For real-time precise orbit determination (POD) of low earth orbit (LEO) satellites, high-accuracy global navigation satellite system (GNSS) orbit and clock products are necessary in real-time. Recently, the Japanese multi-GNSS advanced demonstration of orbit and clock analysis (MADOCA) precise point positioning (PPP) service and the new generation of the Australian/New-Zealand satellite-based augmentation system (SBAS) aided PPP service, provide free and precise GNSS products that are directly broadcast through the navigation and geostationary earth orbit (GEO) satellites, respectively. With the high quality of both products shown in this study, a 3D accuracy of centimeters can be achieved in the postprocessing mode for the reduced-dynamic orbits of small LEO satellites having a duty cycle down to 40%, and at sub-dm to dm-level for the kinematic orbits. The results show a promising future for high-accuracy real-time POD onboard LEO satellites benefiting from the precise free-of-charge PPP corrections broadcast by navigation systems or SBAS.
Precise orbit determination (POD) is the procedure for determining the orbit of a satellite with high accuracy. Compared with global navigation satellite systems (GNSS), the low Earth orbit (LEO) satellites have some different features in space, mainly due to perturbations caused by dynamic forces related to their altitudes. Methods for POD of LEO satellites have been developed over the last decades. Nowadays, postprocessed precise orbits are used in different space applications such as radio occultation, satellite altimetry, and interferometric syntheticaperture radar missions. The advancements in technology decrease the size of LEO satellites and developments in theory increase their orbital accuracy. In recent years, onboard POD has become a hot topic in the navigation and positioning field, which is crucial for, e.g., formation flying of small satellites and GNSS-aided LEO megaconstellations for positioning purposes. This contribution reviews LEO POD methods based on undifferenced GNSS observations in both postmission and real-time data processing. It comprises the quality control step, the models used and their limitations, processing algorithms, and different validation methods. To have a clear insight into the current and the future state of the LEO POD, the most recent developments and important achievements are also discussed.
Global Navigation Satellite Systems’ radio occultation (GNSS-RO) provides the upper troposphere-lower stratosphere (UTLS) vertical atmospheric profiles that are complementing radiosonde and reanalysis data. Such data are employed in the numerical weather prediction (NWP) models used to forecast global weather as well as in climate change studies. Typically, GNSS-RO operates by remotely sensing the bending angles of an occulting GNSS signal measured by larger low Earth orbit (LEO) satellites. However, these satellites are faced with complexities in their design and costs. CubeSats, on the other hand, are emerging small and cheap satellites; the low prices of building them and the advancements in their components make them favorable for the GNSS-RO. In order to be compatible with GNSS-RO requirements, the clocks of the onboard receivers that are estimated through the precise orbit determination (POD) should have short-term stabilities. This is essential to correctly time tag the excess phase observations used in the derivation of the GNSS-RO UTLS atmospheric profiles. In this study, the stabilities of estimated clocks of a set of CubeSats launched for GNSS-RO in the Spire Global constellation are rigorously analysed and evaluated in comparison to the ultra-stable oscillators (USOs) onboard the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC-2) satellites. Methods for improving their clock stabilities are proposed and tested. The results (i) show improvement of the estimated clocks at the level of several microseconds, which increases their short-term stabilities, (ii) indicate that the quality of the frequency oscillator plays a dominant role in CubeSats’ clock instabilities, and (iii) show that CubeSats’ derived UTLS (i.e., tropopause) atmospheric profiles are comparable to those of COSMIC-2 products and in situ radiosonde observations, which provided external validation products. Different comparisons confirm that CubeSats, even those with unstable onboard clocks, provide high-quality RO profiles, comparable to those of COSMIC-2. The proposed remedies in POD and the advancements of the COTS components, such as chip-scale atomic clocks and better onboard processing units, also present a brighter future for real-time applications that require precise orbits and stable clocks.
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