European Space Agency Università di Roma “la Sapienza” Dipartimento di Ingegneria Aerospaziale ed Astronautica THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR

European Space Agency

Università di Roma “la Sapienza”Dipartimento di Ingegneria Aerospaziale ed Astronautica

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 2/20

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


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.)

De

clin

ati

on

Range and Doppler


ΔDOR (1)

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

≤15°

Solar wind

Ionosphere

Troposphere

Baseline


ΔDOR (2)

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)

cosB

c

B


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


Signals

TM carrierTM Harm.

Order +2

Ch. BW 50kHz

TM Harm.

Order -14

TM Harm.

Order +20

Ch. BW 50kHz Ch. BW 50kHz Ch. BW 50kHz

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 = 0.510-26 W/m2Hz)

– Fs=2MHz; 2bit

Accuracy:– Spanned bandwidth: 10 MHz– Signals SNR– Integration time

Ch. BW (2MHz) Ch. BW (2MHz) Ch. BW (2MHz) Ch. BW (2MHz)

QS

QSOBSW

QS

NSTB

/0/

/2

2

Spanned bandwidth BW ≈ 10 MHz


Receiver chain

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

X-bandD/C

L-bandD/C 1

SwitchingMatrix

RHCLHC

540-640 MHz

55-85 MHz

UCPU

Estrack LAN

IFMS

CH3

CH2

CH1

36Mbits/s

GDSP1X

Y

Input 1CFE

X

Y

X

Y

CH4

GDSP3 36Mbits/s

GDSP4 36Mbits/s

GDSP2 36Mbits/s

8.4– 8.5GHz

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


CENTRAL CORRELATOR

D/C

ESU

EXTRACK LAN

DATI

RICEVITORE

D/C

ESU

EXTRAK LAN

DATI

RICEVITORE

IFMS(EOLP GDSP)

IFMS(EOLP GDSP)

Correlator interfaces

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

Station 1

Inputs

Outputs

FD

AIS/W

Correlator

Station 2

FD


S/C correlation process (1)

Digital PLL:reconstruction (using FD data) of signal phases and transmitter frequency for all channels



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



Ambiguity resolution using a least square fit

)(2 12

12

ffff

S

S: residual with respect to the model = model error + differential effects at the two stations

Cost function


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


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


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) 10 min

Tobs_sc = integration time (s/c) 5 min

Sc = correlated flux 1 Jy

Bw = Spanned Bandwidth

(X band) 10 MHz

Bw = Spanned Bandwidth

(Ka band) 160 MHz

Error budget

DELAYSESA X-Band(Tm Harm.)

ESA Ka-Band(DOR Tone)

Type [nsec] [nsec]

Clock Instability 0.004 0.004

Earth Orientation 0.052 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 0.039

S/C observation accuracy 0.324 0.044

Solar Plasma 0.003 0.0002

Station Location 0.012 0.012

Troposphere 0.177 0.177

TOTAL (RMS) 0.531 0.196

QS

QSOBSW

QS

NSTB

/0/

/2

2


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


VEX results

Q = quasar (JPL id)

PEQ = probe - Earth - quasar angle

time = 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.


VEX residuals


MEX results and residuals

NDate

MM/DDOrder Q

DDOR time

Hh:mm:ss.msΔtime (μs) 1σ (ns) Post fit

residual (ns)

1 1/29 SQSQSQSQS

S072

S088

S072

13:21:50.90

13:42:19.90

14:06:42.90

14:42:15.90

0.312

0.312

0.312

0.310

0.488

0.488

1.691

0.533

0.168

0.196

-0.679

-0.041

2 3/7 SQSQS S67813:18:56.90

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


Conclusions and future activities

Conclusions

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

Documents

European Space Agency Università di Roma “la Sapienza” Dipartimento di Ingegneria Aerospaziale ed Astronautica THE SOFTWARE CORRELATOR FOR ESA DELTA-DOR