The implementation of the communication delay algorithm has been tested and validated by comparing against manual calculations of the delay.
Figure 3.6.2: Communication duration
Figure 3.6.2 shows the total window duration required for uplink from the rover and downlink to the rover. Shown are plots of the communication duration for one command cycle consisting of a varying uplink volume and fixed downlink volume. This shows the relative link capability of the three types of communication architectures. As expected, DTE communication duration is the longest at the lowest data rate of 8 kbps, followed by communication via a high orbit relay satellite at 64kbps and the fastest link is the low orbit relay at 256kbps. Note that uplink and downlink data rates are equal for each type of communication architecture.
The communication delay is the total delay including communication window availability. It includes uplink duration from Mars, and if a response is expected from Earth, it also includes round-trip
propagation delays, downlink duration, human response time on Earth and command execution time by the rover before the next command cycle starts. Figure 3 shows the total communication delay for one command cycle with varying uplink data volume from Mars, and a fixed downlink volume of 100 bits expected from Earth. DTE communication is available anytime during the day. However, it is assumed that the DSN is only available for four hours per day for this mission. Thus the window duration is limited to four hours. Low orbit relay satellites typically have overflights every twelve hours, with a duration of seven to twelve minutes per overflight, depending on the rover’s latitude. High orbit relay satellites will be available more frequently than low orbit satellites, with overflights centered approximately six hours apart and a window duration of seventy-two minutes per overflight. The longest Earth-Mars round-time propagation delay of approximately forty minutes is used. It is assumed that operators on Earth will only be making “tactical” choices with a response time of two hours. These “tactical” choices refer to strategic re-planning of a whole day’s activities that may require the overnight command cycle to be neglected in delay calculations. Another simplification that is assumed is that in case of communication via a relay satellite, there are no inherent delays associated with the relay satellite.
Figure 3.6.3: Total communication delay for a command cycle
Figure 3.6.3 indicates that the longest delays are associated with DTE. Although DTE has the longest communication window opportunity, the relatively low data rate results in longer delays for large data volumes. Low orbit relay, which has the highest data rate, is only available for short window durations, and although it represents an improvement over DTE, it does not provide the shortest delays. Better performance is achieved by communicating via a high altitude relay satellite. This is available more frequently than low orbit satellites and provides data rates much faster than DTE, thus achieving a relatively better performance than the other two architectures. This justifies future plans for high altitude telecommunication satellites orbiting Mars.
Note that Figure 3.6.2 and Figure 3.6.3 are generated for one command cycle. Typically, there are multiple command cycles associated with each of the operation-intensive activities. This means that the total delay is the cumulative delay of communicating and executing all command cycles. The level of autonomy has a direct impact on the number of command cycles and data volumes, which in turn affect the communication delays.
Figure 3.6.2 and Figure 3.6.3 also assume that nighttime operation capability exists. If the rover does not have nighttime operation capability, this will increase the communication delay associated with high altitude relay, although the high altitude relay will still provide relatively better performance that DTE and low orbit relay architectures.
Figure 3.6.4: Total communication delay for a command cycle
Figure 3.6.4 is a finer scale of Figure 3.6.3 and shows variations of delay with smaller increases of data volume. There are discontinuous jumps in the delays, due to finite communication windows. For instance, for data volumes around 20 Mb, DTE has the shortest delay because the command cycle delay is less than one window duration. However for low orbit relay, the command cycle cannot be
accomplished within the duration of one communication window, which is approximately seven minutes long. This means waiting for the next available communication window, which is half a day apart, thus the discontinuous jumps in communication delay by half a day. Similarly, high orbit relay has
discontinuous jumps of approximately six hours, which explains why this communication architecture has longer delays than DTE for smaller data volumes. Notice that as the data volume increases, crossover occurs and DTE becomes less efficient, since the relatively low DTE data rate essentially starts giving rise to longer delays.
It is difficult to compare these results to MER or other Mars missions because of the assumptions underlying the modeling. Furthermore, data on command cycles and associated delays is generally unavailable for these missions.
Figure 3.6.5: Odyssey data volume from MER per sol. The data rate is 128 kbps. [4]
Figure 3.6.6: Uplink duration
The only comparison for validation was made with the Odyssey data volume to be received from MER per sol at a rate of 128kbps. Note that Odyssey is a low altitude science orbiter. Figure 3.6.6 shows uplink data duration as a function of data volume. For low orbit, the data rate assumed is double the data rate used to uplink MER data to Odyssey, based on Whetsel’s estimations. Figure 3.6.5 shows that a
maximum of approximately 100Mb can be uplinked per sol at 128kbps. Figure 3.6.6 indicates
approximately 300Mb per day at 256kbps, which is the equivalent of approximately 150Mbits at 128kbps.
The discrepancy is attributed to the assumption that all relay satellite resources are available for the Mars rover mission. In reality, Odyssey has a limited memory availability (100Mb per sol for MER) that
constrains the amount of data that can be uploaded per sol. Additionally, the uploaded data is buffered, and emptying the buffer relies on the availability of the DSN, which may further constrain the amount of
data that may be uploaded to a relay satellite. This demonstrates a limitation of the Communications module, which is the resource availability assumption.
Note that the Communications model incorporates the affect of rover latitude to determine the
approximate window duration for each of the different communication architectures. It also provides a nighttime operation option. Restriction to daylight operation results in longer total delays.