European Solar Polar Orbiter Mission


Telemetry, Tracking and Command (TT&C)



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Telemetry, Tracking and Command (TT&C)


No significant technology requirements were identified within this sub-system. However, some important trades were required to define an optimal solution between sail and spacecraft requirements which vary the system design from a non-sail delivered solar polar orbiter spacecraft.
Data latency is nominally set at less than 1 week, which coupled with a continuous science data acquisition stream of 3.5 kbps means that we acquire approximately 2.1 Gbit of data between downlinks. Assuming each downlink is 8 hrs we can then define a required minimum science telemetry downlink rate of 73.5 kbps. Maximum slant range for the Science Operations mode is approximately 1.46 AU. At this slant range we find that the required spacecraft transmitter power is just over 28 W. Therefore, in order to provide a margin for return of engineering data at all times we set the spacecraft transmitter power at 30 W, giving a design telemetry rate (downlink) of 77.8 kbps and a command rate (uplink) of 38.9 kbps. The minimum margin for engineering data is 4.3 kbps, however this margin increases to over 386 kbps at the minimum slant range of 0.6 AU.
As discussed previously, the expected pointing accuracy while attached to the sail is low due to sail flexing, thus limiting communications to X-Band frequencies and lower while the spacecraft is attached to the sail. We can however consider adoption of a dual X and Ka-Band system which would allow increased data rates, reduced frequency of downlinks and a reduction in power requirements during the science phase of the mission, or a combination of each. Using a 35 m dish in the Ka-band, at 20 kW to match ESA ground station network (ESTRACK) configuration, and increasing the spacecraft pointing accuracy during downlinks to 36 arcseconds, as required for Ka-band, we can analyze such an option. AOCS mass marginally increases assuming one 1.5 m HGA downlink of 8 hrs per week in a 2-year mission due to the increased pointing requirements. The spacecraft transmitting power can be significantly decreased during the Ka-Band HGA downlinks as the available command rate otherwise increases to over 8 Mbps and the telemetry rate to just less than 1 Mbps, assuming conditions such as antenna elevation and weather confidence are similar. However, in order to maintain sufficient link margins and data rates within the other communication modes, we require to maintain spacecraft transmitting power at 30 W within the X-Band limited modes of cruise and emergency, negating the potential power saving available during the science operations mode. Furthermore, during the science operations mode the spacecraft is in a power rich environment and thus power savings are not of significant benefit. We note however that adoption of Ka-Band within the science operations mode would allow a Solid State Power Amplifier (SSPA) to be flown, as spacecraft transmitting power requirements are only 5 W for one 8 hr downlink per week, allowing increased sub-system reliability at the expense of additional mass. It is found that the adoption of a dual X and Ka-band communications architecture, with SSPA, increases the spacecraft total mass by 6.4 kg, which in turn increases sail side length by almost 2 m and total mass at launch by 11.9 kg. The selected spacecraft communications architecture is limited to X-band frequencies, which reduces HGA surface tolerance design limits and removes any potential requirement for a two-way capable Ka-band small deep space transponder (SDST) as current SDST technology is limited to two-way X-band and downlink only in Ka-band.
Maintaining the spacecraft transmitting power at 30 W allows the calculation of the attainable data rates within the other communication modes. We see in Table 3 the available data rates in each communication mode, at the maximum design slant range. Each communication mode strives to minimize ground segment costs and requirements, hence utilizing as small a ground station as possible, as detailed in Table 3. Note the design HGA size is 1.5 m diameter.
Table 3 Available data rates, at the maximum design slant range defined by trajectory analysis

Communication Mode Name

Antenna – Ground Station

Mode Design Slant Range

Minimum Data Rate

Command

Telemetry

Cruise

LGA –

15

m

1.80

AU

0.03

kbps

0.04

kbps

Space Weather

LGA –

5

m

1.46

AU

0.00

kbps

0.01

kbps

Science Operations

HGA –

35

m

1.46

AU

38.9

kbps

77.8

kbps

Emergency

LGA –

35

m

1.80

AU

0.03

kbps

0.21

kbps

The use of alternative ground stations would clearly allow for increased data rates, however sufficient data rates are attainable within each mode. It is considered optimal that data latency be set at the nominal value of 1 week, with a transmitter power rating of 30 W within all communication modes. Data can however be returned at increased frequencies and so the data latency setting is a nominal design value which is used to define on-board memory requirements within the data handling sub-system.


The 0.48 AU orbit is not ideal for dedicated Earth related space weather observations. However, the 0.48 AU orbit allows the observation of coronal mass ejections directed towards Earth for much of the time. The instrumentation on-board can therefore provide an additional contribution to space weather forecasts. The spacecraft could provide up to a 1-day warning of large solar proton events. The space weather communication architecture selected within this paper is based on beacon-mode technology. A simple tonal system is used to indicate whether or not an event has occurred. The beacon mode requires an onboard system that can communicate with Earth 24-hours a day and at least 3 ground antenna, hence the selection of ESTRACK 5 m stations as shown in Table 3, which are currently available and would require minimal further investment. Upon detection of an event, emergency use of a 35 m ESTRACK antenna commands the spacecraft to transmit a special downlink load. The additional cost and system requirements for operating the beacon mode are included in the baseline mission.


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