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504
Development of a smart docking system for small satellites
Alex Caon1,a*, Luca Lion1,b, Lorenzo Olivieri1,c, Francesco Branz2,d,
Alessandro Francesconi2,e
1C.I.S.A.S. - Centre of Studies and Activities for Space "G. Colombo", Via Venezia 15, Padova
(Italy)
2Department of Industrial Engineering, University of Padova, via Venezia, 1, Padova (Italy)
aalex.caon@unipd.it, bluca.lion.1@phd.unipd.it, clorenzo.olivieri@unipd.it,
dfrancesco.branz@unipd.it, ealessandro.francesceoni@unipd.it
Keywords: Docking system, Autonomous system, Space Rider, Space Rider Observer
Cube
Abstract. DOCKS is a smart docking system for space vehicles developed by the Department of
Industrial Engineering, University of Padova, within the framework of the Space Rider Observer
Cube (SROC) mission. The design and development of SROC is being conducted by a consortium
of Italian entities under contract with the European Space Agency (ESA). The SROC mission is
designed to be a payload on the ESA Space Rider (SR) spaceship. The main objective of the
mission is to demonstrate the critical capabilities and technologies required to execute a
rendezvous and docking mission in a safety-sensitive context. The space system is composed by a
nanosatellite (approximately 12U CubeSat) and a deployment/retrieval mechanism mounted inside
the payload bay of SR. During the mission, SROC will be released by SR, will perform inspection
manoeuvres on SR and, at the end of the mission, will dock back inside the bay of SR, before re-
entering Earth with the mothership. The docking functionality is provided by DOCKS. DOCKS is
suitable for use onboard micro- and nanosatellites and merges a classical probe drogue
configuration with a gripper–like design, to manage the connection between the parts. The system
is equipped with a suite of sensors to estimate the relative pose of the target and with a dedicated
computer, making it a smart standalone system. A laboratory prototype has been assembled and
functionally tested, aiming at the validation of the capability to passively manage misalignments
during the docking manoeuvre.
Introduction
The docking system (DOCKS) has been developed in the framework of the Space Rider Observer
Cube (SROC) mission. The purpose of the mission is to demonstrate the capabilities and
technologies required for rendezvous and docking with a target vehicle [1]. A brief description of
the operations performed by SROC is:
1. Launch and early operations. SROC is stored inside the bay of Space Rider (SR). Once in
orbit, SROC is pushed away from SR in order to begin its operative phases.
2. Proximity Operations. SROC is in a relative orbit with respect to SR in order to perform
its observations.
3. Docking and Retrieval Phase. SROC approaches SR and docks with it in order to be re-
stored inside the bay and return to Earth safely.
DOCKS overview
The DOCKing System (DOCKS) has been developed to be a standalone docking mechanism with
an integrated set of sensors and a computer. In the following, all the parts of DOCKS will be
described.
Aeronautics and Astronautics - AIDAA XXVII International Congress Materials Research Forum LLC
Materials Research Proceedings 37 (2023) 504-507 https://doi.org/10.21741/9781644902813-110
505
Docking mechanism
The mechanical connection between SROC and SR is provided by a docking mechanism that is
composed by two main parts (Fig. 1). The first (DOCKS-A on SROC) is the active part with the
mechanism, and the second (DOCKS-B on SR) is the counter part of the docking mechanism (the
drogue) and the LEDs that allows the relative navigation.
The active part of docking mechanism is composed by two parts (shown in Fig. 2): a centring
cone and three claws that provide the rigid mechanical connection.
The centering cone has the purpose to force the alignment between SROC and SR when
coupling with the drogue. In fact, the shape of the probe allows to tolerate 10 mm of lateral
misalignment and 10 deg of pitch (and yaw) misalignment. In addition, the pins on the rim of the
probe force the roll alignment, when they couple with the grooves on the drogue.
The hard docking is achieved with the three claws that ensure the rigid connection, by closing
around the rim of the drogue. The claws are activated by a four-bar linkage in order to prevent
linear actuations that could produce friction or jam the mechanism.
Sensor suite and estimation performances
As illustrated in Fig. 1, DOCKS-A is provided with three different sensors to measure the relative
pose of DOCKS-B.
Figure 1: DOCKS-A and DOCKS-B. They are mounted on SROC and on SR respectively. In
DOCKS-A it is also represented it frame of reference.
Figure 2: The centring cone and claws of DOCKS.
Aeronautics and Astronautics - AIDAA XXVII International Congress Materials Research Forum LLC
Materials Research Proceedings 37 (2023) 504-507 https://doi.org/10.21741/9781644902813-110
506
1. A navigation camera. Its purpose is to measure the entire relative pose of DOCKS-B. The
NavCam with its computer, recognizes the pattern of LEDs on DOCKS-B and reconstruct
its pose [2]. However, at distances lower than 50 mm, the camera is out of focus making
the measurement unreliable.
2. A set of four Time-of-Flight sensors. They are employed to measure the distance and the
relative pitch and yaw angles from 100 mm up to contact (as explained in Fig. 3) [3].
3. A matrix sensor. It is used to measure the relative position along the y and z axes (which is
not measurable with the ToF sensors). The sensor employs a matrix of 5x5 phototransistors
on DOCKS-A and an infrared LED on DOCKS-B. Depending on the relative position, the
LED activates different pattern of phototransistors [4].
The ToF sensors are affected by a noise of approximately 4 mm at distances below 30 mm,
causing an uncertainty on the measure of the relative angles of more than 5 deg. To improve the
estimation, a Kalman filter has been applied to the measurements of the ToF sensors [5]. The tests
performed on the sensor suite provided the estimation error reported in Tab.1.
Table 1: Estimation errors
Error
X [mm]
Y [mm]
Z [mm]
Pitch [deg]
Yaw [yaw]
NavCam
Avg.
2.0
2.0
2.0
1.5
1.5
Std. dev.
0.5
0.5
0.5
1.0
1.0
ToF +
matrix
Avg.
0.14
1.16
1.41
0.52
0.11
Std. dev.
0.43
0.13
2.13
0.64
0.68
Tests on DOCKS
DOCKS has undergone to a series of tests in order to verify its capabilities of DOCKS to tolerate
relative misalignments and to self-align DOCKS-A to DOCKS-B. To this purpose, an ad-hoc
experimental setup has been developed. DOCKS-A is mounted on the end-effector of a robotic
arm, which has the purpose of moving DOCKS-A with an accuracy of 2 mm [6]. DOCKS-B is
mounted on a frame DOCKS-B on a frame that blocks all the movements, except for the degree
of freedom under tests for the self-alignment, as illustrated in Fig. 4.
At the beginning of the tests, the zero position has been defined as the perfect alignment
between DOCKS-A and B. The tests have been conducted as follows: (1) a displacement has been
imposed on DOCKS-B, (2) DOCKS-A has been moved along a linear trajectory to the zero
position forcing the alignment between the parts. The test is considered successful if the claws
close properly on the rim of the drogue without any residual displacement.
Figure 3: Measurement of the Time-of-Flight sensors.
Aeronautics and Astronautics - AIDAA XXVII International Congress Materials Research Forum LLC
Materials Research Proceedings 37 (2023) 504-507 https://doi.org/10.21741/9781644902813-110
507
The results of the tests proved the capability of DOCKS to manage misalignment of 8.0 mm
along the y and z axes, 9.0 deg around the yaw and pitch axes, and 10 deg around the roll axis.
Conclusions
This paper presents a brief description of an autonomous docking system, since it is equipped with
(1) a set of sensors that, whith a Kalman filter, are able to estimate the relative pose of the target;
(2) three actuators and an electromagnet to execute the soft and the hard docking; and (3) an
integrated computer that applies the required algorithms to manage the sensors and actuators.
In addition, DOCKS has been designed to manage misalignment between DOCKS-A and
DOCKS-B. To this aim, its centring cone with three features matches its counterpart on the target
and forces the alignment between the parts. The test executed on DOCKS, proved that it is able to
tolerate misalignment that are five to eight times the estimation errors of the sensors.
References
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Figure 4: the experimental setups for the misalignment along the y axis, around the roll axis and
around the yaw axis.
