THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR L. Iess, R. Abelló, A THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR L. Iess, R. Abelló, A. Ardito, G. Comoretto, M. Lanucara, R. Maddè, M. Mercolino, G. Rapino, M. Sensi, P. Tortora 28 June, 2006 4th Radionet Engineering Forum Workshop: Next Generation Correlators for Radio Astronomy and Geodesy

THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR Summary Summary: DDOR technique for the navigation of interplanetary probes: Measurement description ESA DDOR system description Processing Spacecraft signal correlation process EGRS signal correlation process Error budget Description of tests performed Test results Conclusions and future activities THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR

Navigation of interplanetary probes Orbit Determination of interplanetary probes: DOPPLER: ± 50 μm/s in radial velocity (X-band) RANGING: ± 1m radial range (X-band)  ± 50 nrad (7.5 km at 1 A.U.) ;  ± 100 nrad (15 km at 1 A.U.) with 12 hours of Doppler data DDOR: Improvement of the orbit solution Observation time ≈ 90 min for each pass Orbit solution after crucial manoeuvres (fly-by, orbit insertion) angular accuracy: ≤ 15 nrad ( 2.3 km at 1 A.U.) Declination  Range and Doppler THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR

THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR ΔDOR : Delta Differential One-way Ranging DOR : difference between the times of arrival at two ground stations (max 21 ms) Calibrated effects: on board oscillator frequency offset, dry troposphere Uncalibrated effects: Clock-offset between the stations (main effect), ESA DSA clock offset max 6 μs Instrumentation noises: phase ripple Residual media effects Δ : Calibration of differential effects Acquisition of an EGRS signal, whose position is well known Difference between S/C and EGRS DOR Calibration of S/C DOR measurement ΔDOR requires: Max angular separation: 15° Signals are recorded in the same bandwidth ΔDOR: evaluation of the angular position of a probe in the S/C-baseline plane Solar wind ≤15° Troposphere Ionosphere Baseline THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR

THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR ESA Deep-Space-Antenna: 35m B= Cebreros – New Norcia, 11˙621 Km Accuracy of the measurement: Simultaneous visibility: ≈90 min (10°-30° elevation) Accuracy requirement: 1 ns (4.5 Km at 1 AU)  B THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR

Correlator characterization Software correlator for DDOR measurements Input: S/C, EGRS signals and model Output: residual differential one way range Channels: 4 Spanned bandwidth: 10 – 20 MHz Quantization and bandwidth for each channel: 8 bit/50 KHz for S/C, 2 bit/2 MHz for quasar Baseline: Cebreros – New Norcia Time for a DDOR session: ~2 hours THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR

THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR Signals S/C signal Telemetry harmonics and DOR tones ESA: TM subcarrier harmonics (262 Khz) Fs=50kHz; 8bit QUASAR signal: White noise completely embedded in the receiver noise (0.5 Jy = 0.510-26 W/m2Hz) Fs=2MHz; 2bit Accuracy: Spanned bandwidth: 10 MHz Signals SNR Integration time TM carrier TM Harm. Order +2 Ch. BW 50kHz Order -14 Order +20 Ch. BW (2MHz) Spanned bandwidth BW ≈ 10 MHz THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR

THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR Receiver chain 8.4– 8.5 GHz 540-640 MHz 55-85 MHz X-band D/C RHC L-band D/C 1 Switching Matrix IFMS LHC C F E GDSP1 X Y  Input 1 CH1 D/C chain: analog signal X_Band D/C, B=100MHz (@590MHz) L_Band D/C, B=30 MHz (@70MHz) IFMS (Intermediate Frequency Modem System) A/D conversion (I e Q samples ) 2 GDSP (Generic Digital Signal Processor) for ΔDOR measurements Four channels, Bch=1kHz – 2 MHz Limits: filter response, maximum bit-rate (36 Mbps) Maximum spanned bandwidth = 28 MHz CH2 36Mbits/s CH3 X Y  CH4 GDSP2 36Mbits/s UCPU X Y  GDSP3 36Mbits/s GDSP4 36Mbits/s Estrack LAN Harmonics: from order -14 (15 dBHz) to + 20 (10 dBHz) BW = 10 MHz Typical observation: S/Q/S =5/10/5 min.  ≈ 2.6 GB data volume THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR

Correlator interfaces Station 1 Inputs Outputs FD AI S/W Correlator Station 2 Station1, Station2: data streams from the two stations 50kHz for the spacecraft, 4 channels 2MHz for the quasar, 4 channels FD (input): Dynamical model used in the correlation process, provided by the Flight Dynamics Team at ESOC: state vector for S/C and delay for the quasar AI: ancillary information used in the computation of the error budget COD (output): output file containing the DOR results for S/C and quasar separately, used by FD for computing the final orbit solution THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR

S/C correlation process (1) Digital PLL: reconstruction (using FD data) of signal phases and transmitter frequency for all channels THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR

S/C correlation process (2) Generation of model phases using FD data The signal is beaten to zero frequency (stopped) using the model phase Correlation of the stopped phasors of the two stations for each channel, obtaining four initial differential phases THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR

S/C correlation process (3) Ambiguity resolution using a least square fit Cost function S: residual t with respect to the model = model error + differential effects at the two stations THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR

EGRS correlation process (1) Signal phases are rotated to compensate the Earth rotation and the two data streams are delayed using the predictions provided by the FD model Correlation of the two data streams for each channel in blocks of one second; if the SNR is low, the integration time for each block must be increased THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR

EGRS correlation process (2) The correlation functions are synthesized to build a multi-band correlation function, whose peak represents the quasar DOR G G is the residual delay with respect to the model and represents the clock offset between the two stations DDOR: τS - τG THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR

THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR Error budget Parameter Value unità B = Baseline 11621 km D = Antenna diameter 35 m T_sys = Noise temperature 55 K δ = S/C-station-probe angle 10 deg Tobs_q = integration time (quasar) min Tobs_sc = integration time (s/c) 5 Sc = correlated flux 1 Jy Bw = Spanned Bandwidth (X band) MHz Bw = Spanned Bandwidth (Ka band) 160 DELAYS ESA X-Band (Tm Harm.) ESA Ka-Band (DOR Tone) Type [nsec] Clock Instability 0.004 Earth Orientation 0.052 Instrumental Phase Ripple 0.265 0.017 Ionosphere 0.088 0.006 Quasar observation accuracy 0.252 0.016 Quasar Position 0.039 S/C observation accuracy 0.324 0.044 Solar Plasma 0.003 0.0002 Station Location 0.012 Troposphere 0.177 TOTAL (RMS) 0.531 0.196 THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR

THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR Test plan Tests have been performed using ESA probes Venus Express Cruise (12/20, 12/23) Prior to orbit insertion maneuver (3/11, 3/13, 3/23, 3/31, 4/9) Jitter of the signal Inaccurate knowledge of probe’s position Mars Express Martian orbit (1/29, 3/7) Clean signal Precise model THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR

THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR VEX results Q = quasar (JPL id) PEQ = probe - Earth - quasar angle dtime = differential clock correction between the two stations (New Norcia and Cebreros) 1σ = (assumed) standard deviation of the ΔDOR measurement error Validation of the correlator: DOR results have been processed by the Flight Dynamic Group at ESOC; residuals with respect to the orbit solution including also range and Doppler data have been computed. THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR

THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR VEX residuals THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR

MEX results and residuals Date MM/DD Order Q DDOR time Hh:mm:ss.ms Δtime (μs) 1σ (ns) Post fit residual (ns) 1 1/29 SQSQSQSQS S072 S088 13:21:50.90 13:42:19.90 14:06:42.90 14:42:15.90 0.312 0.310 0.488 1.691 0.533 0.168 0.196 -0.679 -0.041 2 3/7 SQSQS S678 13:18:56.90 13:40:39.90 -0.977 -0.978 0.905 1.169 -0.114 -0.421 MEX residuals < VEX residuals better signal better model of probe’s state vector THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR

Conclusions and future activities The first software correlator for ESA DDOR has been implemented and tested. The correlator has been used for the VOI (Venus Orbit Insertion); ESA residuals are slightly larger than JPL ones, demonstrating the quality of the ESA DDOR system Future developments Improvement of the correlator capabilities (different quantization levels, sampling rates and channel number) JPL format compatibility VLBI format compatibility THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR