|Instrument: GRACE KBR : K-Band Ranging system|
Earth Remote Sensing Instruments
Instrument Class: Active Remote Sensing
Instrument Type: Altimeters
Instrument Subtype: Radar Altimeters
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[Source: GFZ/Potsdam, http://www.gfz-potsdam.de/ ]
The K-band ranging (KBR) system is the key science instrument of GRACE which measures the dual one-way range change between both satellites with a precision of about 1 Ám per second.
The hardware consists of
- a single horn antenna for transmission and reception of the dual-band (24 and 32 GHz) k- and ka-band microwave signals,
- an ultra-stable oscillator (USO) serving as a frequency reference,
- a microwave assembly for up-converting the reference frequency , down-converting the received phase from the other satellite to approximately 2 MHz and for amplifying and mixing the received and the reference carrier phase and
- an instrument processing unit (IPU) used for sampling and digital signal processing of the k-band carrier phase signals and the data of the GPS space receiver, the accelerometer (ACC) and the star camera assembly (SCA).
Both KBR are completely identical, except of the frequencies, which are shifted by 500 KHz to avoid cross-talk between transmitted and received signals and to offset the down-converted signal from zero frequency. Each satellite transmits carrier phase signals on two frequencies allowing for ionospheric corrections. The 10 Hz samples of the phase change at the two frequencies for each satellite are down-linked to ground. The appropriately decimated linear combination of the sum of these phase measurements at each frequency gives the ionosphere-corrected measurement of the range change between the satellites. To reduce measurement errors, the KBR temperature has to be controlled to 0.2 K. Additionally multipath effects are reduced by stringent spacecraft pointing requirements (< 1 mrad) and representative antenna and spacecraft front panels.
Instrument Owner: USA/NASA