ISO 17809:2014

Space data and information transfer systems - Delta-differential one-way ranging (Delta-DOR) operations

Delta Differential One-Way Ranging (Delta-DOR) operations are applicable to space agencies that operate deep space missions that require accurate determination of the spacecraft position in the plane of the sky. For operations where these requirements do not capture the needs of the participating agencies, Delta-DOR operations may not be appropriate. ISO 17809:2014 addresses rationale, requirements and criteria that Delta-DOR operations processes should be designed to meet.

Systèmes de transfert des données et informations spatiales — Exploitation de mesures différentielles de distance par triangulation (Delta DOR)

General Information

• Status: Published
• Publication Date: 22-Jun-2014
• ICS: 49.140 - Space systems and operations
• Technical Committee: ISO/TC 20/SC 13 - Space data and information transfer systems
• Drafting Committee: ISO/TC 20/SC 13 - Space data and information transfer systems
• Current Stage: 9060 - Close of review
• Start Date: 02-Dec-2024

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 17809
First edition
2014-07-01

Space data and information transfer
systems — Delta-differential one-way
ranging (Delta-DOR) operations
Systèmes de transfert des données et informations spatiales —
Exploitation de mesures différentielles de distance par triangulation
(Delta DOR)



Reference number
ISO 17809:2014(E)
©
ISO 2014

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ISO 17809:2014(E)

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©  ISO 2014
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ii © ISO 2014 – All rights reserved

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ISO 17809:2014(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO
member bodies). The work of preparing International Standards is normally carried out through ISO technical
committees. Each member body interested in a subject for which a technical committee has been established has
the right to be represented on that committee. International organizations, governmental and non-governmental, in
liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described in the
ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the different types of ISO
documents should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights.
ISO shall not be held responsible for identifying any or all such patent rights. Details of any patent rights identified
during the development of the document will be in the Introduction and/or on the ISO list of patent declarations
received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not constitute an
endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity assessment, as
well as information about ISO's adherence to the WTO principles in the Technical Barriers to Trade (TBT) see the
following URL: Foreword - Supplementary information
ISO 17809 was prepared by the Consultative Committee for Space Data Systems (CCSDS) (as CCSDS 506.0-M-
1, April 2011) and was adopted (without modifications except those stated in Clause 2 of this International
Standard) by Technical Committee ISO/TC 20, Aircraft and space vehicles, Subcommittee SC 13, Space data and
information transfer systems.
iii

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INTERNATIONAL STANDARD ISO 17809:2014(E)

Space data and information transfer systems — Delta-
differential one-way ranging (Delta-DOR) operations
1 Scope
Delta Differential One-Way Ranging (Delta-DOR) operations are applicable to space agencies that operate
deep space missions that require accurate determination of the spacecraft position in the plane of the sky.
For operations where these requirements do not capture the needs of the participating agencies, Delta-DOR
operations may not be appropriate.
This International Standard addresses rationale, requirements and criteria that Delta-DOR operations
processes should be designed to meet.
2 Requirements
Requirements are the technical recommendations made in the following publication (reproduced on the
following pages), which is adopted as an International Standard:
CCSDS 506.0-M-1, April 2011, Delta-Differential One Way Ranging (Delta-DOR) Operations
For the purposes of international standardization, the modifications outlined below shall apply to the specific
clauses and paragraphs of publication CCSDS 506.0-M-1.
Pages i to vi
This part is information which is relevant to the CCSDS publication only.
Page 1-3
Add the following information to the reference indicated:
[2] Document CCSDS 502.0-B-2, November 2009, is equivalent to ISO 26900.2012.
[3] Document CCSDS 503.0-B-1, November 2007, is equivalent to ISO 13536:2010.
Page D-1
Add the following information to the reference indicated:
[D6] Document CCSDS 505.0-B-1, December 2010, is equivalent to ISO 17107:2011.
Page D-2
Add the following information to the reference indicated:
[D11] Document CCSDS 301.0-B-4, November 2010, is equivalent to ISO 11104:2011.
[D12] Document CCSDS 910.11-B-1, August 2009, is equivalent to ISO 18439:2013.
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ISO 17809:2014(E)


3 Revision of publication CCSDS 506.0-M-1
It has been agreed with the Consultative Committee for Space Data Systems that Subcommittee
ISO/TC 20/SC 13 will be consulted in the event of any revision or amendment of publication CCSDS 506.0-M-
1. To this end, NASA will act as a liaison body between CCSDS and ISO.
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ISO 17809:2014(E)

Recommendation for Space Data System Practices
DELTA-DIFFERENTIAL
ONE WAY RANGING
(DELTA-DOR)
OPERATIONS
RECOMMENDED PRACTICE
CCSDS 506.0-M-1
MAGENTA BOOK
April 2011

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ISO 17809:2014(E)
CCSDS RECOMMENDED PRACTICE FOR DELTA-DOR OPERATIONS
AUTHORITY



Issue: Recommended Practice, Issue 1
Date: April 2011
Location: Washington, DC, USA

This document has been approved for publication by the Management Council of the
Consultative Committee for Space Data Systems (CCSDS) and represents the consensus
technical agreement of the participating CCSDS Member Agencies. The procedure for
review and authorization of CCSDS documents is detailed in the Procedures Manual for the
Consultative Committee for Space Data Systems, and the record of Agency participation in
the authorization of this document can be obtained from the CCSDS Secretariat at the
address below.


This document is published and maintained by:

CCSDS Secretariat
Space Communications and Navigation Office, 7L70
Space Operations Mission Directorate
NASA Headquarters
Washington, DC 20546-0001, USA
CCSDS 506.0-M-1 Page i April 2011

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ISO 17809:2014(E)
CCSDS RECOMMENDED PRACTICE FOR DELTA-DOR OPERATIONS
STATEMENT OF INTENT
The Consultative Committee for Space Data Systems (CCSDS) is an organization officially
established by the management of its members. The Committee meets periodically to address
data systems problems that are common to all participants, and to formulate sound technical
solutions to these problems. Inasmuch as participation in the CCSDS is completely
voluntary, the results of Committee actions are termed Recommendations and are not in
themselves considered binding on any Agency.
CCSDS Recommendations take two forms: Recommended Standards that are prescriptive
and are the formal vehicles by which CCSDS Agencies create the standards that specify how
elements of their space mission support infrastructure shall operate and interoperate with
others; and Recommended Practices that are more descriptive in nature and are intended to
provide general guidance about how to approach a particular problem associated with space
mission support. This Recommended Practice is issued by, and represents the consensus of,
the CCSDS members. Endorsement of this Recommended Practice is entirely voluntary
and does not imply a commitment by any Agency or organization to implement its
recommendations in a prescriptive sense.
No later than five years from its date of issuance, this Recommended Practice will be
reviewed by the CCSDS to determine whether it should: (1) remain in effect without change;
(2) be changed to reflect the impact of new technologies, new requirements, or new
directions; or (3) be retired or canceled.
In those instances when a new version of a Recommended Practice is issued, existing
CCSDS-related member Practices and implementations are not negated or deemed to be non-
CCSDS compatible. It is the responsibility of each member to determine when such Practices
or implementations are to be modified. Each member is, however, strongly encouraged to
direct planning for its new Practices and implementations towards the later version of the
Recommended Practice.
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ISO 17809:2014(E)
CCSDS RECOMMENDED PRACTICE FOR DELTA-DOR OPERATIONS
FOREWORD
Through the process of normal evolution, it is expected that expansion, deletion, or
modification of this document may occur. This Recommended Practice is therefore subject
to CCSDS document management and change control procedures, which are defined in the
Procedures Manual for the Consultative Committee for Space Data Systems. Current
versions of CCSDS documents are maintained at the CCSDS Web site:
http://www.ccsds.org/
Questions relating to the contents or status of this document should be addressed to the
CCSDS Secretariat at the address indicated on page i.
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ISO 17809:2014(E)
CCSDS RECOMMENDED PRACTICE FOR DELTA-DOR OPERATIONS
At time of publication, the active Member and Observer Agencies of the CCSDS were:
Member Agencies
– Agenzia Spaziale Italiana (ASI)/Italy.
– Canadian Space Agency (CSA)/Canada.
– Centre National d’Etudes Spatiales (CNES)/France.
– China National Space Administration (CNSA)/People’s Republic of China.
– Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR)/Germany.
– European Space Agency (ESA)/Europe.
– Federal Space Agency (FSA)/Russian Federation.
– Instituto Nacional de Pesquisas Espaciais (INPE)/Brazil.
– Japan Aerospace Exploration Agency (JAXA)/Japan.
– National Aeronautics and Space Administration (NASA)/USA.
– UK Space Agency/United Kingdom.
Observer Agencies
– Austrian Space Agency (ASA)/Austria.
– Belgian Federal Science Policy Office (BFSPO)/Belgium.
– Central Research Institute of Machine Building (TsNIIMash)/Russian Federation.
– China Satellite Launch and Tracking Control General, Beijing Institute of Tracking
and Telecommunications Technology (CLTC/BITTT)/China.
– Chinese Academy of Sciences (CAS)/China.
– Chinese Academy of Space Technology (CAST)/China.
– Commonwealth Scientific and Industrial Research Organization (CSIRO)/Australia.
– CSIR Satellite Applications Centre (CSIR)/Republic of South Africa.
– Danish National Space Center (DNSC)/Denmark.
– Departamento de Ciência e Tecnologia Aeroespacial (DCTA)/Brazil.
– European Organization for the Exploitation of Meteorological Satellites
(EUMETSAT)/Europe.
– European Telecommunications Satellite Organization (EUTELSAT)/Europe.
– Geo-Informatics and Space Technology Development Agency (GISTDA)/Thailand.
– Hellenic National Space Committee (HNSC)/Greece.
– Indian Space Research Organization (ISRO)/India.
– Institute of Space Research (IKI)/Russian Federation.
– KFKI Research Institute for Particle & Nuclear Physics (KFKI)/Hungary.
– Korea Aerospace Research Institute (KARI)/Korea.
– Ministry of Communications (MOC)/Israel.
– National Institute of Information and Communications Technology (NICT)/Japan.
– National Oceanic and Atmospheric Administration (NOAA)/USA.
– National Space Agency of the Republic of Kazakhstan (NSARK)/Kazakhstan.
– National Space Organization (NSPO)/Chinese Taipei.
– Naval Center for Space Technology (NCST)/USA.
– Scientific and Technological Research Council of Turkey (TUBITAK)/Turkey.
– Space and Upper Atmosphere Research Commission (SUPARCO)/Pakistan.
– Swedish Space Corporation (SSC)/Sweden.
– United States Geological Survey (USGS)/USA.
CCSDS 506.0-M-1 Page iv April 2011

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ISO 17809:2014(E)
CCSDS RECOMMENDED PRACTICE FOR DELTA-DOR OPERATIONS
DOCUMENT CONTROL

Document Title Date Status
CCSDS Delta-Differential One Way Ranging April 2011 Original issue
506.0-M-1 (Delta-DOR) Operations,
Recommended Practice, Issue 1



CCSDS 506.0-M-1 Page v April 2011

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ISO 17809:2014(E)
CCSDS RECOMMENDED PRACTICE FOR DELTA-DOR OPERATIONS
CONTENTS
Section Page
1  INTRODUCTION . 1-1

1.1  PURPOSE . 1-1
1.2  SCOPE AND APPLICABILITY . 1-1
1.3  CONVENTIONS AND DEFINITIONS. 1-2
1.4  COMMON DELTA-DOR TERMINOLOGY . 1-2
1.5  STRUCTURE OF THIS DOCUMENT. 1-3
1.6  REFERENCES . 1-3

2  OVERVIEW . 2-1

2.1  GENERAL . 2-1
2.2  THE DELTA-DOR TECHNIQUE . 2-1
2.3  ADVANTAGES OF DELTA-DOR . 2-4
2.4  LIMITATIONS OF DELTA-DOR . 2-5

3  DEFINITIONS FOR INTERAGENCY DELTA-DOR . 3-1

3.1  OVERVIEW . 3-1
3.2  ROLES OF PARTICIPATING AGENCIES IN OPERATIONAL SCENARIOS . 3-1
3.3  ROLES OF PARTICIPATING AGENCIES IN VALIDATION SCENARIOS . 3-1
3.4  DEFINITION OF THE OPERATIONAL SCENARIOS . 3-1
3.5  DEFINITION OF THE VALIDATION SCENARIOS . 3-2
3.6  DEFINITION OF THE INTERFACES . 3-2
3.7  DEFINITION OF PARAMETERS INTERVENING IN THE MEASUREMENT . 3-4

4  DESCRIPTION OF OPERATIONAL SCENARIOS . 4-1

4.1  OVERVIEW . 4-1
4.2  SCENARIO 1 . 4-1
4.3  SCENARIO 2 . 4-3
4.4  SCENARIO 3 . 4-5
4.5  SCENARIO 4 . 4-7

5  DESCRIPTION OF VALIDATION PROCESS . 5-1

5.1  OVERVIEW . 5-1
5.2  INTEROPERABILITY VALIDATION STEP 1—TRAJECTORY
PREDICTION AND OBSERVABLE MODELING . 5-1
5.3  INTEROPERABILITY VALIDATION STEP 2—DATA COLLECTION . 5-3
5.4  INTEROPERABILITY VALIDATION STEP 3—DATA PROCESSING . 5-5

CCSDS 506.0-M-1 Page vi April 2011

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ISO 17809:2014(E)
CCSDS RECOMMENDED PRACTICE FOR DELTA-DOR OPERATIONS
CONTENTS (continued)
Section Page
6  INTERAGENCY DATA EXCHANGE PRODUCTS AND PROCEDURES . 6-1

6.1  GENERAL . 6-1
6.2  SERVICE REQUEST EXCHANGE SPECIFICATION . 6-1
6.3  ORBITAL EPHEMERIS MESSAGE EXCHANGE SPECIFICATION . 6-5
6.4  RAW DATA TRANSFER/EXCHANGE SPECIFICATION . 6-5
6.5  METEO DATA EXCHANGE SPECIFICATION . 6-6
6.6  REDUCED DELTA-DOR DATA TRANSFER/EXCHANGE SPECIFICATION . 6-7

7  RADIO SOURCE CATALOGUE SPECIFICATION . 7-1

ANNEX A ITEMS FOR AN IMPLEMENTING ARRANGEMENT (IA)
(NORMATIVE) . A-1
ANNEX B SECURITY (INFORMATIVE) .B-1
ANNEX C ABBREVIATIONS AND ACRONYMS (INFORMATIVE) . C-1
ANNEX D INFORMATIVE REFERENCES (INFORMATIVE) . D-1
Figure
2-1  Delta-DOR Observation Geometry . 2-2
2-2  Error Ellipses in the Mars Targeting Plane . 2-5
3-1  High-Level Delta-DOR Flow . 3-3

Table
3-1  Definition of Cross-Support Scenarios . 3-2
3-2  Definition of Cross-Support Validation Scenarios . 3-2
6-1  Definition of Delta-DOR Service Request Parameters . 6-2


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ISO 17809:2014(E)
CCSDS RECOMMENDED PRACTICE FOR DELTA-DOR OPERATIONS
1 INTRODUCTION
1.1 PURPOSE
This Recommended Practice specifies a set of standard practices and message formats for use
in the navigation technique known as Delta Differential One-Way Ranging (Delta-DOR). It
has been developed via consensus of the Delta-DOR Working Group of the CCSDS Systems
Engineering Area (SEA).
Delta-DOR is a Very Long Baseline Interferometry (VLBI) technique that can be used in
conjunction with Doppler and ranging data to improve spacecraft navigation by more
efficiently determining spacecraft angular position in the plane of sky. The establishment of
interoperability for acquiring and processing Delta-DOR data at ground stations of different
agencies, the standardization of service requests for Delta-DOR, the standardization of an
exchange format for raw data, and standardization of interfaces for exchange of supporting
products are key enablers for interagency execution of Delta-DOR operations.
The Recommended Practice addresses aspects of the technique that require standardization in
order to enable Delta-DOR interoperability between space agencies, e.g., configuration
requirements for interagency Delta-DOR measurement; interagency exchange of
measurement data; parameters that are necessary in order to correlate and process the data at
one of the agencies; interagency transfer of the generated observables; and the end-to-end
flow of control. It is believed that such standards will reduce development and operations
costs while improving navigation capabilities by increasing the number of intercontinental
ground station baselines.
There are essentially three parts to providing Delta-DOR services, the first being the definition
of the RF domain signals and reception, the second being the definition of the input and output
data products, and the third being the definition of the method for requesting service and
transferring data products. The first of these is allocated to the CCSDS Space Link Service
(SLS) Area (reference [1]); the second is allocated to the Mission Operations and Information
Management Services (MOIMS) Area (reference [3]); the third will be developed as SLE
Service Request extensions which will be allocated to the Service Management Working Group
within the Cross Support Services (CSS) Area (reference [D12]).
The purpose of this Magenta Book is the production of a set of recommendations for
facilitating interagency Delta-DOR operations that can both be useful now and evolve to
meet future needs. The present document is intended to provide a set of standard practices to
be used for setting up Delta-DOR measurements among different agencies, covering all of
the required elements and describing how they are combined to provide the desired service.
1.2 SCOPE AND APPLICABILITY
Delta-DOR operations are applicable to space agencies that operate deep space missions that
require accurate determination of the spacecraft position in the plane of the sky. For
CCSDS 506.0-M-1 Page 1-1 April 2011

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ISO 17809:2014(E)
CCSDS RECOMMENDED PRACTICE FOR DELTA-DOR OPERATIONS
operations where these requirements do not capture the needs of the participating agencies,
Delta-DOR operations may not be appropriate.
This Recommended Practice addresses rationale, requirements and criteria that Delta-DOR
operations processes should be designed to meet.
1.3 CONVENTIONS AND DEFINITIONS
Conventions and definitions of Delta-DOR concepts are provided in reference [D2], Delta-
DOR Operations—Technical Characteristics and Performance. This reference provides a
detailed description of the Delta-DOR technique, including guidelines for DOR tone spectra,
guidelines for selecting reference sources, applicable foundation equations, and a discussion
of error sources and measurement accuracy that are not germane to the recommendations
presented in this document.
The following conventions apply throughout this Recommended Practice:
– the words ‘shall’ and ‘must’ imply a binding and verifiable specification;
– the word ‘should’ implies an optional, but desirable, specification;
– the word ‘may’ implies an optional specification;
– the words ‘is’, ‘are’, and ‘will’ imply statements of fact.
1.4 COMMON DELTA-DOR TERMINOLOGY
Part of the standardization process involves the determination of common interagency
terminology and definitions that apply to interagency Delta-DOR. The following
conventions apply throughout this Recommended Practice:

Term Meaning
baseline The vector joining two tracking stations
channel A slice of the frequency spectrum that contains a spacecraft or quasar
signal
scan An observation of a radio source, typical duration of a few minutes
spanned bandwidth The widest separation between downlink signal components
P /N Total Power to Noise Spectral Density ratio
T 0
P /N Tone Power to Noise Spectral Density ratio
Tone 0
meteo data Meteorological Data (as a minimum: pressure, temperature, relative
humidity must be considered; slant total electron content might also
be provided)
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ISO 17809:2014(E)
CCSDS RECOMMENDED PRACTICE FOR DELTA-DOR OPERATIONS
1.5 STRUCTURE OF THIS DOCUMENT
In addition to this section, this document contains the following sections and annexes:
– Section 2 provides a general overview of Delta-DOR technique.
– Section 3 provides a set of definitions for the interagency Delta-DOR.
– Section 4 describes the Delta-DOR interoperability scenarios.
– Section 5 discusses the interagency Delta-DOR validation process.
– Section 6 discusses the interagency data exchange products and procedures.
– Section 7 discusses the generation and maintenance of the radio source catalog.
– Annex A lists a number of items that should be covered in an interagency
Implementing Arrangement (IA) prior to commencing regular Delta-DOR operations.
There are several statements throughout the document that refer to the necessity of
such a document; this annex consolidates all the suggested IA items in a single list in
the document.
– Annex B discusses security considerations applied to the technologies specified in
this Recommended Practice.
– Annex C is a list of abbreviations and acronyms applicable to Delta-DOR Operations.
– Annex D contains a list of informative references.
1.6 REFERENCES
The following documents contain provisions which, through reference in this text, constitute
provisions of this Recommended Practice. At the time of publication, the editions indicated
were valid. All documents are subject to revision, and users of this Recommended Practice
are encouraged to investigate the possibility of applying the most recent editions of the
documents indicated below. The CCSDS Secretariat maintains a register of currently valid
CCSDS documents.
[1] Radio Frequency and Modulation Systems—Part 1: Earth Stations and Spacecraft.
Recommendation for Space Data System Standards, CCSDS 401.0-B-20. Blue Book.
Issue 20. Washington, D.C.: CCSDS, April 2009.
[2] Orbit Data Messages. Recommendation for Space Data System Standards, CCSDS
502.0-B-2. Blue Book. Issue 2. Washington, D.C.: CCSDS, November 2009.
[3] Tracking Data Message. Recommendation for Space Data System Standards, CCSDS
503.0-B-1. Blue Book. Issue 1. Washington, D.C.: CCSDS, November 2007.
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ISO 17809:2014(E)
CCSDS RECOMMENDED PRACTICE FOR DELTA-DOR OPERATIONS
[4] Delta-DOR Raw Data Exchange Format. Draft Recommendation for Space Data
System Standards, CCSDS 506.1-R-1. Red Book. Issue 1. Washington, D.C.:
CCSDS, April 2011.
[5] “Radio Source Catalog.” May 26, 2006. Module 107 in DSN Telecommunications
Link Design Handbook. DSN No. 810-005, Rev. E. Pasadena California: JPL,

NOTE – Informative references are provided in annex D.

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ISO 17809:2014(E)
CCSDS RECOMMENDED PRACTICE FOR DELTA-DOR OPERATIONS
2 OVERVIEW
2.1 GENERAL
This section provides a high-level overview of the Delta-DOR technique, its advantages, and
disadvantages.
2.2 THE DELTA-DOR TECHNIQUE
Very Long Baseline Interferometry (VLBI) is a technique that allows determination of
angular position for distant radio sources by measuring the geometric time delay between
received radio signals at two geographically separated stations. The observed time delay is a
function of the known baseline vector joining the two radio antennas and the direction to the
radio source.
An application of VLBI is spacecraft navigation in space missions where delay
measurements of a spacecraft radio signal are compared against similar delay measurements
of angularly nearby quasar radio signals. In the case where the spacecraft measurements are
obtained from the phases of tones emitted from the spacecraft, first detected separately at
each station, and then differenced, this application of VLBI is known as Delta Differential
One-Way Ranging (‘Delta-DOR’ or ‘∆DOR’). (See figure 2-1.) Even though data
acquisition and processing are not identical for the spacecraft and quasar, both types of
measurements can be interpreted as delay measurements and they have similar information
content and similar sensitivity to sources of error (reference [D2]). The data produced in
such a measurement session are complementary to Doppler and ranging data.
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ISO 17809:2014(E)
CCSDS RECOMMENDED PRACTICE FOR DELTA-DOR OPERATIONS
Spacecraft
Quasar
θ
Correlator
Baseline B
c = speed of light
τ B·cos(θ)/c τ

Figure 2-1: Delta-DOR Observation Geometry
To enable a Delta-DOR measurement, a spacecraft must emit several tones or other signal
components spanning at least a few MHz. The characteristics of the tones are selected based
on the requirements for phase ambiguity resolution, measurement accuracy, efficient use of
spacecraft signal power, efficient use of ground tracking resources, and the frequency
allocation for space research.
The Delta-DOR technique requires that the same quasar and spacecraft be tracked essentially
simultaneously during the same tracking pass, at two distinct radio antennas. Thus a viewing
overlap between the two antenna complexes is required; the degree of overlap is dependent
upon the relative station locations, and varies for each pair of antenna complexes. Normally,
a Delta-DOR pass consists of three or more scans of data recording, each of a few minutes
duration. A scan consists of pointing the antennas to one radio source and recording the
signal. The antennas must slew to another radio source for the next scan, and so on. The
observing sequence is spacecraft-quasar-spacecraft, quasar-spacecraft-quasar, or a longer
sequence of alternating observations, depending on the characteristics of the radio sources
and the objectives of the measurement session. A minimum of three scans is requi
...