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1 American Institute of Aeronautics and Astronautics

Visiting Vehicle Radio Frequency Ranging System Performance in International Space Station

Multipath Environment

Dr. Shian U. Hwu1 Barrios Technology Inc., Houston, TX 77058

Kanishka Desilva2 Jacobs Sverdrup, Houston, TX 77058

Matthew Upanavage3 ERC Inc., Houston, TX 77058

Catherine C. Sham4 NASA Johnson Space Center, Houston, TX 77058

Quin D. Kroll 5 NASA Johnson Space Center, Houston, TX 77058

A multipath analysis is performed for a visiting space vehicle Radio Frequency (RF) ranging system in the International Space Station (ISS) multipath environment. The computational electromagnetic modeling technique is applied to compute the multipath signal strength and time delay which are required to determine the ranging error at various locations along the visiting vehicle approach trajectory. Various ISS flight configurations are investigated for the visiting vehicle approach trajectory. Multipath is shown to be mild along the nominal visiting vehicle trajectory and nominal ISS flight configurations. However, significant RF ranging errors are observed with different ISS flight configurations and visiting vehicle rendezvous trajectories. Careful planning and quantitative assessment for a visiting vehicle RF ranging system operation in the Space Station multipath environments are necessary for successful system operations and mission safety.

Nomenclature dB = decibel m = meters Mcps = million chips per second ns = nanoseconds

Acronyms CEM = Computational Electromagnetic Modeling GTD = Geometrical Theory of Diffraction IP = International Partners ISS = International Space Station JEM = Japanese Experiment Module PN = Pseudorandom Noise RF = Radio Frequency SM = Service Module

1 Sr. Engineering Specialist, Avionics Systems Analysis Section, JE-6WA, and AIAA Senior Member. 2 Project Manager, Avionics Systems Analysis Section, JE-6WA. 3 Engineer, Avionics Systems Analysis Section, JE-6WA. 4 Branch Chief, Systems Evaluation & Verification Branch, EV7. 5 Communication Systems Simulation Lab Manager, Systems Evaluation & Verification Branch, EV7.

AIAA Guidance, Navigation and Control Conference and Exhibit18 - 21 August 2008, Honolulu, Hawaii

AIAA 2008-7269

This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.


I. Introduction ANY visiting space vehicles to the International Space Station (ISS) are equipped with a Radio Frequency (RF) ranging system. There are concerns that the Space Station multipath environment may degrade the

visiting vehicle RF ranging system measurement accuracy. The ranging errors could potentially be significant because the Space Station is a very large and complex space vehicle and can cause a severe multipath environment for the visiting vehicle during rendezvous. Typically, a high-gain antenna with narrow beamwidth performs better in a multipath environment. The antenna boresight is pointed at the target and the multipath signals are mostly arrived off the antenna boresight and will be attenuated by the narrow beam antenna pattern. However, a high-gain antenna with narrower beam provides smaller coverage area than a low-gain Omni-type antenna with wider beam. The RF ranging system typically uses Omni-type antennas for better spherical coverage and may not provide effective antenna gain discrimination against multipath signals.

The RF ranging system hardware experimental measurements in the ISS environment are very difficult due to the size and complexity of the Space Station. The computer simulations are practical and efficient for assessing the RF ranging system measurement errors due to the ISS multipath environment. Calculations of ranging error due to multipath require determining multipath to direct signal power ratios. The computational electromagnetic modeling tool computes the multipath signals by knowing the reflection coefficients at specular reflection points and the antenna gain pattern along the signal traveling direction. At a given location along the rendezvous trajectories (a) the multipath to direct signal ratio and (b) the multipath signal delay are computed. The ranging error is then estimated from a set of curves with parameters of (a) and (b) for the RF ranging system receiver.

Bello and Boardman1 analyzed the RF ranging error due to multipath for an aircraft using satellite-based air traffic control systems. The results indicate that the multipath is shown to cause larger ranging errors than previously indicated. The multipath signals were estimated analytically without the modeling of the actual aircraft structure. The computational electromagnetic modeling (CEM) techniques were not used for the multipath signal computations due to the computer resource limitation at the time. Sutton et al,2 analyzed multipath and ranging experiment results at L-band with analytical calculations for a KC-135 aircraft RF ranging system. The reflections off the ocean surface were analyzed.

In this study, we analyzed the RF ranging error due to multipath for a rendezvous visiting space vehicle in the ISS multipath environment. The multipath signals reflected off the ISS structures were computed rigorously with the actual ISS structure model for various ISS flight configurations. The rigorous CEM technique was applied in the multipath signal computations.

II. ISS Multipath Environment The International Partner’s (IP’s) visiting vehicle S-band communication and ranging system will be used to

communicate with ISS and provide range and range rate measurements during rendezvous to the ISS. The ISS has fixed (modules) and moving (solar and radiator panels) metallic structures that can cause multipath

in terms of signal reflections and diffractions (Figure 1). The ISS multipath environment for a rendezvous visiting vehicle is location and time dependent due to solar panels and thermal radiator rotation with time.

The RF range measurement on a visiting vehicle is based on a pseudorandom noise (PN) sequence. A PN code is coherently transmitted by modulating a carrier at S-Band. The receiver turns around the PN code and retransmits the signal. The transmitter computes the round-trip delay from the PN epoch. The PN code chip rate is 3 million chips per sec (Mcps). The chip length is about 333 nanoseconds (ns) and is equivalent to 100 meters (m) path length traveled by an RF signal. The multipath signals reflected off the faraway objects with a longer time delay than the chip time interval of 333 ns will not be seen by the RF ranging system receiver due to the local PN dispreading.

The maximum distance from the ISS ranging system S-Band antenna to the Space Station far-out structure is about 80 m. The maximum time delay of the single bounce multipath signal to the direct signal is 267 ns which is shorter than the one PN chip time interval of 333 ns. The multipath signals reflected off the Space Station structure are not distinguishable by the receiver within one chip interval. Consequently, all the single-bounce multipath signals and multiple bounces with less than 100 m additional path lengths will not be rejected by the receiver, thus they are indistinguishable from the direct signal and will be seen by the RF ranging receiver.

M


III. ISS Multipath Modeling Computer simulations are practical and efficient for assessing the RF ranging system measurement errors in the

ISS multipath environment due to the size and complexity of the Space Station. The computational technique has to be capable of providing multipath signal level and time delay information to allow calculation of ranging error along a visiting space vehicle rendezvous trajectory. Based on the RF ranging system operating frequency and electrical size of the Space Station structures, the Geometrical Theory of Diffraction (GTD)3-5 is used for the RF ranging error assessment. The GTD is a versatile electromagnetic modeling technique for electrically large structures in terms of wavelengths. The GTD is an effective and efficient method at S-band frequencies to compute the signal strength and the time delay of the multipath signals reflected off the Space Station structure (Figure 2).

In GTD simulations, the multipath to direct signal power ratios can be computed by taking into account the direction of direct and indirect signals arriving with respect to the antenna gain pattern. The reflection coefficients at various specular reflection surfaces are computed with respect to the geometrical shape and material of the reflecting surface in the multipath signal computations. The time delay and power spectra of the arrival multipath signals can be computed along visiting vehicle rendezvous trajectories for various ISS flight configurations.

At high frequencies, the scattering fields depend on the electrical and geometrical properties of the scatterers in the immediate neighborhood of the point of reflection and diffraction. In the field computation, the incident, reflected, and diffracted fields are determined by the field incident on the reflection or diffraction point multiplied by a dyadic reflection or diffraction coefficient, a spreading factor, and a phase term. The reflected and diffracted field at a field point r’, Er,d(r’), in general have the following form 6-9:

Er,d (r’) = Ei(r) Dr,d Ar,d(s) e-jks . (1) where Ei(r) is the field incident on the reflection or diffraction point r, Dr,d is a dyadic reflection (Dr) or diffraction (Dd) coefficient, Ar,d(s) is a spreading factor for reflection or diffraction, and s is the distance from the reflection or diffraction point r to the field point r’. Dr,d and Ar,d can be found from the geometry of the structure at reflection or diffraction point r and the properties of the incident wave there.

The RF signal propagation ray paths through the environment from the transmitter antenna to the receiver antenna, including the interactions with surrounding Space Station structures by each ray are obtained through the ray tracing algorithm. The geometrical data for each path consists of the coordinates of the endpoints of one or more connected line segments. The signal interaction with surrounding ISS environment causes a reflection, or diffraction along the RF signal propagation ray paths. The travel time for each propagation path can be calculated by the total geometrical path length and the signal propagation velocity which is the speed of light in free space.

Figure 1. ISS multipath environment.


IV. Simulation Results

A. Nominal Case Figure 3 shows the multipath signal ray tracing for a nominal visiting vehicle rendezvous trajectory. The colors

show the power levels of the multipath signals. The red color represents the strongest signal levels. The yellow represents the medium signal levels. The green represents the least signal levels. Different Space Station structural elements cause multipath at different range distances along the trajectory. Each structure element, depending on its position and orientation, contributes to multipath only at certain range segment along the rendezvous trajectory. When the solar panel is rotated at different angle, the multipath affective area may change to a different region, as shown in Figure 4. Thus, the Space Station multipath environment for a visiting vehicle is a dynamic problem.

When the visiting vehicle is at a close range from the Space Station, the strong reflections are mainly from the ISS truss segment and the solar panel on the Russian segment modules. The multipath signal ray tracing plot at close range is shown in Figure 5. The reflections off a flat panel are typically stronger than off a curved surface such

Figure 3. The multipath signal ray tracing for a nominal visiting vehicle along the trajectory.

Reflector (Plate or Cylinder)

Transmitting Antenna

Ed

Er

Ei

E=Ei+Er+Ed

Figure 2. The GTD field computation.


as the ISS Destiny module (US Lab) due to the energy spreading off a curved surface. Thus, a structure element with a large flat surface, such as solar panels, thermal radiators and truss segment faces, is more of a concern than cylindrical structures, such as various modules.

Figure 6 shows the power spectrum vs. time delay plot for the visiting vehicle at a location along the nominal

trajectory about 30 m from the ISS Japanese Experiment Module (JEM) module. At this location, strong multipath signals exist due to reflections from the truss segment front face, as shown in Figure 5. The primary multipath signal is only 11 decibel (dB) weaker than the direct signal and delayed by 63 ns due to the 21 m additional propagation path. The range error is about 2 m, based on the ranging system transponder susceptibility data. This case demonstrated that mild multipath and range errors exist for nominal visiting vehicle rendezvous trajectory and nominal Space Station flight configurations.

Figure 4. The reflections off the solar panel on the Russian module affect different locations with changing panel positions.

Figure 5. Multipath signal ray tracing plot, at close range along a nominal visiting vehicle trajectory, shows the reflections off the truss and the solar panel on the Russian modules.


Figure 7 shows the power spectrum vs. time delay plot for the visiting vehicle at a location along the nominal trajectory about 50 m from the ISS JEM module. At this location, there is no strong multipath signal. This location is outside the ISS truss segment front face reflection region. There are no significant multipaths from other ISS reflection objects at this location. The multipath signals are about 25 dB weaker than the direct signal. The range error is insignificant based on the RF ranging system transponder susceptibility data.

Figure 7. The power spectrum vs. time delay plot at location 50 m from the JEM module.

Figure 6. The power spectrum vs. time delay plot at a location 30 m from the JEM module.


Only mild range errors were observed along most of the visiting vehicle nominal rendezvous trajectory as the multipath signals were relatively weak compared to the direct signal. The following factors can be attributed to the mild multipath and ranging error for the nominal visiting vehicle rendezvous trajectory and nominal Space Station flight configuration. The ISS JEM module antenna pointing direction is designed such that there are no large Space Station structures in the direction of the antenna boresight to produce multipath strong enough to cause large range errors. The ISS main solar arrays are located above the ISS JEM module aft antenna and farther away from the antenna boresight. The solar panels were seen by the ISS JEM module aft antenna back lobes, which are much weaker than the boresight radiation. The multipath signals reflected off the Space Station structure have longer propagation path lengths, thus the reduced multipath signal strength relative to the direct signal.

Previous IP and NASA studies 10,11 indicate the Russian Service Module (SM) solar panels may cause blockage to the visiting vehicle S-band communication antennas at long range along the rendezvous trajectory. The JEM module is in the front side of the Space Station. The line-of-sight can be blocked by the Russian SM solar panels when the visiting vehicle is aft of the Space Station. To avoid interruption of the visiting vehicle ranging system communication link, the SM solar panels are recommended to park at a horizontal position.10,11 Similarly, the RF range errors in long range distances may be expected due to the reflections off the Russian SM solar panels if the panels are not parked at a horizontal position. If the solar panels are to rotate to different positions with time, the multipath affected area may change along the rendezvous trajectory.

B. Hypothetical Case A hypothetical scenario was created to demonstrate that a severe range error can exist in ISS multipath

environment. This extreme case includes a hypothetical visiting vehicle rendezvous trajectory, antenna patterns, and antenna locations. An Omni directional antenna was placed at the ISS JEM module forward antenna location. A receiver route, also using an Omni directional antenna, was place along the +X axis. The ISS main solar arrays were set with α angle of 0° and β angles of 0° which is parallel to the ISS JEM module forward antenna pointing direction. The multipath signal ray tracing plot, as shown in Figure 8, indicates strong reflections occur from the Space Station main solar panels.

Figure 8. Ray tracing plot, along a hypothetical trajectory of an extreme case, shows the reflections off the main solar panels.


Figure 9 shows the power spectrum vs. time delay plot at a location about 40 m from the ISS JEM module. At

this location, strong multipath signals exist due to reflections off the ISS main solar panels, as shown in Figure 8. The primary multipath signal is only 3.6 dB weaker than the direct signal and delay by 72 ns due to the longer propagation path. The range error is about 16 m. This hypothetical case demonstrated that the severe multipath and large range errors are possible if the right condition exists in ISS environment.

Signal Time Delay at a Location with Strong Multipath Signals and Large Range Error

-70

-60

-50

-40

-30

100 200 300 400 500Time of Arrival (ns)

Pow

er (d

Bm

)

Figure 9. The power spectrum vs. time delay plot at location 40 m from the JEM module.

Signal Time Delay at a Location with Weak Multipath Signals and Small Range Error

-110

-90

-70

-50

-30

100 200 300 400 500 600 700Time Delay (ns)

Pow

er (d

Bm

)

Figure 10. The power spectrum vs. time delay plot at 60 m from the JEM module.

The multipath signals are at least 30 dB weaker than the direct signal.


Figure 10 shows the power spectrum vs. time delay plot at a location about 60 m from the ISS JEM module. At

this location, there is no strong multipath signal. This location is outside the ISS main solar panel reflection region. There are no significant multipaths from other ISS reflection objects. The multipath signals are at least 30 dB weaker than the direct signal. The range error is neglible.

V. Conclusion Ray tracing and geometrical theory of diffraction have been applied to model the multipath in the Space Station

environment for a visiting space vehicle. The RF ranging errors were analyzed for the visiting vehicle travel along the rendezvous trajectory.

Results reveal that ranging errors are larger while the visiting vehicle is within the reflection zone of large Space Station objects such as solar panels and front face of the truss segment. This fact is due to the strong specular reflections off a large flat surface. The range error is larger when the visiting vehicle is inside a strong reflection zone than outside of the reflection zone. The results from this study point out the influence of the Space Station solar panel, and thermal radiator positions and orientations during rendezvous and docking, and are useful for the visiting vehicle mission planning.

Based on the visiting vehicle ranging system antenna location and pointing direction, visiting vehicle nominal R-bar approach trajectory, orientation of the Space Station solar panels, the visiting vehicle ranging system will not encounter severe multipath and range error during rendezvous. Computer simulations indicating mild range errors are expected for the nominal visiting vehicle trajectory and nominal solar panel configuration. However, significant RF ranging errors were observed in the extreme case study with different ISS flight configurations and visiting vehicle rendezvous trajectories. Careful planning and quantitative assessment for a visiting vehicle RF ranging system operation in the Space Station multipath environments are necessary for successful system operations and mission safety.

Typically, the signal variations due to multipath along a trajectory decrease with increasing range to the Space Station. This indicates the multipath signals are weaker relative to the direct signal at larger distances from the ISS structures. Multipath will only be present at a certain area in the major reflection zones and will reduce and disappear with the visiting vehicle moving along the trajectory.

Although the visiting vehicle will not encounter severe multipath and range error for the nominal visiting vehicle approach trajectory, risk can be further minimized with Space Station solar panels and thermal radiators at specific positions. To minimize the multipath from the reflections off the solar panels, it is recommended that solar panels for both U.S. and Russian segments be temporally parked at positions such that the panels are ‘edge-on’ to the RF ranging system antenna during visiting vehicle rendezvous. This is particularly important for the Russian SM solar panels. Previous IP and NASA/Johnson Space Center studies have shown that the SM solar panels can cause interruption of the visiting vehicle ranging system communication links due to blockage. As a result, the SM solar panels are recommended to be parked at a horizontal position to preserve the ISS-visiting vehicle ranging system communication links and minimize the ranging error during visiting vehicle rendezvous and docking.

Acknowledgment The study described in this paper was carried out under contracts with Johnson Space Center of the National

Aeronautics and Space Administration (NASA). The review and comments by William X. Culpepper of NASA/JSC were appreciated.

References 1 Bello, Phillip A. and Boardman, Charles J., "Effect of Multipath on Ranging Error for an Airplane-Satellite Link," IEEE Transactions On Communications, Vol. Com-21, No. 5, p. 564-576, May 1973. 2 Sutton, Robert W., Schroeder, Edgar H., Thompson, Allan D., and Wilson, Stephen G., "Satellite-Aircraft Multipath and Ranging Experiment Results at L Band," IEEE Transactions On Communications, May 1973. 3 Kouyoumjian, R. and Pathak, P., “A uniform geometrical theory of diffraction for an edge in a perfectly conducting surface,” Proc. IEEE, Vol. 62, pp. 1448-1461, 1974.


4 Marhefka, R. and Silvestro, J., “Near zone – basic scattering code user’s manual with space station applications,” NASA CR-181944, Dec. 1989. 5 Marhefka, R., "The other NEC (BSC) an asymptotic complement," Antennas and Propagation Society Symposium, 2004. IEEE, Vol. 3, pp. 2911-2914, 20-25 June 2004. 6 Luebbers, R. "Propagation prediction for hilly terrain using GTD wedge diffraction," IEEE Transactions on Antennas and Propagation, Vol. 32, No. 9, pp. 951- 955, Sep 1984. 7 Yang, C. Y., Wu, B. C., and Ko, C. J., "A Ray-Tracing Method for Modeling Indoor Wave Propagation and Penetration," IEEE Transactions on Antennas and Propagation, Vol. 46, no. 6, June 1998, pp. 907-919. 8 Hwu, S.U., Loh, Y.C., Sham, C.C., “Propagation Characteristics of International Space Station Wireless Local Area Network,” Proceedings of IEEE Radio and Wireless Conference, pp. 407-410, Sep. 2004. 9 Hwu, S.U., Upanavage, M., Sham, C.C., “Lunar Surface Propagation Modeling and Effects on Communications,” Proceedings of the 26th AIAA International Communications Satellite Systems Conference (ICSSC), June 2008. 10 "ISS/Visiting Vehicle Proximity Communication Analysis Progress," Report at Visiting Vehicle CDR#2 C&T Splinter Telecon, Feb. 23, 2006. 11 Hwu, S.U., "ISS GTD Model and Antenna Pattern Test Case Analysis" NASA/JSC Report, EV7-06-4127, March 13, 2006.

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