OPS Forum Delta-DOR 22.04.2005

ESA Delta DOR ESOC, 22 nd April 2004

 DOR definitions (1)

 -DOR stands for Delta-Differential One Way Ranging

DOR is the measure of the difference in signal arrival time between two stations. The observable is an uncalibrated time-delay between the two antennas.

“ Delta” is respect to a simple DOR, and refers to quasar calibration of the S/C DOR.

Since the quasar signal is recorded on the same BW of the S/C channels, ideally any errors which are station or light path dependent will cancel.

 DOR definitions (2)

The extent to which these error sources cancel depends on the angular separation of the two sources being observed. The maximum angular distance between S/C and Quasar is 15 deg.

Thus, one is able to evaluate a potentially error-free relative station delay, which leads to an accurate determination of the S/C position in the plane of the sky

The measurement accuracy is given by:

Longer baselines, better accuracies

 DOR generic signal structure

S/C signal

Set of TM harmonics (possibly unmodulated) or dedicated DOR tones

Low order TM harmonics (and DOR tones) are typically not embedded in noise, high-order harmonics normally are

Accuracy on S/C signal time-delay determination is:

Quasar signal

Random noise-like signal

Quasar signal is totally embedded in antenna noise

Accuracy on Quasar signal time-delay determination is:

“ f max –f min ” is the “total spanned bandwidth”, i.e. the frequency span between the most distant tones, used in the observation.

Current station architecture

XDC output BW: 100 MHz

LDC output BW: 30 MHz

IFMS input BW: 28 MHz

IFMS can handle only 2 35 Mb/s sampled inputs

IFMS CPU and connectivity not suitable to handle the expected data rate generated by the DDOR process

8400 – 8500 MHz X-band D/C L-band D/C 1 Switching Matrix IFMS1 C F E GDSP RHC LHC 540-640 MHz 70 MHz Each input BW: 28 MHz L-band D/C 2 GDSP Test Test UCPU Estrack LAN

Driving requirements (1)

Current ESA Deep Space Missions :

MEX, Rosetta, VEX transponders do not generate dedicated DOR Tones

Use of low and high order even TM Harmonics.

Recommended maximum spanned BW of about 15 – 20 MHz. This means using up to the 40 th harmonic (around 44 dBc).

This is within the input BW of current IFMS

JPL Missions :

Current in-flight JPL missions have transponder with dedicated DOR tones at  3 MHz,  19 MHz (X-band). Maximum spanned BW: 38MHz

This is outside the maximum input BW of the IFMS (28MHz)

Band Re-centring :

Different parts of the spectrum have to be tuned to the current inputs of the IFMS

The relevant parts of the spectrum can be then properly processed in chunks (Band re-centring)

X-band F C 3MHz 19MHz 19MHz 3MHz 20MHz 20MHz Input 1 Input 2

Station configuration for Band re-centring LDC1 LDC2 IFMS1 CFE LDC3 IFMS3 FMS2 CFE CFE Input1 Input1 Input1 Input2 Input3 Input2 Input2 Input3 Input3 A B C A..B.C A B C SWITCHING MATRIX X 2 1 Y 2 1  2 1 X 1 1 Y 1 1  1 1 X 3 1 Y 3 1  3 1 X 1 2 Y 1 2 X 1 1 Y 1 1  1 1 X 2 1 Y 2 1  2 1 X 3 1 Y 3 1  3 1 X 1 2 Y 1 2 X 1 2 Y 1 2

Ka-band

Future ESA Deep Space Missions (BepiColombo) :

Transponders CCSDS compliant (DOR Tones at  4 MHz,  20 MHz in X-band, at  4 MHz,  20 MHz,  80 MHz in Ka-band, i.e. BepiColombo).

Maximum spanned BW 160 MHz (in Ka-band).

This is also outside current IFMS input BW (again, band re-centring has to be used but this will not cover the maximum spanned BW).

80MHz F C 4MHz 20MHz 20MHz 80MHz 4MHz 28MHz 28MHz Input 2 Input 1

Implementation approach

All Station, IFMS, Storage Device, Link modifications are highlighted in red

CORRELATOR D/C IFMS (EOLP GDSP) ESU DSA1 Estrack Lan D/C IFMS (EOLP GDSP) ESU DSA2 Estrack Lan

 DOR observation (1) Sequence

Observation Sequence: S/C – Quasar – S/C (or vice versa)

DOR Tone or TM Harm. F C Ch. BW (50kHz) DOR Tone or TM Harm. DOR Tone or TM Harm . DOR Tone or TM Harm. Ch. BW (50kHz) Ch. BW (50kHz) Ch. BW (50kHz)

S/C Signal: Telemetry Harmonic (or DOR Tone)

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

Quasar Signal: White Noise

(1) IFMS modifications

IFMS GDSP Modifications

4 channels with BW from 1kHz to 2 MHz

Quantization 1,2,4,8,16 bits

Synchronization of GDSPs internal clock

2 GDSPs will be modified, to allow reception of up to 8 channels

IFMS Internal LAN

Private VLAN for  DOR data routing (up to 36 Mbits/s)

IFMS C F E CH4 CH3 CH2 CH1 36Mbits/s GDSP2 36Mbits/s GDSP1

(1) Data rates and volumes

Data Rates & Volumes (each GDSP)

S/C Observation :

Data rate: 50kHz sampling I & Q, 8 bits quantization, 4 channels = 3.2 Mbits/s.

Data Volume: 10 minutes observation = 0.24 GB

Quasar Observation :

Data rate: 2MHz sampling I & Q, 2 bits quantization, 4 channels = 32 Mbits/s.

Data Volume: 10 minutes observation = 2.4 GB

Need for an External Storage Unit (ESU)

 DOR observation (2) Data collection

Redundant ESUs

Each ESU collects and formats data coming from any IFMS

ESUs are physically located in the station MER

Redundant connectivity between IFMSs and ESUs is provided by station LAN

ESU1 IFMS1 GDSP1 GDSP2 IFMS2 GDSP1 GDSP2 IFMS3 GDSP1 GDSP2 ESU2 LAN Switch AER AER LAN Switch MER MER Existing station LAN (100 Mb/s)

 DOR observation (3) Data type

Data type is “open loop”

Time-tagged I and Q samples on a selected polarization collected in files (1 file per minute)

Observation configuration (fixed during observation) parameters in a Primary Header

Parameters that may vary during observation are stored in a Frame Header. The size of each frame is of 1526 bytes.

Primary Header Frame Header and Data

 DOR observation (4) Data transfer

Data collected at the two station are transferred to a centralised facility (ESOC) to perform the correlation process.

In the worst case sequence of observation (Quasar – Spacecraft – Quasar), the data volume will be of up to 5 GB (using 4 channels), and up to 10 GB (using 8 channels).

Data transfer from the stations to the correlator has to be completed within 8 hours (in critical phases).

Link speed requirement: 10 GB / 8 hours = 2.77 Mbits/s

(4) Data transfer requirements

CEB and NNO Deep space stations will be equipped with 2Mbit/s lines

Assuming a maximum operational throughput of about 1Mbit/s, the remaining 1Mbit/s can be assigned to DDOR data transfer.

The extra capacity (about 2Mbit/s in the worst case), during DDOR data transfer, shall come from ad-hoc leased lines.

The feasibility of this solution has already been assessed with OPS-ONC

 DOR observation (5) Correlator I/F

Correlator interfaces definition

Station1, Station2 : open loop data

FD (input): orbital data to help the correlation process

FD (output): the final product of the correlation

CP: configuration parameters

The correlator will be implemented on a Linux workstation placed in an operational area

Inputs Outputs FD Station 1 CP S/W Correlator Station 2 FD

 (5) Signal structure

S/C signal Correlation

Set of TM harmonics

Low order TM harmonics are typically not embedded in noise, high-order harmonics are.

Signal characteristic permits phase extraction

Quasar signal Correlation

Noise-like signal

Quasar signal is totally embedded in receiver noise

Signal characteristic forces to go for a direct correlation method

 DOR observation (6) S/C signal correlation

S/C signal Correlation (1)

Phase extraction by means of a S/W frequency estimator on the lowest TM even harmonic (highest in S/N)

Update of the expected phase (produced after an input by FD), with the estimation obtained above.

Drive the phase extraction of all other tones with the model (properly Doppler-scaled) built on the lowest TM harmonic

At this point, we have the phase sequences of each TM harmonic at each Station, sufficiently corrected for phase uncertainties.

Correlation of the phase of each channel of Station 1 with the phase of the corresponding channel of Station 2

The correlation result as such is “modulo 2  ” ambiguous

 DOR observation (7) Ambiguity resolution

S/C signal Correlation (2)

In order to solve such ambiguity, some operation is performed on each pair of observed channels.

The right delta-delay is the one identified by the slope (S) straight line.

The S/C DOR delay (  S/C ) is then calculated as follows:

  

 DOR observation (8) Quasar signal correlation

Each data stream (channel) from each station is delay- and Doppler- compensated (using the model provided by FD). The delay is mostly given by Earth rotation

After delay- and Doppler- correction, each data stream of Station 1 will be correlated with the corresponding data stream of Station 2 for a range of delays (few  s) around the expected value (provided by FD)

The analysis of the observed data is split in observation periods (“accumulation periods”) of 1 s, in order to keep a tolerable level of error in Doppler compensation

Correlation is performed for a suitable integration time in order to maximise the signal-to-noise ratio

Delay resolution is improved by the use of available multi-band recordings (enlarging the total spanned bandwidth)

The result of the correlation is then added to the value given by FD

As a result, one obtains a Quasar DOR (  Q )

 DOR observation (9) Correlator final output

The final output of the correlator is a file containing three DOR measurements (in the Table only the most important parameters are reported).

 S/C  S/C  Q

 DOR observation (10) Orbit determination

To obtain a single  DOR observation the two S/C observations are linearly interpolated to the time of the single Quasar observation. Direct differencing of the observations can then be made.

The obtained time is used to correct FD estimation of the delay we are looking for.

The corrected delay is then used to calculate the angle between the orthogonal to the baseline and the S/C direction

Results are then calibrated for media effects

Overall accuracies are mainly driven by:

Maximum spanned BW

SNR of S/C and Quasar signals

Summary of the work to be done

Use of 3 rd IFMS in ESA Deep Space facilities as primary  DOR processor

Modification of the IFMS internal DSPs (GDSP)

Availability of all IFMSs in Deep Space facilities for redundant  DOR measurements

Private VLAN for  DOR data routing (IFMS-ESU up to 36 Mbps)

Use of a PC (ESU) with a fast Hard Disk for  DOR data storage (up to 8 GB per  DOR observation)

Enhancement of link capacity (station-correlator) for the availability of data in near real time

S/W Correlator development limited to MEX, VEX, Ros support only

Validation plan

First system tests possible during Correlator SAT.

Test Campaign on current ESA Deep Space missions (MEX, Rosetta).  DOR results will be compared versus standard tracking techniques (Doppler tracking and ranging).