Ground Track Pattern Research Articles

Land ice height-retrieval algorithm for NASA's ICESat-2 photon-counting laser altimeter

The Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) and its sole scientific instrument, the Advanced Topographic Laser Altimeter System (ATLAS), was launched on 15 September 2018 with a primary goal of measuring changes in the surface of the Earth's land ice (glaciers and ice sheets). ATLAS is a photon-counting laser altimeter, which records the transit time of individual photons in order to reconstruct surface height along track. The ground-track pattern repeats every 91 days such that changes in ice sheet surface height can be estimated through time. In this paper, we describe the set of algorithms that have been developed for ICESat-2 to retrieve ice sheet surface height from the geolocated photons for the Land Ice Along-Track Height Product (ATL06), and demonstrate their output and performance using a synthetic dataset over various land-ice surfaces and under different cloud conditions. We show that the ATL06 algorithm is expected to perform at the level required to meet the ICESat-2 science objectives for land ice.

Impact of Groundtrack Pattern of a Single Pair Mission on the Gravity Recovery Quality

For future gravity satellite missions, aliasing of high frequency geophysical signals into the lower frequencies is one of the most challenging obstacles to recovering true gravity signals, i.e., to recover the truth. Several studies have investigated the impact of satellite groundtrack pattern on the quality of gravity recovery. Among those works, the concept of sub-cycle has been discussed as well. However, most of that research has focused on the impact of sampling patterns on global solutions up to a fixed maximum spherical harmonic coefficient, rather than the associated coefficient defined by the Colombo-Nyquist and modified Colombo-Nyquist rules. This work tries to look more closely into the influence of sampling patterns on the gravity recovery quality for global and regional studies when the Colombo-Nyquist and modified Colombo-Nyquist rules apply. For the regional study, the impact of groundtrack patterns of different satellite constellation scenarios are investigated for a hydrological basin in central Africa. The quality of the gravity products are assessed by different metrics, e.g., by spatial covariance representation. The potential meaning of the sub-cycle concept in terms of global and local impacts is also investigated by different repeat-orbit scenarios with even and odd parities. Different solution scenarios in terms of the original and modified Colombo-Nyquist rules will be discussed. The results of our study emphasize the impact of maximum harmonics of the recovery, the influence of sub-cycle on the local gravity recovery and the mission formation impact on the recovery error.

Evaluation of multi-mode CryoSat-2 altimetry data over the Po River against in situ data and a hydrodynamic model

Abstract Coverage of in situ observations to monitor surface waters is insufficient on the global scale, and decreasing across the globe. Satellite altimetry has become an increasingly important monitoring technology for continental surface waters. The ESA CryoSat-2 altimetry mission, launched in 2010, has two novel features. (i) The radar altimeter instrument on board of CryoSat-2 is operated in three modes; two of them reduce the altimeter footprint by using Delay-Doppler processing. (ii) CryoSat-2 is placed on a distinct orbit with a repeat cycle of 369 days, leading to a drifting ground track pattern. The drifting ground track pattern challenges many common methods of processing satellite altimetry data over rivers. This study evaluates the observation error of CryoSat-2 water level observations over the Po River, Italy, against in situ observations. The average RMSE between CryoSat-2 and in situ observations was found to be 0.38 meters. CryoSat-2 was also shown to be useful for channel roughness calibration in a hydrodynamic model of the Po River. The small across-track distance of CryoSat-2 means that observations are distributed almost continuously along the river. This allowed resolving channel roughness with higher spatial resolution than possible with in situ or virtual station altimetry data. Despite the Po River being extensively monitored, CryoSat-2 still provides added value thanks to its unique spatio-temporal sampling pattern.

A ground track-based approach to design satellite constellations

Following an approach based on ground track analysis, original and compact relationships which permit the construction of ground track patterns and the determination of satellite arrangements able to generate appropriate track distance and revisit frequency over a given area are presented. These equations are valid in the general case of elliptical orbit and can easily be implemented in computer codes devoted to the design of single and multi-plane satellite constellations.

CryoSat-2 Altimetry Applications over Rivers and Lakes

Monitoring the variation of rivers and lakes is of great importance. Satellite radar altimetry is a promising technology to do this on a regional to global scale. Satellite radar altimetry data has been used successfully to observe water levels in lakes and (large) rivers, and has also been combined with hydrologic/hydrodynamic models. Except CryoSat-2, all radar altimetry missions have been operated in conventional low resolution mode with a short repeat orbit (35 days or less). CryoSat-2, carrying a Synthetic Aperture Radar (SAR) altimeter, has a 369-day repeat and a drifting ground track pattern and provides new opportunities for hydrologic research. The narrow inter-track distance (7.5 km at the equator) makes it possible to monitor many lakes and rivers and SAR mode provides a finer along-track resolution, higher return power and speckle reduction through multi-looks. However, CryoSat-2 challenges conventional ways of dealing with satellite inland water altimetry data because virtual station time series cannot be directly derived for rivers. We review the CryoSat-2 mission characteristics, data products, and its use and perspectives for inland water applications. We discuss all the important steps in the workflow for hydrologic analysis with CryoSat-2, and conclude with a discussion of promising future research directions.

Satellite constellations in sliding ground track orbits

This paper deals with repeating ground track orbits, taking into consideration the case of a single satellite and of constellations composed of one or more orbital planes. First, the orbital relationships able to guarantee a uniform ground track pattern on the Earth's surface are provided. Then, the sliding ground track concept, based on the possibility to shift from a ground track pattern to another by small corrective manoeuvres, is introduced. This technique, which can be applied to both single satellite and satellite constellations, allows the fulfillment of several objectives in the course of the same mission.

Quality assessment of sub-Nyquist recovery from future gravity satellite missions

Drawing on experience from Gravity Recovery and Climate Experiment (GRACE) data analysis, the scientific challenges were already identified in several studies. Any future mission should focus on improvement in both precision and resolution in space and time. For future gravity missions which use high quality sensors, aliasing of high frequency time-variable geophysical signals to the lower frequency signals is one of the most serious problems. The aliasing problem and the spatio-temporal resolution are mainly restricted by two sampling theorems describing the space-time sampling of satellite missions: (i) a Heisenberg-like uncertainty theorem which states that the product of spatial resolution and time resolution is constant, and (ii) the Colombo–Nyquist rule (CNR), which requires the number of satellite revolutions in a repeat period to be at least twice a given maximum spherical harmonic degree. The CNR holds under the assumption of equal ground-track spacing, and limits the spatial resolution of the gravity solution.This study investigates the quality of sub-Nyquist time interval recoveries (when the time intervals are shorter than what is required by the Colombo–Nyquist rule) of different orbit configurations and formation flights, in particular, the dependence of the gravity field accuracy on the measurement duration and ground-track patterns of the satellite formations. It is shown that the fulfillment of the modified CNR, the mission altitude and avoidance of large unobserved gaps by satellite ground-track patterns have the strongest effect on the quality of the recoveries, while the sub-cycle concept does not appear to play an imortant role. It is also found that the modified CNR holds for architectures including two satellite pairs when accounting for the orbital revolutions of both pairs. Moreover, the quality of the solution in the double pair scenario, consisting of a near-polar and an inclined inline pairs, exceeds that of a single near-polar inline pair solution with twice the observations. One important reason is the East–West information of the inclined pair adds to the North–South measurements of the near-polar mission.The simulations of the gravity recoveries of this study are based on a quick-look tool. The tool assumes nominal repeat orbits for satellite missions. Based on the quality assessment of the recoveries and the technical concerns of advanced formation flights as e.g. Pendulum or Cartwheel, a search space for optimal satellite mission scenarios is suggested.

The Use of Resonant Orbits in Satellite Geodesy: A Review

Dynamic resonance, arising from commensurate (orbital or rotational) periods of satellites or planets with each other, has been a strong force in the development of the solar system. The repetition of conditions over the commensurate periods can result in amplified long-term changes in the positions of the bodies involved. Such resonant phenomena driven by the commensurability between the mean motion of certain artificial Earth satellites and the Earth’s rotation originally contributed to the evaluation and assessment of the Stokes parameters (harmonic geopotential coefficients) that specify the Earth’s gravitational field. The technique constrains linear combinations of the harmonic coefficients that are of relevant resonant order (lumped coefficients). The attraction of the method eventually dwindled, but the very accurate orbits of CHAMP and GRACE have recently led to more general insights for commensurate orbits applied to satellite geodesy involving the best resolution for all coefficients, not just resonant ones. From the GRACE mission, we learnt how to explain and predict temporary decreases in the resolution and accuracy of derived geopotential parameters, due to passages through low-order commensurabilities, which lead to low-density ground-track patterns. For GOCE we suggest how to change a repeat orbit height slightly, to achieve the best feasible recovery of the field parameters derived from on-board gradiometric measurements by direct inversion from the measurements to the harmonic geopotential coefficients, not by the way of lumped coefficients. For orbiters of Mars, we have suggestions which orbits should be avoided. The slow rotation of Venus results in dense ground-tracks and excellent gravitational recovery for almost all orbiters.

Evaluation of Russian Arkon-2 Earth observation satellite

Remote sensing satellites are customarily operated in stable near-circular, sun-synchronous orbits that have an average height typically within the range 450–915 km. The operational height selected for an individual satellite depends on factors which include, among others, the degree of detail to be seen on the ground, the ground track pattern and the revisit time required. By comparison, military optical reconnaissance satellites usually operate at heights lower than those for remote sensing, and they may be manoeuvred during flight to alter their ground track pattern according to the coverage required of specific areas. The launch of the Russian Arkon-2 remote sensing satellite during 2002 into a 1505 × 1771 km non-sun-synchronous orbit was therefore, by convention, unusual. The combination of height and the 2 m resolution on the ground claimed for the camera system raised a number of questions regarding design of the imaging system and the extent of coverage that could be expected. A very limited amount of information on the Arkon system has been provided by the NPO Lavochkin Association, and this has enabled some assessment to be made of the imaging system, together with the extent and quality of coverage that could be expected from this unusual remote sensing satellite.

Taking advantage of inclination variation in resonant remote-sensing satellite orbits

The inclination of remote-sensing satellites, which are generally placed in sun-synchronous orbits, varies as a function of the nominal equatorial crossing local mean solar time selected for a given mission. The Indian Remote-Sensing satellites will have an inclination reduction of about 0.034° per year and for most of the satellites, the local time chosen was around 10:30 hours at descending node. In practice, the initial inclination is biased appropriately so that the expensive out-of-plane maneuvers could be taken up after few years of mission operations, depending on the deviations permitted in the local time for a given mission. However, the scenario differs when the mission objectives require an almost exact repeat orbit of 14 or 15 per day. In such a situation, the satellite orbit, which passes through a 14th or 15th order resonance, undergoes a nearly secular increase in orbit inclination. This paper presents a detailed analysis carried out for such an orbit, based on Cowell's approach. Long-term predictions have been carried out by considering all major forces that perturbs the satellite orbit. Observed behavior of orbit, based on the daily definitive orbit determination is also presented. The variation in inclination and the cause is clearly brought out. Further, it is demonstrated that the selection of longitude for nominal ground track pattern has an impact on the inclination variation. A proposal is made to take advantage of such expected inclination variation so that initial inclination bias can be chosen appropriately. Ground track longitude can be chosen to take advantage, subject to the mission coverage requirements. The paper contains the results of an exhaustive analysis of the actually observed orbit resonance. It is felt that the work has both theoretical and operational importance for remote-sensing missions.

Improved spatial resolution of ocean surface topography from the T/P‐Jason‐1 altimeter mission

On 7 December 2001, the French/U.S. altimetric satellite Jason‐1 was launched as a follow‐on to the TOPEX/Poseidon Mission (hereafter referred to as T/P). The two satellites are now flying in tandem with a track separation half that of the T/P mission, offering a much improved sampling capability for the study of ocean circulation and tides.Since 1992, T/P has been measuring the surface topography of the global oceans along a 10‐day repeat orbit, establishing a decade‐long data record that has been used in a wide range of oceanographic and geophysical studies [Fu and Cazenave, 2001]. The primary objective of Jason‐1 is to continue the data record of TIP, by carrying the same instrument payload and measuring sea surface height along the ground track pattern established by T/P, with a measurement performance at least as good as that of T/P [Ménard et al., 2000].

The Orbits and Ground Track Patterns of Soviet Photographic Remote Sensing Satellites

The Orbits and Ground Track Patterns of Soviet Photographic Remote Sensing Satellites

Mars Observer trajectory and orbit design

The Mars Observer launch, interplanetary, Mars orbit insertion, and mapping orbit designs are described. The design objective is to enable a near-maximum spacecraft mass to be placed in orbit about Mars. This is accomplished by keeping spacecraft propellant requirements to a minimum, selecting a minimum acceptable launch period, equalizing the spacecraft velocity change requirement at the beginning and end of the launch period, and constraining the orbit insertion maneuvers to be coplanar. The mapping orbit design objective is to provide the opportunity for global observation of the planet by the science instruments while facilitating the spacecraft design. This is realized with a sun-synchronous near-polar orbit whose ground-track pattern covers the planet at progressively finer resolution.