# Evaluation of the GPS Precise Orbit and Clock Corrections from MADOCA Real-Time Products
## 1 Introduction  
Quasi-Zenith Satellite System (QZSS) is a regional satellite navigation system covering the entire Asia-Oceania regions. The first Quasi-Zenith Satellite (QZS-1, nicknamed 'Michibiki') was launched on September 11, 2010 and operated by the Japan Aerospace Exploration Agency (JAXA) [1]. The Cabinet Office, Government of Japan (CAO) took control of QZS-1 since 28 February 2017 [2], then developed and launched additional three satellites in 2017. On 1 November 2018, CAO claimed that QZSS started to provide services officially [3], which is currently composed of three quasi-zenith orbit (QZO) satellites and one geostationary orbit (GEO) satellite and will be extended to a seven-satellite system in 2023 [4]. The QZO satellite can provide continuous signal coverage at a high elevation angle and improve the visibility of QZSS satellites over Japan even if there are many urban canyons and mountainous areas [5]. The QZSS satellites could act as complementary navigation satellites by broadcasting exactlythe same navigation signals as GPS satellites [6] and also extend to satellite-based augmentation system (SBAS) by transmitting high-accuracy GNSS satellite orbit and clock correction via satellite linkage for real-time precise point positioning (RTPPP) applications [7]. On RTPPP applications, the accuracy of satellite orbit and clock products is a prerequisite because they are introduced to the PPP observation equations as known parameters [8], thus the errors on the orbit/clock would be propagated to the positioning results and affect the positioning accuracy. Since the QZS-1 was successfully launched, evaluation researches had been started for users in the Asia-Oceania area [7; 9; 10]. However, in the early period, the QZSS real-time orbit and clock products are not generated based on global distributed ground stations, and the positioning performance shows obvious spatial difference [9]. Furthermore, the update rate of 180 s of QZSS real-time products in the early period is not frequent enough; the interpolation on orbit and clock products would thus introduce uncertainty to the real time positioning results [11; 12]. After the successful transfer of QZS-1 control to CAO, the trial service of Centimeter Level Augmentation Service (CLAS), called CLAS Experiment (CLAS-E), started to provide centimeter level augmentation information on March 28, 2017 [13]. These satellite-based augmentation messages are mainly high rate precise satellite orbit and clock products, aiming to meet the requirement of the real time PPP application for users who can receive L6E messages either from QZSS satellite directly or via internet [14]. The augmentation message of CLAS-E is using MADOCA products generated by the software named Multi-GNSS Advanced Demonstration tool for Orbit and Clock Analysis (MADOCA) developed by JAXA. The current MADOCA real-time orbit and clock products are calculated based on real-time data streams from 53 CORS stations distributed as in Figure 1[15]. This research aimed to perform a comprehensive evaluation of the MADOCA real-time augmentation service from the aspect of GPS satellite products availability, accuracy, and positioning performance. Data latency evaluation is ignored as it highly dependent on the user's hardware or internet performance. The paper is organized as follows: This section is a brief review of the QZSS system including its development and augmentation service; and then, the data sets and evaluation strategy for the MADOCA real-time orbit and clock products is introduced in Section 2; in Section 3, the products availability is evaluated; and accuracy of the MADOCA real-time products are analyzed by comparing these with the IGS final products in Section 4; Section 5 discusses the calculation method of the SISRE metrics and obtains the corresponding statistical results; in Section 6, the simulated RTPPP with over 20 IGS station data was applied to evaluate the real-time positioning performance in different areas; finally, conclusions are summarized in Section 7.