GNSS in Space: Part 1

Formation Flying Radio Frequency Missions, Techniques, and Technology

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Formation flying (FF) creates large spaceborne instruments by using several smaller satellites in close formation. The concept requires very accurate relative positioning and orientation of the spacecrafts, the complexity of which is largely outweighed by the enormous benefit of the extended instrument size compared to traditional one-satellite configurations.

The easiest way to perform formation flying with relative attitude and positioning in space is to use signals broadcast by GNSS satellites. Yet this ideal configuration, which could enable a relative positioning of better than one centimeter in certain cases, is limited to formation flying mission in low earth orbit (LEO).

Such low-altitude operations can exploit the full visibility of the GNSS constellations, which is needed to perform the integer carrier phase ambiguity resolution required to achieve centimeter-level accuracies. Use of GNSS is the easiest way because most LEO satellites are already provided with a GPS or GLONASS receiver for orbit and time determination.

Thanks to GNSS constellations, formation flying can be made at higher altitudes with a perigee of up to 25,000 kilometers, if GNSS receivers equipped with low acquisition and tracking thresholds are selected. Such receivers have tight coupling between the signal processing and the onboard orbital Kalman filter delivering pseudovelocity and pseudorange aiding to the open or closed delay locked loops and phase locked loops.

Even in this case, however, the relative positioning accuracies can be degraded by up to a few meters, or even more sometimes, at a high-altitude orbital apogee.

As a result of these GNSS accuracy limitations, a dedicated formation flying radio frequency (RF) technique is needed that is accurate and equipped with omnidirectional features emerged for high and very high altitude orbital missions.

But the use of GNSS-like signals and techniques appears to still be advantageous, because they allow cost effective accurate measurements, thanks to the widespread use of GNSS techniques, even for spacecraft navigation and synchronization.

The first part of this two-part column will describe the PRISMA, PROBA-3, and Simbol-X missions and provide an overview of other future FF missions. Part 2 will discuss those missions’ positioning, orientation, and metrological requirements, focusing on the formation flying RF (FFRF) techniques and instrumentation.

Institutional Roles
Mastery of formation flying is an area of priority for the French Space Agency (Centre National d’Etudes Spatiales or CNES) and the European Space Agency (ESA) and is driving the agencies’ long-term investment in the development of associated technologies and skills in cooperation with other European partners.

The goal of this continuous effort is to prepare for future FF missions of which the CNES Simbol-X project and ESA’s PROBA-3, Darwin, and Xeus projects are representative examples.

Mainly within this European framework, CNES and ESA have collaborated and contributed through R&D activities to the preliminary design and implementation of a new sensor for coarse metrology—the (FFRF) metrology subsystem. This RF-based sensor, for which breadboard development was initiated by ESA in 2001, is based partly on existing spaceborne GPS technology.

Proposed by CNES in an early version in 1991 for the Hermes space plane project, the FFRF equipment was initially designed and developed by a private manufacturer. ESA and CNES officially selected the FFRF equipment as a coarse metrology sensor, FFRF being mandatory for first-stage formation acquisition on all future European non-LEO formation flying missions.

This FFRF generic technology developed by ESA, CNES and CDTI will be validated onboard the PRISMA Swedish mission, and around 2012 it will be used for PROBA-3, an ESA mission demonstrating FF capabilities and making scientific observations. In 2013, it will be onboard Simbol-X, a multilateral CNES mission implementing an X-ray telescope of two satellites.

The PRISMA Mission
To validate FFRF in flight, CNES will participate in the experimental PRISMA mission, a technology test bed for formation flying and rendezvous (RdV) of two LEO microsatellites undertaken by the Swedish National Space Board (SNSB). As the first European mission to demonstrate FF, PRISMA is an invaluable opportunity for CNES to acquire valuable FF experience as early as 2009.

The French contribution to PRISMA—Formation Flying In-Orbit Ranging Demonstration (FFIORD) —consists of implementing the FFRF subsystem on the two PRISMA satellites as a passenger experiment. The FFRF procurement is a partnership between CNES and Spain’s Centre for the Development of Industrial Technology (CDTI).

In addition to in-orbit validation of the sensor, FFIORD will include closed-loop operations with FFRF, thus allowing genuine autonomous formation flying scenarios and associated guidance, navigation and control (GNC) algorithms to be tested in real-life conditions.

The PRISMA satellites have arrived from Sweden to the test facilities in Toulouse, France. Testing in the vaccuum-solar chamber took place in late November 2008. Launch of PRISMA is expected during the summer 2009 as a secondary payload into a sun-synchronous LEO orbit of about 700 kilometers.

Expected duration of the mission is approximately eight months. All flight operations will be controlled by the Swedish Space Corporation (SSC) via the Kiruna ground station.

The mission will consist of two spacecraft, shown in Figure 1 (at the top of this article): the Main spacecraft of 140 kilograms with full three-axis reaction wheel–based attitude control and three-axis delta-V capability, and a second simplified Target spacecraft of 40 kilograms with coarse three-axis attitude control, based on magnetometers, sun sensors, and magnetic torquers. Various sensors, depending on the experiment and the satellite distance, will measure the relative positions of the space vehicles, using the following:

• Differential GPS. The DGPS system provided by DLR (German Aerospace Center) is the primary relative positioning measurement system on PRISMA (made possible by the LEO nature of this mission). Experiments will mainly be carried out for intersatellite distances greater than 30 meters. The system is based on a fully redundant set of 12-channel, single-frequency Phoenix GPS receivers developed by DLR and antennas on each spacecraft.

• Visual-based sensor (VBS). Based on a star tracker used in many microsatellite missions, this sensor provided by the Technical University of Denmark (DTU) will be used to identify the Target spacecraft as a nonstellar object at distances up to 500 kilometers and to track Target down to very close range, typically 10 meters during a sequence of autonomously scheduled approach maneuvers.

• The RF metrology subsystem. Designed to handle first stage omnidirectional RF metrology, the FFRF provided by CNES will mainly function at intersatellite distances from 3 meters to 30 kilometers.

PRISMA Goals
The primary goals for PRISMA are technology demonstrations and maneuver experiments examining GNC and sensor technology for a family of future missions that will use RdV and/or FF. The main demonstrations are as follows:

• GNC maneuvering experiments with a high level of autonomy: autonomous formation flying, homing and rendezvous, proximity operations, and final approach and recede operation. The experiments are run mainly by SSC with important contributions from DLR.

• GPS-based navigation system experiment from DLR, which will evaluate real-time differential GPS as a sensor for autonomous formation flying

• evaluation of the VBS as a multirange, range-tracking and RdV sensor

• demonstration flight test of the FFRF subsystem in open- and closed-loop configuration.

Further, the mission also has some secondary goals mainly oriented toward testing of new technologies appropriate for future small-satellite missions. These include 1) in-flight testing of high performance green propellant (HPGP) and cold gas microthruster propulsion systems, 2) onboard software development with MATLAB/Simulink and autocode generation, and 3) validation of new ground support equipment with multisatellite support capability.

PROBA-3: The Next Step
The ESA PROBA-3 mission is foremost an FF demonstration mission and secondly am FF scientific mission. The first objective is to validate the GNC system, relative position and pointing determination systems (RF, and coarse and fine optical), and actuators. This mission will also validate formation acquisition, maneuvering, and collision avoidance algorithms as well as test different operational scenarios and formation flying architectures.

The second objective is to observe the solar corona, a luminous plasma ”atmosphere” that surrounds the sun. The total light output from the solar corona is less than one-millionth of that radiated by the disk of the sun. This enormous disparity in apparent brightness gives rise to the need to use an “occulter” to block light coming from the solar disk so as to be able to observe the corona better.

PROBA-3 comprises two independent, three-axis stabilized spacecrafts flying close to each other with the ability to accurately control the attitude and separation of the two spacecraft in a closed loop. The spacecrafts will fly in a high earth orbit (HEO) divided between periods of accurate formation flying, when payload observations will be possible, and periods of free flight.

The length of the formation control period will result from a trade-off analysis involving the amount of fuel needed to maintain the orbits when in formation. The formation control part (around the apogee) will be used to demonstrate formation flying for astronomical and scientific missions as well as to observe the solar corona.

During the perigee (portion of orbit nearest to the Earth), the spacecraft will revert to normal, gravitationally determined orbits to reduce fuel consumption, with the thrusters then being used only for collision avoidance. The perigee pass will also demonstrate formation flying configurations required for LEO earth observation missions.

PROBA-3 Guidance & Control
The relative positions of the two spacecraft are determined by the S-band RF metrology subsystem, which functions for separations between five meters and eight kilometers, with an accuracy of centimeters. Increased accuracy for separations up to 500 meters will be obtained using optical laser techniques having both coarse and fine sensors to refine the relative position measurements.

The combined system is expected to achieve a relative positioning accuracy of about 100 micrometers over a separation range of 25–500 meters. Multiple thrusters, using either cold gas or ion technology, will be used to maintain the required relative positions.

Both spacecraft will carry GPS receivers to provide timing synchronization and relative navigation during perigee passage. GPS signals can be used at least 25,000 kilometers from Earth — and even more if carrier-to-noise spectral density (C/N₀) threshold reduction techniques are used. So, the signals should be useful for at least one-third of the proposed orbit and possibly all of it. Each spacecraft will employ a star tracker for absolute attitude determination.

Concerning the observation of the solar corona, the FF control system allows the use of a two-component space system with the external occulter on one spacecraft and the optical instrument on the other spacecraft at approximately 100–150 meters apart. The stability of the formation is linked to the requirement of keeping the optical pupil plane in the shadow of the occulter.

Lateral positioning accuracy is about ±2.5millimeters and longitudinal is about ±250 millimeters. The absolute attitude of the satellite hosting the external occulter is required to be about ±40 arcseconds. With the proposed PROBA-3 arrangement, accurate measurements will likely be possible from 1.05–3.2 solar radii.

Future FF Missions Overview
A remarkably large number of formation-flying and rendevouz missions are planned over the next 10 years or so by European and other nations or groups of nations. Tables 1–3 summarize these missions as well as the probability of their being realized by 2020.

Table 4 provides a classification of the FF missions with respect to the objectives of the mission (space science, earth observation, and technology demonstrations).

Conclusion
GNSS constellations are the most practical way to perform RF formation flying in LEO, and autonomous two-way transmission of GNSS-like S-band signals is a better way to perform FFRF in HEO or within the Lagrange points. PRISMA is a unique opportunity in Europe, both technically and programmatically, to validate under real conditions the basic feature of any non-LEO future FF mission—the RF-based autonomous metrology, using GPS-C/A-like signals and techniques.

By mid-2009, an autonomous RFFF sensor shall be flying onboard the PRISMA satellites. This sensor will use GPS-like signals in S-band. Later, in 2012, the ESA PROBA-3 and CNES Simbol-X spacecrafts will demonstrate the technology in scientific missions in HEO orbit.

Additional Resources
[1] Bourga, C., et al., “Autonomous Formation Flying RF Ranging Subsystem,” Proceedings of ION GNSS 2003, Portland, Oregon, USA, September 2003
[2] Cledassou, R., Ferrando, P., “Simbol-X: An Hard X-Ray Formation Flying Mission,” Focusing Telescope in Nuclear Astrophysics Gamma Wave Workshop, Bonifacio, France, September 2005
[3] Garcia-Rodríguez, A., Formation Flight (FF) Radio-Frequency (RF) Metrology. ESA/ESTEC Technology Dossier, Issue 1.2, Noordwijk, the Netherlands July 2008
[4] Godet, J. et al., “Improving Attitude Determination of Satellites,” Internationnal Workshop on Aerospace Applications of the GPS, Breckenridge, Colorado, USA, February 2000
[5] Harr, J., et al., “The FFIORD Experiment: CNES’ RF Metrology Validation and Formation Flying Demonstration on PRISMA,” 3rd International Symposium on Formation Flying, Missions and Technologies, Noordwijk, Netherlands, April 2008
[6] Issler, J., et al., “Lessons Learned from the Use of GPS in Space; Application to the Orbital use of GALILEO,” Proceedings of ION GNSS 2008, September 2008
[7] Lestarquit, L., et al., “Autonomous Formation Flying RF Sensor Development for the PRISMA Mission,” Proceedings of ION GNSS 2006, Fort-Worth, Texas, USA, September 2006
[8] Persson, S. et al., PRISMA: An Autonomous Formation Flying Mission, Small Satellite Systems and Service Symposium, Chia Laguna, Sardinia, Italy, September 2006
[9] PROBA-3 http://www.esa.int/techresosurces/index.html

To view the figures and graphs referenced in this article, please download the PDF at the top of the page.

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Manufacturers

PRISMA is a Swedish National Space Board (SNSB) mission, undertaken as a multilateral project with additional contributions from CNES, the German DLR, and the Danish DTU. The prime contractor is the Swedish Space Corporation (SSC), responsible for design, integration, and operation of the space and ground segments, as well as implementation of in-orbit experiments involving autonomous formation flying, homing and rendezvous, and three dimensional proximity operations. It employs Phoenix GPS receivers developed by DLR that incorporates the GP4020 chip from Zarlink Semiconductors, Ottawa, Ontario, Canada.

The FFRF subsystem development is currently in phase C/D, with Thales Alenia Space-F as the prime contractor on both the subsystem and FFRF terminal level. FFRF terminals incorporate components and software of the TAS-F TOPSTAR 3000 spaceborne GPS receiver.
In turn, TAS-F is relying on the following subcontractors:

• Thales Alenia Space España (TAS-E, Spain) for development of the RF modules of the FFRF terminal (RF front end, RF transmitter section, RF receiver section)

• GMV (Spain) for development of the navigation software, including implementation of PVT algorithms

• Thales Avionics (France) for development of the FFRF terminal signal processing software

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