Performance Analysis of a UHF/VHF Communication System on ... · 2014 COSGC Space Research...

Performance Analysis of a UHF/VHF Communication System on an Orbiting Small Satellite

Caitlyn Cooke

[email protected] DANDE Software Systems Lead

Colorado Space Grant Consortium, Boulder, Colorado 80309

William Sear [email protected]

DANDE Command and Data Handling Lead Colorado Space Grant Consortium, Boulder, Colorado 80309

April 7th, 2014

Abstract

Over the course of the DANDE satellite mission the satellite’s communication system has not been thoroughly reviewed in order to determine the quality of its connection to the COSGC ground station over time. This paper will focus on what

factors, either environmental, ground station based, or satellite based, have influenced the ability of the DANDE mission operations team to uplink and downlink data with the satellite. Particular emphasis will be placed on the amount of data downlinked by the operations team, the signal strength and position of receipt of DANDE beacons, the overall stability of

the satellite platform, and the general practices of the mission operations team.

2014 COSGC Space Research Symposium Page 1

1. Introduction The Drag and Atmospheric Neutral Density Explorer (DANDE) started in 2007 as a student developed Nano-Satellite

project at the Colorado Space Grant Consortium. DANDE’s mission is to study the effects of space weather and atmospheric drag on a small satellite platform. This involves coupling the data produced by a neutral mass spectrometer and a 6-axis accelerometer system to correlate the effects of neutral particles in the thermosphere to the motion of the satellite through space. DANDE consists of eight electrical subsystems, namely the neutral mass spectrometer (NMS), the accelerometer system (ACC), command and data handling (CDH), electrical power systems (EPS), thermal systems (THM), attitude control systems (ADC), communications (COM), and the separation mechanisms (SEP). The DANDE satellite launched on a Space-X Falcon 9 rocket from Vandenberg Air Force Base on September 29th, 2013 and is now in the operations phase of project lifetime. The communications subsystem plays a critical role in the acquisition of information from the DANDE satellite on orbit. The system operates in the UHF/VHF band, with an uplink frequency of 145.86 MHz and a downlink frequency of 436.75 MHz, and a baud rate of 9,600 bits per second. The supporting ground station consists of a Yagi antenna, a radio transceiver, and a terminal node controller, which provide the path to send and receive AX-25 formatted data using a standardized KISS communications protocol. The terminal node control funnels data packets to and from the InControl software package, designed by L-3 Communications, which provides the operator user interface for communication with the DANDE satellite on orbit. During the operations phase of the DANDE project, many unexpected challenges have been presented in the acquisition of data from the satellite, which have led to an extensive analysis of the performance of the communications system, and the factors that influence the uplink and downlink of data.

Over the course of the on-orbit mission lifetime, the amount of data collected from the DANDE satellite has been significantly lower than originally predicted by pre-launch system analysis. Based on the system’s data generation rate, data downlink rate, and amount of anticipated pass time, the team presented a prediction of the anticipated abilities of the system with regards to data acquisition. After almost four months of communicating with the satellite on orbit, the amount of data downlinked from the system is only 4 percent of this original prediction. Many factors have the potential to influence this number; including the performance of the flight transmitter and receiver, the reliability of the ground station equipment, the techniques of the mission operators, the parameters of the overhead passes, and the health of the entire flight system. Though the performance of the communication equipment is the main focus of the analysis performed, multiple subsystems and operating processes have also been considered in order to provide a broader picture of what was primarily responsible for the inaccuracies of the original data acquisition prediction. From the team’s knowledge of the DANDE system and the data collected during the operations phase of the project, many questions have been answered regarding the performance and expectations of a student-built small satellite on-orbit. These findings will allow future small satellite both in Space Grant and elsewhere to be designed and operated with more capable communications systems that will better support mission objectives.

2 Data Generation and Downlink Rates

The analysis of the communications system surrounds the trends seen in the data downlinked from the system. The DANDE Command and Data Handling system collects a total of 20 health and status data buffers, including space craft power, thermal, communication, and attitude data in the nominal standby system mode. Considering collection rates configured in the data collection, it was calculated that the space craft would produce a total of 9.2MB of compressed subsystem health and status data per day. Over the course of integration testing during a system-wide test referred to as the “Day in the Life Test”, the team performed testing on the downlink capabilities over an RF connection. By transmitting data across the lab, it was determined that the link provided an average data downlink rate of approximately 450kB/s. Taking into account that the space craft is only predicted overhead 11.8 minutes per day, this equates to only 319kB of data maximum downlinked data in one day.

Knowing ahead of time the limits of the communication system downlink abilities, the team took many steps to attempt to reduce the volume of data produced on the space craft in order to put more emphasis on the meaningful data products. The primary solutions to this problem were to average the data points over an extended time series and save it as one data point in a file, as well as slowing down overall data generation rates. For example, during the Day in the Life testing phase, the data collectors were configured to collect thermal data from all 10 thermal buffers once every five seconds. It was later determined that the thermal properties of the space craft were not capable of varying a significant amount during that collection period, thus the collection rates were reduced to approximately once every 30 seconds. Techniques such as these improved the operations team’s ability to target desired and meaningful data products and be able to downlink what was needed in a reasonable amount of passes, and kept the data storage memory at a lower level.

Despite the steps taken to control the generated amount of data in order to be able to retrieve necessary data products, the amount of data downlinked from the system over the course of the mission was still significantly lower than the


already minimal hypothesized budget. Figure 1 below shows the amount of data downlinked during each pass over a two month time period, with pass 100 on November 7th, 2013, and pass 280 on January 9th, 2014.

Figure 1. Data Downlink Rate Over Time

This shows that the behavior on orbit is significantly different from the pre-launch prediction. If the data presented in

this plot is averaged over the course of the mission, it equates to 11.9 bytes per sec for an average of 20.7 minutes of pass time per day. The total amount of downlinked data was 14,780 bytes per day, which is only 4% of the anticipated downlink capabilities. Even on the best pass, the system only achieved 112.3 bytes per second for 8 minutes, which is about 25% of the anticipated nominal rate. This discrepancy severely reduced the amount of data that the DANDE mission operators were able to acquire and view throughout the lifetime of the mission which reduced the amount of science data that could be collected, left significant gaps in the knowledge of the overall system performance, and increased the team’s response time to critical errors due to lack of knowledge of the factors that led to the error. We also note the decrease in overall data acquisition over the course of the mission lifetime. In the following sections, possible factors that lead to this discrepancy will be discussed further. 3. Communications System Data Products

The communications subsystem on DANDE satellite reports a number of data products collected as buffers on the space craft and downlinked using the ground station interface. Among these data products is a series of information regarding the performance of the on-board transmitting unit, the receiving unit, and the operating temperature of the subsystem. In addition to the space craft generated buffers, data was also collected by the ground station equipment, providing information about the amount of data downlinked, the pass parameters, and the receive signal strength. This information provides significant insight into the performance of the system on orbit. 3.1. Ground Software Influences

The ground station and ground software capabilities are one of the most crucial part of the success of the DANDE communications system overall. Without a reliable way to receive and process the incoming data, the performance of the onboard communications system is meaningless. The ground software configuration introduced several inefficiencies in the teams’ ability to effectively downlink data, of which a few will be addressed here. One major factor is related to the file transfer protocol that is implemented within the L3 InControl software control system, used to command and downlink data from the system. The file transfer protocol in place is called Zmodem, which is a commonly used open source protocol leveraging TCP/IP. During Day in the Life testing, it was noted that that this protocol has a standard packet size in which is transmits data over the RF link. This directly influenced the efficiency of the downlink rates as it related to the size of the data files. This meant that there was a specific file size that was to be targeted on the space craft in order to efficiently fill the Zmodem packets for downlink. Unfortunately, there was a failure in the DANDE system configuration that only allowed data files that contained one data point per file. This configuration generated files that were much below the targeted efficient file size, which means that there is significant wasted overhead in transmitting

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Zmodem packets that are not filled with the maximum amount of data. This decrease in efficiency had a significant effect on the amount of data the team was able to downlink over the course of the mission.

Another major factor injected by the ground software configuration is regarding the command execution timer. This mechanism is responsible for providing a timeout when commands fail being sent up to the system. When transmit or receive signal strength is not consistent throughout the pass, as we will explore further in the next section, there is a possibility of not being able to successful gather data and send the space craft commands consistently throughout the pass. When a command is initiated, it should execute on the space craft within a number of seconds. The operator will be notified of the success and can continue to the next task. When the command fails to send though, the InControl software will sit in idle for a preconfigured amount of time. The command timer keeps track of this and then ends the command attempt after this preconfigured amount of time. In the configuration for the DANDE mission, this time was set much too high, causing the commanding system to sit in idle much longer than necessary. This leads to a significant amount of overhead in simply attempting to send commands, which wastes valuable pass time and lowers the amount of overall data that can be gathered.

Other factors that could play a role in the decrease in data acquisition are regarding operator interface. In some instances, commands in the catalog could take a significant amount of time to find, or must be developed from scratch during the pass. There were also errors seen in the execution of preconfigured commands, where they were not programmed correctly, or required an unnecessary password to be typed by the operator in order to execute. Flaws in the efficiency of the user interface such as these lead to a decrease in data acquisition because pass time was not used in a productive manor.

3.2. Signal Strength and Pass Parameters The receive signal strength of the ground station equipment affects the ability of the TNC to decode the incoming data, and thus directly influences the ability to receive and process data. Figures 3 and 4 below are plot graciously provided by one of the team’s loyal amateur radio operators, who listened frequently for DANDE overhead from his amateur ground station in Australia. Though we acknowledge that there are a lot of unknown factors of this setup that could potentially invalidate this data, it does provide insight into receive signal strength, which the DANDE team was unable to collect. The plots show the receive signal strength in dBm versus time in the pass, with the elevation of the space craft overlaid in green. The blue bars indicate a beacon packet that was received, but was not strong enough to be decoded, while the red bars are packets that were able to be decoded. From this information, we see that the quality of the pass is a major contributing factor in the ability to receive and process data. The figure on the left shows a very successful pass, with high elevation. Once the spacecraft rose above about 20 degrees, the amateur HAM radio operator’s ground station was able to decode the beacon packet. On the other hand, the figure on the right shows a very unsuccessful pass in which the space craft never rose particularly high enough to be able to decode the data consistently. This figure also incorporates another major factor, in which an obstacle (a tree near his ground station setup) blocked the receive antenna and lowered the receive signal strength further.

Figure 2. DANDE Pass on 11th October at 88

Degrees Elevation [1] Figure 3. DANDE Pass on 4th November at 19

Degrees Elevation [1]


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The DANDE team saw similar trends with the on-sight ground station equipment. Passes that were at high elevation, and closer range tended to be more successful, as the receive signal strength was higher and provided a more reliable connection. This reliable connection is crucial to decreasing ground software efficiency problems, as discussed in the previous section. Figures 4 and 5 below show data collected at our ground station here in Boulder. The plots indicate the range and the elevation of the space craft at which we received the first successfully decoded beacon packet. We see from this data that these orbital parameters do have a significant effect of the acquisition of data. Overall, the operations team saw similar trends of not being able to receive beacons successfully until a certain elevation was reached. In Boulder, the team also had factors such as the mountains to the West, which caused the beacon acquisition to be higher during passes that originated from this direction. Figure 6 shows an analysis of the pass time versus the amount of data downlinked. The pass time in directly related to the speed at which the space craft flies over, the elevation and the range. As we can see from the plot, there is an ideal pass time for maximum data downlink, which suggests that there is an ideal set of orbital parameters that influence this. More notably though, we point out that there does not seem to be a trend of degradation of the communication system throughout the mission lifetime. As the space craft gets further into mission lifetime, we actually see in increase in the ability to receive beacon data at farther ranges and lower elevations, suggesting that system degradation was not a contributing factor in the decrease of downlinked data from the system over the lifetime of the mission.

Figure 4. Range of First Beacon vs. Time Figure 5. Elevation of First Beacon vs. Time

Figure 6. Pass Length versus Downlink Data

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In examining the amateur radio operator’s plots further, we can see some other trends in receive signal strength that could be of interest. Figure 7 on the left shows a pass that was conducted before DANDE’s separation event and figure 8 on the right shows a pass that was conducted after the separation event. The separation mission milestone involves ejecting a large metal plate that is used to connect the round satellite to the launch vehicle. With this plate removed, the DANDE space craft becomes a sphere with a known center of mass and surface area, which are two major design features needed for successful science operation, further discussion of this is outside the scope of this analysis. What is noteworthy about this event is the separation event causes a large amount of mechanical movement of the entire space craft, which has the ability to significantly change the attitude state of the space craft. In the first plot we see almost an alternating pattern between the red and blue bars, possibly suggesting the space craft transmit antenna was being pointed towards and away from the ground station with a certain frequency. After the separation event, we see a modified pattern, in which the frequency of the low signal periods is spread out further across the length of the pass. This stark change manifests itself in several plots given to us by the amateur radio operator that are not shown here. This could possible suggest that the orientation and attitude state of the space craft is partially responsible for the inconsistent receive signal seen over the course of the mission. It is necessary to note that without the completion of the attitude stabilization and adjustment phases, the orientation of the space craft is not known throughout the length of the mission, but this data does provide some potential evidence that the attitude of the space craft and the directionality of the antenna could have been a substantial factor in receiving a consistent signal.

Figure 7. DANDE Pass on 30th October at 36

Degrees Elevation [1] Figure 8. DANDE Pass on 28th December at 35

Degrees Elevation [1]

3.3. COM Subsystem and Thermal Data All of the useful thermal data gathered from the communications subsystem comes from the data points contained within the beacon. The data for both the receiver and transmitter are shown below.

Figure 9 COM Receiver Temperature over

Mission Life


Figure 10 COM Transmitter Temperature over Mission Life Both figure 9 and 10 show similar trends in temperature data with values peaking at 35 degrees Celsius and bottoming out at around 8 degrees Celsius. Given that these values as well within the tolerances of the components on this board it can be concluded that temperature has not played any significant role in changing the COM subsystems environment. The data reported from the communications system itself also did not show any trends that could have suggested that the system was underperforming, or degrading over mission lifetime. 3.4. Anomalous Periods of Operation

Over the course of the on orbit mission lifetime, there were some major anomalies seen in the performance of the onboard flight communications equipment. Around the 13th of October, 2013, approximately 2 weeks into the on-orbit lifetime of the DANDE satellite, the team noted a significant change in the beacon emitted from the system. Nominally, this beacon signal was configured to transmit for a length of 250 milliseconds. On October 13th, the length of the transmitted beacon packet increased to approximately 3 seconds. This anomaly was verified with the COSGC ground station, as our TNC was actively decoding for a significantly longer period than normal. It was also verified by a waterfall plot received from another one of DANDE’s amateur radio operators, which confirmed the 3 second length. The anomaly was detected directly after a major solar storm, suggesting that radiation damage could have been the cause of the failure. This anomaly was never seen during testing, and there was not sufficient data gathered during the anomalous period to draw any further conclusions about the cause of the failure, or the effects on the data downlink performance. One major side-effect though was a significant increase in overall system power draw, which drained the batteries to dangerously low levels and sent the space craft into an error state. The toggling of power to the communications system fixed the issue, suggesting further a single event upset due to radiation effects.

During the next major solar storm seen by the system around the beginning of January, we lost the ability to contact the DANDE space craft all together. Leading up to the loss of contact event, there were also other factors that could have been responsible for the silencing, such as power system temperature, but the lack of data during these periods made it difficult to draw any steady conclusions. Due to the upset seen in mid-October, the possibility of another failure due to radiation cannot be ruled out. Overall, it seems that the communication subsystem was susceptible to radiation, and this shortcoming did lead to undesirable performance of the space craft as a whole. 4. The Command and Data Handling Subsystem The Command and Data Handling subsystem of the DANDE satellite is DANDE’s primary computer system. All subsystem data and other information is stored and managed by this subsystem and all communication into and out of the DANDE satellite is mediated by it. The CDH subsystem is primarily categorized by the mission operations team using the CDH load average (a measure of how many processes CDH can handle vs. how many are waiting to run), the number of file-system errors, the number of files present on DANDE, the number of process zombies, and DANDE’s current RAM usage. For the purposes of analyzing the COM system only the CDH load average and number of files present on DANDE will be discussed as the number of file-system errors and RAM usage were steady over time and the number of process zombies cannot affect the COM subsystem. 4.1. Load Average and the Number of Files on DANDE

The COM system’s primary purpose is to communicate and receive data on behalf of the Command and Data Handling (CDH) system which serves as the primary computer on the DANDE satellite. As the CDH subsystem has to manage a large number of computations, the relative speed with which CDH can respond to COM greatly impact the ability of the DANDE MOPS team to communicate with the satellite. The primary insight into the speed of computation on the CDH system is a parameter referred to as the load average, which is a measure of the number of processes waiting to be executed at one time by the flight computer RTOS scheduler. A load average less than 1 is ideal because this signifies that on average, all tasks get serviced when they ask to be by the operating system. Over the course of the mission the 15 minute load average has steadily increased, as shown in figure 11. This consistent upward trend of the 15 minute load average over mission life is consistent with gradual system degradation over time. Another very important trend in the 15 minute load average is that it increases linearly with the number of files present in DANDE’s file system as can be seen in the below figure 12. This trend is due to the beacon. Every time DANDE sends a beacon it executes a list command on the file system as a whole, thus causing the load average when preparing a beacon to increase dramatically due to the increased number of files to sort through. This trend and the general increase in the 15 minute load average over mission life could possibly interfere with the COM systems ability to communicate with the ground, but there is no useful data that was collected over mission life that can prove or disprove this hypothesis.


Figure 11 15 Minute Load Average vs.

Mission Life

Figure 12 15 Minute Load Average vs.

File Count

4.2. Data Collector File Consolidation Due to an unfortunate error prior to launch the DANDE satellite’s data collector does not have the ability to concatenate single data points into larger files thus exacerbating the problem of increased file counts discussed in the above section 4.1. In addition to the load average increase effecting entire system performance, the presence of large amount of files on the system made it difficult, or in some cases impossible for the operators to target specific data ranges in order to collect the most relevant and important data. Entire passes were lost on commands that were designed to search through the file system real-time, find the requested data range, compress all of the requested files into one downlinkable entity, and initiate the transfers. With an extremely large amount of files, this process could take on the order of minutes, and by the time the downlink file was generated, the pass would be over and the file would be lost again in the file system. This feature was in the process of being overhauled by the mission operations team prior to loss of contact and no solution was ever implemented or pursued. 4.3. Trends in CDH Data at Loss of Contact Prior to loss of contact with the DANDE satellite the CDH system was stable as far as our beacon data could confirm. DANDE had a load average of 0.87 which is higher than desired but not unduly worrying as DANDE has routinely been operated up to a load average of 1.00 while on orbit. The relatively large value of the load average is due to the high number of files present on DANDE at that time. At loss of contact DANDE had 24,822 files present on the system and the MOPS team has been trying to keep the number of files present on the satellite under 35,000 as this number has been predicted to cause a load average of 1.00 on DANDE [2]. As this number is still within reasonable bounds it is not too concerning. Given that both the load average and number of files present on DANDE were within reasonable tolerances no connection between CDH and loss of contact with the DANDE satellite can be drawn. 5. Electrical Power System DANDE’s electrical power system (EPS) is the subsystem that provides power to all subsystems on the satellite. As EPS provides power to the COM system its stability is critical for successful communication with the DANDE satellite. For the purposes of analyzing the COM system only the battery voltages and 5 volt power supply current will be considered as they are the only two elements of EPS that directly support the COM system. 5.1. Battery Data DANDE stores power from its solar arrays in two battery boxes that are fed directly from the solar arrays. The voltages of these two battery boxes and their temperatures over mission life can be seen in the below figures 13, 14, 15, and 16.


Figure 13. Battery 1 Half Voltage

Figure 14. Battery 2 Half Voltage

Figure 15. Battery Box minus Temperature

Figure 16. Battery Box plus Temperature

In the above figures a plot of the illumination time (time spent in the sun per orbit) has been over laid on the data to show how illumination times affect the battery voltage. In periods of low sun the battery does not as high as it does not receive enough energy from the solar panels. However, in the most recent high sun period the battery voltages have dropped dramatically. There is currently no explanation for why this occurs but these low voltages do not correlate to anything useful. This data shows that there is some instability in the batteries during periods of high illumination which could relate to our communications difficulties. 5.2. Regulated 5-Volt Line Data

While the battery data shows that there is instability in the electrical system it does not imply that there is instability in the power supply to the COM system. The COM system is supplied by the 5 volt line which is regulated so as to always be providing 5 volts to every system connected to it. The current on the 5 volt line over DANDE mission life is shown in the left figure. Two lines can be seen throughout the majority of mission life. Each of these lines shows the current levels when DANDE is in sun (the higher current) and when DANDE is in shadow (the lower current). More interesting is the drop in current prior to initial loss of current. It is possible that the PLL (Phase Lock-Loop) in the COM System suffered from a low power state and failed to


operate, thus preventing us from communicating with the satellite. Unfortunately, this theory cannot be proved as no other data supports this result. 6. Known Operator Error Over the course of DANDE’s mission life operator actions on the ground caused several problems in communicating with the satellite. On several occasions hardware issues with the ground station occurred which caused passes to either be conducted suboptimal or be lost entirely. During the early stages of the mission operators would often not properly connect the gateway and lose pass time connecting before sending commands. It was also possible for them to neglect resending failed commands if they were not paying attention, thus losing pass time without communicating with DANDE. Another common issue was that typed Linux commands that were sent to DANDE would often be incorrectly formatted, which would cause confusion and delay in the middle of a pass as the command would need to be skipped, or quickly fixed if it needed to be executed before other commands. Further training in commanding DANDE and testing against the ground based test satellite has prevented any errors of this kind since the first few months of operations. On several occasions mission operators neglected to properly review the ground station and cabling changes were not fixed prior to a pass starting. In most cases these changes were quickly reverted. In two cases the connections to the antenna were removed and a pass with DANDE was lost before mission operators recognized that the radio was not connected to the antenna. These failures to properly assess the state of the ground station resulted in lost communication time with DANDE which affected the ability of the mission operations team to downlink data from the satellite. As a result of these issues the DANDE team has gone to great lengths to enforce stricter training regimes that has prevented any loss of pass time due to ground station issues in the last few months before contact with DANDE was lost. 7. Conclusion

There are many factors that had the potential to affect the overall performance of the DANDE communications over the lifetime of the on-orbit mission. Upon review each potential factor in detail; including ground station configuration, spacecraft hardware and software performance, and operator proficiency, a few of these have been identified to be major contributors to the teams’ inability to gather the expected amount of data over the course of the mission.

Overall, the DANDE flight communications hardware itself did not exhibit any unexpected performance on-orbit or have any thermally related performance faults, aside from the increased susceptibility to anomalous single event upsets due to radiation effects. We did not observe any degradation in the overall performance of the communication hardware over the lifetime of the mission. There was however some negative impacts due to issues with other subsystems on the flight side, mainly associated with the performance of the flight computer. The inefficiencies in file management and the relations of this to system task execution significantly lowered the efficiency of our data downlink process. In the future, increased efforts to test the system in a more flight like configuration, matching the amount of files we saw accumulated on flight, would help the team recognize these system characteristics on the ground where steps can be taken to mitigate them. The orbital parameters also had a significant effect of the integrity of the uplink and downlink signal and the ability to process information. The integrity of the signal is essential to consistent uplink and downlink of data. To some extent, it was expected that at low elevations there would be greater difficulty acquiring the satellite, but these factors were not taken into account when making the initial prediction of how much pass time could be dedicated to data downlink. This significantly impacted our expectations of the performance of the system on-orbit.

Though the flight communications hardware performed as expected, the ground communications equipment had some design characteristics that significantly lowered the amount of data acquisition from the satellite. Some of these characteristics, such as the command timeout length, were known before flight, and some were discovered as on-orbit operations were executed, such as the Zmodem file size deficiencies and the inefficiencies of the interaction with the user. Overall, we believe that the largest factor that influenced the amount of information obtained from the space craft is centered on operator techniques and standard procedures. In addition to the known procedural errors addressed in section 6, there were also issues in communication between operators. During some passes, operators were unsure of what was important to downlink on that particular pass and would send commands and perform actions that did not contribute to the overall success of the mission. Even with operator inefficiency aside, the initial downlink prediction was calculate based on the assumption that all pass time could be dedicated to the acquisition of data, which clearly is in error. A significant amount of pass time was spent uplinking information to the satellite, executing commands, and investigating on-orbit errors. All of these tasks take away from the amount of time that can be spent downlinking data, but are standard operations that must be performed and taken into account. When making estimations for the performance of the space craft, factors such as this can go unnoticed. In future on-orbit operations, operator proficiency and pass time task allocation must be taken into account to achieve an accurate estimate of data acquisition. If all of these factors were


known and addressed prior to launch, some could have potentially been addressed to improve the overall efficiency of communications system and downlink abilities.

8. References [1] C. Hurst, DANDE Pass Data [2] G. Frank, DANDE LEOR Analysis [3] M. Trowbridge, DANDE Load Average Analysis

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