ESA Undertakes First Satellite-to-satellite Laser Communications Test
In unrelated but parallel efforts, the European Space Agency (ESA) has announced that it has successfully established an in-orbit, satellite-to-satellite, laser data link and has validated the communications link between the ESA Huygens probe and the American National Aeronautics and Space Administration's (NASA) Cassini orbiter that will begin exploration of Saturn and its moon system during the period 2004 to 2008. Taking these in the order given, the satellite-to-satellite laser data link test took place on 21 November 2001 when the Astrium designed and built SILEX laser terminal was used to establish contact between the ESA's Artemis communications satellite and the SPOT 4 observation vehicle. During the course of the trial (which ESA bills as a 'world first'), Artemis was in a temporary parking orbit 31,000 km above the Earth, while the SPOT 4 satellite was at an altitude of 832 km. In all, a link was established four times during successive SPOT 4 orbits. Functionally, the SILEX terminal aboard Artemis activated an integral optical beacon to scan the area in which the SPOT vehicle was expected to be. With contact made, the SPOT 4 satellite's terminal responded by transmitting a laser beam towards Artemis. When detected, the Artemis terminal stopped its scanning sequence and the optical link between the two vehicles was maintained for pre-programmed periods of between four and 20 minutes. During the link ups, data was transmitted from SPOT 4 to ground stations at Redu, Belgium and Toulouse, France via Artemis at a rate of 50 Mbps. The SPOT-Artemis link-up was preceded by a SILEX validation exercise that involved the ESA facility on Tenerife. For the future, the intention is to use the new link to transmit SPOT 4 imagery to the SPOT ground station at Toulouse in real-time via Artemis rather than having to store acquired data aboard the SPOT vehicle until it comes within line-of-sight of Toulouse during each orbit.
Scheduled to arrive at Saturn on 1 July 2004, the Cassini-Huygens combination is intended to undertake a four-year investigation of Saturn and its moon system, with the Huygens probe being used to penetrate the atmosphere of Saturn's largest moon Titan during January 2005. During this event, Huygens will use Cassini as a data relay and the dynamics of the exercise (the two vehicles will be more than 65,000 km apart and travelling in opposite directions during the descent) have the potential to create significant shifts in communications signal strength and frequency which could be outside the operating bandwidth of the Cassini receiver subsystem. Such problems could be further exacerbated if the Huygens probe encounters stronger than anticipated winds as it drops through Titan's atmosphere. Accordingly, a joint ESA - Jet Propulsion Laboratory (Pasadena, CA) team has used the period 16 to 21 November 2001 to simulate the data stream envisaged for the Titan mission. During the simulation, a nominal mission scenario and several deviations from that scenario were undertaken to test the robustness of the Cassini-Huygens link.
NEC/Siemens Launch Europe's First Fully Operational 3G/UMTS Network
Japan's NEC and Germany's Siemens Information and Communications Mobile group have announced the launch of what they term Europe's 'first fully operational' third generation (3G) Universal Mobile Telecommunications System (UMTS) network on the Isle of Man. The consortium members describe the new architecture as being a platform for client Manx Telecom (a subsidiary of the UK's mmO2 provider) to investigate the commercial potential of the multimedia capabilities of 3G/UMTS.
The system being implemented is described as being a complete UMTS infrastructure that includes a core network, complete technology for a UMTS Terrestrial Radio Access Network (UTRAN) and the Roke Manor Research-developed Isle of Man Air application. Of these, the UTRAN consists of 30 Node B basestations (grouped into 28 sites), with the system as a whole making the maximum use of existing Global System for Mobile communications (GSM) antenna masts. All the architecture's basestations are linked via a central control unit (the Radio Network Controller - RNC) and are integrated via the core network into existing networks. The Isle of Man application makes use of Frequency Division Duplex (FDD)/Wideband Code Division Multiple Access (WCDMA) transmission processes (providing radio coverage throughout the region) and data transmission takes the form of circuit and packet switching. Here, circuit switching is used where there is a permanent connection between two mobile radio subscribers, with packet switching being employed where there is no exclusive transmission channel. Data to be transported is divided into packets (as for Internet protocol), facilitating maximum use of available bandwidth. Mobile subscribers can be permanently connected to the network as only data packets addressed to them arrive at their terminals.
The use of FDD or WCDMA mode enables the system to operate at a theoretical data transfer rate of 64 Kbps in the uplink (user to network) and 384 Kbps in downlink (network to mobile subscriber) modes. Applications available comprise the already noted Isle of Man Air (location specific data), Manx Telecom's mobile office, BT Openworld and MT video content, NEC sourced video telephony, enhanced Wireless Application Protocol (WAP), Imagecom sourced live video and Gameplay's gaming services. Overall, the Isle of Man 3G/UMTS network covers 85 percent of the island's landmass and is fully integrated with its existing communications infrastructure. Manx Telecom was awarded its 3G/UMTS licence in December 1999 and selected the NEC/Siemens consortium to implement the network during the following February. At the time of going to press, appropriate handsets were scheduled for delivery to a representative group of the island's population during early 2002 with the aim of establishing the commercial viability of the network's various applications.
Intermap Lands NEXTMap Britain Contracts
North American contractor Intermap Technologies has been awarded contracts valued at $3.2 M covering the supply of high accuracy digital maps of the United Kingdom (UK), with the entire effort being designated as the NEXTMap Britain programme. With a data acquisition phase scheduled for the period December 2001 to March 2002, NEXTMap Britain makes use of digital elevation models (including surface and terrain models) derived from information provided by Intermap's airborne STAR-3i interferometric Synthetic Aperture Radar (SAR). Developed by the Environmental Institute of Michigan (ERIM) and the Jet Propulsion Laboratory (Pasadena, CA), STAR-3i is mounted in a modified Learjet 36 aircraft and takes the form of an X-band (9.5675 GHz centre frequency) SAR interferometer. As such, the sensor incorporates a two-element antenna array that is mounted in a solid Invar pedestal with 1 m separation between the antennas. The antenna pedestal is steerable in azimuth, thereby facilitating data collection to port and starboard. Motion measurement data is provided by a Honeywell H770 Inertial Reference Unit (IRU) which, in post processing, is coupled to an Ashtech Z-12 Global Positioning System receiver and its complementary basestation. The IRU is mounted on the antenna pedestal to eliminate lever arm errors between the host aircraft and the antenna phase centres. The sensor's interferometric channels share a single receiver chain and operate in a ping-pong mode to provide an effective doubling of baseline length. Other system specifications include a bandwidth of 67.5 MHz, a pulsewidth of 22.6 microseconds, range and azimuth resolutions of 2.5 m, an effective baseline of 1.841 m, a slant range of 7500 m, a ground swath of 10 km (nominal) from an altitude of 9144 m and a data rate of 13.75 Mbytes/s. The NEXTMap Britain project will map the entirety of the UK's landmass, with the ground surface being modelled as a series of elevation points five meters apart. Once processed, the resultant map data will be sold under licence through Intermap's Global Terrain data store and other authorised distributors in the UK.