GPS Radio Occultation and Ultraviolet Photometry-Colocated (GROUP-C) Early Orbit Testing Results

After routine flight, capture, and installation, GROUP-C underwent Early Orbit Testing to verify its performance prior to science operations.

August 1, 2022

Naval Research Laboratory, Washington, DC

(Left) The refurbished TIP for GROUP-C included optical, electrical, and surface treatment changes for the STP-H5 mission. (Right) The GROUP-C TIP filter wheel is larger in diameter, printed from black Ultem^® plastic, and includes both an open aperture and a sapphire filter for red-leak monitoring. The outline of the original, smaller filter wheel is in blue.

The GPS Radio Occultation and Ultraviolet — Colocated (GROUP-C) experiment was originally conceived in 2010 as a CubeSat mission, combining a compact GPS occultation receiver and high-sensitivity far-ultraviolet (FUV) photometer experiment to be flown as a Space Test Program experiment. The concept was to incorporate a commercial off-the-shelf GPS receiver and a small second-generation FUV photometer to replicate the space weather portion of the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC/FORMOSAT-3) mission at lower cost. In the same timeframe, the Air Force Space and Missile System Center initiated the Space Environment NanoSatellite Experiment (SENSE) to demonstrate several CubeSat technologies for space environment sensing, which included the Compact Tiny Ionospheric Photometer (CTIP) and the Compact Total Electron Content Sensor (CTECS).

The SENSE effort eased the urgency of demonstrating CubeSat technology using GROUP-C. When the opportunity arose in 2013 for GROUP-C to fly on Space Test Program Houston Pallet #5 aboard the International Space Station (ISS) with the volume, power, and weight constraints removed, the experiment more narrowly focused upon demonstrating second-generation space environment sensors.

Space experiments have relied on 135.6 nm emission to characterize the F-region nighttime ion density for decades, operating on the principle of spectrally isolating the useful 135.6 nm recombination emission from unwanted nightglow. Nearly twenty years ago, NRL introduced the concept for a new class of compact, high-sensitivity ionospheric 135.6 nm sensors designed for the COSMIC mission. The COSMIC retrievals of ionospheric density and height from occultation data typically assume ionospheric spherical symmetry with no horizontal gradients in the vicinity of the occultation. The presence of horizontal ionospheric gradients can introduce errors into the inversion and lead to inaccurate retrieval results. However, these ionospheric gradients can be characterized using horizontal photometry and supplement GPS occultation results, but the sensor sensitivity should be at least 100 counts s^-1 R^-1 to have sufficient signal-to-noise to impact and improve the GPS retrievals. The COSMIC TIP sensors fulfilled their role as pathfinders for nighttime ionospheric photometry; however, these low-cost, first-generation instruments exhibited some shortcomings of the sensor design and its performance on the COSMIC microsatellites.

The first problem revealed during preflight tests was red leak, which refers to weak residual sensitivity of the 135.6 nm FUV photometer to detect unwanted longer wavelengths, including visible light (“redder” than ultraviolet). On-orbit, the flaw manifested itself as observations of city lights and moonlit clouds that contaminated nighttime ionospheric signals. The second problem involved scattered light, both external to the TIP sensor assembly and within the TIP optical train. Aboard the crowded, compact satellite bus, TIP viewed the nadir through a jagged porthole, and an antenna from another experiment was close to the field-of-view. Inside the instrument several components included stainless steel and iridited aluminum surfaces, which have poor FUV reflectivity, but significant reflectivity in the visible. The impact of this visible scattered light was that the photometer could not operate near the twilight, and the scattered light potentially increased red-leak sensitivity.

The GROUP-C experiment included an upgraded, second-generation TIP module, a refurbished qualification unit from the COSMIC program. Several modifications were made to the TIP optical train to mitigate the scattered light and red-leak problems identified during the COSMIC mission (Figure Left). First, a larger, visibly black filter wheel and black anodizing of the detector housing was implemented to reduce visible scattering and red-leak. A filter overlay was generated using 3D printing of vacuum-rated black Ultem^® polyetherimide plastic with only two filter apertures: one aperture open to collect FUV signal, and the other a sapphire window for monitoring red-leak contamination (Figure Right). Sapphire passes radiation longward of 145 nm, which allows monitoring the contamination alone, without the 135.6 nm ionospheric signal. By differencing the measurements through the open aperture (135.6 nm + red leak) and the sapphire filter (red leak only), the FUV ionospheric signal can be determined. Finally, aboard the STP-H5 pallet TIP did not view through a jagged hole nor have a dipole antenna near the field-of-view.

This work was done by Scott A. Budzien, Andrew W. Stephan, Todd E. Humphreys, Steven P. Powell, Brady W. O’Hanlon, and Rebecca L. Bishop, for the Naval Research Laboratory. For more information, download the Technical Support Package (free white paper) here under the Test & Measurement category. NRL-0081