{
  "slug": "aria-stratospheric-beam-is-an-aperture-problem",
  "title": "A £1.3m contract points a 5.8 GHz beam up at the stratosphere. The design is settled on the ground.",
  "date": "2026-09-21",
  "summary": "ARIA is funding RF power beaming to a solar aircraft at about 20 km. The binding constraint is not transmitter power, it is the width of the ground array, and the same choice decides the exposure question. At 5.8 GHz a 30 m array needs tens of kilowatts, and a 217 m array needs under one.",
  "author": "Ido Yahalomi",
  "tags": [
    "market",
    "standards",
    "power-beaming",
    "spectrum"
  ],
  "provenance": "documented",
  "words": 1720,
  "headings": [
    {
      "id": "what-was-actually-bought",
      "level": 2,
      "text": "What was actually bought"
    },
    {
      "id": "the-aperture-not-the-wattage-sets-the-price",
      "level": 2,
      "text": "The aperture, not the wattage, sets the price"
    },
    {
      "id": "the-same-choice-decides-the-exposure-question",
      "level": 2,
      "text": "The same choice decides the exposure question"
    },
    {
      "id": "the-good-news-is-that-there-is-a-regulator-at-all",
      "level": 2,
      "text": "The good news is that there is a regulator at all"
    },
    {
      "id": "what-it-does-not-say-about-orbit",
      "level": 2,
      "text": "What it does not say about orbit"
    },
    {
      "id": "what-would-change-my-mind",
      "level": 2,
      "text": "What would change my mind"
    },
    {
      "id": "sources",
      "level": 2,
      "text": "Sources"
    }
  ],
  "html": "<p>On 15 September 2026 Space Solar announced a £1.3m research contract from the UK&apos;s Advanced</p>\n<p>Research + Invention Agency to develop RF power beaming to high-altitude platforms, at 5.8 GHz.</p>\n<p>The interesting number is not the money and not the wattage. It is how much ground the</p>\n<p>transmitter needs.</p>\n<p>Figures below are labelled documented where a source states them and estimated where I have</p>\n<p>calculated them from stated assumptions.</p>\n<h2 id=\"what-was-actually-bought\">What was actually bought</h2>\n<p>Documented, from the company&apos;s own news page dated 15/09/2026: a £1.3m contract, one of 18</p>\n<p>funded R&amp;D Creators in ARIA&apos;s Enduring Atmospheric Platforms programme, to take rectenna work</p>\n<p>from TRL 3 to TRL 7 over 36 months, at 5.8 GHz, ending in an integrated flight demonstration</p>\n<p>aboard a PHASA-35. The named partners are Prismatic Ltd, which develops that aircraft, and</p>\n<p>MuWave Limited, which is to develop the ground transmitter. Space Solar&apos;s release quotes co-CEO</p>\n<p>Martin Soltau saying &quot;Energy is one of the central constraints for HAPS.&quot; The release gives no</p>\n<p>rectenna area, no range and no efficiency.</p>\n<p>ARIA&apos;s own documents give the target the work is aimed at. The call for proposals, version 1.0</p>\n<p>dated 3 December 2025, sets a primary metric of 300 W delivered continuously to a payload in the</p>\n<p>sky for a full week while holding line of sight to a fixed point on the ground, with a 20 kg</p>\n<p>payload as the tertiary metric. The programme runs 3.5 years. The proposed budget in that</p>\n<p>document is £50m, split £6.1m to enabling technologies, £37.7m to system integration and</p>\n<p>testing, £3.1m to deployment planning and £3.1m to programme partners; the programme&apos;s public</p>\n<p>page now gives £70m. Orbital systems are explicitly out of scope, air-breathing very low Earth</p>\n<p>orbit satellites included.</p>\n<p>One small thing for anyone quoting the forward target. The programme thesis, version 2.0, puts</p>\n<p>the future goal at 3 kW over one week and gives it as 0.5 MWh, which is right. The call for</p>\n<p>proposals gives the same target as 840 kWh, which is not. Three kilowatts for 168 hours is</p>\n<p>504 kWh. Use the energy figure that matches the power figure you are quoting.</p>\n<p>Two limits on the award itself. ARIA has not published a Creators list for this programme: the</p>\n<p>path that carries one for its sibling programme returns HTTP 404 here, checked on 20 September</p>\n<ol>\n<li>And the company&apos;s release is first-party for the award and for the frequency, but it is</li>\n</ol>\n<p>the only account of either. Nothing below depends on a second source for those two facts.</p>\n<h2 id=\"the-aperture-not-the-wattage-sets-the-price\">The aperture, not the wattage, sets the price</h2>\n<p>A ground-to-stratosphere power link is not a far-field radio link and Friis will mislead you.</p>\n<p>Take a 100 m ground array at 5.8 GHz. Its far field begins at roughly 2D²/λ, which is about</p>\n<p>387 km, so a receiver at 20 km sits deep inside the near field, where the beam can be focused</p>\n<p>and where the right expression is the classical beam-efficiency result for two apertures: with</p>\n<p>τ² = A_t·A_r / (λR)², the captured fraction is approximately 1 − exp(−τ²).</p>\n<p>Everything in this section is estimated. The assumptions are: 5.8 GHz, so λ = 51.7 mm; a</p>\n<p>vertical range of 20 km, which is the best case, since a slant path is longer; a rectenna of</p>\n<p>20 m² on the underside of the aircraft, which is generous for a 35 m wing that BAE Systems</p>\n<p>publishes as weighing 150 kg all-in and which already carries photovoltaics on top; and a</p>\n<p>rectenna converting RF to DC at 70%. λR is then about 1,030 m².</p>\n<p>A 30 m ground array is 707 m². That gives τ² ≈ 0.013 and a captured fraction of about 1.3%. To</p>\n<p>land 300 W of DC in the payload you radiate about 33 kW.</p>\n<p>A 100 m array is 7,850 m². Capture rises to about 14%, and the radiated power falls to about</p>\n<p>3.1 kW.</p>\n<p>A 217 m array is 3.7 hectares. Capture reaches about 50%, the radiated power is about 860 W,</p>\n<p>and the wall-plug draw is about 1.2 kW if the transmitter converts DC to RF at 70%. That is an</p>\n<p>end-to-end efficiency near 25%.</p>\n<p>Seven times the array width buys a fortyfold cut in radiated power. That is the whole design</p>\n<p>conversation, and it is a trade of land against watts. Electricity is not what is being traded.</p>\n<p>Even the wasteful case draws about 47 kW, which at 25p per kilowatt-hour is about £12 an hour,</p>\n<p>against the programme&apos;s own gross operating cost target of under £500 an hour. That target is</p>\n<p>ARIA&apos;s threshold for the platform, not a forecast of anything, and the electricity line is not</p>\n<p>where it will be won or lost.</p>\n<h2 id=\"the-same-choice-decides-the-exposure-question\">The same choice decides the exposure question</h2>\n<p>ICNIRP&apos;s 2020 radiofrequency guidelines set, for 2 to 300 GHz, a whole-body incident power</p>\n<p>density reference level of 10 W/m² for the general public and 50 W/m² for occupational</p>\n<p>exposure, each averaged over 30 minutes. IEEE C95.1-2019 sits alongside them. Those are the</p>\n<p>numbers a site is assessed against, not a reassurance about beam density being low.</p>\n<p>Now put the estimates beside them. The 30 m array radiating 33 kW is about 47 W/m² averaged</p>\n<p>across its own face. That is just under the occupational reference level, about 4.7 times the</p>\n<p>general public one, and a tapered illumination is higher than the average on axis. It implies a</p>\n<p>fenced, controlled, access-managed site, with the exposure case made at the array rather than</p>\n<p>at the aircraft.</p>\n<p>The 100 m array radiating 3.1 kW is about 0.4 W/m² across its face, a factor of 25 below the</p>\n<p>general public level. The 217 m array is lower again by more than an order of magnitude.</p>\n<p>So the aperture decision and the exposure decision are the same decision. A small array is a</p>\n<p>regulated industrial site. A large one is a field with a low-power emitter in it and a planning</p>\n<p>problem instead. At the far end, 300 W into 20 m² is about 15 W/m² at an uncrewed airframe,</p>\n<p>which is a hardware and airframe question rather than a human exposure one. The column of air</p>\n<p>between the two is inside UK controlled airspace, which is why ARIA&apos;s solicitation offers</p>\n<p>Creators segregated airspace at a UK test range during defined testing windows, and names the</p>\n<p>Civil Aviation Authority.</p>\n<h2 id=\"the-good-news-is-that-there-is-a-regulator-at-all\">The good news is that there is a regulator at all</h2>\n<p>This beam points up and stays inside one jurisdiction, and that is a materially better</p>\n<p>regulatory position than the one a satellite downlink is in. 5.8 GHz is not regulatorily</p>\n<p>available for space-to-Earth: the Radio Regulations, Edition of 2024, carry no space-to-Earth</p>\n<p>power flux-density row for the band, and wireless power transmission has no status in the</p>\n<p>Regulations at all. None of that binds a ground-to-air link. What binds it is Ofcom, and ARIA&apos;s</p>\n<p>own milestone list requires TA3 teams to reach a critical design review with plans for spectrum</p>\n<p>licensing and buy-in from the CAA and Ofcom.</p>\n<p>The international instruments are thinner than the size of the beam suggests. The only ITU-R</p>\n<p>Recommendation naming frequency ranges for beam wireless power transmission is SM.2151-0</p>\n<p>(09/2022), still listed In force (Main) on the ITU&apos;s editions page when I checked it on 20</p>\n<p>September 2026, and its title scopes it to mobile and portable devices and sensor networks. Tens</p>\n<p>of kilowatts aimed at an aircraft is not that. A draft revision is in the approval queue:</p>\n<p>Administrative Circular CACE/1203 of 26 August 2026 records Study Group 1 adopting it, with</p>\n<p>Member States asked to reply by 26 October 2026, and the annex says it adds 24.1-24.15 GHz to</p>\n<p>Table 1 on the strength of the impact studies in Report ITU-R SM.2505. The scope in the title is</p>\n<p>unchanged.</p>\n<p>The broader question is queued behind a conference. WRC-31 preliminary agenda item 2.2, under</p>\n<p>Resolution 910 (WRC-23) and assigned to Working Party 1A, covers possible bands for non-beam and</p>\n<p>beam wireless power transmission, and its published framing is avoiding harmful interference</p>\n<p>caused by such transmission rather than enabling it. A 36-month project starting now finishes</p>\n<p>years before that item reaches a conference. The demonstration will exist before the band</p>\n<p>question is answered, which is an argument for doing the demonstration under a national licence</p>\n<p>with published conditions, and for publishing what those conditions turn out to be.</p>\n<h2 id=\"what-it-does-not-say-about-orbit\">What it does not say about orbit</h2>\n<p>Run the same formula at 400 km and λR becomes about 20,700 m², so the aperture product required</p>\n<p>for a given capture fraction grows by a factor of 400. The 217 m ground array that reaches 50%</p>\n<p>at 20 km becomes about 4.3 km across at 400 km against the same 20 m² receiver. Stratospheric</p>\n<p>power beaming is not a scale model of orbital power beaming. It is a different problem that</p>\n<p>happens to use the same hardware vocabulary, and the honest reading of this contract is as a</p>\n<p>test of rectennas, phased arrays and beam control at a range where the arithmetic closes.</p>\n<p>BeamDesk has no involvement in this programme and no relationship with any organisation named</p>\n<p>here. Everything above is from their published material and from the issuing bodies&apos; own</p>\n<p>documents.</p>\n<h2 id=\"what-would-change-my-mind\">What would change my mind</h2>\n<p>A measured end-to-end DC-to-DC efficiency at kilometre-scale range, published with the aperture</p>\n<p>areas and the illumination taper, would test the whole of the second section. If it beats these</p>\n<p>estimates materially, the assumption that broke is probably the rectenna area or the taper, and</p>\n<p>I would want to know which.</p>\n<p>If the flight demonstration meets the metric in bursts rather than continuously, or fractionally</p>\n<p>across several platforms, which the programme explicitly permits, then the land-against-watts</p>\n<p>trade is not the one being made and the exposure arithmetic changes with it.</p>\n<p>If ARIA publishes its Creators list and more than one award is power beaming, particularly an</p>\n<p>optical one, then treating this as an RF bet is too narrow and the story is RF against laser at</p>\n<p>the same altitude.</p>\n<p>If Ofcom authorises the transmission with published EIRP or power density conditions, that is a</p>\n<p>concrete national answer to a question the ITU has parked until WRC-31, and it would be the most</p>\n<p>useful document anyone in this field could read this decade.</p>\n<h2 id=\"sources\">Sources</h2>\n<p><a href=\"https://www.spacesolar.co.uk/space-solar-wins-1-3m-aria-contract-to-help-power-enduring-stratospheric-flight-over-britain/\" rel=\"noreferrer noopener\">https://www.spacesolar.co.uk/space-solar-wins-1-3m-aria-contract-to-help-power-enduring-stratospheric-flight-over-britain/</a></p>\n<p><a href=\"https://www.spacesolar.co.uk/category/news/\" rel=\"noreferrer noopener\">https://www.spacesolar.co.uk/category/news/</a></p>\n<p><a href=\"https://aria.org.uk/opportunity-spaces/scoping-our-planet/enduring-atmospheric-platforms\" rel=\"noreferrer noopener\">https://aria.org.uk/opportunity-spaces/scoping-our-planet/enduring-atmospheric-platforms</a></p>\n<p><a href=\"https://aria.org.uk/media/xiukdjjv/enduring-atmospheric-platforms-programme-solicitation_accessible.pdf\" rel=\"noreferrer noopener\">https://aria.org.uk/media/xiukdjjv/enduring-atmospheric-platforms-programme-solicitation_accessible.pdf</a></p>\n<p><a href=\"https://aria.org.uk/media/et5hsubs/programme-thesis-enduring-atmospheric-platforms.pdf\" rel=\"noreferrer noopener\">https://aria.org.uk/media/et5hsubs/programme-thesis-enduring-atmospheric-platforms.pdf</a></p>\n<p><a href=\"https://www.baesystems.com/en/product/phasa-35\" rel=\"noreferrer noopener\">https://www.baesystems.com/en/product/phasa-35</a></p>\n<p><a href=\"https://www.itu.int/rec/R-REC-SM.2151/en\" rel=\"noreferrer noopener\">https://www.itu.int/rec/R-REC-SM.2151/en</a></p>\n<p><a href=\"https://www.itu.int/dms_pub/itu-r/md/00/cace/cir/R00-CACE-CIR-1203!!PDF-E.pdf\" rel=\"noreferrer noopener\">https://www.itu.int/dms_pub/itu-r/md/00/cace/cir/R00-CACE-CIR-1203!!PDF-E.pdf</a></p>\n<p><a href=\"https://www.itu.int/en/ITU-R/study-groups/rcpm/Pages/wrc-31-preliminary-studies.aspx\" rel=\"noreferrer noopener\">https://www.itu.int/en/ITU-R/study-groups/rcpm/Pages/wrc-31-preliminary-studies.aspx</a></p>\n<p><a href=\"https://www.itu.int/pub/R-REG-RR\" rel=\"noreferrer noopener\">https://www.itu.int/pub/R-REG-RR</a></p>\n<p><a href=\"https://www.icnirp.org/cms/upload/publications/ICNIRPrfgdl2020.pdf\" rel=\"noreferrer noopener\">https://www.icnirp.org/cms/upload/publications/ICNIRPrfgdl2020.pdf</a></p>\n<p><a href=\"https://standards.ieee.org/ieee/C95.1/4940/\" rel=\"noreferrer noopener\">https://standards.ieee.org/ieee/C95.1/4940/</a></p>"
}