RESURS-P LEO On Board Resurs-P LEO Satellite GLONASS for Precise Navigation in Space As the Russian Federation continues to implement its GLONASS multiple access (CDMA) navigation pay- modernization program, which will include newly designed spacecraft loads onboard the GLONASS-M satel- lites # 755–761 — only the first of which and signals, the Russian Federal Space Agency is evaluating methods has been launched. In the meantime, the for improving the system’s performance based on the current GLONASS developers’ team conducted generation of GLONASS-M satellites. This effort includes assessing the analysis and experimental testing of the options to improve performance of the performance of the network of ground reference stations and options GLONASS services based on the current for its improvement as well as using the current GLONASS FDMA GLONASS-M satellites transmitting the signals to precisely determine the orbits of a remote sensing satellite. traditional frequency division multiple access (FDMA) signals. he current stage of GLONASS Analysis of GLONASS evolution is aimed at meet- Evolution Prospects ing future user requirements As a result of the GLONASS Federal Tof which the most important is the Program in 2002–2011 resulted in the VICTOR ASHURKOV, ANDREY VOLKOV improved accuracy of positioning. creation of a considerable scientific and RUSSIAN FEDERAL SPACE AGENCY (ROSCOSMOS) During the implementation of technological backlog. Technological the GLONASS Space Segment Mod- solutions for augmenting GLONASS NIKOLAY TESTOYEDOV ernization Program (2012–2015), the were implemented primarily based ISS RESHETNEV COMPANY GLONASS team is facing the situation on modernized software to enable the ANDREY TYULIN in which it is not feasible to launch new improvement of system performance. RUSSIAN SPACE SYSTEMS COMPANY navigation satellites because the existing The most significant of these augmenta- SERGEY SEREDIN, SERGEY KARUTIN, constellation is comprised of GLONASS- tions is the ground network of reference VLADIMIR MITRIKAS, AND IVAN SKAKUN M satellites operating beyond their stations for the System of Differential THE CENTRAL RESEARCH INSTITUTE OF guaranteed design lifetime. Nine more Correction and Monitoring (SDCM). MACHINE BUILDING (ROSCOSMOS) GLONASS-M satellites are in ground A trade-off analysis for GLONASS ARKADII TIULIAKOV, DMITRII FEDOROV storage. services development identified the RUSSIAN INSTITUTE OF RADIONAVIGATION In these circumstances the decision need to create a global ground network AND TIME (RIRT) was taken to install L3 code division to improve the accuracy of orbit and 54 InsideGNSS SEPTEMBER/OCTOBER 2015 www.insidegnss.com 90˚ clock prediction when implementing techniques for estimating the combined 60˚ ephemeris and time parameters using one-way measurements by the ground 30˚ segment. Associated efforts initiated in 2007 0˚ resulted in deployment of a ground network with 19 reference stations in –30˚ the Russian Federation and 4 stations Latitude (degrees) abroad for the SCDM, which enable –60˚ observation of the GLONASS satellites in both hemispheres. The main disad- –90˚ vantage of the network is the lack of –180˚ –135˚ –90˚ –45˚ 0˚ 45˚ 90˚ 135˚ 180˚ overlap for satellite visibility zones from Longitude (degrees) the stations in the Russian territory and FIGURE 1 Ground network used for GLONASS orbit and clock data estimation the stations abroad. This results in the need to attract additional international stations on the territory of the Indian 100 Ocean to provide for orbit and clock data estimates along the whole satellite track. Despite these limitations, the pre- requisites for improving the current GLONASS performance still exist. 10 *1.E-14 That is why the task of estimating the potentially achievable GLONASS per- formance in terms of accuracy becomes utterly vital. The main issue is identi- fying the minimum number of refer- ence stations needed and determining 1000 10,000 100,000 the dependence of orbit and clock data errors on the periodicity of their update on board a satellite. FIGURE 2 Allan Deviation estimates of the onboard atomic frequency standards GLONASS Performance We selected a 45-day period starting tioning that the clock corrections were Improvement Options February 2, 2015 for the estimation of of generated using an adaptive model We conducted an experimental study to potential GLONASS performance. Figure without high-accuracy referencing to assess options for improving GLONASS 1 shows the network of selected SDCM the GLONASS time scale. performance. To do this we used the reference stations expanded with several The equivalent user range error adaptable high-accuracy hardware and IGS stations. These stations provide a (UERE, disregarding propagation errors software of the Information and Analy- data stream at one-hertz frequency. and user receiver biases) was estimated sis Center (IAC) for Positioning, Naviga- We conducted the orbit and clock against the postprocessed precise orbit tion and Timing residing in the Central data estimation using two-day interval and clock data generated by IAC at Research Institute of Machine Building measurements with six-hour periodic- 15-minute intervals. We estimated of the Russian Federal Space Agency. ity. The estimated parameters included UEREs for all satellites in the constella- Based on International GNSS Service the GLONASS satellites ephemerides, tion without limitations. (IGS) weekly estimates, the IAC deter- the stochastic corrections to satellite Figure 2 presents the estimates of mined the orbital error of GLONASS and reference station timescale param- Allan deviation for the clocks on all satellites to be about 0.02–0.025 meter eters, the troposphere model parame- operational satellites in the GLONASS (RMS). In the technological cycle ters, the coordinates of the stations, and constellation. employed in IAC we also estimate the the Earth rotation parameters. The following five optional modes systematic errors of the reference sta- At the end of each solution the pre- of operation for onboard and ground tions’ receivers. The delay of the IAC dicted orbit and clock data for a 24-hour capabilities were studied when estimat- final data is seven days. interval were generated. It is worth men- ing UERE (Table 1): www.insidegnss.com SEPTEMBER/OCTOBER 2015 InsideGNSS 55 RESURS-P LEO Orbit and Clock Data • estimating, predicting, and uploading orbit and clock data faster calculation of Upload Periodicity, hour/ Orbit and Clock Data every 6 hours with 6-hour delay from the latest measure- the orbit and clock Prediction Interval, hour σ , m ment (orbit and clock data prediction at the 6–12 hour data and its delivery 0 6 / 6–12 0.8 interval to users. • estimating, predicting, and uploading orbit and clock data 6 / 1–6 0.5 every 6 hours with an hour delay from the latest measure- GLONASS Use 1 / 2–3 0.45 ment (orbit and clock data prediction at 1–6 hour interval for Precise Orbit 2 / 1–3 0.4 Determination instead of the 1–7 hour interval) 1 / 0.25–1.25 0.3 • estimating, predicting, and uploading orbit and clock of LEO Satellites data every hour with 2-hour delay from the latest mea- For decades before Table 1 UERE estimation results for GLONASS-M constellation surement (orbit and clock data prediction at the 2–3 hour the global navi- interval) gation satellite systems came into being, the position of any • estimating, predicting, and uploading orbit and clock data satellite had been determined through a dynamic method every 2 hours with an hour delay from the latest measure- incorporating a priori specified models (geopotential, tidal ment (orbit and clock data prediction at 1–3 hour interval) deformations, atmosphere, and so on). Such models were vali- • estimating, predicting and uploading orbit and clock data dated along with the satellites orbits, while solution accuracy every 15 minutes with an hour delay from the latest mea- was estimated based on measurement residuals or laser ranging surement (orbit and clock data prediction at 0.25–1.25 data. With the advent of GNSS, onboard carrier phase mea- hour interval). surements and precise point positioning (PPP) technology in The obtained results demonstrate that the accuracy of particular have enabled a change in the approach to the solu- user position solutions can be improved by employing the tion of the aforementioned tasks. orbit and clock data determination technique based on the Such solutions are based on high-accuracy GNSS satellite measurements from both the limited ground network and the orbit and clock data, resulting from the sophisticated FDMA navigation signals. The improved orbit determination processing of one-way navigation measurements obtained and clock synchronization accuracy is achieved through the from global networks of commercial geodetic-grade receivers. One team with one goal: accurate and reliable GNSS positioning anywhere 56 InsideGNSS SEPTEMBER/OCTOBER 2015 www.insidegnss.com In the 1990s such solutions were carried located in Pasadena, Ottawa, Bern, clock data is the only thing needed to out only for geodynamic research Potsdam, and Darmstadt) united by the accomplish this. Still, one substantial in analysis centers (such as those International GNSS Service (IGS) with restriction on carrier phase positioning its Central Bureau located at the NASA is the need for a continuity of phase Jet Propulsion Laboratory (JPL). During measurements at a relatively high rate that period methods and models of providing cycle-slip detection. Therefore, processing phase measurements were it seems reasonable that the evolution of being improved using a posteriori GNSS measurement techniques could approaches. support orbit determination of low By that time it had already become Earth orbiting (LEO) satellites. The clear that GNSS phase measurements position dilution of precision (PDOP) could potentially enable the at a LEO altitude of 500 kilometers is determination of user position in real almost the same as that at the ground time with significantly better accuracy. level, while atmospheric influences, the FIGURE 3 Resurs-P satellite exterior High-accuracy GNSS satellite orbit and major error source for a ground-level user, are substantially lower.