GPS Solutions

, 14:109

Metop-GRAS in-orbit instrument performance

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

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.

Keywords

Radio occultation Open loop Metop GRAS Ocean reflection 

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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Copyright information

© Springer-Verlag 2009

Authors and Affiliations

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

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