# Comparing GRACE-FO KBR and LRI Ranging Data with Focus on Carrier Frequency Variations
## 1 Introduction  
The Gravity Recovery and Climate Experiment (GRACE) satellites have measured temporal variations in Earth's gravity field from 2002 until 2017. Its successor mission GRACE Follow-On (GRACE-FO), launched in 2018, continues the valuable dataset of monthly gravity field maps [1], which are used for instance in the fields of climate research [2], hydrology [3] and seismology [4]. Prominent GRACE(-FO) results have quantified the ice mass loss in regions such as Greenland and Antarctic over the past two decades [5] and allowed to attribute the effect of mass influx into the oceans in the observed global mean sea level rise [6]. Some changes in the gravity field are likely caused by groundwater depletion [7]. The GRACE(-FO) results lead to more than 2600 publications, and many of them are frequently cited in reports of the Intergovernmental Panel on Climate Change (IPCC) [8, 9]. The primary measurement containing the gravity field information in a GRACE-like mission is the biased inter-satellite range, which was measured to micrometer precision in GRACE using a microwave K-band ranging system (KBR) [10, 11]. GRACE-FO in contrast hosts two ranging instruments, the KBR as well as a novel and more precise laser ranging instrument (LRI, [12, 13]). The ranging data of either KBR or LRI, or a combination of both, are processed in the course of gravity field retrieval together with various othermeasurements, such as attitude and orbit data as well as with measurements of non-gravitational accelerations obtained from accelerometers. LRI and KBR measure the inter-satellite distance variations in GRACE-FO in parallel. Future missions such as the European-lead Next Generation Geodesy Mission (NGGM, [14]) and the US-lead Mass Change Mission [15], with counterpart GRACE-I(carus) [16] in Germany, are currently being studied or developed. Since these new missions will solely rely on laser-based ranging, it is important to understand what is currently limiting the ranging data quality. The GRACE-FO data discussed here are publicly available in terms of raw observations (level1a), in processed observations (level1b) and in terms of gravity field products (level2) [17]. The unique setup of two almost independent ranging systems allows us to study the differential instrument behavior and small instrument errors at frequency regions, which are usually dominated by signal several orders of magnitude larger than the expected noise. The KBR and LRI units on both satellites each emit and receive electromagnetic radiation and utilize heterodyne interferometry, meaning that the received radiation is interfered with a local oscillator field, derived from the transmitted radiation. Due to a frequency difference of both interfering fields, a beatnote is produced in the kHz domain in the KBR [10; 11] and in the MHz domain in the LRI [18]. The ranging information is encoded in the phase of the measured beatnote, which is the primary science measurement. Both KBR and LRI exhibit similar or even the same types of noises and errors, but these have different magnitudes due to different implementations and differences in wavelength. Another distinguishing feature is the fact that microwave radiation is more susceptible to free electrons in the ionosphere, requiring dual-band measurements in KBR in order to mitigate ionospheric disturbances [10]. Since KBR and LRI send radiation in both directions, intrinsic fluctuations in the phase of the wave source can be highly suppressed via a self-comparison. In KBR, this suppression is achieved in post-processing, when noisy phase measurements from both satellites are combined with proper delays. In LRI, the suppression of laser phase noise occurs in the optical domain via a transponder approach [18]. Hence, the beatnote phase as measured with a photodiode and phase meter is already less noisy. The higher sensitivity of two to three orders of magnitude in LRI w.r.t. KBR [19] is mainly achieved by the shorter wavelength of laser radiation.