A Dynamic Deep Space Communication Link Analysis
Tool for the Deep Space Network (DSN)1,2
Yogi Y. Krikorian, Milton K. Sue, Giadira V. Leon, Lamont Cooper, Sieu K. Do, Debra L. Emmons,
Donald J. Dichmann, John P. McVey, and Eric T. Campbell
The Aerospace Corporation
P.O. Box 92957
Los Angeles, California 90009-2957
310-336-1793
Yogi.Y.Krikorian@https://www.doczj.com/doc/6c3750639.html,
1
0-7803-8870-4/05/$20.00 ?2005 IEEE 2
IEEEAC paper #1113, Version H, Updated December 20, 2004 3
DSMS Telecommunications Link Design Handbook,
Document No. 810-005, Rev. E, Jet Propulsion Laboratory, Pasadena, CA.
Abstract —A dynamic deep space communication link analysis tool is described in this paper. This tool, developed by The Aerospace Corporation, provides the capability to analyze coverage and data throughput for communication links between a spacecraft and the Jet Propulsion Laboratory’s (JPL) Deep Space Network (DSN). The tool determines the link margin and data throughput over time during the trajectory of a spacecraft as monitored by the DSN. The analysis takes into account several dynamic effects in calculating the link budget, including transmit and receive antenna gains and space loss. This tool consists of a database derived from JPL’s DSMS Telecommunications Link Design Handbook,3 from which relevant link budget parameters are extracted for all of the 70-m and 34-m antennas. Some of the relevant DSN link budget parameters include receive and transmit gain, as well as system noise temperature. Since these parameters vary, depending on operating conditions, the database takes into account various conditions, such as cumulative weather distribution, month of operation, elevation angle, particular DSN antenna being used, and uplink and downlink frequency. By utilizing the corresponding DSN link budget parameters and the spacecraft’s trajectory and link budget parameters, one is able to determine the amount of coverage and data throughput versus time.
T ABLE OF C ONTENTS
1. I NTRODUCTION .................................................1 2. A STRODYNAMICS T RAJECTORY M ODELING ...2 3. C OMMUNICATIONS M ODELING .......................2 4. A NALYSIS ..........................................................2 5. R ESULTS ...........................................................4 6. C ONCLUSION ....................................................5 REFERENCES ...................................................5 B IOGRAPHY . (5)
1. I NTRODUCTION
Deep space communication’s link analyses are usually performed using static link budgets. For a recent study, a tool was developed at The Aerospace Corporation that allows for dynamic deep space communication link analysis. This tool consists of several components: a database derived from JPL’s DSMS Telecommunications Link Design Handbook, an Astrodynamics analysis program called Satellite Orbit Analysis Program (SOAP), and a driver program called Autosoap. SOAP is a tool that allows for the calculation of positions and velocities of satellites, ground stations, planets, moons, and the Sun. For our purposes, SOAP is mainly used to compute whether there is visibility between a satellite and a given ground station. Given that there is visibility, we then use SOAP to calculate the distance between the satellite and ground station and to make sure that any minimum elevation angle requirements are met. Autosoap is an interface tool to SOAP that allows the user to easily run multiple deep space communications scenarios. Autosoap takes input in the form of three text files. The first text file contains information on the trajectory of a spacecraft and the spacecraft’s relevant communications parameters. The second text file contains the extracted DSN communications parameters from the database. The last text file contains information about the time and dates to perform the calculations. Given these input files, Autosoap will insert the necessary link budget equations and link budget parameters into a SOAP file, have SOAP run the file and perform the necessary calculations,
and format the output so one is able to determine the amount of coverage and data throughput for the duration of interest. This tool allows a team of engineers to quickly and accurately analyze various scenarios for spacecraft missions and determine whether the scenarios can meet mission requirements. If mission requirements cannot be met in a particular scenario, the quick turnaround time realized by the tool allows for analysis of alternative scenarios that may be able to meet mission requirements.
To illustrate the tool’s capabilities, we have performed three example analyses. The first one is of a satellite traveling from Earth to Venus. The second one is of a satellite traveling from Earth to Mars. The final example is of a satellite orbiting Mars. All of these are hypothetical examples, and they do not represent any specific current or future satellite mission.
2.A STRODYNAMICS T RAJECTORY M ODELING
In order to perform a dynamic spacecraft communications link analysis, we needed to compute a spacecraft trajectory. We used two sample interplanetary trajectories, one between Earth and Venus, and another between Earth and Mars. Using a genetic algorithm optimization routine developed at The Aerospace Corporation, we constructed a trajectory that closely matched the launch and arrival dates and velocity changes. A patched conic approximation was used to compute the interplanetary trajectory.4 To meet the launch and arrival dates between each of the trajectories, we applied Battin’s method to solve the Lambert problem.5 The trajectory was then written as a binary file in the SPICE (Spacecraft Planet Instrument C-Matrix Events) format. SPICE is a trajectory format, created at JPL, which allows for easy portability between many astrodynamics software packages.6 The SPICE trajectory file was imported into SOAP, an astrodynamics software tool developed at The Aerospace Corporation that performs orbit geometry computations and space visualization. Using SOAP we were then able to dynamically compute the communications link budget between the spacecraft and the DSN network on Earth. Unlike the first two examples, the third example is of a satellite orbiting Mars.
3.C OMMUNICATIONS M ODELING
For the first example, the spacecraft leaves Earth on August 4, 2010, and arrives at Venus on November 6, 2010 (see Figure 1). During this cruise phase, we determine whether a medium-gain antenna (MGA) can effectively communicate with DSN’s 34-m beam waveguide (BWG) 4C. D. Brown, Spacecraft Design, 2nd ed. AIAA, 1998.
5 R. H. Battin, An Introduction to the Mathematics and Methods of Astrodynamics. AIAA, 1987.
6https://www.doczj.com/doc/6c3750639.html,/naif.html antennas for at least 8 hours a week at 125 bps downlink and 30 bps uplink. The second example is similar to the first, with the exception that the spacecraft leaves Earth on October 26, 2009, and arrives at Mars on June 10, 2010 (see Figure 2). The third example is slightly different from the other two. In this example, this satellite is orbiting Mars, and we will determine whether it can send back 20 GB of data over a period of one year using a high-gain antenna (HGA) to communicate with the 34-m BWG antennas (see Figure 3). We set the downlink data rate at 75 kbps and the uplink data rate at 64 bps. See Table 1 for the spacecraft antenna parameters used, Table 2 for the communications link parameters used, and Table 3 for the DSN antenna parameters used. The gains and system temperature for the DSN antennas are extracted from the database based on 0.95 cumulative distributions of the weather and the uplink and downlink frequencies, as well as a 20° minimum elevation angle for link closure. The 70-m DSN antennas are listed because we also consider the 70-m in case the 34-m BWG antennas cannot provide the required coverage.
4.A NALYSIS
The satellite trajectories along with the link parameters listed below are inserted into SOAP, via Autosoap, for computation of link closure and data throughput versus time. Autosoap generates the necessary SOAP files and runs SOAP to perform all the necessary calculations for uplink and downlink to the 34-m BWG antennas. As a precautionary measure, we also have Autosoap initiate the computations of the uplink and downlink to the 70-m antennas. As an example, Figure 4 is a visual representation of what it looks like after all the input is inserted into SOAP. Figure 4 represents the trajectory of the satellite traveling from Earth to Venus with the icon labeled “Venus Sat,” representing the satellite. Each of the three DSN sites is also labeled. As the spacecraft continues along its trajectory or orbit, SOAP continually computes the link margin for the spacecraft to ground link. As an example, the equation for the downlink to the Canberra DSN site is: Satellite to Canberra Link Margin =
Sat Tx Power (dBm)
+ Sat Tx Gain (dBi)
+ Sat Tx Circuit Loss (dB)
+ Sat Tx Pointing Loss (dB)
+ Downlink Modulation Loss (dB)
+ 10*log10(Downlink Data Rate (bps))
+ Sat to Canberra Space Loss (dB)
+ Canberra Rx Gain (dBi)
+ Canberra Rx Losses (dB)
– (–198.6 + 10* log10(Canberra Rx System Temp (K))
Equation 1
SOAP is set up so that it computes the link margin only if the spacecraft is visible to the ground station and is above the ground station’s minimum elevation angle. To add
some safety margin to all the calculations, we consider a link closed only if the link margin is at least 3 dB. Once the calculations are completed by SOAP, Autosoap formats the data and outputs it into a text file.
Figure 1 – Example 1 trajectory
Figure 2 – Example 2 trajectory
Figure 3 – Example 3 orbit
Figure 4 – Example 1 Ground View
Table 1. Satellite antenna parameters
Ant. Type Tx power (W) Tx gain (dBi) Tx circuit loss (dB) Tx pointing loss (dB) Rx gain (dBi) Rx loss (dB) Rx System Temp
(K)
MGA 20 10 -1 -0.75 10 -7 600 HGA 20
36
-1
-0.75
36
-7
600
Table 2. Communications link parameters
Downlink
Uplink
Case Required Eb/N0 (dB) Data Rate (bps) Modulation Loss (dB) Frequency (MHz) Required Eb/N0 (dB) Data Rate (bps) Modulation Loss (dB) Frequency
(MHz)
1 1.24 125 -2.6 8425 9.6 30 -4.
2 7170 2 1.24 125 -2.6 8425 9.6 30 -4.2 7170
3 1.2
4 75000 -2.6 842
5 9.
6 64 -4.2 7170
Table 3. DSN antenna parameters
Ant. Designation Ant. Type Location Tx power (W) Tx gain (dBi) Tx circuit loss (dB) Tx pointing loss (dB) Rx gain (dBi) Rx loss (dB) Rx System
Temp (K) DSS 14 70-m Goldstone,
CA 20000 72.93 0.45 0.1 74.28 -0.1
24.58 DSS 25 34-m BWG Goldstone,
CA 20000 66.88 0.4 0.1 68.22 -0.1 38.83 DSS 34 34-m BWG Canberra, Australia 20000 66.74
0.4
0.1
68.08 -0.1
47.97
DSS 43 70-m Canberra, Australia N/A N/A N/A N/A 74.25 -0.1 28.61 DSS 55 34-m BWG Madrid, Spain 20000 66.75 0.6 0.1 68.09 -0.1 32.97 DSS 63
70-m
Madrid, Spain
20000
72.72
0.45
0.1
74.08
-0.1
28.16
5. R ESULTS
The output provided by Autosoap is in the form of a table
listing things such as the total data that can be transmitted (for both uplink and downlink), the number of times the link can be closed, the minimum, average, and maximum durations that the link is closed, and the minimum, average, and maximum amount of data that be transmitted during a closed link. Depending on the example, different results will be of interest. The first two examples (the Earth-to-Venus example and the Earth-to-Mars example) require the number of times the link can be closed as well as the duration of the link closure to be evaluated. The last example (the satellite orbiting Mars example) requires only the total data that can be transmitted on the downlink to be evaluated.
From the results listed in Figure 5, we can see that the Canberra site is the only one that can support an 8-hour track. From the data, we can see that there is at least one time that the link is closed a bit short of 8 hours, but the average time for closed links is well over the 8-hour period. Since there are 94 closed links for Canberra, there is an approximately 8-hour track time every day during the satellite’s cruise. Also, the constraint was set that a minimum elevation angle of 20 degrees had to be met for link closure. Should the minimum elevation angle constraint be lowered to 10 degrees, the link closure requirement most likely would be met. If necessary, weekly analysis could be performed to ensure that Canberra truly meets the requirement of 8-hour tracks per week.
From Figure 6 it can be seen that the Goldstone site can easily support 8-hour tracks, however, the duration of the cruise is roughly 227 days and Goldstone does not have that many closed links on the downlink. Madrid has some fairly long tracks, but they are all slightly short of 8 hours. It appears that even with the usage of Madrid as an additional tracking site, the 34-m BWG antennas would be unable to support an 8-hour track every week. Looking at Figure 7, we can see that the 70-m antenna at Goldstone would definitely be able to support 8-hour tracks every week. To support 8-hour tracks every week, the 34-m BWG at Goldstone can be used most of the time, and the 70-m at Goldstone can be used when the 34-m BWG is not capable of closing the downlink with the spacecraft. If desired, analysis can be done to determine exactly which weeks the 70-m would be needed. All that would need to be done is to examine the trajectory weekly, instead of analyzing the whole trajectory at once.
The 34-m BWG results for example 3 are not shown because the downlink cannot be closed for the entire duration of the analysis period. Figure 8 depicts the downlink and uplink with the 70-m antennas. Unlike the previous two examples, we are not looking for 8-hour tracks per week for this example. Example 3 is about whether the orbiting spacecraft can downlink 20 GB of data over one year. Figure 8 shows that any of the 70-m antennas can achieve over 20 GB of data transmission.
Figure 5 – Earth-to-Venus results
Figure 6 – Earth-to-Mars results
Figure 7 – Earth-to-Mars results
Figure 8 – Orbiting Mars results
6.C ONCLUSION
The dynamic deep space communication link analysis tool described provides a fast, simple method to verify whether or not a given scenario meets mission requirements. We provided three examples—two using interplanetary spacecraft, one using a orbiting spacecraft—to illustrate the power of this tool. For the first two examples, the mission requirement was an 8-hour track per week. The proposed scenario was to use the 34-m BWG antennas to support this requirement. However, from the analysis performed, it was shown that, for example two, a 70-m antenna was also needed to meet the requirement. For the orbiting Mars example, the proposed scenario was to use the 34-m BWG antennas to meet the mission requirement of sending back 20 GB at a given data rate. However, for the given data rate, the 34-m BWG antennas were unable to close the downlink. Since we also performed the analysis with the 70-m antennas, it was easy to show that with the use of the 70-m antennas the mission requirement could be met. REFERENCES R.H. Battin, An Introduction to the Mathematics and Methods of Astrodynamics, AIAA, 1987.
C.D. Brown, Spacecraft Mission Design, 2nd ed., AIAA,
1998.
DSMS Telecommunications Link Design Handbook, Document No. 810-005, Rev. E, Jet Propulsion
Laboratory, Pasadena, CA.
https://www.doczj.com/doc/6c3750639.html,/advmiss/index.html
https://www.doczj.com/doc/6c3750639.html,/naif.html
B IOGRAPHY
Yogi Y. Krikorian has over 14 years of
experience in communications
engineering, including 5 years in
commercial industry at Hughes Space
and Communications Company. While
at Hughes, Mr. Krikorian worked as a
Payload System Engineer on the ICO
Global Communication Satellite Program. Mr. Krikorian also served as Manager of Applications Engineering at Elanix, Inc. in Westlake Village, CA, where he provided technical expertise and support on SystemView, a PC-based software simulation program for communications systems. Other commercial experience includes serving as Senior Technical Engineer and Director of Engineering at RJS, Inc. in Santa Fe Springs, CA.
Mr. Krikorian rejoined The Aerospace Corporation in August 2000, after spending 8 years at Aerospace (1987–1995), during which time he earned his master’s degree in electrical engineering from the California Institute of Technology. Mr. Krikorian recently analyzed, simulated, and presented a detailed briefing on the susceptibility of the ICO commercial satellite to pulsed radar frequency interference for GMSK and QPSK modems. Mr. Krikorian also received a Letter of Commendation for support of the STAR 37S Nozzle Anomaly Corrective Action Study, for which he analyzed nozzle radiographs using digital image processing to measure nozzle wall thickness. Mr. Krikorian
is currently involved in communication link and data throughput analysis for the Mars Satellite Orbiters and Mars Ground Rovers in conjunction with the Jet Propulsion Laboratory (JPL) and NASA.
Dr. Donald Dichmann is an
Engineering Specialist at The
Aerospace Corporation. He has
worked in the astrodynamics field for
over 15 years, and was instrumental in
the development of the spacecraft trajectory design tools Swingby and STK/Astrogator. He received his Ph.D. in applied mathematics from the University of Maryland.
Milton K. Sue is a Member of the
Technical Staff at The Aerospace
Corporation. He received his
master’s degree in electrical
engineering from the University of
Southern California and joined The
Aerospace Corporation in 2003.
Currently, Mr. Sue is working on
characterizing the performance of various modems in different communications channels. Prior to working at Aerospace, Mr. Sue worked at a start-up analyzing fiber-optic communications systems.