iac paper final6 - dlr...program funded by the german space agency dlr and the swedish ssc, which...

61st International Astronautical Congress, Prague, CZ. Copyright ©2010 by the International Astronautical Federation. All rights reserved.

IAC-10-E2.1.10 Page 1 of 10

IAC-10-E2.1.10

IN-FLIGHT VERIFICATION OF A NOVEL METHOD FOR THE TRACKING OF ROCKETS

K. Makowka

Technische Universität München, Germany, [email protected]

S. Jäkel

Technische Universität München, Germany, [email protected]

This paper presents a new beam tracking method using a small and lightweight antenna for the tracking of

moving objects. The method is based on conical motion of an antenna around the vector to the anticipated target

and the measurement and processing of the received signal strength. Hence, no active and expensive systems as

radar support are required, as the tracking is performed directly to the received RF signal. The tracking algorithm

principle is based on movement towards the position of maximum signal strength. The mathematical background

and mechanical constraints are investigated. Advantages and disadvantages of this method compared to other

types of beam tracking are analyzed in this paper. The developed system has been successfully tested on the

REXUS 7 and 8 sounding rockets as a part of the Verification of Concepts for Tracking and Orientation

(VECTOR) experiment. Results gathered during simulation and testing are compared to recorded data from both

rocket flights.

I. BACKGROUND

The developed tracking software was an essential

part of the VECTOR experiment1 within the REXUS

sounding rocket program. REXUS is an experiment

program funded by the German space agency DLR and

the Swedish SSC, which gives university students the

possibility to build their own experiments on sounding

rockets. Those rockets reach altitudes of approximately

100km including a short microgravity phase2. The

launches take place at Esrange Space Center near

Kiruna in northern Sweden. VECTOR stands for

Verification of Concepts for Tracking and Orientation

and is an experiment by a student team of the

Technische Universität München. In addition to the

tracking experiment part, VECTOR had the aim to

verify a new real-time video compression method using

an FPGA by taking pictures from the upper atmosphere,

compressing and transmitting them to the VECTOR

ground station, which consisted of the receiving

Lightweight Inter-Satellite Antenna (LISA)3 and the

VECTOR ground station network4.

The intent of the developed Beam Tracking

Algorithm (BTA) is the autonomous repositioning of

the small S-band antenna LISA towards the moving

sounding rocket, while simultaneously ensuring a stable

RF connection between the rocket-borne radio

transmitter and LISA. This small planar array antenna

(see Fig. 3) was originally built to be used as a

lightweight (m=5kg) and therefore agile antenna to be

utilized for on-orbit servicing (OOS) scenarios. Such

scenarios give the opportunity to enhance the time of

operation of important scientific or military satellites by

repairing or replenishing them on orbit within a

servicing mission5. OOS scenarios include the use of a

robotic manipulator attached to the servicer satellite in

order to operate on the client. While operation utilizing

enhanced autonomous algorithms may be used in most

of the cases, human supervision and eventually human,

low-level manual control via a respective telepresence

system might be needed. Such telerobotic systems

require long-duration and stable link conditions in order

to allow the human operator on ground to steer the

teleoperator in space with a good transparency of the

control system and therefore high performance utilizing

a multimodal (optic, haptic, acoustic etc.) feedback

system6,7.

Since not only the movements of the robotic

teleoperator but also unforeseen, coincidental

interactions between the manipulator and the client may

occur, high and partly unpredictable angular velocities

of the satellite platform are induced. Therefore, the use

of an autonomous beam tracking system in order to

accurately point the antenna and ensure a stable

communication link is highly required8.

Several systems have been investigated and are

already used on satellite missions such as Phased Array

(PA)9 and monopulse Autotracking systems10. While PA

antenna systems steer the antenna main lobe

electronically and therefore allow a very fast scanning

frequency, their scanning range is limited to 60° in

every direction. Monopulse tracking systems on the

other hand give the possibility for a very exact, fine

scale tracking but also need a lot of supplementary and

cost intensive equipment.

The presented tracking method within this

publication is based on a mechanical conical scanning

and therefore does not require any additional RF

hardware as it is solely using the currently measured

signal strength which is always available when tracking

objects with an on-board transmitter. The purpose of the

VECTOR experiment was to deal as a technology

demonstrator, investigating the feasibility of such a

tracking method with a small and lightweight antenna

system such as LISA. The tracking algorithm was tested

during the REXUS 7 sounding rocket flight without data

transmission and finally demonstrated with

simultaneous data downlink during the REXUS 8 flight.

61st International Astronautical Congress, Prague, CZ. Copyright ©2010 by the International Astronautical Federation. All rights reserved.

IAC-10-E2.1.10 Page 2 of 10

II. ALGORITHM PRINCIPLE

The tracking algorithm is based on conical motion of

the antenna around the vector to its anticipated target

while measuring and processing the signal strength

distribution on the cone. By calculating the direction of

maximal signal strength, the center position of the next

scanning cone is determined. The resulting antenna

motion, while performing the presented tracking

algorithm, can be described as a spiral movement and is

therefore referred to as Dynamic Spiral Tracking (DST).

Fig. 1: DST scanning cone with parameters and

reference coordinate systems

Fig. 1 depicts exemplarily such a DST scanning

cone introducing the four parameters describing it. The

first two parameters are azimuth und elevation of the

DST cone center, described in the shown North-East-Up

(NEU) coordinate system. The NEU frame is inertially

fixed to the earth’s surface, while the UVW frame turns,

following the attitude of the scanning cone with axis U

being collinear to the cones symmetry axis and pointing

into the anticipated target direction. The two other

parameters are the opening angle αDST and the angular

velocity of the conical movement ωDST=2πfDST .

Fig. 2: DST scanning cone parameters and transition

While the antenna performs the conical DST motion,

the received signal strength ΩV is correlated to the

prevailing antenna alignment direction, as shown in Fig.

2. After the scanning cone is completed, the direction

φDST is determined based on the signal strength

measurement. Applying an angular step size ζDST,k in the

previously calculated direction yields the center position

of the next DST scanning cone, represented by the

angles ψDST,k+1 and θDST,k+1.

III. PARAMETERS AFFECTING

PERFORMANCE

There are three main parameters that can be

modified within this tracking concept: The DST

frequency fDST, the DST cone opening angle αDST and the

step size ζDST. αDST has to be large enough to provide a

sufficient signal fluctuation due to the DST scanning

motion in order to have the ability to unambiguously

distinguish the maximum signal strength from the rest

of the signal. On the other hand αDST should be kept

minimal since it increases the angular distance to the

target and thereby reduces link quality. In addition, a

high αDST provides high mechanical loads on the

antenna steering motors, which should be avoided. The

opening angle was set to 6° in the implemented tracking

software, which is a satisfactory compromise between a

reliable signal fluctuation, low mechanical loads and a

good link stability regarding the antenna’s 3dB angle of

8°.

The step size ζDST is the angular distance between the

center of a given DST scanning cone and the center of

the next scanning cone. ζDST constitutes the most

important parameter within the given experiment setup

because the other parameters are highly restricted by

maximum motor performance. In addition, ζDST affects

the tracking quality more severely than the other two

parameters. Even if the direction of antenna re-

alignment after a scanning cone is ideal, an

inappropriate step size would cause either a too large or

too small jump in this direction. Therefore it is plausible

to choose the step size similar to the expected angular

velocity of the target. In fact, it is advisable to set the

step size to slightly larger values than the target’s

angular velocity since it has to be considered that the

tracking algorithm will not provide a perfect alignment

after each scanning cone. Within the given tracking

software the step size was predefined as a function of

time where this function was partly gained by extracting

the angular velocities from past REXUS sounding

rocket flights and mainly by parameter studies in the

tracking simulation environment.

Simulations with a dynamically adapted step size

reacting to the present signal change have been

conducted but appeared too unstable for this application.

Since only one rocket launch had been initially planned,

the use of a dynamically adapted step size control

constituted an unjustifiable risk. However, the final

objective is to obtain a preferably autonomous tracking

system, therefore the assumption of an approximate

flight path constitutes a simplification that will be

eliminated from the tracking system in future

developments.

The time span the antenna needs to complete one

scanning cone is determined by the DST frequency fDST.

αDST

ωDST

ψψψψDST

ϴϴϴϴDST

U

V

W

Up

East

North

ζDST

αDST

(ψDST, k, ϴϴϴϴDST, k)

φDST(t)

Ωv(φDST)

(ψNEU(φDST), ϴϴϴϴNEU(φDST))

(ψDST, k+1, ϴϴϴϴDST, k+1)

61

A high

reducing the necessary step size

performed scanning cones have the advantage that the

target

completed

realignment will not always be ideal due to errors in

measurement and the capability of the us

equipment

and correlated positioning corrections

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is

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two degree of freedom steering mechanism including

two

Fig.

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Fig.

61

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st International Astronautical Congress, Prague, CZ. Copyright ©2010 by the International Astronautical Federation. All rights r

A high



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completed



equipment


adequate antenna

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International Astronautical Congress, Prague, CZ. Copyright ©2010 by the International Astronautical Federation. All rights r

A high



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International Astronautical Congress, Prague, CZ. Copyright ©2010 by the International Astronautical Federation. All rights rInternational Astronautical Congress, Prague, CZ. Copyright ©2010 by the International Astronautical Federation. All rights rInternational Astronautical Congress, Prague, CZ. Copyright ©2010 by the International Astronautical Federation. All rights rInternational Astronautical Congress, Prague, CZ. Copyright ©2010 by the International Astronautical Federation. All rights rInternational Astronautical Congress, Prague, CZ. Copyright ©2010 by the International Astronautical Federation. All rights r

effect on the antenna’s motion in this

fraction of the vertical azimuth movement is transferred

to the antennas change in azimuth alignment direction,

depending on the current elevation of the DST cone

center.

ψ

θ

transformation and yield the actual azimuth and

elevation motor positions as a function of the current

position on the DST circle

Fig.

significant increase of the actual azimuth motor

movement with rising DST center elevation

be seen in

elevation shaft positions

with equal DST

elevation levels. While the azimuth movement

o

the time to complete one circle stays the s

in increasing

frequency is desirable, since it improves the algorithms

ability to correct alignment errors

previous

DST frequency must be reduced at high eleva

Otherwise the maximum motor loads would be

exceeded.

close to the tolerable limits, the motors

endure torque without producing stepping mistakes

decreases significantly. Those stepping mistak






center.

NEUψ

NEUθ




Fig.



be seen in


with equal DST


obviously increases with a larger DST center elevation,


in increasing



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center.

NEU

NEU




Fig.



be seen in


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previous



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center.

NEU

NEU

Eqs




Fig.

for

DST center elevations

This coordinate transformation results in a



be seen in


with equal DST




in increasing

Regarding the tracking algorithm, a high DST



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center.

=

=

Eqs




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for





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Eqs




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depending on the current elevation of th

iac paper final6 - dlr...program funded by the german space agency dlr and the swedish ssc, which...

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