1. 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, th Radionet Engineering Forum Workshop: Next Generation Correlators for Radio Astronomy and Geodesy

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR
Receiver chain
8.4– 8.5
GHz
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
L_Band D/C, B=30 MHz
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 (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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

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

19. 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

20. 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