Telecoms
EXPERIMENTAL SYSTEMS DEVELOPMENT
Joe Waight, Principal Experimental Officer
SATELLITE BEACON RECEIVER EXPERIMENTS
MILLIMETRE WAVE WORK: PAST AND PRESENT
The successful experimental campaigns carried out since the summer of 1996 to receive the Italsat 39.592 GHz (F40) satellite beacon were due to the implementation of a highly developed and adaptable groundstation receiver designed by members of the group, back in 1993. Key features of this groundstation are its portability and low weight, with the mechanical design and overall size governed mainly by the 500mm diameter high performance lens horn antenna. This hardware was developed for indoor siting only.
During the 3 years prior to the summer 1996 campaign at UPC-ETSIT in Barcelona, Spain, the groundstation underwent extensive testing at the Portsmouth laboratory.
The photograph, Figure 1 shows the first basic groundstation with receiver units, front end (suspended in a dismantleable cradle) and IF (lower right), and laptop PC data-logger/controller with controller interface. This was limited to receiving only the F40 beacon and logging In-phase and in-Quadrature data at base band. Note the long elevation adjuster screw on the side of the platform. The tracking of Italsat was sometimes a lengthy process, especially in the azimuth plane, since very fine angular movements of the platform were needed to ensure good pointing. In the elevation plane this was not so much of a problem since the adjuster screw provided continuous, free movement of the microwave front end. Eventually this manual elevation adjustment was to be replaced by a stepper motor under computer control using coordinates data provided by Fucino ground control and updated regularly.
Different voltage-controlled signal sources in the beacon downconversion and I and Q lowpass filters were tested in the IF receiver stage (1st IF = 70 MHz, 2nd IF = 10.7 MHz) for the purpose of extracting phase noise scintillation and signal level information in the presence of beacon frequency cyclic drift and signal fade (clouds and rainfall). In parallel with the hardware development a considerable amount of effort was put into datalogging software development with particular emphasis on the control of the second local oscillator (80.7MHz VCO). This was required in order to maintain the satellite beacon downconversion within the 1kHz 3dB bandwidth of a 10.7 MHz Bessel bandpass filter preceding the I/Q demodulator. The groundstation, during this period, included only one 486 PC to run these software activities, including dataloging from an independent IF tracking receiver, radiometer and drop-counting raingauge. In the months leading to the Barcelona experimental programme (from June 96 to December 97) a 40 GHz radiometer was integrated into the experimental hardware. The radiometer was provided by the RAL Chilbolton Observatory in Hampshire. The close collaboration has since then continued.
The photograph, Figure 2, shows the final receiver system with the microwave front end, radiometer and a final year student of ETSIT controlling the data logging in the Barcelona laboratory. The microwave front end antenna was inclined at approximately 40 degrees of elevation and pointed due South.
An interesting point about the Portsmouth groundstation platform is its adaptability to retro-fit hardware. For instance, the initial 40 GHz radiometer with its 150mm diameter horn lens antenna was constructed on a single metal plate and then bolted to the side of the receiver microwave front-end with the critical aspect being the alignment of the two antennae axes. The only common connection with the beacon receiver hardware was the mmwave (low) local oscillator feeding the radiometer front-end mixer. During the CLARE campaign, in which Portsmouth participated for remote sensing the earth space path, a second radiometer, working at 37 GHz (F37), was added to the side of the groundstation, working entirely independently of the 40 GHz hardware. This used the original 150mm horn whilst the F40 radiometer 'shared' the 500mm horn antenna, via an orthomode transducer, with the beacon receiver.
The photograph, Figure 3, shows the re-engineered beacon receiver front end hardware (centre) and the adaptations of the main lens horn antenna to use it as a radiometer with a narrow pencil beam. The 150mm lens antenna and its F37 radiometer front end are obscured in the photo by the F40 casing on the left hand side, but the orthomode transducer feeding the F40 front end is just visible at the 500mm lens antenna flange. On the right hand cradle upright a stepper motor has replaced the manual elevation adjuster for automatically tracking the satellite.
Another important addition was a satellite tracking mechanism needed at the time when the ageing Italsat began to drift in orbit in the vertical sense. A second PC was introduced to handle this problem, using a prediction algorithm and data supplied by RAL and Fucino ground control to periodically correct the groundstation antenna elevation. Groundstation Schemes As the groundstation has evolved with changes in experimental requirement, it has retained its portable identity. The research group's experience in this area has shown that complex remote sensing experiments can be set up by a one or two person team using compact systems, easily transportable to different locations. Success with the Portsmouth first portable groundstation has paved the way for a new era of remote sensing experiments.
FUTURE MILLIMETRE WORK: THE STENTOR SATELLITE
It is anticipated that Portsmouth's new work using the French Stentor satellite, due to be launched by CNES in December 2000, will involve the construction of three groundstations with hardware and experimental techniques based on those employed in the Italsat remote sensing experiments. Brief descriptions are as follows but will be subject to change:
- A 20.7GHz beacon receiver together with (i) a radiometer which will use the same beacon receiver antenna, and (ii) a separate reference (calibration) radiometer
- A 41.4GHz beacon receiver and a radiometer using the common Cassegrain or a lens horn antenna (choice dictated by carrier received signal level)
- Communications experiment using the 20.7GHz receiver of (1)
Rainfall rate data will be recorded at the chosen sites for each of the groundstations. The proposed sites are Kourou, French Guyana for No 2 above, and Portsmouth and/or RAL Chilbolton UK for No 1 above.
PROGRESS
- Investigations of antenna and front-end components for dual antenna platforms
- Investigations of antenna and front-end components for single antenna platforms:
- Righthand Circular Polarisation transmission - conversion to linear polarization (V component)
- for beacon, and linear polarization (H component) for radiometry
- Reuse of Italsat groundstation components: Testing the 39.592GHz balanced mixer at the 41.4GHz beacon frequency
The photograph, Figure 4, shows the Italsat groundstation first downconversion mixer (foreground) under test in the Portsmouth laboratory, using, as the r.f. input, 41.4GHz to simulate the STENTOR beacon frequency. It is intended that as much of the original Italsat hardware as possible will be used in the proposed 41.4GHz receiver and radiometer. The spectrum analyser in the background displays the 70MHz I>F. resulting from mixing the r.f. with a 41.33GHz local oscillator signal. The hardware to the right is the Italsat groundstation local oscillator waveguide run. This has been erected for testing the millimetre wave multiplier at the STENTOR frequency.