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For LEO sensor systems there are several factors to consider when determining an optimum for overpass time and the spatial coverage per orbit:

• Overpass time at the same time each day or multiple times per day?

• In the tropics early in the morning is preferred as clouds have not yet developed

• But, depending on season sun glint needs to be avoided which is most severe at tropical latitudes.

• At high latitudes solar noon is the best overpass time to get as much signal from the water bodies as possible.

• How many sensor platforms are being considered? One or 2 or 5 or 100+?

• Near polar orbit or also considering an International space station type quasi-circum- equatorial orbit?

• Will the sensor be pointable in space (see 3.6)?

Some sort of compromise will be need to be achieved between all of these variables. A coral reef focused imaging system ( also suitable for all tropical aquatic ecosystems) would focus on an early morning overpass time, but a high latitude boreal lake sensor system would focus on a solar noon overpass time guaranteeing highest possible sun zenith angle.

For GEO sensor systems these issues do not apply as the sensor is fixed over a fixed position on the equator (but with higher slant angles at high North and South latitudes). However the distance of a GEO stationary satellite has significant consequences for lens aperture, the imaging detector array, data transmission etc.

3.3.2 Sun glint avoidance and mitigation strategies

Here we discuss sun glint avoidance and sensor based mitigation strategies, whereas the post launch image based sun and sky glint removal strategies will be discussed in chapter 4.

Currently, the presence of sun glint during the acquisitions depends on the characteristics of the platform: swath, time of overpass, orbit inclination, depointing capabilities, spatial sampling, etc. Thus, before the platform and its orbit are defined, recommendations have to be to maximise avoidance of sun glint.

Earth observation sensors that have been designed for use over water bodies have opted for either tilt or roll for sun glint avoidance to increase the fraction of glint free acquisitions. For example, OLCI has a 12 degrees westward roll to avoid sun glint. For wide swath sensors however, across track glint avoidance is limited by the incidence angle threshold for accurate retrievals of water-leaving

radiance (MODIS uses a 55 degrees upper limit for sun zenith and 75 for sensor zenith angles

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As with SeaWIFS and OCTS, PACE is proposing a ± 20 degrees aft (Northern hemisphere on the descending node) and forward (Southern hemisphere) pitch in order to minimize sun glint (PACE Mission SDT Report, 2012). This implies that the instrument is tilted as it comes over the sub-solar point with a staggered pattern to ensure data is acquired over the tilting latitudes (generally, the equator) to avoid a gap using off-nadir data from adjacent orbits. Alternative approaches can also achieve equivalent glint avoidance (Gregg and Patt, 1994). Sensors that neither roll nor tilt, such as MERIS (operated from 2003 to 2012), were affected by a considerable fraction of sun glint

(Steinmetz et al., 2011).

Figure 3.2 (after Meister et al., 2011) Illustration of how choosing sensor tilt (for SeaWiFS) significantly decreases the sun glint effects in the imagery. Global map of SeaWiFS (top) and MODIS Aqua (bottom) glint coefficients for March 22nd 2006. MODIS is not tilted. Glint coefficients larger than 0.005 in reflectance terms) are classified as high glint in NASA ocean colour processing and coloured pink in the images. Glint coefficients from 0.001 to 0.005 are classified as moderate glint and coloured red to white. The tilting of the SeaWiFS sensor significantly decreases the amount of data affected by sun glint.

The LEO sensor we are considering key for aquatic ecosystem processes will need to be carefully designed to avoid sun glint as much as possible given that low latitudes earth observation will be a crucial component of this sensor mission with its abundance of shallow water tropical ecosystems such as coral reefs, tropical seagrasses etc. Sun glint avoidance thus will be a combination of (for polar orbiting LEO) overpass time and tilt of the sensor. In the case of orbits such as that of the ISS (from about 51.6 ° N to 51.6 ° S with a 3 to 5 day cadence and associated varying overpass times) this becomes more intricate to solve.

3.3.3 Polarization

Accurate knowledge of polarization sensitivity prior to launch is critical. Top of atmosphere

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but tend to be less than 50 % (Meister et al., 2011). This linear polarization primarily arises from atmospheric Rayleigh single scattering with a maximum perpendicular to the direction of light propagation and at longer wavelengths (Chandrasekhar, 1961). Light reflected by water is

horizontally polarized and maximum at the Brewster angle (53.1°) while the upwelling light tends to be vertically polarized (Sabbah and Shashar, 2007). Hence, coastal imagers have demanding

polarization sensitivity requirements coming from the high spatial heterogeneity, high contrast and significant water and atmospheric polarization (Van Gorp et al., 2010).

The impact of polarization on retrieved geophysical parameters is a result of the convolution of the top of atmosphere radiance polarisation with the instrument polarization sensitivity, which for water colour sensors typically ranges by design from 1 to 4 %. Most of this sensitivity can be attributed to polarization sensitive instrument components such as gratings, specular reflectance on optical surfaces at high incidence angles and dichroics. Such low polarization sensitivity can still lead to errors in the atmospheric correction resulting in significant errors (as much as 10 % at 443 nm) in the retrieved water leaving radiance (Gordon et al., 1997).

HICO’s design-level polarization sensitivity was for example estimated at 4 % @900 nm, 2 % @450- 650 nm and higher around 350 nm (Lucke et al., 2011). The polarization sensitivity of MERIS was about 3 % without a depolarization scrambler but reduced to only 0.25 % when one was introduced (Qian, 2016). The PACE upper limit for polarization sensitivity is 1 % while it is recommended that it be characterized to within 0.2 % to reduce the uncertainty in TOA radiances due to polarization to less than 0.1 % for a large majority of global ocean cases (Meister et al., 2011). Ocean colour instruments, such as CZCS, SeaWiFS and MERIS all opted for polarization scramblers in order sufficiently reduce polarization sensitivity. Polarization scramblers, such as the dual Babinet, however have a negative impact on the spatial resolution and, to a lesser extent, on signal to noise ratio (Collett, 2005, Caron et al., 2012).

Thus for the proposed aquatic ecosystem Earth observation sensor we are selecting the PACE requirements of an upper limit for polarization sensitivity of 1 % while it is recommended that it be characterised to within 0.2 % to reduce the uncertainty in TOA radiances due to polarization to less than 0.1 % for a large majority of water bodies. Regardless of the polarization response, it is

generally accepted that this instrument behaviour should be characterized to 0.5 % uncertainty and that characterization should be applied on-orbit to remove the effects of response to polarization.