The Coordinated Ionospheric Reconstruction Cubesat Experiment (CIRCE) is a joint US/UK mission consisting of two 6U CubeSats actively maintaining a lead-follow configuration in the same low Earth orbit with a launch planned for the 2020 timeframe. These nanosatellites will each feature multiple space weather payloads. From the US, the Naval Research Laboratory will provide two 1U Triple Tiny Ionospheric Photometers (Tri-TIPs) on each satellite, observing the ultraviolet 135.6 nm emission of atomic oxygen at nighttime. The primary objective is to characterize the twodimensional distribution of electrons in the Equatorial Ionization Anomaly (EIA). The methodology used to reconstruct the nighttime ionosphere employs continuous UV photometry from four distinct viewing angles in combination with an additional data source, such as in situ plasma density measurements, with advanced image space reconstruction algorithm tomography techniques. From the UK, the Defence Science and Technology Laboratory (Dstl) is providing the In-situ and Remote Ionospheric Sensing suite consisting of an Ion/Neutral Mass Spectrometer, a triple-frequency GPS receiver for ionospheric sensing, and a radiation environment monitor. We present our mission concept, simulations illustrating the imaging capability of the Tri-TIP sensor suite, and a range of science questions addressable via these measurements.
The second generation Tiny Ionospheric Photometer (TIP) is a compact, high-sensitivity, nighttime ionospheric photometer designed for small satellites. TIP launched February 19, 2017 to the International Space Station as part of the GPS Radio Occultation and Ultraviolet Photometry—Colocated (GROUP-C) experiment to test advanced sensing objectives. The TIP optical design improves upon previously-flown photometers and employs a filter wheel to measure signals. The third generation sensor is a 1U Cubesat-compatible Triple Tiny Ionospheric Photometer (Tri-TIP), manifested to fly on the dual 6U Coordinated Ionospheric Reconstruction CubeSat Experiment (CIRCE) in early 2020. The Tri-TIP design builds upon several technologies demonstrated aboard TIP, but utilizes a beam splitter to simultaneously monitor signal, red-leak, and background signals. This paper compares the pre-flight and on-orbit performance of TIP with pre-test theoretical results for Tri-TIP.
The U.S. Naval Research Laboratory (NRL) has developed the Triple Tiny Ionospheric Photometer (Tri-TIP), an ultraviolet remote-sensing instrument based on the TIP. Tri-TIP measures emissions of atomic oxygen (Oi 135.6 nm) to determine plasma density in the nighttime ionosphere. The Tri-TIP design shrinks TIP to a 1U CubeSat form-factor and simplifies the mechanical design with a three-channel photometer system to isolate the target wavelength without a filter wheel. A heated strontium fluoride (SrF2) filter eliminates incoming light at wavelengths shorter than 135.6 nm. The filtered light is divided between two matched photometers by a beam splitter with a magnesium fluoride coating over aluminum (AlMgF2) deposited on 50% of the surface in a polka-dot pattern. The third photometer monitors dark count noise for later subtraction. One Tri-TIP configuration uses a beam splitter with a sapphire (Al2O3) substrate, which is opaque to wavelengths shorter than ∼140 nm, to later subtract contaminating emissions at wavelengths longer than 140 nm. A second Tri-TIP configuration uses a MgF2 substrate beam splitter to simultaneously measure Oi 135.6 nm from two adjacent fields-of-view. The performance of both beam splitters has been tested at NRL, and the results are presented.
We have developed the Triple Tiny Ionospheric Photometer (Tri-TIP) as a CubeSat-compatible 1U sensor to obtain high-sensitivity measurements of the far-ultraviolet (FUV) OI 1356 Å airglow for remote sensing of ionospheric density. The Tri-TIP concept evolved from heritage sensors flown on the COSMIC/FORMOSAT-3 (CF3) constellation, and more recently as part of the GPS and Radio Occultation and UV Photometry – Colocated (GROUP-C) experiment on the International Space Station. The concept for all of these sensors is to isolate this emission using heated strontium-fluoride filters to eliminate shorter wavelength emissions such as O I 1304 Å and H I 1216 Å and cesium iodide photocathodes to reduce sensitivity longward of ~1800 Å. There are no other spectral features in this FUV portion of the airglow spectrum at night. However, the nadir-viewing sensors on CF3 observed significant long-wavelength emissions from city lights and moonlit clouds that contaminated the data. The GROUP-C instrument included a sapphire filter that could be alternated with the strontium fluoride to measure and remove this spectral “red leak” from the observations. The Tri-TIP design pairs a heated strontium fluoride filter in line with a sapphire beam splitter that feeds the UV (with spectral leak) and long-wavelength (spectral leak only) signals to two matched photomultiplier tubes (PMTs). A third PMT monitors the signal contribution from high-energy particles and dark current. We present the results from laboratory tests of these components that ensure the high-sensitivity performance of this new optical configuration for ionospheric remote sensing and imaging from a CubeSat platform.