Vincent Latendresse, Jonathan Lavoie, Roman V. Kruzelecky and Wes Jamroz
MPB Communications Inc, Pointe Claire, Quebec, Canada
Dina Katsir and Keren Shabtai
Acktar Ltd., Israel
Isbrucker Consulting Inc., Sturgeon Point, Ontario, Canada
GHGSat Inc. Montreal, Quebec, Canada
GHGSat is building and will launch and operate the world’s first nanosatellite capable of monitoring greenhouse gas (GHG) and air quality gas (AQG) emissions from any industrial site in the world. The instrument which is designed, integrated and tested by MPB Communications will be used to monitor carbon dioxide (CO2) and methane (CH4) from a low earth orbit of about 500 km. The instrument was designed to be very small (about 20x20x20 cm), very light (<10 kg) and consume very little power (< 20 W) in order to fit in the limited size, weight and power budgets of a nanosatellite.
The instrument will monitor greenhouse gases with a precision in parts per million / billion, and a resolution of < 50 meters. The high resolution and the sensitivity require a large front aperture in order to collect enough optical signal, however, this large front aperture (on order of 10 cm diameter) with the size of the instrument imply a considerable stray light risk. The signal from the field of view of the instrument is very small compared to the total light that can enter the instrument; according to our models, it is about 1.7ª104 times greater. The size of the instrument also prevented us from using a pupil at an intermediary image plane in the system, technique that is often used to block stray light for remote sensing missions, since there is no such plane in it.
Stray light was a major concern during the GHGSat instrument design phase and many precautions had to be taken. The instrument is equipped with a baffle and many vanes in order to cut the unwanted light from outside the required field of view. Blocking the light is not enough if the light can be reflected and end up back on the optical path. For that reason it was necessary to coat everything near the optical path with the Acktar Vacuum BlackTM coating to enhance the stray light performance.
Total integrated scattering (TIS) and bidirectional reflectance distribution function (BRDF) measurements of the Acktar coatings were taken in our wavelength range of interest to verify its performance. The total scattered light is very low for low angles of incidence (near normal) and increases when getting closer to grazing angles.
The vacuum black coating from Acktar was selected for its reflectance properties in our wavelength range of interest (around 1600 nm), but it also has very good properties for use in the space environment. An important feature is that the coatings are thin and conform perfectly to sharp baffle edges. The Acktar coatings have essentially no outgassing. The coatings have a wide range thermal stability and are resistant to ATOX and space radiation.
With all the precautions in place and the Acktar coating on the surfaces near the optical path, the stray light concern has been mitgated and the launch of the nanosatellite is now scheduled for the third quarter of 2015.