by Jean-Luc George van Eeckhoutte (Hiltron Communications) and Peter Kneissl (ESA Microwave GmbH)
High-Accuracy 3D Laser Scanning and Evaluation of Teleport Antennas
Satellite teleports have massively expanded in number since the mid 1980s. Many reflectors currently in use date back to those early years and are now performing with reduced efficiency. The performance of a satellite antenna and signal chain is largely dependent on the geometry of the reflector. These data are not always available from the manufacturer.
One of the main options for teleport managers is to upgrade existing antennae to higher frequencies, typically from C-band to Ku-band or Ka-band). This saves the need to buy a new antenna and allows services in the lower bands to be maintained.
We recently introduced a globally-available service allowing high-accuracy 3D laser scanning and evaluation of teleport antennas. Based on technology developed by our subsidiary partner, ESA Microwave GmbH, this resource allows reflectors of practically any size to be measured quickly and accurately.
Using data obtained from laser scanning ,we can simulate the behaviour of an antenna at specific frequencies. Thanks to the experience of ESA Microwave and the versatile tools developed during the last 20 years we are then able to re-design, manufacture, and test the existing feed, including the feed horn, in order to achieve the expected performance. The new feed can be designed as a single band, an extended band, or a multi-band feed at any frequency bands from S to Ka.
The antenna refurbishment can be complemented by the Hiltron offerings with an antenna control unit including a tracking function and LNB/LNA/HPA redundancy systems, as well as with the monitoring and control facility.
3D laser scanning versus photogrammetry
The 3D laser scanning technique we use enables reflectors to be checked in very high resolution as part of a complete performance evaluation. It is far more accurate than the commonly used photogrammetry technique and can be performed while the antenna is actually in operation. It is also much more efficient than photogrammetry which requires manual attachment of measurement targets to the reflector, a time-consuming process and one which results in only a few hundred points being measured. Figure 1 shows 3D laser scanning of a large satellite antenna in progress.
Photogrammetry demands the use of a cherry picker or crane and at least two people must be on site. The antenna must be taken out of service during process. Several hundred adhesive targets must be applied by hand on to the reflector. The cost is quite high and the process is time consuming. As a result, only a few hundred points will be measured.
Our system allows a million surface reference points per second to be captured with a geometric accuracy of less than 1 mm. The measurements can be taken quickly and safely by a single technician using easily transported equipment which in many instances can be operated entirely from a ground level tripod. The resultant information is integrated into a approximately 60 million points. This data cloud is then used to create a computer-aided design model.
Alignment, registration and surface condition
After digitisation and computation of the reflector and sub-reflector topography, the ingested data are used to calculate alignment, registration and any required fine-tuning such as surface restoration.
The 3D laser scan measurements represent the actual state of the reflector surface. To calculate further parameters, several sections are defined on the surface and viewed more closely over the entire antenna. A top/down reflector position is adopted.
Deviation from the measured surface geometries is usually compared to the specifications calculated and determined by the antenna manufacturer to determine the reference accuracy.
Since the target value is not indicated, a best-fit paraboloid is calculated based on individual sections and measured individual points, thus constructed as a CAD paraboloid.
Comparison of the measured laser scanning surfaces and the best-fit CAD paraboloid constructed allows accurate minimum and maximum deviations of the measured antenna to be obtained. The built-up paraboloid is simply a set-point hypothesis for quantifying differences. These discrepancies can then be used to specify an RMS value.
shows an area comparison and determination of a deviation measurement data/set point. To obtain meaningful values as reference points for determining the deviation of the entire active antenna surface, the reflector was subdivided and the previous individual sections (section 1, 2, 3, 4) were calculated with their thousands of individual points and their deviation from the nominal main reflector contour. A distinction is made between the outer area of the antenna reflector and the inner area. The inner surface of the antenna is more affected by the occupation of the radiation characteristics by gaps such as possible deformations of the reflector.
When re-engineering and upgrading an existing antenna system, a clear statement must be made regarding the antenna gain-to-noise temperature (G/T). Precise parameters are necessary for the exact determination of the G/T. These requirements become more demanding at the higher operating frequencies.
In addition to the high-resolution acquisition of the reflector geometry, the antenna system is analysed. The actual perofrmance parameters are measured electrically and mechanically and integrated into a model for the upgrade. This includes not only the ideal design of reflector, sub-reflector and horn but also the feed system losses under ‘worst case’ conditions. Among other things, the resulting losses in the rotary feedthroughs and waveguide switches are studied as a function of the antenna position. This holistic approach makes full use of the combined skills of ESA Microwave and Hiltron Communications.
The 3D laser scanning service is available to new and existing Hiltron customers for any brand and model of satellite antenna up to 35 metres in diameter. Measured specifications and related performance parameters are delivered to the antenna operator or owner together with recommendations clarifying whether the antenna would benefit from upgrading, conversion or fitting with a multi-band feed system.