Feature: Satellite technology
and aviation professionals, as well as consumers. With thousands of satellites planned for each LEO network, the sheer scale of these new networks makes a denial of service much more difficult. While the signal strength offered by LEO satellites is slightly better than traditional GNSS, essentially they could provide a like-for- like PNT service as existing GPS or Galileo satellites. Crucially, LEO offers much more
redundancy and flexibility, so PNT users may have more choice in navigational positioning. From critical infrastructure to defence, this provides a much more reliable GNSS service. Whereas established GPS satellites use
MEO satellites that orbit at an altitude of 2,000-3,600km, LEOs are much closer to the planet, located between 160 and 2,000km above the Earth. On the face of it, this change in distance between Earth and satellite is a mere detail, but in practice this shiſt in position requires a new type of satellite, with a different engineering focus, including network communications.
Distance Te current stable of GPS satellites mostly send out communications that may not be data rich but must be extremely accurate. Each satellite sends a continuous stream of pseudorandom bits (zeros and ones) that repeats every millisecond. Each satellite has its own individual pattern of bits, which provides the recipient device
with a very precise time. Fundamentally, thanks to triangulation and mathematic algorithms, those using timing signals can get either 2D (land-based) or 3D (air-based) positioning, depending on the number of satellites. LEO satellites come with several
challenges. Historically, PNT signals have occupied the L band, allowing a relatively low frequency with good penetration to the surface; this, however, is more problematic in dense urban environment, canyons and mountains. Lower orbits reduce transmission path
loss, which is normally offset by LEOs operating in higher frequency bands (typically AU, E or V). Given that these are significantly higher than L band, it results in a potential overall increase in path loss. Tis means that, normally, a GPS antenna will be polarised (but non- directional) and will achieve the required signal gain to get reception. But, at higher bands, antenna gain and directionality are more difficult. Also, managing timing and position
of satellites in a crowded environment of satellites becomes more complex. At the same time, triangulation normally requires multiple simultaneous connections to achieve accurate positioning. Management of the PNT signals is also more complex at the satellite end. Tousands of devices, all in slowly-decaying orbits, require highly sophisticated soſtware to analyse the data.
Two approaches PNT needs less complex modulation, as the data rates are very low compared to satellite broadband. In practice, this means lower signal-to-noise ratio (SNR) can be tolerated and still achieve good reception of the PNT signal. Te simplest solution here is an antenna with some directionality. Operators could segment the sky into, for example, four quarters. Here, the antenna would not be steerable, but would monitor a specific section of the sky. Although with limited performance compared to broadband antennas, this solution is relatively cheap. Te introduction of active steerable
antennas with narrower beam widths would drastically improve SNR and be more impervious to denial-of-service attacks. Tis would result in higher costs, yet commercial enough, similar to the 5G mmWave steerable antennas. Tis higher performance type of
antenna is based on work undertaken for consumer products. Tese more- sophisticated antennas are resistant to unwanted attacks and provide better interference tolerance compared to older technology. At Novocomms we have been running an R&D programme looking at the development of a new generation of array solutions with improved security, whilst achieving commercial viability.
New technologies Tere are several technologies that are already focusing on hybrid arrays and new materials, and are showing huge potential in antenna development. Smaller, lighter and lower-power arrays are definitely a key part of the future of satellites providing PNT services – but at a cost. The defence and aviation sectors may pay for the added security and higher performance delivered by the new LEO satellites, but not so the general and consumer services, transport operators and car drivers. We may be entering an era of two-tier satellite service provision, with the defence, security and aviation industries using LEOs, and the rest continuing to use the established MEO/ GNSS services.
46 July/August 2024
www.electronicsworld.co.uk
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