ESA Document - Emits - ESA

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HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 207 of 422
• Higher pointing accuracy is required, compared with X-band and S-band. For example, 4
times more with respect to X-band and 16 times with respect to S-band.
• There is more bandwidth availability with respect to X-band, since nowadays very few
spacecrafts are using Ka-band.
• During superior solar conjunction, Ka-band carrier suffers 15 % less amplitude
scintillation (changes in frequencies) and 20 % less spectral broadening than X-band for
the same Sun-Earth–S/C angle. Therefore, it is recommended since it will allow higher
data rate than X-band. [RD44]
Considering laser communications versus Ka-band [RD50], the following factors are important:
• Higher transmission speed.
• Less technologically mature than Ka-band.
• Usable with reduced data rate, or not usable at all, for SEM (Sun-Earth-Mars) angles
below 10 degrees. Therefore, it has less availability than Ka-band.
• More G/S availability since in case of clouds or rain, laser communications are not usable
but Ka-band could work at a reduced data rate.
• Laser communications are difficult use for uplink from the G/S since they require a
complex adaptative optics. Downlink adaptative optics are not mandatory, but if they are
used, they are very different to the uplink ones. The conclusion is that the same telescope
(G/S) cannot be used for both uplink and downlink.
3.3.7.2.2 Operations during first days of LEOP and contingency situations
It is assumed that, in these conditions, near omni-directional coverage is desirable and low gain
antenna(s) will be used. Additionally, Earth orbit relay satellites working in X-band should be
used.
The main advantage of using X-band is that LGAs based on waveguide (W/G) technology can
handle power levels up to 100 W. However, typical S-Band LGAs (quadrifilar helix) can handle
maximum 10 W.
Since in case of contingency, as much data rate as possible will be required, X-band would be
better to obtain 100W of transmitted power. Taking into account the reference mission date
(around 2025), it is considered that by that date X-band uplink and downlink capability will
already be available in most stations.
3.3.7.2.3 Modulation for deep-space data transmission
To design the RF (Radio Frequency) link Mars-Earth, using the CCSDS recommendation
[RD46], and because this mission is a deep-space mission (CCSDS category-B) with high data
rate requirements (over 2 Mbps) two modulations have been considered; GMSK and T-OQPSK.
For these missions, ESA has decided to only implement GMSK in the future; therefore it is the
used one in this design. As recommended as well by CCSDS in [RD46], a coded GMSK with
BTb = 0.5 has been chosen. Reasons for this election are the low Eb/N0 required and the
equalization so that end-to-end losses for this modulation are in the order of 0.1 to 0.15 dB (i. e.
insignificant).
3.3.7.2.4 Relay satellite