TTC 2013 6th ESA International Workshop on Tracking, Telemetry and Command Systems for Space Applications
ESA-ESOC, 10 - 13 September 2013
IMPROVING TRACKING SYSTEMS FOR DEEP SPACE NAVIGATION L. Iess1, F. Budnik2, C. Colamarino3, A. Corbelli4, M. Di Benedetto5, V. Fabbri6, A. Graziani7, R. Hunt8, N. James9, M. Lanucara10, R. Maddè11, M. Marabucci12, G. Mariotti13, M. Mercolino14, P. Racioppa15, L. Simone16, P. Tortora17, M. Westcott18, M. Zannoni19 1 Sapienza, Università di Roma, Italy,
[email protected] 2 European Space Agency (ESA), Germany,
[email protected] 3 Thales Alenia Space Italia, Italy,
[email protected] 4 ALMASpace S.r.l., Italy,
[email protected] 5 Sapienza Università di Roma, Italy,
[email protected] 6 ALMASpace S.r.l., Italy,
[email protected] 7 University of Bologna, Italy,
[email protected] 8 BAE Systems, United Kingdom,
[email protected] 9 BAE Systems, United Kingdom,
[email protected] 10 European Space Agency (ESA), Germany,
[email protected] 11 European Space Agency (ESA), Germany,
[email protected] 12 Sapienza Università di Roma, Italy,
[email protected] 13 University of Bologna, Italy,
[email protected] 14 European Space Agency (ESA), Germany,
[email protected] 15 Sapienza Università di Roma, Italy,
[email protected] 16 Thales Alenia Space Italia, Italy,
[email protected] 17 University of Bologna, Italy,
[email protected] 18 BAE Systems, United Kingdom,
[email protected] 19 University of Bologna, Italy,
[email protected] I. INTRODUCTION Doppler, ranging and Delta-Differential One-Way Ranging (ΔDOR) are the main observable quantities used in deep space navigation. For the current ESA X-band tracking systems, their accuracy is at level of, respectively, 0.1 mm/s at 60 s integration time, 1 to 5 m, and 6 to 15 nrad. Similar accuracies are provided by NASA’s DSN systems, with the notable exception of ΔDOR, whose performances are now consistently at the level of 1-2 nrad. In 2010 the ESA’s General Studies Programme funded a study aimed at defining a roadmap for the enhancement of the radio observables accuracy in each of the three tracking techniques by about one order of magnitude. In particular the target accuracies were set to 0.01 mm/s at 60 s integration time for Doppler, 20 cm for ranging (two-way) and 1 nrad for ΔDOR measurements. The study was undertaken by a consortium formed by Sapienza University of Rome, ALMASpace (a spinoff company of the University of Bologna), BAE Systems and Thales Alenia Space Italy. The first step of this study was the error budget consolidation of each technique, achieved by analyzing a data set from the Rosetta and Cassini mission. Rosetta data consist of X-band ranging and Doppler observables from Nov. 2009 to Oct. 2010, when the spacecraft was in the initial cruise phase, en route to the Comet 67/P Churyumov-Gerasimenko. The data set begins just two days after the last Earth gravity assist, on Nov. 13, 2009. The Cassini data set comprises X- and Ka-band Doppler observables at 34-32 GHz, acquired by NASA Deep Space Network (DSN) during the first Gravitational Wave Experiment (GWE1, Nov. 26, 2001 to Jan. 4, 2002) and the Solar Conjunction Experiment [1, 2] (SCE1, Jun. 6, 2002 to Jul. 5, 2002). During these radio science experiments the spacecraft was in cruise towards Saturn. Those multi-frequency observations are extremely useful for better isolating dispersive noise sources (interplanetary plasma and, to a less extent, ionosphere). The radio-metric data set is complemented by measurements obtained from JPL’s Advanced Media Calibration system (AMC) [3, 4], a suite of instruments (including a microwave water vapor radiometer or MWR) for high accuracy calibration of the tropospheric phase delay (dry and wet) during radio science experiments (GWE1 and SCE1). In addition, six years of Doppler and range data at X-band (from Jan. 2005 to Mar. 2011) acquired during the Cassini Saturnian phase have been used for the validation of the error models. In this work we provide a consolidated error budget for the different tracking techniques, identify the main noise sources and outline the technology upgrades needed to improve the current navigation accuracies. I. DOPPLER OBSERVABLES The frequency shift experienced by the radio link used for deep space communications is exploited in Doppler observables as a measure of the spacecraft-to-Earth relative velocity along the line of sight. Both ESA and
TTC 2013 6th ESA International Workshop on Tracking, Telemetry and Command Systems for Space Applications
ESA-ESOC, 10 - 13 September 2013
NASA navigation systems rely on X band links while the use of the Ka band is currently confined to radio science experiments. The analysis of the available Cassini and Rosetta Doppler data allowed the consolidation of the error budget through the identification of the main error sources, as well as their statistical characterization. The robustness of the error budget has been proved by comparing the root summed squared value of all random contributions and the noise of both Rosetta and Cassini residuals. Fig. 1 shows the comparison between the observed noise and error model for Cassini data at 60 s integration time (expressed in mm/s). Each point of the plots represents the RMS of the noise for a single tracking pass, ~ 6 hours long. The Rosetta data and error model are fully consistent with the Cassini results. Fig. 2 shows the current total error budget for Rosetta, together with the predicted noise attainable with the improved configurations suggested later in the paper. As expected, the main noise contribution in the Rosetta and Cassini data is due to interplanetary plasma, exhibiting a strong dependence from the solar elongation angle (also called Sun-Earth-Probe or SEP) [5]: near solar conjunction, the range rate error can be as large as ~ 7 mm/s. Near solar oppositions the plasma noise decreases by roughly two orders of magnitudes, becoming an error source comparable or even smaller than tropospheric, mechanical and numerical noise. Since fluctuations of the refractive index of the interplanetary plasma, and to a less extent, the ionosphere are proportional to the inverse of the square of the carrier frequency, the noise can be either mitigated by using higher frequency links or even removed with multi-frequency observations. Therefore switching from an X band to a Ka band system allows reducing the plasma effect by a factor of ~ 18 in terms of Allan deviation. However, this solution could be unsatisfactory when the signal propagates deep in the solar corona (SEP 0.001 Hz revealed
TTC 2013 6th ESA International Workshop on Tracking, Telemetry and Command Systems for Space Applications
ESA-ESOC, 10 - 13 September 2013
seasonal fluctuations with boundary level of ~ 3-10·10−14 at 60 s integration, with minima occurring at winter nighttimes and maxima at summer daytimes. We have identified the numerical noise as an additional and partially unexpected significant contribution in Doppler observables, introduced by orbit determination codes. Numerical noise origins in storage and arithmetic of numerical variables (e.g. time and distances) with floating point representation. It appears when computed observables are generated, and the dominating terms arise in the solution of light-time problem; this error is proportional to the reciprocal of the Doppler count time and, roughly, to the projection of the spacecraft-Earth relative velocity along the line of sight. A more detailed description of the statistics of numerical noise in OD codes, carried out in the frame of ASTRA study, is given in [9]. To solve this issue, a recompilation of existing code in quadruple precision can be foreseen. The reduction of numerical noise of a factor of ~ 100 is expected, but on the other side, this solution causes longer computational time, incompatible with real time operations. A second possibility, even if with a limited reduction of numerical noise (by a factor of 10), entails an improved representation of time variables and does not impact on the computational time. However, an increase of the required storage and a quite significant implementation and certification effort are required. Other minor contributions in the Doppler error budget are due to thermal, electronics and mechanical noise. Thermal noise is inversely proportional to the integration time and depends on both the square root of the inverse of signal to noise ratio in uplink and downlink [10]. Because of the very weak signal transmitted back by the spacecraft (tens of Watts), only the downlink contribution can be considered, even if current improvements on space communications and the large use of highly directional High Gain Antennas (HGA) makes thermal noise usually important only at 1-10s integration times. Electronics components both on ground and onboard the spacecraft introduce phase instabilities on the signal carrier. However the main contribution comes generally from the onboard DSTs at frequencies larger than 10−4 ÷ 10−3 Hz. Rosetta’s DST is a first generation digital device which has a relatively poor stability (Allan deviation of 4.7·10-14 at 60 s integration time, or 0.015 mm/s range rate accuracy). For comparison, BepiColombo’s analog DST performs better, having an Allan deviation of 5.2·10-15 at 60 s integration time, or 0.002 mm/s range rate accuracy. Same level of contribution is expected to come in extreme cases from the ground antenna thermal and mechanical noise, due to wind and gravitational loading, and deformations from time-varying thermal gradients across the antenna structure. The level of ESA’s antennas mechanical noise has been estimated on average at the level of ~ 6.7·10-15 (0.002 mm/s) at 60 s integration time [11]. Once the main error contributions will be mitigated, further improvement may be attained by means of multistation tracking [12]. This technique requires the use of a small listen-only antenna in addition to the main one, enabling the two-way coherent link. A proper combination of two- and three-way Doppler observables allows replacing the local noise of the large antenna (antenna mechanical and troposphere) with the corresponding ones of the smaller antenna that can be stiffer and suitably located. II. RANGE OBSERVABLES An accurate breakdown of the error budget for range measurements is more difficult than for Doppler observables, because ranging is affected both by random and systematic effects. Fig.3 shows the error budget for both random and systematic contributions. Thermal noise in the receiving chain (both at the ground station and the onboard transponder) produces a short term jitter in range measurements, depending on the SNR of the range signal, the highest frequency component of the range signal and the integration time1 [13]. Current ranging system, based upon characteristic tone frequency of 1 MHz, can attain range jitter at the level of 0.5 m with 70 m antennas, or larger with 34 m antennas. Attaining accuracies of 10 cm or better requires the adoption of higher frequencies, pseudo noise (PN), and regenerative system. In the PN ranging [14], a code sequence is phase modulated onto the carrier and is uplinked. This code is received at the spacecraft and regenerated onboard and sent back. At the ground station, the receiver locks to the code, resolves the distance ambiguity correlating it, and extracts the ranging observable. Since PN ranging is regenerative, the downlink ranging SNR can be considerably higher than an equivalent transparent approach, with a consequent improvement in range jitter. The basic tone/code ranging system in use at ESA ground stations has recently been augmented by a PN ranging code system, based on balanced Tausworthe sequences, which is suitable for regeneration on-board the spacecraft. At present the PN clock tone component is limited to the same maximum frequency as the conventional ranging tone (1.5 MHz corresponding to a maximum chip rate 3 Mcps). This is a restriction 1
For the ESA ranging system, the integration time must be replaced with the inverse of ground station loop bandwidth
TTC 2013 6th ESA International Workshop on Tracking, Telemetry and Command Systems for Space Applications
ESA-ESOC, 10 - 13 September 2013
resulting from the limited uplink bandwidth of the current IFMS equipment. It is expected that a new G/S signal processor, the Telemetry and Telecommand Processor (TTCP) will enter service around 2015. This will have a significantly higher uplink bandwidth of at least 80 MHz and it will support wideband PN, with chip rates of 24 Mcps, thus permitting a reduction in range jitter for a given tone, loop bandwidth and SNR. The path delay due to interplanetary plasma has a twofold nature: a slowly varying contribution due to the average electron density in solar wind and solar corona (essentially varying with SEP), and a fluctuating part due to plasma turbulences. The average contribution can be computed from a density model of the solar corona and interplanetary plasma, as in the Baumbach-Allen model [15]. The total electron content can be expressed analytically, as function of SEP and the path delay can be computed. As for Doppler measurements, using higher frequency link allows reducing the plasma range delay, while further reductions can be achieved with a multilink system. In particular, with triple two-way coherent links it is possible to achieve a complete removal of the dispersive noise, regardless of the solar elongation angle, while a less effective calibration is provided by various incomplete link systems. The fluctuations of the plasma delay cannot be directly inferred from the available database, but the plasma frequency shift has been integrated over each tracking pass, in order to assess the short-term variability. For large SEP, the ionosphere is identified as the main responsible of the signal variability, since the range delay variations are dependent on local hour, therefore the range noise due to plasma fluctuations in the interplanetary medium are negligible and almost certainly below 10 cm for a X/X radio link. On the other hand, near conjunction, the total electron content from Baumbach-Allen model has large variations over a pass duration. By detrending the integrated, dispersive frequency shifts, the short-term variation due to interplanetary plasma does not exceed 4cm for X band link. For troposphere, a calibration relying upon measurements of the ground pressure, combined with the assumption of hydrostatic equilibrium and lack of horizontal pressure gradients, is generally sufficient for current ranging systems (accurate to 1m). For advanced ranging systems, such as the one envisaged for BepiColombo geodesy experiments, a monitoring of the wet delay is desirable. The calibration of the total troposphere path delay can be carried out by processing data obtained from dual-frequency GNSS receivers [16], with a software based on a least square filters or Extended Kalman Filter to process data in post processing or Real Time Kinematic environment. However, the use of MWR allows better estimation of the wet path delay. Systematic errors at ground station are generally referred as station bias, comprising several effects, some of them not completely identified and characterized. Every ground station used for deep space navigation is equipped with group delay calibration system. Unfortunately, calibration cycles are carried out only before and after a tracking pass. The ground station delay as measured by the receiver changes with the elevation angle due to changes in the standing wave and multipath patterns. Group delay calibrations are therefore of limited use and benefit (inconsistencies with Doppler data are frequent), forcing navigation teams to estimate station biases either as stochastic or deterministic parameters. The estimates of range biases (of the order of 1-5m) in the orbit determination process could however hide or absorb also media effects and errors of solar system ephemerides. In order to produce an absolute ranging measurement it is necessary to obtain a range observable between two defined points in the electronics and then correct this so that the range is defined between a precisely defined mechanical point in the ground antenna structure, typically the intersection of azimuth and elevation axes. Currently, the correction consists of a pre or post pass Long Loop (LL) range measurement augmented by a geometrical correction relating the LL path to the antenna phase centre which is then corrected to the axis intersection. Tests performed for this study in a ESA ground stations have demonstrated that the LL calibration value appears to be affected by multipath effects which lead to apparent loop calibration errors of several ns and which are strongly dependent on antenna orientation, frequency plan and, potentially, environmental factors. Any error in the LL calibration will lead to a bias in the absolute range derived for that pass. In addition to LL, ESA ground stations have a second calibration path referred to as the Medium Loop (ML). In this loop the signal is turned around by coupling power from the High Power Amplifier (HPA) through a translator into the Low Noise Amplifier (LNA). Essentially, this path includes all of the GS electronics but not the antenna BWG or structure. Further tests showed that the ML calibration path is free from the problematic variations with frequency and antenna pointing suffered by the LL path. This suggests that the ML path may be more suitable for ranging calibration than the LL path. Differently from Doppler, numerical noise is not an issue for ranging. According to the IEEE 754 standard, only 52 bits are used for the mantissa in the double precision representation of a number. The maximum relative error, called ε or machine epsilon, is about 1.1·10−16. At large distances, e.g. 10 AU, it corresponds to ~ 0.15 mm range error and it is therefore negligible on current ranging systems. III. ΔDOR OBSERVABLES
TTC 2013 6th ESA International Workshop on Tracking, Telemetry and Command Systems for Space Applications
ESA-ESOC, 10 - 13 September 2013
The main benefit of ΔDOR measurements is to directly provide the angular position of the spacecraft in the plane of the sky that is orthogonal to the line-of-sight (along which Doppler and ranging provide information about). A couple of distant ground stations simultaneously observing the same source (VLBI configuration) must be used to obtain a measure. Accuracy, at nanoradians level, is guaranteed by comparing the spacecraft DOR (difference in signal arrival time between the two stations) with a reference DOR provided by a quasar whose position is well known. This ensures to cancel out all common errors, primarily the clock offset (synchronization error) between the stations. For this to work, the quasar and the spacecraft must be angularly close each other (maximum 10 degrees) to preserve signal path commonality, and the signals must be recorded on the same spanned bandwidth and with a limited temporal separation. On the other side, any differential effect between the spacecraft and quasar contributes to the error budget. Other errors come from the signal correlation accuracy (thermal noise), and from inertial position uncertainty of both the quasar and the involved ground stations. An error budget includes both random and systematic contributions and can be evaluated applying well proven formula reported in [17, 18, 19]. The shortage of available observations (generally one per day and only during critical mission phases) and the extreme difficulty in disentangling noise sources prevent inference of each error contributions from analysis of experimental data. The current end-to-end accuracy for ESA implementation of ΔDOR reaches the level of about 12 nrad (Fig. 4) for a typical configuration with X-band signals and a total spanned bandwidth of 10 MHz. Increasing the spanned bandwidth would improve the overall performances as two of the main contributions, quasar signal correlation and phase dispersion, scale down proportionally (spacecraft thermal noise decreases as well). Those errors are strictly related in the sense they depend on the noise-like nature of the quasar signal. Quasar observations are wideband to increase the accuracy of the group delay determination. The quasar signal is recorded in distinct separated channels (2-bit quantization over 4 channels with 2 MHz bandwidth is used in the ESA system) and the correlation process measures the average group delay across the total spanned bandwidth. Therefore, for a given quasar power spectral flux, ground stations G/T, and quantization level, the quasar thermal noise decreases with increasing channel and spanned bandwidths. On the opposite, the spacecraft signal is narrowband (being either a discreet tone or a TM harmonic). The signal is recorded on channels at the same centre frequencies as the quasar, but with a smaller bandwidth. The spacecraft group delay is obtained by measuring the slope of the signal phase across the channels by linear fit. Any non-linear phase response (phase ripple) in the receiving electronics will affect the spacecraft and the quasar signals in different ways thus introducing a phase dispersion error. The spacecraft thermal noise error will benefit of a wider spanned bandwidth as well. The maximum spanned bandwidth for ESA is currently limited to less than 28 MHz by the ground station L-band down-converter and by the SNR of the spacecraft signal2. However, successful tests [20] demonstrated that configuring the ground station to simultaneously use a couple of L-band down converters and using DOR tones for the spacecraft signal allow for larger spanned bandwidth (about 40 MHz) with no need of new hardware designs. Switching to Kaband can further increase the maximum bandwidth. Moreover going up in frequency has also the advantage of proportionally reducing the noise contribution from dispersive media (Earth ionosphere and interplanetary plasma). The Ka-band DOR tones can be easily implemented with current DST technology, as already foreseen for the BepiColombo mission, while only Kaband receiving capabilities are required at ground stations. Despite the proposed wider bandwidth, there is still benefit in mapping the ground station electronics group delay performance to address phase ripple errors at sub nanoradians level. As discussed in [21], this error is stable over time scales of 1.5 hours and is relatively insensitive to antenna pointing, allowing for a calibration scheme based on quasar-only recording at high SNR immediately before and/or after the ΔDOR observation. Alternatively, off-line group delay mapping of the receiving path can be realized exploiting the test signal injection mode of existing calibration hardware (RFTU) at the ESA ground station. Performing ranging measurements for the relevant channel frequencies and bandwidth provides measurements of the downlink group delay ripple. This calibration scheme, if applied to the transmission path, is also relevant for Doppler measurements to address errors resulting from uplink pre-steering. Almost complete cancellation of the phase ripple error can be achieved by changing the spacecraft signal from a sequence of narrowband DOR tones to a spread-spectrum signal. This would mean that the same wideband configuration can be applied for measuring the quasar and spacecraft DOR, and any group-delay variation would affect both sources in the same way, thus cancelling in the ΔDOR measurement. From the link budget point of view, a spacecraft such as Rosetta, has an 2
All currently flying ESA deep space spacecrafts do not have the possibility of transmitting dedicated DOR tones (the first will be BepiColombo), but telemetry subcarrier harmonics are used for ΔDOR tracking. The limited telemetry signal power does not allow using harmonics with a separation of more than 10 MHz. Moreover, L-band down-converters at the ESA ground stations have a bandwidth limit of 28 MHz.
TTC 2013 6th ESA International Workshop on Tracking, Telemetry and Command Systems for Space Applications
ESA-ESOC, 10 - 13 September 2013
EIRP of 57 dBW using the HGA at X-band and could generate an equivalent power spectral flux of about 1 Jy at 2 AU and 40MHz of bandwidth, comparable to that of a strong quasar. This approach would require no change at the ground station but the software correlator. Some modification is instead required by onboard hardware. However, the same binary coding schemes currently used for PN ranging can be applied for the new spread-spectrum DOR signal. The third major contribution to the error budget comes from differential tropospheric delays. This error depends on the spacecraft–quasar angular separation and the tracking elevation. The latter is generally small (about 10-15 degree) because of the geometric constraint for simultaneous visibility of the spacecraft with a very long baseline. While minimization of the angular separation is strongly constrained by the limited availability of quasars3, the calibration of the dry and wet path delay can be obtained from the same MWR measurements proposed for accurate Doppler and Ranging observations. Given the expected performance of the MWR system it is feasible to reduce this error below the target level of 1 nrad (given a spacecraft quasar angular separation of about 6 degrees). No modification of the radiometer hardware is required. A best performing system, entailing a wide spanned bandwidth along calibration systems for the phase ripple and the differential tropospheric errors, would allow reducing the largest error sources well below the target accuracy of 1 nrad. In this case the limit to the end-to-end performances would come from accurate knowledge of the ICRF quasar location that is currently about at the same level. IV. CONCLUSIONS The analysis of the available data from the Rosetta and Cassini missions led to the identification of five main noise sources on current Doppler, range and ΔDOR systems: interplanetary and ionospheric plasma noise (affecting Doppler, range and ΔDOR); tropospheric noise due to water vapor (affecting significantly Doppler and ΔDOR, and marginally also range); numerical noise (affecting only Doppler); multipath effects and time variable phase delays in the ground and onboard antenna systems (affecting range); and group delay variations across the measurement bandwidth (affecting ΔDOR). We have also identified methods and strategies to lessen these errors and improve by one order of magnitude the accuracy of the three radio metric techniques. The improvements entail modifications, quite substantial in some cases, to both the existing ground and spacecraft equipment. Fig. 2, 3 and 4 showed different options for architectural design of the end-to-end tracking system, with increasing performances and complexity. The first configuration provides some improvements over the accuracies attainable with the current system, as the TT&C flight hardware of the incoming BepiColombo mission. Tropospheric calibrations (now based on seasonal models) are accomplished by means of commercial microwave radiometers, while numerical noise is reduced by recompiling existing codes in quadrupole precision. The variability in the station group delay is diminished by changing from a long to medium loop calibration. Internal calibrations based on BWG geometry and a priori tables provide reduction of phase ripple error by a factor of 5 in ΔDOR measurements. The second and third options allow reaching almost completely the study goal, entailing modification also in the flight hardware. Ka-band uplink is required (although not in the multi-frequency configuration). Numerical noise is now reduced thanks to an improved time representation in OD codes. The ground station bias is calibrated by means of an automated mapping of loop group delay (based on wideband PN ranging), while the bandwidth for ΔDOR open loop recording increases to 152 MHz bandwidth for better quasar SNR at Ka-band. In option 3, advanced microwave radiometers are employed for tropospheric calibration, and calibration of ground station bias is made with real-time monitoring and spread spectrum ranging. Finally, the last configuration provides the ultimate extend in terms of both precise spacecraft navigation and scientific applications, entailing a multi-frequency link system at X and Ka band, augmented for spread spectrum ranging and ΔDOR.
3
The quasar power flux plays an important role in determining its suitability for Delta-DOR observations since the group delay accuracy decrease proportionally with the quasar power flux. The irradiated flux for a quasar is not constant but is generally lower at Ka-band rather than at X-band. However, the larger bandwidth enabled by Ka-band has the net effect of increasing the quasar SNR of about 9 dB, thus widening the number of usable quasars.
TTC 2013 6th ESA International Workshop on Tracking, Telemetry and Command Systems for Space Applications
ESA-ESOC, 10 - 13 September 2013
2-way Doppler noise of Cassini X-band Doppler data, Tc=60s (northern hemisphere) 1.0e+03 SEP RMS of Doppler residuals Wet Troposphere Plasma G/S S/C Numerical noise Total noise model
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30 1.0e-03
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Fig. 1. RMS of Cassini Doppler residuals with Tc=60s, compared with model
2-way Doppler noise, Tc=60s (Rosetta) 1.0e+03 SEP Nominal OPTION 1 OPTION 2 OPTION 3 OPTION 4 Study goal
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Fig. 2. Error budget breakdown for Doppler system at ESA. Current configuration (grey) agrees with the RMS values of Rosetta data. Option 1 (purple) assumes the state of the art for onboard DST and ground station, an X/X and X/Ka link, commercial radiometer and a recompilation of OD code in quadruple precision. Option 2 (red) foresees an upgrade of the ground station with Ka-band uplink, and an improved representation of time in OD code. Option 3 (blue) uses advanced water vapour radiometer for tropospheric calibration. Option 4 (green) has a complete plasma noise calibration with multi-frequency link.
TTC 2013 6th ESA International Workshop on Tracking, Telemetry and Command Systems for Space Applications
ESA-ESOC, 10 - 13 September 2013
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Fig. 4. Error budget breakdown for ΔDOR system at ESA. Errors refer to the Cebreros-New Norcia baseline; about 11600 km. Current configuration (grey) assumes a total spanned bandwidth of 10 MHz at X-band, a quasar power spectral flux of 0.6 Jy and spacecraft TM harmonics with P/N0 of 19 dBHz. Option 1 to 3 shows the effect of the proposed improvements. Option 1 (purple) assumes tropospheric calibration with commercial MWR, off-line calibration of G/S group delay and DOR tones at X-band with a spanned bandwidth of 40 MHz. Option 2 (red) improves the previous one by switching to Ka-band and 152 MHz of spanned bandwidth. Option 3 (light-blue) assumes spread spectrum signals for the spacecraft allowing for real-time group delay calibration, and improved MWR design for better tropospheric calibration.
TTC 2013 6th ESA International Workshop on Tracking, Telemetry and Command Systems for Space Applications
ESA-ESOC, 10 - 13 September 2013
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