FIGURE 1: PRISMA satellites (SSC image)
GNSS in Space: Part 1
Formation Flying Radio Frequency Missions, Techniques, and Technology
Working Papers explore the technical and scientific themes that underpin GNSS programs and applications. This regular column is coordinated by Prof. Dr.-Ing. Günter Hein. Contact Prof. Hein at Guenter.Hein@unibw-muenchen.de
Using two or more small satellites can sometimes be better than one, especially when trying to create a large spaceborne instrument for scientific research or experiments. But coordinating the alignment of the components of such instruments on separate space vehicles requires highly accurate orientation and positioning. Carrier phase GNSS can provide such precision for spacecraft operating below the altitude of GPS satellites, and GNSS-like techniques can be employed for spacecraft operating in higher orbits.
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.
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
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:
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 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 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
Table 4 provides a classification of the FF missions with respect to the objectives of the mission (space science, earth observation, and technology demonstrations).
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.
To view the figures and graphs referenced in this article, please download the PDF at the top of the page.
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.
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