GPS Solutions

, 14:109 | Cite as

Metop-GRAS in-orbit instrument performance

  • Magnus Bonnedal Email author
  • Jacob Christensen
  • Anders Carlström
  • Anders Berg
Original Article


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.


Radio occultation Open loop Metop GRAS Ocean reflection 



Closed loop (L1 or L1/L2 carrier tracking)


Dual frequency


GNSS receiver for atmospheric sounding


Noise power density


Open loop (only code tracking)


Precise orbit determination


Phase-locked loop


Rubidium atomic frequency standards


Radio frequency


Radio occultation


Radio occultation performance estimator


Ray tangent height


Real-time navigation




Single frequency


Straight line tangent altitude


Space vehicle




Ultra-stable oscillator



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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Copyright information

© Springer-Verlag 2009

Authors and Affiliations

  • Magnus Bonnedal
    • 1
    Email author
  • Jacob Christensen
    • 1
  • Anders Carlström
    • 1
  • Anders Berg
    • 2
  1. 1.RUAG Space ABGöteborgSweden
  2. 2.Chalmers University of TechnologyGöteborgSweden

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