Chapter 6 Follow-On Mission Trade Analysis and Design
6.2 Follow-On Mission Definition Trade Analysis Results
In this Section we discuss follow-on mission major trade analysis results performed during the advanced study mission definition phase. The major trade analysis results include the mission orbit properties, the orbit inclination angle, the sounding data distribution, the
proposed follow-on constellation spacecraft configuration, and the number and density of occultation data points. Then we discuss the data latency analysis that will impact to the overall space system architecture design and ground communication network. At the end we show the follow-on mission system architecture and preliminary spacecraft conceptual design [32], [36], [67].
6.2.1 Mission Orbit Properties
The follow-on mission requires the satellite at low-Earth-orbit from 500 km to 900 km.
The engineering consideration on the altitude is mainly for the constellation deployment period. Constellation deployment period is a function of inclination angle, eccentricity, and difference of the parking orbit altitude. If the altitude difference of parking orbit and mission orbit is larger, it will be sooner for the mission to achieve its final constellation.
Therefore, we propose 500 km as the parking altitude and 800 km as the mission altitude.
As for the shape of the orbit, a circular orbit is preferred for simplification. The optimal performance of the radio occultation payload is to have highest gain pointing to the Earth surface. However, if there is a requirement from scientific payload, it is probably feasible to have one satellite with an elliptical orbit with the difference of apogee and perigee less than 150 km, which is the capability of GOX on F3.
6.2.2 Orbit Inclination Angle
The following four important factors depend on the orbit inclination angle:
(1) Number of ground stations: general speaking, if the satellite is at high inclination angle orbit, it requires fewer ground receiving stations to achieve the full data dumps per revolution.
(2) Constellation period: the constellation period depends on the cosine the inclination angle. Therefore the inclination angle can not be too close to 90o.
inclination angle is as shown in Figure 6-1. It is understandable that the number of occultation is higher if the inclination angle is higher since the GNSS system is orbiting at a higher inclination angle.
(4) Data distribution and spatial density: the topic will be analyzed further since the mission requires the data to be distributed homogeneously over the globe.
The analysis of inclination angle vs. measurement distribution has been studied and published by authors [32], [36], [67]. It is realized the inclination angle of 72o of F3 will make the measurements in low latitudes a little bit sparse. Therefore, there will be a need to add some satellites at a low inclination orbit.
6.2.3 Sounding Data Distribution and Spatial Density
We define the “equivalent area covered by one occultation” or “horizontal spatial density” as the average area in square km associated with a single sounding, e.g., one sounding per N km (x N km). As we take a closer look at the dependence of data distribution and density with inclination angle, a high inclination angle favors the data collection at high latitudes and a low inclination angle favors the data distribution at low latitudes. Taking a 72o inclination as an example (see Figure 6-2), the data distribution at low latitudes is sparser than at high latitudes. Within the latitude zone of -10o to +10o, there is one sounding per 1530 km x 1530 km and within the latitude zone of 80o to 90o (northern and southern hemisphere), there is one sounding per 800 km x 800 km .
Figure 6-3 shows our analysis for inclination angles of 0o, 12o, 24o, 60o, 72o, 90o and 98.6o. 98.6o is corresponds to a 800 km sun-synchronous orbit. One can see the trend for 72o, 90o, and 98.6o are similar and the trend for 0o, 12o, and 24o are similar. Therefore, the approaches for global distribution homogenously are (1) to pick the inclination in the middle;
(2) to choose a satellite constellation combined with high inclination and low inclination.
For this project, we start with the latter approach because F3 is a constellation with 72o
inclination angle and it is running well in terms of payload, spacecraft, and data centers.
6.2.4 Follow-on Spacecraft Constellation
We propose the following constellation of 12 satellites (Figure 6-4 shows 12 satellites constellation) as follows: 8 of them will be at high inclination angle (72o for this analysis) and 4 of them will be at low inclination angle (24o for this analysis). The satellites at high inclination angle will be stacked in one (or two) launch vehicle(s) and be placed to the parking orbit. The operations team will then perform the thrust burns so that their orbital plane can be separated through the differential precession rate with the differential orbit altitude. The satellite at low inclination angle will go through the similar launch and constellation deployment process. The final constellation of 12 satellites constellation would be 8 high-inclination-angle satellites at 8 orbital planes which are marked as pick lines in Figure 6-4, and 4 low-inclination-angle satellites at 4 orbital planes which are marked as blue lines in Figure 6-4.
6.2.5 Occultation Points
With the various uncertainties on the follow-on project, we also calculate the number of occultation points with 12 satellites in the constellation. They are listed in Table 6-1.
Figure 6-5 shows the daily occultation point distribution with 12-satellite constellation for the F3 follow-on mission. The calculation is based on 28 GPS satellites, 27 GALILEO satellites, and 21 GLONASS satellites with the assumption of 350 effective atmospheric profiles per LEO per day if the satellites perform similarly to the F3 satellites. Please note that the estimation is based on the following ideal conditions: no spacecraft emergency, no anomaly on ground segment, and no errors from operation segment.
6.2.6 Data Latency
The data latency depends on the number and locations of the available ground stations in
the world. In the analysis, the ground stations, which are located at Fairbanks, Tromso, and McMurdo, used for F3 are assumed to receive the data from the high-inclination-angle satellites of the follow-on mission. For the low-inclination-angle satellite, we tentatively use TT&C stations located in Taiwan, Banglore, and Mauritius for the RO number calculation and latency analysis. These three low-latitude ground stations can also to support data dumps from the high-inclination-angle satellites. To maximize the use of the ground stations, the argument of latitude of the orbit needs to be phased properly to avoid more than one spacecraft flying over the same ground station at the same time. For a constellation of 12 satellites, the data latency due to storage and dumping is about 36 minutes on the average. If we assume ground network and processing take about another 14 minutes, the total average data latency is about 50 minutes.
6.2.7 Effective Coverage Area
Currently, the F3 constellation can collect about 2500 measurements per day when all six GOX are at 100% duty cycle. After the data are processed, the number of good atmospheric soundings is about 70% of the total measurements. In other words, there are approximately 1600-2200 good soundings per day depending on the GOX duty cycles. For this number of soundings the spatial data density is about one sounding per 550 km x 550 km. It should be noted that the horizontal scale of a tropical cyclone is about several hundred square kilometers.
Therefore F3 may take only one measurement in the area of highest interest. Therefore, the follow-on mission should have significantly more soundings distributed more or less homogeneously over the globe to make the system a significant improvement over F3. The effective spatial data density in the contemplated 12 satellites constellation of the follow-on mission with GNSS capable of receiving GPS, GALILEO and GLONASS signals can be reduced to one soundings per 250 km x 250 km.