Flechtner F.M., Gruber Th., G?ntner A., Mandea M., Rothacher M., Sch?ne T., Wickert J. (Eds.) System Earth via Geodetic-Geophysical Space Techniques

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Global Atmospheric Data from CHAMP and GRACE-A 441

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Near-Real Time Satellite Orbit Determination

for GPS Radio Occultation with CHAMP

and GRACE

Grzegorz Michalak and Rolf König

1 Introduction

One of the CHAMP and GRACE mission objectives is to perform radio occultation

measurements of GPS signals propagating through Earth’s atmosphere. From these

measurements it is possible to derive vertical proﬁles of bending angles, temperature

and humidity of the atmosphere on a global scale which can be used by numerical

weather prediction systems (NWP). To fulﬁll this mission objective, rapidly avail-

able precise orbits and clock offsets both for occulting GPS satellites and the Low

Earth Orbiters (LEOs) are required. Efﬁcient occultation data assimilation in the

NWP systems requires a 3 h time line for occultation products. As the orbits are

needed to generate the occultation products, the latency of the orbit products is

required not to exceed 0.5 h. For this purpose a Near-Real Time (NRT) orbit pro-

cessing system was developed to generate precise GPS, CHAMP, GRACE-A and

TerraSAR-X orbits with a mean latency of 15–30 min (Michalak et al., 2007). The

orbit latency is deﬁned here as a difference between the epoch of the orbit gener-

ation and the epoch of the last data point used in the processing. The space-borne

GPS Satellite-to-Satellite Tracking data (SST) and the occultation data can be down-

loaded (dumped) from each LEO once per revolution, approximately every 1.5 h.

Processing of the most recent dump, containing 1.5 h of occultation data, starts

when the NRT orbits become available. The typical processing time of the occul-

tation data is 0.5 h; taking into account additional 0.5 h of NRT orbit latency, the

delay of occultation products generated from the recent dump is between 1 and 2.5 h

(on average 1 h 45 min). This delay meets the 3 h timeline and the data can be suc-

cessfully assimilated by the weather centers (Wickert et al., 2009). Dumping of the

SST data in each revolution is assured for near polar satellite orbits by operating a

GFZ’s northern latitude receiving station in Ny

Ålesund. To enhance reliability and

to test different approaches of orbit generation, three independent NRT processing

G. Michalak (B)

Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences,

Department 1: Geodesy and Remote Sensing, Telegrafenberg, 14473 Potsdam, Germany

e-mail: michalak@gfz-potsdam.de

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F. Flechtner et al. (eds.), System Earth via Geodetic-Geophysical Space Techniques,

Advanced Technologies in Earth Sciences, DOI 10.1007/978-3-642-10228-8_39,

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Springer-Verlag Berlin Heidelberg 2010

444 G. Michalak and R. König

chains were developed delivering the orbits with different accuracies and latencies

which can be used also for other applications according to their accuracy/latency

requirements. Details of the different approaches are given in the following sections.

2 NRT Orbit Processing System

The Near-Real Time LEO orbit processing system is based on so-called two step

approach in which the LEO orbits are estimated using ﬁxed GPS orbits and clocks

estimated before. For this reason the system consists of two separate subsystems.

One is designed to generate GPS NRT orbits and clocks; the second one generates

LEO NRT orbits. The estimation of the GPS orbits and clocks is performed by using

data of 30–60 stations of a globally distributed GPS ground network from IGS. The

modelling standards and estimated parameters are very similar to those used for

the so-called Rapid Science Orbits (Michalak et al., 2003). Since the processing

of the GPS ground data is the most time consuming part, for the NRT application

this processing was split into a long and a short arc. The long 24 and 12 h GPS

arcs (see details below) are generated on an hourly basis and parallel to this the

short 3 h arcs extending the long arcs are generated every 15 min. The generation

of the short arcs is fast and gives access to GPS orbits and clocks with low latencies

of approximately 15 min, where waiting and downloading of the GPS data takes

10 min, and processing takes less than 5 min. Subsequent 14 h LEO NRT arcs are

based on the combination of the most recent long and short GPS arcs. The LEO

NRT processing starts as soon as new space-borne GPS satellite-to-satellite (SST)

data become available, downloaded from the satellite or “dumped”. Therefore this

way of processing is called dump-related. Typical delay in access to SST dump data

for CHAMP and GRACE is in the range of 5–10 min. To assure high reliability and

to test possible accuracies and latencies, the NRT orbit processing system was split

into three separate chains which generate LEO orbits based on different sets of GPS

orbits. The description of the chains (CHAIN 1, CHAIN 2 and CHAIN 3) is given

below. In addition to CHAMP and GRACE, the TerraSAR-X satellite, launched on

June 15, 2007 was also included in the NRT system. The accuracy of the LEO NRT

orbits is determined by independent Satellite Laser Ranging (SLR) observations.

Accuracies and latencies achieved for all LEOs and for all chains are summarized

in Table 1. The previously running Ultra-rapid Science Orbit (USO) system (König

et al., 2005) delivering LEO orbits with latencies above 2 h was eventually shut off

and replaced by the new NRT system in September 2007. The latencies for USO

and NRT processing for CHAMP are given in Fig. 1.

2.1 CHAIN 1: GPS-Based Processing

In this chain, called “GPS-based”, the 14 h LEO orbits are based on the combination

of 24 and 3 h GPS orbits and clocks, estimated from the GPS ground station data

with 5 min spacing. The 3 h arcs are predictions originating from the long 24 h arcs,

Near-Real Time Satellite Orbit Determination for GPS Radio Occultation 445

Table 1 Summary of the accuracies and latencies of the 14 h NRT LEO orbits for various NRT

processing chains. For calculation of the position and velocity overlap values, the last 2 h of the

LEO arcs are used. Latency is deﬁned as the difference between the orbit generation time and the

epoch of the last available SST observation used in the processing

SLR RMS (cm)

3-D Pos.

overlap (cm)

3-D Vel.

overlap

(mm/s) Latency (min)

CHAMP Period 2006/08/17–2008/12/15

CHAIN 1 (GPS-based) 6.8 11.5 0.11 31

CHAIN 2 (IGU-based) 9.6 18.8 0.18 13

CHAIN 3 (IGU-ﬁxed-30s) 5.1 6.5 0.07 30

GRACE-A Period 2006/08/22–2008/12/15

CHAIN 1 (GPS-based) 7.6 11.0 0.09 31

CHAIN 2 (IGU-based) 10.6 19.2 0.17 13

CHAIN 3 (IGU-ﬁxed-30s) 6.0 7.0 0.06 30

TerraSAR-X Period 2007/08/27–2008/12/15

CHAIN 1 (GPS-based) −−––

CHAIN 2 (IGU-based) 9.8 17.8 0.17 30–60

CHAIN 3 (IGU-ﬁxed-30s) 5.6 5.4 0.06 30–60

but the 5 min clocks are estimated. The latency of the3hGPSorbitsiscurrently

approximately 20 min, where 15 min is the waiting and data downloading time, the

remaining 5 min is the data processing proper. Currently the 3-D accuracy of the

short 3 h GPS arcs, when compared to the estimated part of the Ultra Rapid Orbits

(IGU) of the International GNSS Service (IGS) is 19 cm (see Fig. 2). The LEO

orbits are generated with an average latency of 30 min and an accuracy of 7 cm

Fig. 1 The l atency of the CHAMP orbit generation for two systems: USO and NRT. The NRT

latency comes from the CHAIN 2

446 G. Michalak and R. König

Fig. 2 Position differences between the GPS NRT orbits (3-h arc length) and the estimated part of

the IGU orbits

validated by independent Satellite Laser Ranging (SLR) measurements. This chain

is not active for the TerraSAR-X satellite.

2.2 CHAIN 2: IGU-Based Processing

In this chain, designated as “IGU-based”, the 14 h LEO orbits are based on purely

predicted IGU orbits and clocks. Since the s pacing of the IGU orbits and clocks is

15 min, for processing of the 30s SST data the IGU clocks are linearly interpolated

to 30s intervals. Because there is no ground GPS data processing involved, the LEO

orbits can be generated very rapidly with a mean latency of 13 min, but a comparably

low accuracy of 10 cm (via SLR validation). This low accuracy is mainly caused by

the predicted IGU clocks used as ﬁxed in the LEO processing. The chain i s very

robust because it depends solely on the availability of the IGU orbits and the SST

dump data; no GPS ground data are needed. Accuracies and latencies of the NRT

orbits for CHAMP, GRACE-A and TerraSAR-X satellite generated by this chain are

given in the Figs. 3, 4, and 5.

2.3 CHAIN 3: IGU-Fixed-30s Processing

This chain, designated as “IGU-ﬁxed-30s”, generates LEO orbits based on ﬁxed pre-

dicted IGU orbits, but 30s clocks estimated from GPS ground data. Since estimation

Near-Real Time Satellite Orbit Determination for GPS Radio Occultation 447

Fig. 3 (a) Accuracy of CHAMP NRT orbits generated by CHAIN 2. (b) Latency of CHAMP NRT

orbits generated by CHAIN 2

448 G. Michalak and R. König

Fig. 4 (a) Accuracy of GRACE-A NRT orbits generated by CHAIN 2. (b) Latency of GRACE-A

NRT orbits generated by CHAIN 2

Near-Real Time Satellite Orbit Determination for GPS Radio Occultation 449

Fig. 5 (a) Accuracy of TerraSAR-X NRT orbits generated by CHAIN 2. (b) Latency of TerraSAR-

X NRT orbits generated by CHAIN 2. Due to lacking of the data from polar receiving station Ny

Ålesund, the data are signiﬁcantly delayed. Test dumps over Ny

Ålesund demonstrated very low

latency also for TerraSAR-X

450 G. Michalak and R. König

Fig. 6 (a) CHAMP NRT orbit accuracy generated by the CHAIN 3. (b) CHAMP NRT orbit

latency generated by the CHAIN 3

Near-Real Time Satellite Orbit Determination for GPS Radio Occultation 451

Fig. 7 (a) Accuracy of the GRACE-A NRT orbits generated by CHAIN 3. (b) Latency of the

GRACE-A NRT orbits generated by CHAIN 3