The Visible/Infrared Imager Radiometer Suite (VIIRS), built by Raytheon Santa Barbara Remote Sensing (SBRS) will be one of the primary earth-observing remote-sensing instruments on the National Polar-Orbiting Operational Environmental Satellite System (NPOESS). It will also be installed on the NPOESS Preparatory Project (NPP). These satellite systems fly in near-circular, sun-synchronous low-earth orbits at altitudes of approximately 830 km. VIIRS has 15 bands designed to measure reflectance with wavelengths between 412 nm and 2250 nm, and an additional 7 bands measuring primarily emissive radiance between 3700nm and 11450 nm.
The calibration source for the reflective bands is a solar diffuser (SD) that is illuminated once per orbit as the satellite passes from the dark side to the light side of the earth near the poles. Sunlight enters VIIRS through an opening in the front of the instrument. An attenuation screen covers the opening, but other than this there are no other optical elements between the SD and the sun. The BRDF of the SD and the transmittance of the attenuation screen is measured pre-flight, and so with knowledge of the angles of incidence, the radiance of the sun can be computed and is used as a reference to produce calibrated reflectances and radiances. Unfortunately, the opening also allows a significant amount of reflected earthshine to illuminate part of the SD, and this component introduces radiometric error to the calibration process, referred to as earthshine contamination (ESC). The VIIRS radiometric error budget allocated a 0.3% error based on modeling of the ESC done by SBRS during the design phase. This model assumes that the earth has Lambertian BRDF with a maximum top-of-atmosphere albedo of 1.
The Moderate Resolution Imaging Spectroradiometer (MODIS) has an SD with a design similar to VIIRS, and in 2003 the MODIS Science Team reported to Northrop Grumman Space Technology (NGST), the prime contractor for NPOESS, their suspicion that ESC was causing higher than expected radiometric error, and asked whether VIIRS might have a similar problem. The NPOESS Models and Simulation (M&S) team considered whether the Lambertian BRDF assumption would cause an underestimating of the ESC error. Particularly, snow, ice and water show very large BRDFs for geometries for forward scattered, near-grazing angles of incidence, and in common parlance this is called glare. The observed earth geometry during the period where the SD is illuminated by the sun has just such geometries that produce strongly forward scattering glare. In addition the SD acquisition occurs in the polar regions, where snow, ice and water are most prevalent. Using models in their Environmental Products Verification and Remote Sensing Testbed (EVEREST), the M&S team produced a model that meticulously traced the light rays from the attenuation screen to each detector and combined this with a model of the satellite orbit, with solar geometry and radiative transfer models that include the effect of the BRDF of various surfaces. This modeling showed that radiometric errors up to 4.5% over water and 1.5% over snow or ice. Clouds produce errors up to 0.8%. The likelihood of these high errors occurring has not been determined. Because of this analysis, various remedial options are now being considered.
The act of performing a sequential trigonometric ray trace through an optical system is as simple as applying Snell's Laws of refraction and reflection, along with some form of the grating equation, to the geometric description of the lenses and mirrors which can comprise that system. The orderly incorporation of these physical laws into an algorithm, which at the same time incorporates all possible geometries which may occur in this optical system, will be the result of this paper. The ray/surface equations and other useful numerical techniques will be spelled out in detail so that this paper can be used as a starting point for their own ray tracing program when commercial optical design codes are either not available or inappropriate.
All optical design and analysis software computer codes possess some form of ray tracing capability. Optical design software computer codes have made do with sequential ray tracing algorithms for the past 40 to 50 years. In developing a general non-sequential ray trace tool at TRW, a new paradigm had to be adopted in order to make this new software tool completely general and flexible. This paper compares and contrasts existing sequential ray tracing algorithms and then describes how we went about crafting our own solution to the general non-sequential ray tracing problem
The use of relatively simple functional form optical surface profiles has, for many years, been the standard way in which optical systems have been designed. A spherical surface profile first order design may have some or all of its spherical surface profiles changed to conic and/or polynomial aspheric profiles during the optimization process in order to add needed degrees of freedom to the design. Traditionally, surface profiles have either been picked for their functional simplicity (spheres, conics and/or polynomial aspherics) or they have been custom derived to fit a specific optical geometry. This paper takes a somewhat different design approach for systems in which "standard" optical surface profiles do not yield the required optical performance and for which custom surface profiles, based on optical system geometry, cannot easily be derived. The approach starts with a "best" design based upon spherical, conic and/or aspheric surface profiles. A non-specific functional form surface profile is then added to the standard surface profile. The retention of the 'standard' surface profile maintains a numerical legacy to the starting point design while a non- specific functional form surface profile is added to and reshapes the surface. The resultant more general surface profile yields a design which may more closely meet the system optical performance requirements. This design approach will be demonstrated with an optical design example.
A common approach used in the design of many optical systems includes the use of "standard" spherical, conic and/or polynomial aspheric surface profiles during the optimization process. Many times, the use of these "standard" surface profiles yields optical systems which either meet or exceed the optical system performance requirements. There are other optical systems, however, which cannot be optimized to their required performance levels using these "standard" optical surfaces. This paper will examine one such case and show how a new type of optical surface profile can be derived which will yield "perfect" optical performance. The derived surface equations will be coded into a user-defined surface in the CODE V optical design program, a verification ray trace will be performed and the explicit surface profile will be displayed. This technique will suggest that there may be other geometries for which surface shapes may be derived in order to solve specific optical problems.
This paper describes an earth orbiting Deep Space Relay Satellite System (DSRSS) based on optical coherent detection communication for the user spacecraft to DSRS link. The optical coherent detection DSRSS is considered as a possible augmentation to the Deep Space Network (DSN), after the 70 meter antennas are upgraded to Ka Band near the turn of the century. While significant development is required and technical complexity issues remain, the coherent optical system appears capable of achieving the desired order of magnitude improvement relative to the Ka Band DSN. For example, the coherent optical system provides a capability of 670 kbps at Pluto (40 A.U.), a 10.4 dB improvement over the upgraded DSN.
This paper describes an earth orbiting Deep Space Relay Satellite System (DSRSS) based on optical direct detection communication for the user spacecraft to DSRS link. The optical direct detection DSRSS is considered as a possible augmentation to the Deep Space Network (DSN), after the 70 meter antennas are upgraded to Ka Band near the turn of the century. While development is required, the extrapolation from current technology appears relatively straightforward, and the direct detection system appears capable of exceeding an order of magnitude improvement over the upgraded Ka Band DSN. For example, with a 75 cm aperture on the user spacecraft, the optical direct detection system provides a capability of 1.23 Mbps at Pluto, a 13 dB advantage over the upgraded DSN. The direct detection system can also provide 0.83 Mbps with a 60 cm user aperture, an 11 dB improvement.