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Metop-GRAS in-orbit instrument performance

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Abstract

The GRAS instrument on the Metop-A satellite provides more than 600 radio occultation measurement profiles per day. The instrument is characterized by its wide antenna coverage, high signal-to-noise ratio and an ultra-stable clock reference. The conventional dual-frequency tracking of GPS signals is under dynamic atmosphere conditions complemented by open loop tracking with sampling of the signal at a 1 kHz rate, providing an unprecedented view of the signal spectral environment. This paper presents the instrument performance as derived from analysis of in-orbit measurement data. We show that the noise figure is low enough to enable mapping of external radio noise variations over the earth’s surface. An error propagation model is presented to relate instrument characteristics to bending angle performance. This model is also used to illustrate the relation between filter bandwidth, resolution and measurement noise. The Doppler model, guiding open loop measurements, is found to be accurate to better than 20 Hz with a possibility for improvement to 10 Hz. The high performance at low altitudes enables the presence of surface reflections at the −20-dB level to be identified in more than 50% of the occultations. The potential performance improvements for next generation receivers are discussed.

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Abbreviations

CL:

Closed loop (L1 or L1/L2 carrier tracking)

DF:

Dual frequency

GRAS:

GNSS receiver for atmospheric sounding

NPD:

Noise power density

OL:

Open loop (only code tracking)

POD:

Precise orbit determination

PLL:

Phase-locked loop

RAFS:

Rubidium atomic frequency standards

RF:

Radio frequency

RO:

Radio occultation

ROPE:

Radio occultation performance estimator

RTH:

Ray tangent height

RTN:

Real-time navigation

S/C:

Spacecraft

SF:

Single frequency

SLTA:

Straight line tangent altitude

SV:

Space vehicle

S/W:

Software

USO:

Ultra-stable oscillator

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Acknowledgments

The GRAS data analyzed in this study were provided by EUMETSAT for the period 30 September to 1 November 2007. Navigation data for the same period have been obtained from GeoForschungsZentrum Potsdam (GFZ) (Beyerle et al. 2009) with partial global coverage over the same period. A second set of data from November 2006 to January 2007 were provided by ESA from the GRAS in-orbit verification phase. Part of the work was financed by the ESA study; GRAS Radio Occultation Performance Study, Contract 21995/08/EL.

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Authors and Affiliations

  1. RUAG Space AB, Solhusg, 405 15, Göteborg, Sweden

    Magnus Bonnedal , Jacob Christensen & Anders Carlström

  2. Chalmers University of Technology, 412 96, Göteborg, Sweden

    Anders Berg

Authors
  1. Magnus Bonnedal
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  2. Jacob Christensen
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  3. Anders Carlström
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  4. Anders Berg
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Correspondence to Magnus Bonnedal .

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Bonnedal , M., Christensen, J., Carlström, A. et al. Metop-GRAS in-orbit instrument performance. GPS Solut 14, 109–120 (2010). https://doi.org/10.1007/s10291-009-0142-3

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  • DOI: https://doi.org/10.1007/s10291-009-0142-3

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