Chris Rocken

Real-Time GPS Data Transmission Using VSAT Technology

The University NAVSTAR Consortium (UNAVCO) Boulder Facility is assessing Very Small Aperture Terminal (VSAT) technology for near real-time transmission of GPS data from a remote receiver to a central processing facility. The study is motivated by the need for a robust, cost-effective data communications solutions to transfer GPS data from remote sites where no other communication alternatives exist. Future large-scale plate boundary deformation initiatives using spatially dense networks of GPS will require receivers to be located where the science dictates, and not the power or communications infrastructure. For other applications, such as determining rapid GPS orbits and time transfer, there is a push toward reducing the latency in GPS data used to produce GPS data products and differential corrections (Talaya & Bosch, 1999; Jackson, Meertens, & Rocken, 2000; Muellerschoen, Bar-Sever, Bertiger, & Stowers, 2001), and to support upcoming Low Earth Orbiting (LEO) missions requiring low latency, 1-s GPS data. In this paper we evaluate two Ku-band systems, the Nanometrics Libra VSAT and the StarBand 2-way satellite Internet VSAT. The Nanometrics system test results show that continuous, 1-s GPS data can be streamed from multiple remote stations within the VSAT footprint, quality checked, and delivered for processing with a <2.5-s latency (mean 1.2 s) and a 99.8% reliability. Benefits of the Nanometrics system include global coverage, control of bandwidth allocation and data hub, and the low power draw of the system. Negatives include the cost of hub and remote infrastructure and the need to negotiate landing rights issues on a country-by-country basis. The UNAVCO Facility currently operates a Nanometrics hub and three remote VSAT systems.The StarBand system showed 98.9% reliability with a maximum latency of 10.2 s (mean latency 1.7 s) for 1-Hz GPS data and an average uplink speed of 31.7 kbps. Benefits of the StarBand system include the cost and small profile of the remote antenna. Negatives include coverage limited to coterminous United States and the high power draw of remote systems.

Radio Occultation Data Processing at the COSMIC Data Analysis and Archival Center (CDAAC)

This paper gives an overview of the COSMIC Data Analysis and Archival Center (CDAAC) and presents initial results that were obtained from neutral atmospheric radio occultation data processing of the GPS/MET, CHAMP, and SAC-C datasets. Approximately 45 % of CHAMP and SAC-C retrieved radio occultation profiles reach below 1 km altitude, compared to only 35 % for GPS/MET. All missions exhibit a negative refractivity bias in the lower troposphere of nearly 1 % compared to NWP models. When constrained to the tropics, the CHAMP data show a negative refractivity bias of approximately 1.7 %, and only 20 % of occultation profiles reach 1 km. Current CDAAC LEO orbit determination for CHAMP agrees with JPL orbits at the 30 cm 3D root mean square level, which can result in temperature errors of about 0.3 degrees K at 30 km altitude.

Reply to comment by A. von Engeln et al. on “Monitoring the atmospheric boundary layer by GPS radio occultation signals recorded in the open-loop mode”

[1] We thank von Engeln et al. for their comment [von Engeln et al., 2007] (hereinafter referred to as VE07) on our paper [Sokolovskiy et al., 2006] (referred to as S06 in the comment). VE07 said that S06 used five definitions of the PBL top height without addressing their difference. By performing additional study, VE07 demonstrated that different definitions result in different heights. We believe this is to be expected. In addition, we would like to clarify several points made by VE07. [2] (1) S06 did not use five definitions of the PBL top altitude (also, see next paragraph). (2) The goal of S06 was not studying different definitions of the PBL height, but demonstration that the break (or elbow) point in refractivity profile, associated with the top of PBL (as shown in Figure 1 of S06), is a robust estimator that can be obtained with the use of the open-loop (OL) radio occultation (RO) signals because the OL tracking allows penetration of the retrieved profiles to the surface. It is emphasized by S06 that the break point in refractivity does not necessarily correspond to the point of the maximum refractivity gradient (section 2). [3] Association of the top of PBL with fading of the amplitude of RO signals transformed to impact parameter representation by radio-holographic (RH) methods may not be considered as definition because it has no physical justification. It was used by S06 for only discussion of the results [von Engeln et al., 2005] (referred to as E05 by VE07). It is known that fading of the transformed amplitude denotes the surface or any other height if the signal is lost by receiver at earlier time. E05 write: ‘‘Software updates to the tracking algorithm will modify the relation of the PBL top altitude to the 50% FSI amplitude one’’ (p. 3). This means that the method proposed by E05, which relies on the loss of lock by receiver at the top of PBL, requires calibration, and therefore the results are statistically dependent on the ancillary data used for the calibration. VE07 say that E05 validated their results with about 142,000 occultations. We believe this statement is misleading. Without physical justification of 50% (also, see next paragraph), this is not a validation, but a calibration with the use of ECMWF analysis. We believe this is a very important conclusion, but it was never stated explicitly. [4] By performing additional study, VE07 demonstrated that cut-off heights of the profiles retrieved at different processing centers are different. This is not surprising since different centers may use different algorithms. E05 write: ‘‘Processing stops when the smoothed occultation FSI amplitude is reduced by 50%’’ (p. 2). This raises questions: (1) What smoothing method (window) was applied? (2) Reduced by 50% compared to what? It is known that RH amplitude undergoes strong scintillation in the troposphere, especially in the tropics and, for some occultations, is not stable at higher altitudes. Answers to these questions are necessary for interested readers to reproduce the results of E05.