2-25-08_questions

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What type of solar cells does LRO use? (single/double/triple junction?)
What kind of efficiency do they have?
Triple-junction GaAs. 28% if you operate at the peak-power point. We use a direct energy
transfer power system (where the battery is directly across the bus), so our voltage typically runs
below the voltage of the peak power point, providing less efficiency. A peak-power tracking
power system will run at the peak-power point, but you end up with extra losses through the DCDC converters, and you end up with more parts in series with your power (reducing reliability).
pros and cons both ways
What type of rocket motor does LRO use? What type of propellant?
We are using 20 N and 80 N thrusters. Mono-propellant hydrazine. A bi-propellant system
would have required less fuel mass, but it was more complex, with more plumbing and different
tanks, leading to higher cost and, more significantly, a longer development time. Other missions
have come up with other results.
What types of antennas are typically used on Moon missions for communicating with Earth?
What range of data rates is typical?
(We are debating whether we should use two different antennas and radios: one for
communicating with the lander, the other for communicating with Earth. What is your opinion
on this issue? What advice can you give us?)
I think spacecraft should always have omni-directional antennas for emergency situations. For
nominal mission operation, you will need some sort of high-gain antenna in order to get the data
rates of interest without needing too much transmitter power. It is pretty straightforward to
calculate how much antenna gain you need for a particular data rate and transmit power. If you
use concatenated convolutional and Reed-Solomon encoding, you need about 2.5 dB signal-tonoise ratio into the receiver theoretically, with another 2 dB to cover non-theoretical aspects of
receivers. Then you need about 3 dB of margin in the link. I have attached a link spreadsheet I
put together years ago for convolutional encoding without the R-S. That can help you get to the
signal-to-noise ratio. You should be able to find a simple formula to go from gain to antenna
size for a particular wavelength. For a parabolic dish antenna, you can assume that you can
capture 55% of the incident RF energy (55% efficiency).
For S-band, you are not allowed to exceed 5 MHz RF bandwidth, due to frequency usage
restrictions. For higher data rates, you are going to have to use Ka-band (X-band is not allowed
from the moon). LRO's high-gain antenna is 0.75 m in diameter and provides 44 dBi (dB above
isotropic) gain at Ka-band and 21 dBi at S-band. LRO is operating at 100 Mbps with a 40 W
Ka-band transmitter. The Ka-band beam is -1.2 dB at 0.3° off of boresight. The S-band beam is
something like 2.5 degrees wide for about -1.5 dB point. LRO uses a 5 W transmitter for S-band
telemetry at 64 or 128 kbps.
If you use a high-gain antenna for talking to the lander, you will have to point the spacecraft at
the lander. This may not be a problem, since you are imaging anyway. Take a look at the
geometry. You might need a medium-gain system in order to be able to point near-by for
imaging. You could possibly use the same HGA to talk back to earth, since you only need to
collect data when you are over the pole. The gain of the antenna on the spacecraft will impact
how much transmit power the lander will need. I expect it would be too much complication for
the lander to have a high-gain antenna, since it would have to track the orbiter. As you can tell,
it is pretty important to figure out exactly how you plan to operate. You may be able to save
mass by using the same system for communication with the lander as you use for communication
with earth, but you may add additional complexity or have trouble finding a suitable radio.
I hope this helps some. Keep asking questions if I've added more confusion.
What is the transmitter power and antenna gain at White Sands, NM? Would a
ground station in Australia have similar equipment?
Here are key parameters for the WS1 antenna at White Sands:
Part
Units
Main Reflector diameter
S-band revieve frequency range
18.3
2200 to 2300
meters
MHz
S-band transmit frequency range
2025 to 2120
MHz
Ka-band receive frequency range
25.5 to 27.0
GHz
S-band G/T (See Note 1)
28.8
dB/K
Ka-band G/T (See Note 2)
45
dB/K
S-band EIRP
79
dBW
Maximum slew and tracking rate
2
Azimuth range
+400
deg
Elevation range
0 to +180
deg
Mount type
Elevation over azimuth
Note 1 - S-band G/T value is for clear sky and 5* elevation angle
Note 2 - Ka-band G/T is for clear sky and 10* elevation angle
You can assume similar parameters for a hypothetical station in Australia.
o
per second on
each axis
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