Rosetta CONSERT Data as a Testbed for In Situ Navigation of Space Probes and Radiosciences in Orbit/Escort Phases for Small Bodies of the Solar System

: Many future space missions to asteroids and comets will implement autonomous or near-autonomous navigation, in order to save costly observation time from Earth tracking stations, improve the security of spacecraft and perform real-time operations. Existing Earth-Spacecraft-Earth tracking modes rely on severely limited Earth tracking station resources, with back-and-forth delays of up to several hours. In this paper, we investigate the use of CONSERT ranging data acquired in direct visibility between the lander Philae and the Rosetta orbiter, in the frame of the ESA space mission to comet 67P/Churyumov-Gerasimenko, as a proxy of autonomous navigation and orbitography science capability.


Introduction: Rosetta Science and the CONSERT Experiment
Rosetta was an ESA cornerstone mission, launched 2 March 2004, that reached comet 67P/Churyumov-Gerasimenko (hereafter 67P/C-G) on August 2014 [1][2][3][4][5]. This mission dramatically improved our understanding of comets and the formation of the solar system [6][7][8][9][10][11][12][13][14][15][16][17]. One of the key moments of the mission was the tumultuous landing of the small companion lander Philae on 12 November 2014. After about seven hours of free fall descent to the comet surface [4,18], and two unexpected bounces after the failure of the anchoring system, the Philae lander finally came to rest at the so-called "Abydos" site, about 1 km from the planned location, in a non-optimal location and unknown orientation, but was able to operate on battery power for about 60 h [18][19][20][21]. The lander was finally identified lying on its side in a deep crevice in the shadow of a cliff, in an image taken by the Rosetta spacecraft on 2 September 2016.
Among the 10 scientific payloads onboard Philae [22,23] was the Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT), a VHF radar transponder, designed to sound the interior of the comet. Results from the CONSERT served as a key input to help locate Philae's position by determining the small zone where the lander came to a final rest [24][25][26][27]. The CONSERT instrument consisted of two parts: the CONSERT Orbiter (OCN) onboard the Rosetta spacecraft, and the CONSERT Lander (LCN) onboard the Philae lander. The main idea was to use a double-way radio-link with a sufficiently low Table 1. Typical ranging accuracy for CONSERT, S-band and X-band signals from DSN tracking antennas [27,31,32].

Autonomous Navigation of Deep Space Probes
The number of deep space missions are going up as the number of countries involved in deep space research are increasing, and the size and mean cost of such missions are decreasing. Usually, the navigation of these space probes is carried out by two-way radio-links. The reason for this is that it was, until recently, impossible to fly sufficiently miniaturized precise atomic clocks (such as onboard GPS cesium-rubidium clocks) on deep space probes, and so the only way to obtain sufficiently precise Doppler/Ranging measurements was to use a radio-link with an Earth-based atomic clock (maser). The situation will probably change very soon with the design of such a miniaturized clock, the Deep Space mercury-ion trap Atomic Clock (DSAC), that was flight tested first on 25 June 2019 [33,34]. It is expected that a DSAC would incur no more than 1 microsecond of error in 10 years of operations, a 10-100 times improvement above current onboard Ultra Stable Oscillators, halving the load of Earth tracking antennas and allowing nearly autonomous radio navigation for a spacecraft's time-critical events, such as orbit insertions or landings with a one-way Earth uplink. Another radical way to reduce Earth antennas' time is to use optical navigation, with respect to the stars'/planets'/asteroids' background or landmarks on target objects, with no need of an Earth-based reference [35,36]. This has been demonstrated with the DS1 (Deep Space 1) [37], Deep Impact missions [38,39], and latest asteroid missions, such as DAWN and OSIRIS-Rex mission. Besides the pure landmarks/background navigation, X-ray pulsars navigation or a combination of these different approaches could also be used for deep space navigation [40][41][42]. For the HERA mission that is succeeding the unselected AIDA/AIM mission to the same binary asteroid target Didymos, an Inter-Satellite-Link will be used to determine the gravity field of the Dimorphos moon with cubesats subsatellites flying down to 600 m from its surface [43]. On the OSIRIS-REX mission to asteroid Bennu, the gravity field of the asteroid was determined by modeling the trajectory of small particles ejected from the asteroid surface [44].
In this article, we study the use of CONSERT-like ranging data as another type of data able to constrain the navigation of space probes close to small bodies of the solar system. As navigation and gravity field estimation are two inseparable facets of deep space probe radioscience, we also derive the GM of comet 67P/C-G and Philae lander location by treating the CONSERT direct distance measurement as a "lander -orbiter" ranging, and compare our solution with the mass estimate derived from X-band radio-tracking data of the Rosetta spacecraft from Earth [15,45].  [6]. Three sequences of direct visibility ranging data were collected during the FSS phase.

Philae Lander Rest Position
The Philae lander separated from the Rosetta S/C at 08:35 UTC on Nov 12th, 2014, then flew in free fall during a 7 h descent [4,18]. Due to the failure of both the ADS (Active Descent System) and anchoring harpoons, the Philae lander experienced two unexpected bounces until finally coming to rest at the "Abydos" site, at approximately 1 km from the  [6]. Three sequences of direct visibility ranging data were collected during the FSS phase.

Philae Lander Rest Position
The Philae lander separated from the Rosetta S/C at 08:35 UTC on 12 November 2014, then flew in free fall during a 7 h descent [4,18]. Due to the failure of both the ADS (Active Descent System) and anchoring harpoons, the Philae lander experienced two unexpected bounces until finally coming to rest at the "Abydos" site, at approximately 1 km from the originally targeted place [19]. By using CONSERT ranging data during the FSS, the Philae position was constrained to an area of 15 m × 150 m [6], then refined to 22 m × 106 m [27]. From January 2015 through March 2016, the distance from Rosetta to the comet was too large to take high resolution images of the potential rest area of the Philae lander, though contact was sporadically re-established in June 2015 [4,20]. To  [26]. The Philae lander was finally located unambiguously from an OSIRIS Navigation Camera (NAC) high-resolution image (5 cm/pixel) on 2 September 2016, taken from a 2.7 km distance from the comet surface. As the Philae lander could be pinned in the OSIRIS image directly, the coordinates from OSIRIS are regarded as the ones with the highest accuracy, directly related to the accuracy of nucleus shape models. The 67P/C-G shape models were derived using the stereo-photogrammetric (SPG) [47] method at DLR, and stereo-photoclinometry (SPC) [48] technique at LAM. Table 3 lists the Philae lander position estimates from different sources. It can be seen that the coordinates from LAM SPC and SPG SHAP7 are close to each other. For the SPG SHAP7 model, the image resolution varied from 0.2 m/pixel to 3 m/pixel, and its horizontal sampling was about 1-1.5 m while the vertical accuracy was at the decimetre scale [49]. Considering the one-meter size of the Philae lander, the final uncertainty of Philae coordinates is therefore about 2 m. In our following study, we chose the latest estimates from Kofman et al., (2020), in the comet body-fixed 67P/C-G_CK frame, as our a priori nominal values for the lander position.

Lander-Orbiter "One-Way" Range Model and Solution Method and Solve-for Method
The CONSERT ranging observables can be treated as an "instantaneous" S/C-lander one-way range tracking model as the propagation time is ten microseconds with respect to relative velocities, that are of metric magnitude. In this framework, the motion of the Rosetta S/C around comet 67P/C-G is described by the following Newtonian equation of motion [50]: where r is the Rosetta S/C barycentre position vector in the 67P/C-G-centered J2000 reference system, G is the universal gravitation constant, M 67P is the mass of 67P/C-G, M Bi is the mass of other celestial bodies (i.e., sun, planets, asteroids, etc.), ..
R 0 is the acceleration of 67P/C-G in SSB (Solar System Barycenter), and F S is the sum of all other forces acting on the Rosetta S/C (e.g., non-spherical gravity, solar radiation pressure, outgassing from the comet, etc.). Given the initial state vector x 0 , including the state of S/C, GM etc., we could obtain the S/C orbit and STM Φ (State Transform Matrix, i.e., the derivatives of the S/C state vector w.r.t. the initial S/C state etc.) by a standard integration of Equation (1) and its associated partial derivatives [51].
The CONSERT direct range measurement ( Figure 2) is the difference between the LCN center-of-phase position and OCN center-of-phase position in the J2000 frame as: where r s/c is the Rosetta S/C position vector in J2000, r OCN is the OCN center-of-phase coordinate in the Rosetta Spacecraft Frame, M 1 · symbolizes the rotation matrix from the Rosetta Spacecraft Frame to the J2000 frame, r LCN is the LCN center-of-phase in the 67P/C-G body fixed frame, M 2 symbolizes the rotation matrix from the 67P/C-G body fixed frame to the J2000 frame, and δρ is the electronic system delay (20 nanoseconds). We stress that r OCN here is the OCN center-of-phase coordinate, fixed with respect to the Rosetta S/C body, which is modelled from SPICE Rosetta kernels (ftp://spiftp.esac. esa.int/data/SPICE/ROSETTA/kernels, accessed on 19 September 2019). The center-ofphase of LCN is always considered as the Philae reference frame center, so there is no specific CONSERT frame defined on the lander side (Yves, private communication). From a practical point-of-view, this means that r LCN is the Philae lander position mentioned in Section 3.2.
Remote Sens. 2021, 13, x FOR PEER REVIEW 6 of 18 GM etc., we could obtain the S/C orbit and STM Φ (State Transform Matrix, i.e., the derivatives of the S/C state vector w.r.t. the initial S/C state etc.) by a standard integration of Equation (1) and its associated partial derivatives [51]. The CONSERT direct range measurement ( Figure 2) is the difference between the LCN center-of-phase position and OCN center-of-phase position in the J2000 frame as: where / We stress that OCN r here is the OCN center-of-phase coordinate, fixed with respect to the Rosetta S/C body, which is modelled from SPICE Rosetta kernels (ftp://spiftp.esac.esa.int/data/SPICE/ROSETTA/kernels, accessed on 19 September 2019). The center-of-phase of LCN is always considered as the Philae reference frame center, so there is no specific CONSERT frame defined on the lander side (Yves, private communication). From a practical point-of-view, this means that LCN r is the Philae lander position mentioned in Section 3.2. In our model, the CONSERT direct range measurement is a function of time and of a set of eight initial parameters: x 0 (Rosetta S/C initial position [SX0 SY0 SZ0] in the J2000 comet-centered frame), the Philae lander position [LX0 LY0 LZ0] in the comet body-fixed 67P/C-G_CK frame, the CONSERT constant measurement Bias, and the comet GM. We are only solving for an initial position correction of the Rosetta spacecraft and not for a joint position/velocity correction, because, as according to Pablo et al., (2015), the velocity measurement error of Rosetta from Earth tracking is at the level of 1 mm/s. Therefore, during the 22 min tracking period, the maximum position error caused by the velocity error is less than 1.32 m, whereas the CONSERT ranging error is at a level of 6 m (see Sections 4.2 and 4.3). We are also only solving for the lander position, that is considered at rest in the comet-fixed frame. Its velocity in the 2000 comet-centered frame is given through the comet rotation model. In linearized form, the partial derivatives of ρ consert w.r.t. the initial parameters set x 0 define the Jacobi matrix H: The computed delay values ρ consert are then fitted to the CONSERT range observables, mentioned in Section 3.1, through a weighted least-squares inversion to obtain a correction δx to the initial parameters x 0 by: where P −1 0 denotes the inverse of the a priori covariance matrix associated with x 0 , W is the weight (diagonal) matrix containing the inverse of the standard deviations of the CONSERT range measurements, and y is the difference vector between observables and computed value ρ consert . An iterative process is performed until the observables and the model values converge.

Rosetta Orbit Dynamics and Error Source Analysis
As already mentioned in Section 3.1, due to the small number of CONSERT range data in Sequences 1 and 3, we are only focusing on Sequence 2, from 10:20:50.49 UTC to 10:42:22.92 (22 min). The SPICE Rosetta kernels used are listed in Table 4. With the initial S/C state from the SPK kernels, we integrate Equation (1) to model the Rosetta S/C orbit. The dynamical force models [36,39] for the integration include comet 67P/C-G point mass attraction, non-spherical gravitational force (up to second degree and order), solar radiation pressure (SRP), third-body perturbations and comet outgassing drag. The typical magnitudes of acceleration during this arc in the J2000 frame are shown in Table 5. The largest is 67P/C-G point mass gravity acceleration, at about 2.7 × 10 −7 m/s 2 , and the SRP is the second largest, at about 3.1 × 10 −8 m/s 2 . During these 22 min, the relative position between Philae and Rosetta ranges from 47.185 km to 47.342 km (157 m), and the corresponding relative velocity ranges from 0.231 m/s to 0.226 m/s (5 mm/s difference) in the comet-centered J2000 frame, and from 6.531 m/s to 6.51 m/s (22 mm/s difference) in the comet-fixed (67P/C-G_CK SPICE kernel) frame. For reference, the tangential velocity in the comet-fixed frame for a spacecraft in a bounded circular orbit at this altitude is about 0.12 m/s (29 day period).
To investigate the influence of Rosetta S/C dynamic force models to the orbit, we considered two cases: For case 1, all the dynamic force models mentioned above are adopted when integrating the S/C orbit. For case 2, only a 67P/C-G point mass gravity model is used. Figure 3 illustrates the orbit differences between our integrated orbit in this paper and the nominal orbit provided by ESOC (RORB_DV_257_03___T19_00345.BSP). From the upper part of Figure 3, it is clear that during the 22 min period, the case 1 orbit is similar to the ESOC nominal orbit (discrepancies less than 2 mm). With only a comet point mass force (case 2), the orbit differences during the integration interval are less than 30 mm. Considering that the accuracy of CONSERT range is about 6 m (±20 ns), it can be safely Remote Sens. 2021, 13, 3747 7 of 16 concluded that the dynamical models 1 and 2 are both accurate enough for the following parameter estimation. To investigate the influence of Rosetta S/C dynamic force models to the orbit, we considered two cases: For case 1, all the dynamic force models mentioned above are adopted when integrating the S/C orbit. For case 2, only a 67P/C-G point mass gravity model is used. Figure 3 illustrates the orbit differences between our integrated orbit in this paper and the nominal orbit provided by ESOC (RORB_DV_257_03___T19_00345.BSP). From the upper part of Figure 3, it is clear that during the 22 min period, the case 1 orbit is similar to the ESOC nominal orbit (discrepancies less than 2 mm). With only a comet point mass force (case 2), the orbit differences during the integration interval are less than 30 mm. Considering that the accuracy of CONSERT range is about 6 m (±20 ns), it can be safely concluded that the dynamical models 1 and 2 are both accurate enough for the following parameter estimation. In Table 6, we list the a priori values of the parameters related to the model of CON-SERT direct range and their errors. For the comet 67P/C-G rotation model, we used the numerical quaternions contained in the CK kernel (CATT_DV_145_02_______00216.BC), produced by the ESOC/Flight Dynamics (FD). Due to the complexity of comet rotation, In Table 6, we list the a priori values of the parameters related to the model of CONSERT direct range and their errors. For the comet 67P/C-G rotation model, we used the numerical quaternions contained in the CK kernel (CATT_DV_145_02_______00216.BC), produced by the ESOC/Flight Dynamics (FD). Due to the complexity of comet rotation, linked to outgassing, it is difficult to describe, over long periods of time, the comet rotation with the standard IAU formulation. The uncertainty in RA (±0.05 • ) and DEC (±0.03 • ) provided in Table 6 for the comet rotation axis have the same level of influence as the Philae lander position uncertainties (i.e., around 2 m).

Compatibility of CONSERT Measurements with the Tracking Nominal Parameters
To assert the compatibility of the CONSERT measurements with the tracking nominal parameters given in Table 6, we solved for the eight parameters described in Section 4.1. After three iterations, the eight parameters stabilized to the values given in Table 7. For convenience, only the changes of the parameters are shown. The associated correlation matrix of the parameters is given in Figures 4 and 5. Table 7. Parameters corrections and associated formal errors for the parameters, with respect to the least-squares fit corresponding to Equations (1)-(3) and the initial values and parameters given in Table 6.

Estimated Parameter Corrections (m) Deviations (mm)
Rosetta S/C position corrections in the J2000 comet-centered frame   Table 7. The correlations relative to the Rosetta S/C position [SX0, SY0, SZ0] are given in the J2000 comet-centered system of reference. An important result is that GM is uncorrelated with the other parameters, showing that CONSERT-like measurements can be, in principle, used to determine the GM coefficient, if sufficient numbers of measurements are stacked along an orbit with no maneuvers. We also see that the CONSERT instrument bias has a strong correlation with the SZ0 value (see text).   Table 7. The correlations relative to the Rosetta S/C position [SX0, SY0, SZ0] are given in the J2000 comet-centered system of reference. An important result is that GM is uncorrelated with the other parameters, showing that CONSERT-like measurements can be, in principle, used to determine the GM coefficient, if sufficient numbers of measurements are stacked along an orbit with no maneuvers. We also see that the CONSERT instrument bias has a strong correlation with the SZ0 value (see text).   Table 7. The correlations relative to the Rosetta S/C position [SX0, SY0, SZ0] are given in the J2000 comet-centered system of reference. An important result is that GM is uncorrelated with the other parameters, showing that CONSERT-like measurements can be, in principle, used to determine the GM coefficient, if sufficient numbers of measurements are stacked along an orbit with no maneuvers. We also see that the CONSERT instrument bias has a strong correlation with the SZ0 value (see text).  We can see in Figure 4 that the CONSERT instrument Bias has a strong correlation with the SZ0 value. To make it clearer, we mapped the Rosetta S/C initial position [SX0 SY0 SZ0] to radial (R), along-track (Transverse) and cross-track (Normal) directions [SR0 ST0 SN0] in Figure 5. The SR0 value, along the radial direction, shows a very strong correlation with the CONSERT range bias. Figure 6 gives the post-fit residuals of the range measurements with respect to Table 7. The mean value is about 0.004 m and the standard deviation is about 1.5 m. The largest residues are less than 6 m, which corresponds to the CONSERT range measurement noise level. From the residuals figure, we can also see that there are 4 to 5 measurements grouped every 2 min, corresponding to the stroboscopic ranging acquisition mode discussed in Part 3.1.
The Rosetta S/C initial position vector [SX0 SY0 SZ0], SX0 and SY0 have only small variations w.r.t. to the a priori values, with their formal error being close to the a priori formal errors. It is the same for the lander position vector [LX0 LY0 LZ0] and comet GM, reflecting the fact that the CONSERT measurements are fully compatible with the a priori nominal parameters.
We can see in Figure 4 that the CONSERT instrument Bias has a strong correlation with the SZ0 value. To make it clearer, we mapped the Rosetta S/C initial position [SX0 SY0 SZ0] to radial (R), along-track (Transverse) and cross-track (Normal) directions [SR0 ST0 SN0] in Figure 5. The SR0 value, along the radial direction, shows a very strong correlation with the CONSERT range bias. Figure 6 gives the post-fit residuals of the range measurements with respect to Table  7. The mean value is about 0.004 m and the standard deviation is about 1.5 m. The largest residues are less than 6 m, which corresponds to the CONSERT range measurement noise level. From the residuals figure, we can also see that there are 4 to 5 measurements grouped every 2 min, corresponding to the stroboscopic ranging acquisition mode discussed in Part 3.1.  Table 7. Note the typical "burst repartition" of the 51 measurements, that is the result of CONSERT stroboscopic mode (see Appendix A).

In-Situ Navigation Analysis
To investigate the possibility of in situ navigation of a spacecraft, we fixed the lander position [LX0 LY0 LZ0] and CONSERT range bias, and perturbed both the initial state vector (with a 10-sigma error) and the nominal GM (by a 3-sigma error), and fitted with a least-squares fit the Rosetta position [SX0 SY0 SZ0] ( Table 8). The post-fit orbit errors of Rosetta S/C with respect to the nominal orbit provided by ESOC, are shown in the left column of Table 9 (case A). The orbit error is at a one hundred meters level along the along-track direction (T), and is one order of magnitude smaller in the radial (R) and crosstrack (N) directions. This is the usual behavior for orbital errors. Table 8. Evaluation of navigation accuracy (case A). The Rosetta initial position vector has been perturbed with a 10-sigma error and the GM with a 3-sigma error. Post-fit orbit residuals are given in Table 9, first column. The Philae position is kept fixed to the coordinates given in Table 6.

A Priori Sigma Corrections A Posteriori Values
A Posteriori Sigma Figure 6. Post-fit residuals for CONSERT range measurements corresponding to the values of Table 7. Note the typical "burst repartition" of the 51 measurements, that is the result of CONSERT stroboscopic mode (see Appendix A).

In-Situ Navigation Analysis
To investigate the possibility of in situ navigation of a spacecraft, we fixed the lander position [LX0 LY0 LZ0] and CONSERT range bias, and perturbed both the initial state vector (with a 10-sigma error) and the nominal GM (by a 3-sigma error), and fitted with a least-squares fit the Rosetta position [SX0 SY0 SZ0] ( Table 8). The post-fit orbit errors of Rosetta S/C with respect to the nominal orbit provided by ESOC, are shown in the left column of Table 9 (case A). The orbit error is at a one hundred meters level along the along-track direction (T), and is one order of magnitude smaller in the radial (R) and cross-track (N) directions. This is the usual behavior for orbital errors.
Usually, when a spacecraft arrives in the vicinity of a small body, the uncertainty of the body's GM value is quite large [53]. Therefore, we used, in the following two simulations, relatively large a priori discrepancies (10% and 50%) to the nominal GM value, then only solved for the initial state vector of Rosetta [SX0 SY0 SZ0], also with a 10-sigma error to initial state vector. The least-squares fit results are shown in Table 10 (10% relative GM error, case B) and Table 11 (50% relative GM error, case C). The post-fit orbit errors of Rosetta S/C are shown in the second and third columns of Table 9. The orbital error has the same magnitude in both cases, mainly along the along-track direction (T). These results indicate that the duration of the tracking window is simply too small (22 min) to constrain the GM value. This is not an indication of an intrinsic inability for the CONSERT-like measurements to constrain the GM, as Figures 5 and 6 also indicate clearly that the determination of the GM value is decorrelated from the determination of the other parameters.
After several weeks following arrival, the GM value was determined with a small relative error, either by 2-Way Doppler from Earth or by local (mainly optical) measurements. Assuming the GM was considered as constant (the mass of CG-67 decreased by a small amount after perihelion, due to outgassing), we were able to study this nominal navigation case by fixing the GM value to its nominal value and only solving for the 22 min arc initial position [SX0 SY0 SZ0], with a 3-sigma error imposed on their a priori values (case D, a priori initial position values found in Table 11). The Rosetta S/C orbit differences with respect to the ESOC orbit decreased to 35 m with respect to cases A, B and C (last column of Table 9), again along-track (T). It is essential to mention that the corrections to SZ0 from cases A, B and C are larger than those two other directions. This is because the SZ0 has a strong correlation with CONSERT range (see also Tables 6 and 7). Table 8. Evaluation of navigation accuracy (case A). The Rosetta initial position vector has been perturbed with a 10-sigma error and the GM with a 3-sigma error. Post-fit orbit residuals are given in Table 9, first column. The Philae position is kept fixed to the coordinates given in Table 6.  Table 8 Case B, Table 10 Case C,  Table 10. Evaluation of navigation accuracy (case B). Parameter corrections and associated formal errors for the least-squares fit with a 10% relative error on the GM value. Post-fit orbit residuals are given in Table 9, second column. The other parameters are kept constant to the values found in Tables 6 and 8.  Table 11. Evaluation of navigation accuracy (case C). Parameter corrections and associated formal errors for the least-squares fit with a 50% relative error on the GM value (case C). Post-fit orbit residuals are given in Table 9, third column. The other parameters are kept constant to the values found in Tables 6 and 8.

Conclusions
Many papers have been devoted to the autonomous navigation of spacecraft in the vicinity of small bodies of the solar system. Our goal was not to give a full analysis of the autonomous navigation problem (see [35,53]), merely to add to knowledge by demonstrating that CONSERT-like ranging measurements, a flight-proven design, can be used towards an achievable goal of improving accuracy in positioning to a few tens of meters or less. In addition, there is also the possibility to adjust for ancillary (and science-rich) parameters such as the gravitational constant of the small body if observations are conducted during a sufficient amount of time along an orbit with no maneuvers. The advantage of a CONSERT-like method is its ability to provide range measurements, i.e., at absolute scale. Optical measurements are only giving angles, absolute scale must be inferred by indirect means.
phase modulated with pseudo-randomly encoded data is transmitted back-and-forth (see Figure A1) from the orbiter (OCN electronic box, mass 3 kg, power 3 W) towards the lander on the comet surface (LCN electronic box, mass 2.3 kg, power 3 W). An indirect "ping-pong" transmitting procedure was designed by the science team to reduce the required accuracy of the clocks on both sides of the transmission link (frequency stability constraint of ∆f/f = 2 × 10 −7 ) and to stay within stringent constraints on mass and power consumption.
A periodic pseudo-random code of 25.5 microseconds is transmitted from orbiter to lander, and the transmission cycle lasts about 200 milliseconds. The received signal is digitized and accumulated for 26 msec in the lander receiver to increase the signal to noise ratio and to assess the arrival time of the main path. During processing, the signal is compressed to obtain a time/space accuracy corresponding at least to ±50 nanoseconds (it was found to be closer to 20 nanoseconds in actual measurements [54]). After the signal processing on the lander, which determines the position of the strongest path, the lander transmits the same pseudo-random code with a delay corresponding to that of the strongest path. The transmission cycle again lasts about 200 microseconds. The signal propagates back to the orbiter along virtually the same path, since the orbiter velocity with respect to the comet surface is a few meters per second. The signal is received on the orbiter, accumulated for about 26 msec and stored in the memory, in order to be sent to Earth. A complete measurement cycle lasts about 1 s. In this way the OCN measures twice the propagation delay plus the electronic delays on the OCN/LCN (see Figure A2). An in-depth description of the inner workings of the CONSERT instrument as well as the details of the physical implementation can be found in [25].
Remote Sens. 2021, 13, x FOR PEER REVIEW 15 of 18 Figure A1. The CONSERT signal propagation throughout the nucleus between OCN and LCN. The red rays correspond to the direct visibility mode used in this paper (adapted from [6]). Figure A2. The CONSERT "ping-pong" signal propagation scheme (from [27]). Figure A1. The CONSERT signal propagation throughout the nucleus between OCN and LCN. The red rays correspond to the direct visibility mode used in this paper (adapted from [6]).
Remote Sens. 2021, 13, x FOR PEER REVIEW 15 of 18 Figure A1. The CONSERT signal propagation throughout the nucleus between OCN and LCN. The red rays correspond to the direct visibility mode used in this paper (adapted from [6]). Figure A2. The CONSERT "ping-pong" signal propagation scheme (from [27]).