Surrey Satellite takes the small route to the high ground

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Micro-satellites specialist Surrey Satellite Technology hopes soon to be preparing for launch of a new synthetic aperture radar payload that could bring a dramatic new capability to its Earth observation customers.

A SAR-capable spacecraft has been in development by the company since 2009. SSTL partner Astrium and the Surrey Space Centre made payload tests on an airborne platform last year and the payload and platform this February passed its preliminary design review, so a spacecraft could be available for launch in 2013.

SSTL's head of Earth observation Luis Gomes told a London meeting of users of the Disaster Monitoring Constellation (DMC) - including from Algeria, Nigeria, Spain, the UK and China, for which SSTL has built and launched eight satellites since 2000 - that SAR technology spacecraft has the potential to bring a new level of capability to the constellation.

Critically, he says, unlike normal optical payloads, SAR can see through clouds and record images at night, if a user needs a picture of a particular spot on the ground it can definitely be had on the satellite's next pass.

SSTL's SAR technology lead Phil Whittaker adds that the payload will be capable of imaging pixels as fine as 10m (33ft) across, although may more typically be set to a 20m resolution, allowing it to cover a swath 100km (54nm) wide, or 30m/150km. By contrast, one of SSTL's optical satellites capable of taking a 30m-resolution image would cover a swath 600km wide.

SAR sensors are limited by the fact that they must, through a special antenna, illuminate the ground as they pass. But, notes Whittaker, in addition to being able to see at night or through bad weather, radar images pick up different backscatter patterns than recorded by optical sensors. A SAR image may thus, for example, differentiate between types of vegetation more clearly than an optical image.

Whether a SAR satellite is contracted to join the DMC constellation depends on getting a customer to build and launch one. Whittaker notes that, to date, SAR technology has typically been used on its own. But to combine what Gomes says will be a low-cost SAR option with the existing optical imaging spacecraft in the DMC constellation will be a commercially viable option.

The DMC was proposed in 2000 following calls for improved response to man-made and natural disasters, and got its start with the launch of the SSTL-built Tsinghua-1, for China's Tsinghua University, followed by Algeria's AlSat-1, the UK's British National Space Centre-supported UK-DMC-1, NigeriaSat-1 and Turkey's BILSAT-1.

These satellites, four subsequent launches and, imminently, NigeriaSat-2 follow each other in the same polar, low-Earth orbit. While a single satellite might pass over any particular point on the ground anywhere between every few days to every couple weeks depending on the width of the observation swath, with a constellation it is possible to get daily images of any particular point.

DMC users typically need images of their own territory to monitor long-term phenomena - slow-onset disasters such as drought or desertification, for example - but in the event of an acute disaster can call on images from other users' spacecraft to meet urgent imaging needs. By contrast, the US Landsat satellite has made enormous contributions to our understanding of the Earth's geography but is only able to image any given point once every 16 days.

Constellation members also have the opportunity to sell images to non-members. The Nigerian space agency, for example, has signed a data distribution deal with DMC International Imaging, a company set up by SSTL to disaster response and distribute images, using the revenue to fund management of the constellation for humanitarian use during natural disasters.

Like NigeriaSat-2, SSTL's SAR satellite will be about the size of a domestic refrigerator and weigh around 400kg (880lb). The spacecraft can be expected to have a service life of about seven years, by which time propellant may have run out and moving parts worn.

Satellites can be engineered for longer service lives but, says Gomes, the SSTL approach is to keep costs low by using off-the-shelf components where possible in place of vastly more expensive "space-hardened" equipment.

Most SSTL satellites - the company has launched 34 - fall into the 100kg class and are about the size of a washing machine, compared with the bus-sized spacecraft more typical of the Earth observation mission. So-called micro-satellites are much cheaper to launch and can often be orbited as piggyback payloads.

SSTL also, says Gomes, avoids the typical satellite mission cost-trap of freezing design early in a multi-year construction phase. This more flexible approach allows engineers to incorporate the latest technology, an option not available in more rigid design and construction programmes.

Micro-satellites certainly face greater risks of failure than larger, more expensive units but, Gomes sees the lower cost of design, construction and launch as more than making up for risk to many users.

But with launch costs accounting for 40% or more of a typical mission price tag, finding new, low-cost launch options - which could include launches from some of the suborbital vehicles now under development - is the key to giving more users access to space through micro-satellites, he adds.

Market figures presented by SSTL show the Earth observation market to be worth $930 million yearly, with more than 80% accounted for by governments and two-thirds by militaries.

Governments and militaries may be the natural customers for Earth observation capabilities, but neither are good at commercial exploitation of the data. So with barely $165 million spent yearly by private customers Gomes is probably right that finding ways to cut launch costs would expand the market.