject are the

40 Wickert et al.

de vel op ment of soft ware mod ules for the pre cise and rapid

der i va tion of GPS and LEO sat el lite or bits and glob ally dis -

trib uted at mo spheric pro files from GPS SST data, the de -

monstration of a near-real time pro vi sion of at mo spheric

data from CHAMP and GRACE-A with cor re spond ing as -

sim i la tion in global weather mod els from ECMWF, the Met

Of fice and the Ger man Weather Ser vice. Ad di tional cen ters,

as, e.g., JMA, Meteo France, NOAA/NCEP, and En vi ron -

men tal Cen ter (Can ada) also make use of the pro ject data. A

ma jor goal of NRT-RO is to reach an av er age de lay be tween

mea sure ment and pro vi sion of the cor re spond ing at mo -

spheric pro file of about 2 hours. The pro vi sion of NRT

occultation data for weather cen ters started on 28 No vem ber

2005. In this first stage of the pro ject, e.g., an av er age de lay

of par al lel CHAMP and GRACE-A mea sure ments of less

than 4 hours was reached for a pe riod of 41 days in Jan u -

ary/Feb ru ary 2006 (see also Fig. 8). This was the first NRT

dem on stra tion for a two-sat el lite GPS RO con stel la tion. The

data were used for in ves ti ga tions to char ac ter ize the im pact

of CHAMP and GRACE-A bend ing an gles to global wea -

ther fore casts at ECMWF (Healy et al. 2007).

A sig nif i cant pre req ui site for the ini tial NRT ac tiv i ties

at GFZ was and is the pro vi sion of NRT or bit data for the

GPS and CHAMP/GRACE-A sat el lites. For this pur pose a

spe cific data prod uct was de fined al ready in 2003, the Ul tra

Rapid Sci ence Or bit (USO). These data are cal cu lated with a

fixed 3 hourly pe riod for the GPS and LEO sat el lites, in de -

pend ent from the oc cur rence of data down load at the po lar

re ceiv ing sta tion at Ny Ålesund (see sec tion 3.1). The ac cu -

racy of USO is only slightly worse com pared to the RSO

data (see sec tion 3.2); the main dif fer ence in gen er at ing

USO and RSO is a dif fer ent GPS ground sta tion data han -

dling and qual ity con trol set tings to de rive GPS or bit data

(for de tails see, e.g., König et al. 2006). To il lus trate this,

Fig. 9 shows re sults of a com par i son of CHAMP and

GRACE-A USO data with SLR mea sure ments. In 2007 it

yields an av er age RMS of 8.0 cm for CHAMP and 5.7 cm

for GRACE-A.

Al though the pro vi sion of at mo spheric data from

CHAMP based on the 3 hour USO pe riod stim u lated the

use of GPS RO at sev eral Nu mer i cal Weather Pre dic tion

(NWP) cen ters sig nif i cantly, the re sult ing time de lay for

the pro files of around 4 h (see Fig. 8) is in suf fi cient to sat -

isfy the re quire ment for a max i mum la tency of 1 - 4 h for

global pre dic tion (WMO, World Me te o ro log i cal Or ga ni za -

tion, TD No. 913, SAT-21, 28/9/1998). For that rea son the

de vel op ment of an im proved NRT or bit and occultation

pro cess ing sys tem was started. The main pre req ui site for

this de vel op ment was the ad ap ta tion of the pe ri ods of the

or bit and occultation pro cess ing to the times of the LEO

data down load at the po lar re ceiv ing sta tion at Ny Ålesund

around ev ery 1.5 h.

A cru cial task is the de vel op ment of an ap pro pri ate

sys tem for the near-real time cal cu la tion of GPS and LEO

or bits. De tails of the sys tem were de scribed by Michalak

et al. (2007b). Three ap pro pri ate, but al ter na tive pro cess -

ing chains were im ple mented to find the op ti mal method.

The re sult ing qual ity of the data prod ucts and cor re spond -

ing time de lays were eval u ated in de tail. The first, called

IGU-based (In ter na tional GNSS Ser vice Ul tra rapid or -

bits and clocks) chain, pro duces lower ac cu racy LEO or -

bits (~25 cm for po si tion; 0.3 mm s-1 for ve loc ity er ror; 9.7

- 11.5 cm SLR RMS) but with a low la tency of ~13 min

only. The sec ond chain, GPS-based, due to a GFZ based

GPS Ra dio Occultation with CHAMP, GRACE and FORMOSAT-3/COS MIC 41

(a) Elec tron Den sity Ne and Plasma Freqency fp

Fig. 6. Two ex am ples of ini tial ver ti cal ion o spheric pro files (elec tron

den sity and cor re spond ing plasma fre quency fP vs. al ti tude), de rived

from GRACE-A occultation mea sure ments on 8 March 2007 (for de -

tails see text).

(b) Elec tron Den sity Ne and Plasma Freqency fp

es ti ma tion of 5-min GPS or bits and clocks, pro duces more

ac cu rate LEO or bits (12 - 13 cm for po si tion; 0.14 mm s-1

for ve loc ity; ~7 - 9 cm SLR RMS) with a la tency of 33

min. This la tency is mainly driven by wait ing for com -

plete ground data from IGS data base. The third chain,

based on IGU pre dicted or bits, but es ti mated dense 30-s

clocks, pro duces the most pre cise LEO NRT or bits (10 cm

for po si tion; 0.1 mm s-1 for ve loc ity; 4.5 - 6.4 cm SLR

RMS) with a la tency of ~30 min. This chain is po ten tially

the most suit able for high pre ci sion NRT ap pli ca tions and

will be used as the pri mary method for the new NRT

mode. But also the two al ter na tive ap proaches would as -

sure a gen er a tion of occultation prod ucts with a max i mum

de lay well be low three hours and could be used in case of

fail ure of the pri mary chain.

On 17 April 2007 (ac ti va tion of the new NRT mode)

data from the new pro cess ing sys tem for CHAMP were

mon i tored by the Met Of fice (GRAS-SAF). All of the 157

42 Wickert et al.

Fig. 7. First ris ing at mo spheric pro file, de rived from FORMOSAT-3/COS MIC GPS ra dio occultation data (red) us ing the GFZ in ver sion soft ware

applied to ex cess phase and nav i ga tion bit in for ma tion pro vided by UCAR (for de tails see text). The pro file (left: dry tem per a ture; mid dle:

refractivity N) is com pared with UCAR (black) and ECMWF “wet” tem per a ture and refractivity (blue tri an gles). The frac tional refractivity dif fer -

ence (NUCAR - NGFZ) / NUCAR (in %) is shown right. For de tails see text.

Fig. 8. Mean daily time de lay (red tri an gles) be tween the mea sure ment and avail abil ity of cor re spond ing anal y sis re sults at GFZ since 28 No vem ber

2005 un til March 2007 for (a) CHAMP and (b) GRACE-A. Blue and green di a monds in di cate the min i mum and max i mum time de lay for in di vid ual

pro files each day.

(a) (b)

pro files reached the weather cen ter within 2 h 54 min,

amount ing to an av er age de lay of 1 h 44 min (Fig. 10). The

cor re spond ing qual ity mon i tor ing plot is shown in Fig. 11.

The com par i son shows in gen eral only small (< 1%) de vi a -

tions be tween mea sure ments and anal y sis and the re sults are

nearly iden ti cal with those from the “older” NRT mode but

with larger la tency of about a 4 h mean de lay (see Fig. 5). It

is be yond the scope of this pa per to dis cuss the dif fer ences in

more de tail, which can be caused by un cer tain ties of the

mea sure ments but also of the me te o ro log i cal anal y sis. Cur -

rent ac tiv i ties fo cus to the sta bi li za tion of the new pro cess -

ing mode for op er a tional data pro vi sion and also in clu sion of

the GRACE-A into the new NRT mode.

4. AP PLI CA TIONS FOR GPS RO DATA

The avail abil ity of the mea sure ments from FORMO -

SAT-3/COS MIC sig nif i cantly in creases the data base for

var i ous ap pli ca tions of GPS RO data in at mo spheric re -

search (Fig. 12). We in ves ti gated the pe riod from 1 Oc to ber

to 31 De cem ber 2006 in more de tail. The over all amount of

GPS RO data (cal i brated at mo spheric ex cess phases) com -

prises 183976 glob ally dis trib uted mea sure ments (~2000

daily). FORMOSAT-3/COS MIC data form the ma jor share

(152790 mea sure ments, 83.0%), fol lowed by CHAMP

(17991 mea sure ments, 9.8%) and GRACE-A (13195 mea -

sure ments, 7.2%). Even though the FORMOSAT-3/COS -

MIC data clearly dom i nate the occultation data base, the

share of CHAMP and GRACE-A is sig nif i cant (17%). Re -

gions with max i mum num ber of occultations per pixel are

the lat i tude bands around ~25° and ~50° North and South.

The equa tor re gion ex hib its the low est num ber of oc cul -

tations per pixel.

An other, more spe cific, view to the data avail abil ity in

the lower tro po sphere is of fered by Fig. 13. It shows the per -

cent age of pro files reach ing al ti tudes of about 1 and 0.5 km,

re spec tively for a 3 month pe riod start ing from 1 Au gust

2006. The red ar eas in di cate re gions with nearly per fect

penetration down to 0.5 or 1 km above mean sea level

(MSL), re spec tively. For the dark blue or black re gions there

are less or no data avail able. This re flects the global topo -

graphy, e.g., moun tain re gions as the An des, Hi ma la yas,

Rocky Moun tains or Green land and Antarctica. For large

areas the ma jor ity of the FORMOSAT-3/COS MIC pro files

reach deep to the tro po sphere, in the win ter hemi sphere

(here: South) above oceans even down to 0.5 km. These

char ac ter is tics of the FORMOSAT-3/COS MIC data al lows

sig nif i cantly im proved in ves ti ga tions con cern ing the Plan e -

tary Bound ary Layer (PBL, Sokolovskiy et al. 2006b) com -

pared to the CHAMP mis sion (e.g., von Engeln et al. 2006).

Know ing the prop er ties of the PBL, and in par tic u lar its

depth (height of its top), is im por tant for un der stand ing

trans port pro cesses in the tro po sphere, weather pre dic tion

and cli mate mon i tor ing (e.g., Garratt 1994).

In the fol low ing we pres ent se lected ex am ples for ap -

plicat ions and com pare re sults from CHAMP with those

from CHAMP+FORMOSAT-3/COS MIC+GRACE-A.

4.1 Global Tropo pause Pa ram e ters from Ra dioOccultation Data

The tropo pause (TP) re gion is one of the key re gions of

the at mo sphere. It sep a rates the tro po sphere and strato -

sphere that have fun da men tally dif fer ent char ac ter is tics with

re spect to chem i cal com po si tion and static sta bil ity. There -

fore the de ter mi na tion of TP pa ram e ters, such as al ti tude or

tem per a ture, on a global scale is an im por tant goal for many

branches in atmospheric research.

With re gard to the cur rent cli mate change dis cus sion, TP

pa ram e ters have re ceived more at ten tion in re cent years

since they are used to de scribe cli mate vari abil ity and

change. The global mean TP al ti tude shows an in crease in

GPS Ra dio Occultation with CHAMP, GRACE and FORMOSAT-3/COS MIC 43

Fig. 9. Com par i son of Ul tra Rapid Sci ence Or bits (USO) with SLR mea sure ments. (a) CHAMP (9 April 2002 - 12 April 2007). (b) GRACE-A (11

Jan u ary 2006 - 12 April 2007). For 2007 the mean RMS for CHAMP is 8.0 cm and for GRACE-A is 5.7 cm.

(a) (b)

44 Wickert et al.

Fig. 10. Mon i tor ing plot (GRAS-SAF, Met Of fice) for a time de lay of CHAMP near-real time data prod ucts from 17 April 2007. A mean de lay

between mea sure ment and mon i tor ing at the Met Of fice of 1 h 44 min was ob served, all 157 pro files ar rived within 2 h 54 min af ter measurement

(http://mon i tor ing.grassaf.org).

Fig. 11. Mon i tor ing plot (GRAS-SAF, Met Of fice) for the de vi a tion of CHAMP near-real time refractivity pro files from the Met Of fice global model

(blue: bias; red: RMS) for 17 April 2007. The right plot shows the num ber of pro vided data per al ti tude.

reanalyses and ra dio sonde ob ser va tions over the last de -

cades and seems to be a more sen si tive in di ca tor for cli mate

change as the Earth’s sur face tem per a ture (Sausen and

Santer 2003). An other ap pli ca tion area of TP stud ies deals

with the role of the TP re gion in the strato sphere-tro po sphere

ex change. In this con text mul ti ple tropo pauses or TP break

re gions are im por tant be cause in these re gions most of the

ex change pro cesses take place (e.g., Pan et al. 2004).

One im por tant data source for the de ter mi na tion of TP

pa ram e ters are ra dio sondes. De spite good ver ti cal res o lu tion

of the ra dio sonde data, global cov er age is im pos si ble. In

con trast, the RO tech nique of fers both global cov er age but

good ver ti cal res o lu tion as well and is there fore no ta bly ap -

pli ca ble for de tailed at mo spheric mon i tor ing and cli mate

stud ies (Leroy et al. 2006). Since the avail abil ity of GPS RO

data start ing with the GPS/MET ex per i ment (1995 - 1997)

sev eral stud ies con cern ing the up per tro po sphere and lower

strato sphere (UTLS) re gion were per formed us ing the con -

sec u tive RO mis sions (CHAMP, SAC-C, GRACE and

FORMOSAT-3/COS MIC).

First re sults for the trop i cal TP re gion us ing the GPS/

MET data were pub lished by Nishida et al. (2000) and

Randel et al. (2003). Ap pli ca tions of CHAMP RO data to

that re gion were shown by Schmidt et al. (2004) and

Borsche et al. (2007), a global anal y sis of TP pa ram e ters

from CHAMP and SAC-C was made by Schmidt et al.

(2005b). The first cli ma tol ogy and ob ser va tional char ac ter -

is tics of mul ti ple tropo pauses was ac com plished by Schmidt

et al. (2006) and Randel et al. (2007). The re cently launched

RO sat el lite mis sions, es pe cially FORMOSAT-3/COS MIC,

pro vide a data base for TP stud ies with sig nif i cantly higher

tem po ral and spa tial res o lu tion and gen er ally open new per -

spec tives for mon i tor ing the UTLS re gion.

As an ex am ple Fig. 14 shows the global CHAMP ther -

mal lapse rate tropo pause (LRT) al ti tude (Fig. 14a) for Oc to -

ber 2006 for a lat i tude/lon gi tude res o lu tion of 10 de grees.

The def i ni tion of the World Me te o ro log i cal Or ga ni za tion

(WMO) was used for the de ter mi na tion of the LRT (WMO

1957). Ac cord ing to this re port the (first) ther mal tropo pause

is defined “as the low est level at which the lapse rate de -

creases to 2°C km-1 or less, pro vided also the av er aged lapse

rate be tween this level and all higher lev els within 2 km does

not ex ceed 2°C km-1”. The LRT al go rithm ap plied for the

GPS RO data is de scribed in de tail by Schmidt et al. (2005b).

GPS Ra dio Occultation with CHAMP, GRACE and FORMOSAT-3/COS MIC 45

Fig. 12. Avail abil ity of GPS RO data (mea sure ments per pixel) for global in ves ti ga tions be tween 1 Oc to ber and 31 De cem ber 2006. (a) for CHAMP

(lat i tude/lon gi tude res o lu tion 10 ´ 10°); (b) for CHAMP+GRACE-A+FORMOSAT-3/COS MIC (5 ´ 5°). Please note the dif fer ent scale.

Fig. 13. Global avail abil ity (5 ´ 5°) of GPS RO data from FORMOSAT-3/COS MIC (OpenLoop Track ing, UCAR pro files) for the (a) 900 hPa

(~1 km al ti tude) and (b) 950 hPa (~0.5 km al ti tude) pres sure level be tween 1 Au gust and 31 Oc to ber 2006. The given num ber cor re sponds to the frac -

tion of pro files reach ing the re spec tive al ti tude.

(a) (b)

(a) (b)

For Oc to ber 2006 a to tal num ber of 4722 CHAMP tem -

perature pro files is avail able where a TP could be suc cess -

fully de ter mined. White ar eas rep re sent lat i tude/lon gi tude

bins with no data. The typ i cal TP al ti tude struc ture with the

max i mum in the trop ics, the tran si tion zone in the sub-

tropics (20 - 40° on both hemi spheres) and the low est TP

altitude in the po lar re gions is clearly vis i ble. With the ad di -

tional 3882 GRACE pro files for Oc to ber 2006 the ho ri -

zontal res o lu tion is not im proved sig nif i cantly (not shown

here). Figure 14b pres ents the LRT al ti tude from CHAMP,

GRACE, and FORMOSAT-3/COS MIC with a to tal num ber

of 46735 suc cess fully de ter mined TPs. Due to the FOR -

MOSAT-3/COS MIC data the hor i zon tal res o lu tion could be

im proved to 2.5 ´ 2.5 de gree in lat i tude and lon gi tude. This

al lows a much better dis cus sion of the TP struc ture, es pe -

cially in the sub-trop ics.

4.2 Global Wa ter Va por Dis tri bu tion

Wa ter va por plays a cru cial role in weather, cli mate, and

hy drol ogy. It is a key el e ment of the at mo spheric green house

ef fect and the work ing of the hy dro log i cal cy cle; both sup -

port the sustainability of life on Earth. The hy dro log i cal cy -

cle de scribes the move ment of wa ter, in all three phases,

within and be tween the Earth’s at mo sphere, oceans, and

con ti nents. In the va por phase, wa ter can move quickly th -

rough the at mo sphere and re dis trib utes en ergy as so ci ated

with its evap o ra tion and recondensation. The move ment of

wa ter va por through the hy dro log i cal cy cle is strongly cou -

pled to pre cip i ta tion and soil mois ture, which have im por -

tant prac ti cal im pli ca tions. The ba sic op er a tion of the hy -

drologic cy cle is well known, but some de tails are poorly

understood, mainly be cause we do not have suf fi ciently

good ob ser va tions of wa ter va por. In ad di tion, mon i tor ing of

long-term changes in wa ter va por, which are closely linked

to other cli mate vari a tions and trends, is needed to both pre -

dict and de tect changes.

These are more gen eral rea sons to in ves ti gate, if global

wa ter va por dis tri bu tions, de rived from GPS occultation

mea sure ments, may help to ex tend our knowl edge on the hy -

dro log i cal cy cle and its vari a tions.

Refractivity data from CHAMP (pro cessed by GFZ),

GRACE-A (pro cessed by GFZ) and FORMOSAT-3/COS -

MIC (pro cessed by UCAR) data were used to de rive ver ti cal

wa ter va por pro files (spe cific hu mid ity) us ing the DWVP

(Di rect Wa ter Va por Pres sure) method, in tro duced by Heise

et al. (2006). Fig ure 15 shows monthly mean global wa ter

va por dis tri bu tions based on CHAMP and CHAMP+

46 Wickert et al.

Fig. 14. Mean global tropo pause al ti tude (LRT) for Oc to ber 2006 de rived from (a) CHAMP with a hor i zon tal res o lu tion of 10 ´ 10 de gree and from

(b) CHAMP, GRACE-A, and FORMOSAT-3/COS MIC (2.5 ´ 2.5 de gree res o lu tion).

Fig. 15. Global dis tri bu tion of the mean spe cific hu mid ity at 600 hPa, de rived from (a) CHAMP data (5 ´ 10° lat i tude/lon gi tude) and (b)

CHAMP+GRACE-A+FORMOSAT-3/COS MIC (2.5 ´ 5.0°) for Oc to ber 2006.

(a) (b)

(a) (b)

GRACE-A+FORMOSAT-3/COS MIC data. This il lus trates

the ca pa bil i ties of the grow ing RO multi-sat el lite con stel la -

tion for global wa ter va por mon i tor ing es pe cially in the mid

tro po sphere re gion. While CHAMP alone is only ca pa ble of

giv ing rough struc tures of the monthly mean dis tri bu tion

(Fig. 15a), the multi-sat el lite con stel la tion (Fig. 15b) al lows

for much more de tailed in for ma tion. This is vis i ble es pe -

cially in the trop i cal re gion where the CHAMP data are ob vi -

ously too sparse to re pro duce the strong wa ter va por gra di -

ents con nected e.g., with the po si tion of the in ner tro pical

con ver gence zone or cold sea cur rents (e.g., the Hum boldt

Cur rent).

4.3 Global In ves ti ga tions of Ion o spheric Ir reg u lar i ties

The av er age up per al ti tude of the high rate (50 Hz) GPS

occultation mea sure ments is set to a value be tween 120 and

140 km (e.g., CHAMP: 135 km; GRACE-A: 140 km;

FORMOSAT-3/COS MIC: 120 km, see Fig. 16). This al lows

the in ves ti ga tion of an other in ter est ing at mo spheric phe -

nom e non: plasma ir reg u lar i ties of the lower ion o sphere, in

more de tail the for ma tion of thin ion iza tion lay ers (spo radic

E-layer, Es). The ob ser va tion of the global dis tri bu tion, sea -

sonal and di ur nal be hav ior of these ir reg u lar i ties gives in for -

ma tion on at mo spheric dy nam ics, elec tro dy nam ics and cou -

pling pro cesses with the neu tral at mo sphere. It is ex pected

that the char ac ter is tics of the spo radic E oc cur rence will pro -

vide in for ma tion on the im pact of so lar vari abil ity, the cou -

pling to the geo mag netic field and on anthropogenic ef fects

on the up per at mo sphere. A global mon i tor ing of spo radic E

would also al low for an im proved se lec tion of per tur ba -

tion-free ra dio links for com mu ni ca tion and nav i ga tion

(Hocke and Tsuda 2001; Hocke et al. 2001 and ref er ences

therein; Wu et al. 2005). Gen er ally the mech a nism for the

formation of spo radic E-lay ers is well un der stood from

ground-based mea sure ments. Also early de tailed global

maps of spo radic E oc cur rence were gen er ated based on

mea sure ments of a large ionosonde net work dur ing the In -

ter na tional Geo phys i cal Year in 1958 (Hocke et al. 2001).

These maps are mainly based on ground mea sure ments and

suf fer from the fact that only a small amount of data is avail -

able over ocean and po lar re gions which cover more than

three quar ters of the Earth’s sur face. Sat el lite data would al -

low for an im proved global char ac ter iza tion of spo radic E.

GPS ra dio occultation of fers a unique tool for such in ves ti -

ga tions. For the first time the long-term data set of CHAMP

(2002 - 2007) al lows de tailed sea sonal, di ur nal and also the

start of cli ma to log i cal stud ies.

We de rived the lo ca tion of the spo radic E-lay ers us ing

am pli tude vari a tions of the RO sig nal as an in di ca tor

(Wickert et al. 2004b, Ar ras et al. 2008). Fig ure 17 shows

initial re sults of these in ves ti ga tions and dem on strates that

the avail abil ity of the FORMOSAT-3/COS MIC data al lows

the char ac ter iza tion of the global dis tri bu tion with un prec e -

dented res o lu tion in time and space (Ar ras et al. 2008). With

GPS Ra dio Occultation with CHAMP, GRACE and FORMOSAT-3/COS MIC 47

Fig. 16. Dis tri bu tion of max i mum al ti tude of the GPS ra dio occultation

data (high rate, 50 Hz) from CHAMP, GRACE-A and FOR MO -

SAT-3/COS MIC.

Fig. 17. Oc cur rence fre quency (in %) of spo radic E-lay ers, de rived

from GPS ra dio occultation data for au tumn 2006 (Sep tem ber, Oc to ber,

and November). (a) CHAMP (hor i zon tal res o lu tion 10 ´ 10° lat i -

tude/lon gi tude); (b) CHAMP+GRACE-A+FORMOSAT-3/COS MIC

(5 ´ 5°).

(a)

(b)

an oc cur rence fre quency of up to around 50% of all oc -

cultations in par tic u lar re gions, spo radic E is a sig nif i cant

phe nom e non in the lower ion o sphere. It ap pears pref er en -

tially on the sum mer hemi sphere and is not uni formly dis -

trib uted in lon gi tude. With the much higher num ber of

FORMOSAT-3/COS MIC data com pared to CHAMP and

GRACE-A a sig nif i cantly better spa tial res o lu tion is reached.

Thus, it is pos si ble to better iden tify re gions where Es pre -

ferably ap pears (e.g. the south ern hemi spheric Pa cific re -

gion in clud ing Aus tra lia in the south ern hemi spheric sum -

mer). This ob ser va tion in di cates a cor re la tion with the

amount of in ci dent so lar ra di a tion, al though the gap above

the South At lan tic re gion can not be ex plained based on this

fact. The oc cur rence of Es also strongly ex hib its daily and

an nual vari a tions, which will be in ves ti gated in more de tail

in the fu ture with the GPS RO data base. One of the al ready

ob served fea tures is the pref er en tial oc cur rence of mid-

latitudes Es dur ing the day time on the re spec tive sum mer

hemisphere.

5 SUM MARY AND OUT LOOK

The sta tus of the op er a tional data anal y sis of CHAMP

and GRACE-A ra dio occultation mea sure ments at GFZ

were re viewed and com ple mented by an over view of the ini -

tial re sults of the FORMOSAT-3/COS MIC data anal y sis and

ap pli ca tions at GFZ. GPS ra dio occultation data from the

cur rent mis sions are op er a tion ally as sim i lated to im prove

global weather fore casts at dif fer ent cen ters, a mile stone for

the rep u ta tion of the GPS RO tech nique as an es tab lished

remote sens ing tech nique. In par tic u lar, the sig nif i cantly

in creased num ber of glob ally dis trib uted pro files from

FORMOSAT-3/COS MIC (com pared to the cur rent avail able

data) sig nif i cantly ad vances the ap pli ca tion of GPS RO in

at mo spheric re search with rel e vance to weather fore cast ing

and cli mate change re lated stud ies. This was dem on strated

by the der i va tion of global tropo pause char ac ter is tics, wa ter

va por dis tri bu tions and char ac ter iza tion of plasma ir reg u lar -

i ties in the lower ion o sphere based on CHAMP, GRACE-A

and FORMOSAT-3/COS MIC data. The much higher GPS

RO data avail abil ity since 2006 is com ple men tary to the

unique long-term data set from CHAMP, which al lows in -

itial cli ma to log i cal in ves ti ga tions.

GNSS (Global Nav i ga tion Sat el lite Sys tems; in a more

gen eral sense than GPS) ra dio occultation is on its way to be -

come an es tab lished at mo spheric re mote sens ing tech nique.

This role will be strength ened by ad di tional and new re -

ceivers in space, by the fu ture avail abil ity of sig nals from

the GA LI LEO and the cur rently re cov ered GLONASS nav i -

ga tion sat el lite sys tems and also by the avail abil ity of new

GNSS sig nals (e.g. L2C), which will ex tend the po ten tial of

the GNSS ra dio occultation tech nique.

The year 2006 marks an im por tant mile stone on this

suc cess ful way with the launch of FORMOSAT-3/COS MIC

and the be gin of the op er a tional GPS RO data as sim i la tion to

im prove global weather fore casts.

Ac knowl edge ments We thank all en gi neers and sci en tists

of the CHAMP and GRACE team for their work, which is

the base for our in ves ti ga tions. This in cludes, in ad di tion to

col leagues at GFZ, the work of col leagues at JPL (Black -

Jack GPS re ceiver) and at the Uni ver sity of Texas. We also

ac knowl edge UCAR (Boul der, USA) and NSPO (Tai wan)

for the free and rapid pro vi sion of FORMOSAT-3/COS -

MIC data and re lated sup port. Part of the work for the pre -

sented pa per was per formed by the cor re spond ing au thor

du r ing a re search stay at the Na tional Cheng Kung Uni ver -

sity Tainan at Tai wan where he found a very pro duc tive

work ing at mo sphere. ECMWF data were pro vided through

the Ger man Weather Ser vice (DWD). The near-real time

activities with CHAMP and GRACE-A are sup ported by the

GEOTECHNOLOGIEN re search pro gram, funded by the

Ger man Min is try for Re search and Ed u ca tion (Pro ject NRT-

RO, Near-Real Time Ra dio Occultation). The gra phics pro -

vided by the Met Of fice were gen er ated un der the GRAS-

SAF, which is a part ner ship be tween DMI, ECMWF, IEEC,

and the Met Of fice and is partly funded by EUMETSAT.

REF ER ENCES

Anthes, R. A., P. A. Bernhardt, Y. Chen, L. Cucurull, K. F.

Dymond, D. Ector, S. B. Healy, S. P. Ho, D. C. Hunt, Y. H.

Kuo, H. Liu, K. Man ning, C. McCormick, T. K. Meehan,

W. J. Randel, C. Rocken, W. S. Schreiner, S. V. Soko -

lovskiy, S. Syndergaard, D. C. Thomp son, K. E. Trenberth,

T. K. Wee, N. L. Yen, and Z. Zhang, 2008: The COS MIC/

FORMOSAT-3 mis sion: Early re sults. Bull. Amer. Meteor.

Soc., March, 313-333, doi: 10.1175/BAMS-89-3-313. [Link]

Ao, C., W. Schreiner, and J. Wickert, 2003: First Re port on the

CHAMP Ra dio Occultation Intercomparison Study, JPL

Pub li ca tion. 03-016, 10 pp.

Ar ras, C., J. Wickert, G. Beyerle, S. Heise, T. Schmidt, and C.

Jacobi, 2008: A global cli ma tol ogy of ion o spheric ir reg u -

lar i ties de rived from GPS ra dio occultation. Geophys. Res.

Lett., 35, L14809, doi: 10.1029/2008GL034158. [Link]

Beyerle, G., T. Schmidt, J. Wickert, S. Heise, M. Rothacher, G.

König-Langlo, and K. B. Lauritsen, 2006: Ob ser va tions

and sim u la tions of re ceiver-in duced refractivity bi ases in

GPS ra dio occultation. J. Geophys. Res., 111, D12101, doi:

10.1029/2005JD006673. [Link]

Borsche, M., G. Kirchengast, and U. Foelsche, 2007: Trop i cal

tropo pause cli ma tol ogy as ob served with ra dio occultation

mea sure ments from CHAMP com pared to ECMWF and

NCEP anal y ses. Geophys. Res. Lett., 34, L03702, doi:

10.1029/2006GL027918. [Link]

Cheng, C. Z., Y. H. Kuo, R. A. Anthes, and L. Wu, 2006: Sat el -

lite constellation monitors global and space weather. EOS

Trans. AGU, 87, 166 pp, doi: 10.1029/2006EO170003.

[Link]

48 Wickert et al.

Cucurull, L., J. C. Derber, R. Treadon, and R. J. Purser, 2007:

As sim i la tion of global po si tion ing sys tem ra dio oc cul -

tation ob ser va tions into NCEP’s global data as sim i la tion

sys tem. Mon. Wea. Rev., 135, 3174-3193, doi: 10.1175/

MWR3461.1. [Link]

Dunn, C., W. Bertiger, Y. Bar-Sever, S. Desai, B. Haines, D.

Kuang, G. Frank lin, I. Har ris, G. Kruizinga, T. Meehan, S.

Nandi, D. Nguyen, T. Rogstad, J. B. Thomas, J. T., L.

Romans, M. Watkins, S.-C. Wu, S. Bettadpur, and J. Kim,

2003: In stru ment of grace GPS aug ments grav ity mea sure -

ments. GPS World, Feb ru ary 1.

Garratt, J. R., 1994: The At mo spheric Bound ary Layer, Cam -

bridge Univ. Press, New York, 315 pp.

Healy, S. B., and J. N. Thepaut, 2006: As sim i la tion ex per i -

ments with CHAMP GPS ra dio occultation mea sure ments.

Quart. J. Roy. Meteor. Soc., 132, 605-623, doi: 10.1256/

qj.04.182. [Link]

Healy, S. B., A. Jupp, and C. Marquardt, 2005: Fore cast im pact

ex per i ment with GPS ra dio occultation mea sure ments.

Geophys. Res. Lett., 32, L03804, doi: 10.1029/2004GL

020806. [Link]

Healy, S. B., J. Wickert, G. Michalak, T. Schmidt, and G.

Beyerle, 2007: Com bined fore cast im pact of GRACE-A

and CHAMP GPS ra dio occultation bend ing an gle pro -

files. Atmos. Sci. Lett., 8, 43-50, doi: 10.1002/asl.149. [Link]

Heise, S., J. Wickert, G. Beyerle, T. Schmidt, and C. Reigber,

2006: Global mon i tor ing of tro po spheric wa ter va por with

GPS ra dio occultation aboard CHAMP. Adv. Space Res.,

37, 2222-2227, doi: 10.1016/j.asr.2005.06.066. [Link]

Hocke, K. and T. Tsuda, 2001: Us ing GPS to study Plasma Ir -

reg u lar i ties. GPS World, July.

Hocke, K., K. Igarashi, M. Nakamura, P. Wilkinson, J. Wu, A.

Pavelyev, and J. Wickert, 2001: Global sound ing of spo -

radic E lay ers by the GPS/MET ra dio occultation ex per i -

ment. J. Atmos. Sol.-Terr. Phys. , 63, 1973-1980, doi:

10.1016/S1364-6826(01)00063-3. [Link]

Jakowski, N., 2005: Ion o spheric GPS ra dio occultation mea -

sure ments on board CHAMP. GPS Solut., 9, 88-95 doi:

10.1007/s10291-005-0137-7. [Link]

Jakowski, N., A. Wehrenpfennig, S. Heise, Ch. Reigber, H.

Lühr, L. Grunwaldt, and T. K. Meehan, 2002: GPS ra dio

occultation mea sure ments of the ion o sphere from CHAMP:

Early re sults. Geophys. Res. Lett., 29, 1457, doi: 10.1029/

2001GL014364. [Link]

Jensen, A. S., M. Lohmann, H. H. Benzon, and A. Niel sen,

2003: Full spec trum in ver sion of ra dio occultation sig nal.

Ra dio Sci., 38, doi: 10.1029/2002RS002763. [Link]

König, R., G. Michalak, K. Neumayer, R. Schmidt, S. Zhu, H.

Meixner, and C. Reigber, 2005: Re cent de vel op ments in

CHAMP or bit de ter mi na tion at GFZ. In: Reigber, C., H.

Lühr, P. Schwintzer, and J. Wickert (Eds.), Earth Ob ser va -

tion with CHAMP: Re sults from Three Years in Or bit,

Springer Berlin Hei del berg New York, ISBN: 3-540-

22804-7, 65-70, doi: 10.1007/3-540-26800-6_10. [Link]

König, R., G. Michalak, K. Neumayer, and S. Zhu, 2006: Re -

marks on CHAMP orbit products. In: Flury, J., R. Rummel,

C. Reigber, M. Rothacher, G. Boedecker, and U. Schreiber

(Eds.), Ob ser va tion of the Earth Sys tem from Space,

Springer Berlin Hei del berg New York, ISBN: 3-540-

29520-8, 17-26, doi: 10.1007/3-540-29522-4_2. [Link]

Kursinski, E. R., G. A. Hajj, K. R. Hardy, J. T. Schofield, and R.

Linfield, 1997: Ob serv ing the earth’s at mo sphere with ra -

dio occultation mea sure ments us ing the global po si tion ing

sys tem. J. Geophys. Res., 102, 23429-23465, doi: 10.1029/

97JD01569. [Link]

Leroy, S. S., J. G. An der son, and J. A. Dykema, 2006: Test ing

cli mate mod els us ing GPS ra dio occultation: A sen si tiv ity

anal y sis. J. Geophys. Res., 111, D17105, doi: 10.1029/

2005JD006145. [Link]

Michalak, G., J. Wickert, R. König, and M. Rothacher, 2007a:

Pre cise or bit de ter mi na tion of COS MIC/FORMOSAT-3

sat el lites for ra dio occultations. EGU Vi enna 2007, Geo -

phys i cal Ab stract EGU2007-A-08402.

Michalak, G., J. Wickert, R. König, and M. Rothacher, 2007b:

Pre cise sat el lite or bit de ter mi na tion for GPS ra dio oc -

cultation in near-real time (NRT). EGU Vi enna 2007, Geo -

phys i cal Ab stract EGU2007-A-08740.

Montenbruck, O., A. Yago, H. Bock, T. V. Helleputte, J. V. D.

Ijssel, M. Loiselet, C. Marquardt, P. Silvestrin, P. Visser,

and Y. Yoon, 2008: Track ing and nav i ga tion per for mance

of the GRAS in stru ment on MetOp-A. GPS Solut., 12,

289-299, doi: 10.1007/s10291-008-0091-2. [Link]

Nishida, M., A. Shimizu, T. Tsuda, C. Rocken, and R. H. Ware,

2000: Sea sonal and lon gi tu di nal vari a tions in the trop i cal

tropo pause ob served with the GPS occultation tech nique

(GPS/MET). J. Meteorol. Soc. Jpn., 78, 691-700.

Pan, L. L., W. L. Randel, B. L. Gary, M. J. Mahoney, and E. J.

Hintsa, 2004: Def i ni tions and sharp ness of the extra -

tropical tropo pause: A trace gas per spec tive. J. Geophys.

Res., 109, D23103, doi: 10.1029/2004JD004982. [Link]

Randel, W. J., F. Wu, and W. R. Rios, 2003: Ther mal vari abil ity

of the trop i cal tropo pause re gion de rived from GPS/MET

ob ser va tions. J. Geophys. Res., 108, D1, 4024, doi:

10.1029/2002JD002595. [Link]

Randel, W. J., D. J. Seidel, and L. L. Pan, 2007: Ob ser va tional

char ac ter is tics of dou ble tropo pauses. J. Geophys. Res.,

112, D07309, doi: 10.1029/2006JD007904. [Link]

Reigber, C., H. Lühr, P. Schwintzer, and J. Wickert (Eds.),

2005: Earth Ob ser va tion with CHAMP - Re sults from

Three Years in Or bit, Springer, Berlin, Hei del berg, New

York, ISBN 3-540-22804-7, doi: 10.1007/b138105. [Link]

Rocken, C., R. Anthes, M. Exner, D. Hunt, S. Sokolovskiy, R.

Ware, M. Gorbunov, W. Schreiner, D. Feng, B. Herman, Y.

H. Kuo, and X. Zou, 1997: Anal y sis and val i da tion of

GPS/MET data in the neu tral at mo sphere. J. Geophys.

Res., 102, 29849-29866, doi: 10.1029/97JD02400. [Link]

Sausen, R. and B. D. Santer, 2003: Use of changes in tropo -

pause height to de tect hu man in flu ences on cli mate.

Meteorol. Z., 12, 131-136, doi: 10.1127/0941-2948/2003/

0012-0131. [Link]

GPS Ra dio Occultation with CHAMP, GRACE and FORMOSAT-3/COS MIC 49

Schmidt, T., J. Wickert, G. Beyerle, and C. Reigber, 2004:

Tropical tropo pause pa ram e ters de rived from GPS ra dio

occultation mea sure ments with CHAMP. J. Geophys. Res.,

109, D13105, doi: 10.1029/2004JD004566. [Link]

Schmidt, T., J. Wickert, G. Beyerle, R. König, R. Ga las, and Ch.

Reigber, 2005a: The CHAMP at mo spheric pro cess ing sys -

tem for ra dio occultation mea sure ments. In: Reigber, C.,

H. Lühr, P. Schwintzer, and J. Wickert (Eds.), Earth Ob ser -

va tion with CHAMP: Re sults from Three Years in Or bit,

Springer Berlin Hei del berg New York, ISBN: 3-540-

22804-7, 597-602, doi: 10.1007/b138105. [Link]

Schmidt, T., S. Heise, J. Wickert, G. Beyerle, and Ch. Reigber,

2005b: GPS ra dio occultation with CHAMP and SAC-C:

Global mon i tor ing of ther mal tropo pause pa ram e ters.

Atmos. Chem. Phys., 5, 1473-1488.

Schmidt, T., G. Beyerle, S, Heise, J. Wickert, and M. Rothacher,

2006: A cli ma tol ogy of mul ti ple tropo pauses de rived from

GPS ra dio occultations with CHAMP and SAC-C. Geo phys.

Res. Lett., 33, L04808, doi: 10.1029/2005GL024600. [Link]

Schreiner, W., C. Rocken, S. Sokolovskiy, S. Syndergaard, and

D. Hunt, 2007: Es ti mates of the pre ci sion of GPS ra dio

occultations from the COS MIC/FORMOSAT-3 mis sion.

Geophys. Res. Lett., 34, L04808, doi: 10.1029/2006GL

027557. [Link]

Sokolovskiy, S. V., C. Rocken, D. Hunt, W. S. Schreiner, J.

John son, D. Mas ters, and S. Esterhuizen, 2006a: GPS pro -

fil ing of the lower tro po sphere from space: In ver sion and

de mod u la tion of the open-loop ra dio occultation sig nals.

Geophys. Res. Lett., 33, L14816, doi: 10.1029/2006GL

026112. [Link]

Sokolovskiy, S., Y. H. Kuo, C. Rocken, W. S. Schreiner, D.

Hunt, and R. A. Anthes, 2006b: Mon i tor ing the at mo -

spheric bound ary layer by GPS ra dio occultation sig nals

re corded in the open-loop mode. Geophys. Res. Lett., 33,

L12813, doi: 10.1029/2006GL025955. [Link]

Tapley, B. D. and Ch. Reigber, 2004: GRACE (Grav ity Re -

covery and Cli mate Ex per i ment). In: McGraw-Hill Year -

book of Sci ence & Tech nol ogy, McGraw-Hill, New York.,

135-138.

von Engeln, A., J. Teixeira, J. Wickert, and S. A. Buehler, 2006:

CHAMP ra dio occultation de tec tion of the plan e tary

bound ary layer top. In: Foelsche, U., G. Kirchengast, and

A. Steiner (Eds.), At mo sphere and Cli mate: Stud ies by

Occultation Meth ods, Springer Berlin Hei del berg New

York, ISBN 3-540-34116-1, 265-272, doi: 10.1007/3-

540- 34121-8. [Link]

von Engeln, A., C. Marquardt, J. P. Luntama, and J. Wil son,

2007: The GRAS in stru ment on MetOp: Over view. EGU

Vi enna 2007, Geo phys i cal Ab stract EGU2007-A-09276.

Vorob’ev, V. V. and T. G. Krasnil’nikova, 1994: Es ti ma tion of

the ac cu racy of the at mo spheric re frac tive in dex re cov ery

from Dopp ler shift mea sure ments at fre quen cies used in

the NAVSTAR sys tem. Phys. Atmos. Ocean, 29, 602-609.

Wehrenpfennig, A., N. Jakowski, and J. Wickert, 2001: A dy -

nam i cally configurable sys tem for op er a tional pro cess ing

of space weather data. Phys. Chem. Earth, 26, 601-604,

doi:10.1016/S1464-1917(01)00054-X. [Link]

Wickert, J., 2002: The CHAMP-Ra dio occultation ex per i ment:

Al go rithms, Pro cess ing sys tem and first re sults (germ.),

Sci en tific Tech ni cal Re port STR 02/07, GFZ Potsdam.

Wickert, J., C. Reigber, G. Beyerle, R. König, C. Marquardt, T.

Schmidt, L. Grunwaldt, R. Ga las, T. K. Meehan, W. G.

Mel bourne, and K. Hocke, 2001a: At mo spheric sound ing

by GPS ra dio occultation: First re sults from CHAMP.

Geophys. Res. Lett., 28, 3263-3266, doi: 10.1029/2001GL

013117. [Link]

Wickert, J., R. Ga las, G. Beyerle, R. König, and Ch. Reigber,

2001b: GPS ground sta tion data for CHAMP ra dio oc -

cultation mea sure ments. Phys. Chem. Earth, 26, 503-511,

doi: 10.1016/S1464-1895(01)00092-8. [Link]

Wickert, J., G. Beyerle, G. A. Hajj, V. Schwieger, and Ch.

Reigber, 2002: GPS ra dio occultation with CHAMP: At -

mo spheric pro fil ing uti liz ing the space-based sin gle dif fer -

enc ing tech nique. Geophys. Res. Lett., 29, doi: 10.1029/

2001GL013982. [Link]

Wickert, J., T. Schmidt, G. Beyerle, R. König, C. Reigber, and

N. Jakowski, 2004a: The ra dio occultation ex per i ment

aboard CHAMP: Op er a tional data anal y sis and val i da tion

of ver ti cal at mo spheric pro files. J. Meteorol. Soc. Jpn., 82,

381-395, doi: 10.2151/jmsj.2004.381. [Link]

Wickert, J., A. G. Pavelyev, Y. A. Liou, T. Schmidt, C. Reigber,

K. Igarashi, A. A. Pavelyev, and S. Matyugov, 2004b: Am -

pli tude vari a tions in GPS sig nals as a pos si ble in di ca tor of

ion o spheric struc tures. Geophys. Res. Lett., 31, L24801,

doi: 10.1029/2004GL020607. [Link]

Wickert, J., G. Beyerle, R. König, S. Heise, L. Grunwaldt, G.

Michalak, Ch. Reigber, and T. Schmidt, 2005: GPS ra dio

occultation with CHAMP and GRACE: A first look at a

new and prom is ing sat el lite con fig u ra tion for global at mo -

spheric sound ing. Ann. Geophys., 23, 653-658.

Wickert, J., T. Schmidt, G. Beyerle, G. Michalak, R. König, S.

Heise, and Ch. Reigber, 2006a: GPS ra dio occultation with

CHAMP and GRACE: Re cent re sults. In: Foelsche, U., G.

Kirchengast, and A. Steiner (Eds.), At mo sphere and Cli -

mate: Stud ies by Occultation Meth ods, Springer Berlin

Hei del berg New York, ISBN 3-540-34116-1, 3-16, doi:

10.1007/3-540-34121-8_1. [Link]

Wickert, J., T. Schmidt, G. Michalak, G. Beyerle, S. Heise, B.

Tapley, F. Flechtner, and M. Rothacher, 2006b: GPS based

at mo spheric sound ing with CHAMP and GRACE: Pre lim -

i nary re sults of com par a tive data anal y sis. EGU Vi enna

2006, Geo phys i cal Ab stract EGU06-A-03737.

WMO, 1957: Def i ni tion of the tropo pause. WMO Bull., 6,

Geneva.

Wu, D. L., C. O. Ao, G. A. Hajj, M. de la Torre Juarez, and A. J.

Mannucci, 2005: Spo radic E mor phol ogy from GPS-

CHAMP ra dio occultation. J. Geophys. Res., 110, A01306,

doi: 10.1029/2004JA010701. [Link]

Yunck, T. P., C. H. Liu, and R. Ware, 2000: A his tory of GPS

sound ing. Terr. Atmos. Ocean. Sci., 11, 1-20.

50 Wickert et al.