6.1. Setting up the link ends¶

In Tudat, an observation model is referred to a number of link ends. For a one-way range model, for instance, a receiver and transmitter are required, both of which are termed a ‘link end’. An enum termed LinkEndType is available that lists all the possible kinds of link ends. A full list of observation models, as well as the link ends that they require, is given on the page on Setting up Observation Models.

6.1.1. Ground Station Creation¶

Often, you will need to define the positions of ground stations on celestial bodies to/from which observations are made. Presently, the position of a ground station is fixed in the body-fixed frame of the body on which it is located, but modifications to allow a time-varying position (due to tides, continental drift, etc.) are planned.

With the present setup, you must provide only the name and position (as an Eigen::Vector3d) of the ground station to create it. The position may be defined in several coordinate systems, defined by an enum of PositionElementTypes:

cartesian_position For this type, the three entries of the position are simply the x, y and z components in the body-fixed frame.
spherical_position For this type, the three entries of the position are the distance from the body’s center of mass, and the geocentric latitude and longitude on the body (in that order).
geodetic_position For this type, the three entries of the position are the altitude (w.r.t. the body’s shape model), the geodetic latitude and the longitude (in that order).

Now, creating a ground station is done by:

boost::shared_ptr< Body > earth = ...;// Define object for Earth

// Define name of station
std::string stationName = "Graz";

// Position of ground station
Eigen::Vector3d stationPosition;
stationPosition << 4194511.7, 1162789.7, 4647362.5;
PositionElementTypes positionType = cartesian_position;

createGroundStation( earth, stationName, stationPosition, positionType );

Which will create a ground station named "Graz" on the body earth at the given position.

6.1.2. Creating a Set of Link Ends¶

A set of link ends used for a given observable are stored in a LinkEnds type, which is a typedef for std::map< LinkEndType, std::pair< std::string, std::string > >. As you can see, the map value is a pair of strings. The first entry of the string is the body on which the link end is placed, the second entry the reference point on this body (typically the ground station). In the case where the second entry is empty (as is often the case for spacecraft), the body’s center of mass is used. An example of defining link ends is given below:

LinkEnds oneWayLinkEnds;
oneWayLinkEnds[ transmitter ] = std::make_pair( "Earth", "Graz" );
oneWayLinkEnds[ receiver ] = std::make_pair( "LRO", "" );

This defines a link for which the ground station termed Graz on the boy called Earth acts as transmitter, and the body called LRO is used as the receiver (in this case placed at the body’s center of mass).

An example of link-ends for a two-way link from Graz to LRO and back to Graz is:

LinkEnds twoWayLinkEnds;
twoWayLinkEnds[ transmitter ] = std::make_pair( "Earth", "Graz" );
twoWayLinkEnds[ reflector1 ] = std::make_pair( "LRO", "" );
twoWayLinkEnds[ receiver ] = std::make_pair( "Earth", "Graz" );

Where the Graz station now acts as both transmitter and receiver. Similarly, the receiver may be different from the transmitter (in what is typically called a three-way observable in Deep Space tracking ), so:

LinkEnds twoWayLinkEnds;
twoWayLinkEnds[ transmitter ] = std::make_pair( "Earth", "Graz" );
twoWayLinkEnds[ reflector1 ] = std::make_pair( "LRO", "" );
twoWayLinkEnds[ receiver ] = std::make_pair( "Earth", "Matera" );

where the signal is transmitter by Graz station, retransmitter or reflected by LRO, and then received by the Matera station.