francis 4/22/05 1:08 PM Page 2 Cryosat System Guy Ratier, Richard Francis & Constantin Mavrocordatos CryoSat Project, ESA Directorate of Earth TheThe CryoSatCryoSat SystemSystem Observation, ESTEC, Noordwijk, The Netherlands Reinhold Zobl –– TheThe satellitesatellite andand itsits radarradar altimeteraltimeter Head of Earth Observation Programmes Department, ESA Directorate of Earth Observation, ESTEC, Noordwijk, The Netherlands n the preceding article Prof. Duncan Wingham has outlined the genesis of ICryoSat, its scientific objectives and its programmatic constraints. The mission objectives are characterised by the determination of small height-related changes over a three-year period. This imposes requirements on the type of measurements to be made, the physical stability of the system, control of the measurement configuration, and consistency in the data-processing system. The programmatic constraints, on the other hand, may be simply characterised as the need for a relatively short development cycle with a stringent cost ceiling. CryoSat is a fully integrated system in which all of the elements have been developed together within the programme to ensure the control needed to satisfy the mission objectives. However, in the interests of readability, the system’s description has been split over two articles: this one describing the elements that are to be launched into space, and the following one those parts that will remain on Earth. Precision Measurements from Space The CryoSat satellite is the part of the system that makes the measurements. The fundamental measure is the distance from the satellite to the Earth’s surface below, and for this a radar altimeter is used. Given the enormous success of the ERS radar altimeters over icy surfaces, this was a natural choice. CryoSat’s radar altimeter is called SIRAL, a contraction of SAR (Synthetic Artist’s impression of CryoSat at the moment of separation from the launch vehicle’s upper stage Aperture Radar) and Interferometric Radar esa bulletin 122 - may 2005 3 francis 4/22/05 1:08 PM Page 4 Earth Observation Cryosat System Artist’s impression of CryoSat in orbit, flying almost towards the Exploded view of the CryoSat spacecraft viewer. The prominent features visible are the solar arrays, giving a roof-like appearance, and the three star trackers which are mounted on the rigid structure supporting the SIRAL antennas. The structure forming the ‘nose’ at the very front of the satellite is a thermal radiator for the heat generated by the SIRAL instrument CryoSat geometry was arranged such that every orbit has enough sunlight on one or SIRAL interferometer measurement. both of the solar panels to maintain a SIRAL measures the angle of arrival of the positive energy balance onboard. echo in its own reference frame, i.e. with This assumes, of course, that the solar respect to the line joining the centres of the panels have the intrinsic capability to two antennas, the ‘baseline’. Before that generate enough power in the first place, if information can be used to identify the the Sun were shining directly on them. The exact position on the Earth, we must know requirement to fit CryoSat inside the the orientation of that baseline, and in fairing of a ‘small’ launcher put absolute order to meet the mission objectives this constraints on the size of the panels, thus measure must also be precise to within removing one of the key degrees of 30 arcseconds. This is the angle subtended freedom in this equation. That only left one by a football at a distance of 2 km. parameter to ensure sufficient power The scientific mission objectives not generation – the efficiency of the solar only defined the type and accuracy of the cells themselves. Thus CryoSat, as a low- Altimeter, and this indicates where may talk loosely of height above sea level, measurements that had to be made (which cost mission, has ended up pioneering the improvements have been made in the but in this demandingly precise application led to the payload selection), but also their use of new, high-efficiency solar cells in instrument concept. As well as a we refer to height above a ‘reference location and timing. As Prof. Wingham low Earth orbit. More about this later, but conventional mode, which offers ellipsoid’ – an exactly defined, almost explains in the previous article, this ‘where we can say right now that this choice was continuity with earlier missions, SIRAL spherical surface that closely approximates and when’ is encapsulated in the definition Kiruna in Sweden was selected. As well as played by heritage from the CHAMP and still cheaper than introducing fold-out can also operate in so-called ‘synthetic- to the shape of the Earth. To determine the of a specific orbit for CryoSat. The orbital enabling the sharing of resources with other GRACE satellites, which were designed solar panels. aperture mode’. This increases the along- satellite’s position, and thus its height inclination is 92° (and therefore on-going ESA missions, this choice against similar orbital and programmatic The two SIRAL antennas are track resolution, enabling it to more readily above this reference ellipsoid, retrograde), with a mean altitude of about resulted in manageable requirements in constraints. accommodated side-by-side near the front distinguish the narrow ‘leads’ of open measurements from some further payload 720 km, the exact altitude being defined by terms of handling the 3 to 4 consecutive So instead of a rather ponderous of the satellite. They are slightly elliptical water between sea-ice floes. Over the items are needed. the required track-repeat characteristics. orbits per day during which contact with deduction of CryoSat’s design from the in outline in order to fit within the rough terrain at the edges of the major ice CryoSat includes a DORIS Receiver, a This orbit is not one of the class known as the ground station is not possible. mission’s requirements and constraints, launcher’s fairing – which slightly sheets, this increased along-track special radio receiver that picks up signals ‘Sun-synchronous’, for which the orbital The design of the CryoSat satellite was here we will take a ‘reverse engineering’ complicates the scientific data processing, resolution is further augmented by across- from a network of more than 50 plane maintains a fixed orientation with determined, as is always the case, by a look at CryoSat to show how it responds to but has no impact on performance. We track interferometry using the second transmitting stations evenly spread around respect to the Sun, because, as we shall see number of key factors. From the mission- these drivers. We will start with the most have already indicated that both the antenna and receive channel. The derived the Earth. By measuring the Doppler shifts later, this would have impacted on the science objectives came the payload obvious feature, its shape. So why does structural stability and knowledge of this angle of arrival of the radar echoes allows of these signals, the range-rate to each one satellite’s design. complement and its requirements, the orbit CryoSat look like dog kennel? assembly is vital to the mission more precise identification of the point can be determined. The accuracy of the and the minimum lifetime. Programmatic As mentioned earlier, the required orbit performance, and the design has a number from which the echo came. orbit that may be computed from such a Designing the System constraints included the need to operate is not Sun-synchronous. Every day the of special features to ensure this. In the SIRAL makes measurements of the dataset obviously depends on several The full CryoSat system has several parts, from a single ground station, launch on a orbital plane shifts to be almost 3 minutes calm environment of space, the principal range to the surface that are very precise: factors, not least the precision and and although will not describe here those ‘small’ and therefore low-cost launcher, earlier with respect to the Sun; in enemy of stability is heat, which causes each has an uncertainty of just a few tens accuracy of the Doppler-shift that will remain on the ground, we will extensive onboard autonomy, a low-cost 8 months, therefore, it drifts through all expansion. The first line of defence against of centimetres. The averaging of many measurements. In this DORIS excels, and outline the overall architecture as this has design and a decision to forego the normal local times. This means that the direction that is to use materials that are least such measurements brings the system for over a decade the system has been the an impact on the design of all of the approach of building precursor, ‘proof-of- from which sunlight falls on the satellite is susceptible to it. We have mainly used performance to the level needed to satisfy foremost means of making routine, high- elements. Programmatic constraints were concept’ models of the satellite (structural constantly changing. The operation of the Carbon-Fibre Reinforced Plastic (CFRP), the mission objectives. However, the precision orbit determinations. The dominant in this part of the system model, engineering model, etc.). SIRAL instrument demands that its which has a coefficient of thermal precise measurement of range alone is DORIS receiver is augmented by a passive definition and led to the minimum While it is not the case that these driving antennas point towards the Earth’s surface expansion close to zero. It is used for the insufficient. The exact position of the laser retro-reflector, which allows precise configuration required to satisfy the factors led inevitably to the CryoSat design to within a few tenths of a degree, and antennas themselves and also for the satellite at the time of each measurement is range measurements to be made by mission objectives.