TRACKING

Based on an initial set of Keplerian elements, the SatTrack software package (Version 4.0) has been employed to propagate the orbits of the four Quatro spacecraft for a time right after deploy and for a time starting three months later. Questions addressed are the circumstances of passes over suitable ground stations for downlinking of science data, as well as spacecraft orbit determination. The following set of Keplerian elements was used for this analysis: 

Epoch:                 QUATRO 1     QUATRO 2     QUATRO 3     QUATRO 4
21Dec96  0:0:0 UTC

Apogee Radius [RE]       12.00        12.00        13.50        13.50
Perigee Radius [RE]       3.00         3.00         3.50         3.50

Inclination [deg]         7.57        10.89         8.16         8.16
RAAN [deg]              325.45       304.09       307.43       307.43
Arg Perigee [deg]        33.19        54.29        51.14        51.14
Mean Anomaly [deg]        0.00         0.00         0.00        13.00

The passes of all four Quatro satellites have been computed for two ground stations, one at Space Sciences Laboratory in Berkeley, California, and the other at the ESA tracking station in Malindi, Kenya. A time span of 12 days, beginning on March 21, 1997 (1997/80) has been covered. Two filters, one for elevation and one for range were used.

Quatro Passes Over Berkeley and Malindi
1997/80:00:00:00 - 1997/93:00:00:00 UTC
Filter: Elevation above 0 degrees; Range less than 20,000 km
 
           Orbit             Berkeley                        Malindi

QUATRO 1     76                                     80:08:05:00 - 80:10:25:00
             77     81:13:20:00 - 81:13:40:00
             78     82:18:05:00 - 82:20:05:00       81:14:20:00 - 81:15:35:00
             79
             80     84:00:20:00 - 84:01:20:00       85:03:35:00 - 85:05:05:00
             81                                     86:08:55:00 - 86:11:10:00
             82     87:13:45:00 - 87:14:40:00
             83     88:18:45:00 - 88:20:50:00       87:15:15:00 - 87:16:00:00
             84     90:01:00:00 - 90:01:55:00
             85                                     91:04:10:00 - 91:06:05:00

QUATRO 2     76                                     80:08:05:00 - 80:10:20:00
             77     81:13:15:00 - 81:13:45:00
             78     82:18:00:00 - 82:20:05:00       81:14:20:00 - 81:15:35:00
             79     84:00:00:00 - 84:01:20:00
             80                                     85:03:30:00 - 85:05:05:00
             81                                     86:08:55:00 - 86:11:10:00
             82     87:13:45:00 - 87:14:45:00
             83     88:18:45:00 - 88:20:50:00       87:15:15:00 - 87:16:00:00
             84
             85     90:00:50:00 - 90:01:55:00       91:04:10:00 - 91:06:05:00

QUATRO 3     65
             66                                     84:12:10:00 - 84:13:35:00
             67
             68     85:23:30:00 - 86:00:15:00       87:09:25:00 - 87:11:20:00
             69     88:20:30:00 - 88:22:05:00
             70                                     90:06:55:00 - 90:08:50:00
             71     91:18:05:00 - 91:19:40:00


QUATRO 4     65                                     81:13:20:00 - 81:14:20:00
             66     83:00:20:00 - 83:01:00:00       84:10:35:00 - 84:12:20:00
             67     85:21:35:00 - 85:23:05:00
             68                                     87:08:00:00 - 87:09:55:00
             69     88:19:05:00 - 88:20:45:00
             70                                     90:05:35:00 - 90:07:20:00
             71     91:16:45:00 - 91:18:10:00

A preliminary analysis of the above passes shows that two stations separated by almost 180 degrees in longitude on the Earth should provide enough coverage to allow almost all data to be downlinked during times when the spacecraft are within a range of less than 20,000 km. A more careful analysis covering a larger time interval will be needed.

TRACKING PLOTS:

A collection of sample plots created with SatTrack V4.0 show various aspects of the circumstances of passes suitable for downlinking of science data. Data have been processed with the SatTrack Graphics Visualization Tool.

Azimuth/Elevation Polar Plot. This plot shows the sky track of Quatro 1 over a period of 12 days, beginning on 21 March, 1997, as seen from Berkeley.

Elevation Plot. This plot displays the elevation of Quatro 1 as a function of time over the same time interval, also seen from Berkeley.

Range Plot. Range of Quatro 1 as a function of time.

Doppler Shift Plot. The Doppler shift has been calculated for a downlink frequency of 2300 MHz.

Path Loss Plot. The free-space path loss at 2300 MHz, show as a function of time, will be used to analyze link quality. Other aspects of the link margin calculations will take into account the antenna pattern of the ground station antenna and the proximity angle between the spacecraft and the Sun.

The five screen samples shows simulations of the SatTrack V4.0 real-time tracking capabilities for multiple satellites.

Screen Example 1: Graphical User Interface in real-time multisat tracking mode for four objects.

Screen Example 2: Graphical User Interface in real-time single-satellite tracking mode for one selected spacecraft, showing the state vector, location of the sub-satellite point and circumstances of the next available pass.

Screen Example 3: Graphical User Interface in real-time single-satellite tracking mode for one selected spacecraft, showing the osculating orbital elements.

Screen Example 4: Sky View Display with the location of three of the four Quatro spacecraft right after deploy.

Screen Example 5: World Map Tracking Chart. The location (nadir view) of all four Quatro spacecraft is shown in this simulation. The ground track is plotted for Quatro 1 for three orbits. Also shown are the zero-elevation acquisition contours (in red) for two ground stations in Berkeley and Malindi, as well as the footprint of the selected probe, Quatro 1.

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Please send comments/suggestions to:
vassilis@ssl.berkeley.edu