International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B1. Istanbul 2004 
up-to-date computing products cheaply available on the market 
to be used in spacecrafts and thus alleviate the computing 
power on-board. 
In certain cases there have been also advancements in the: 
suppliers' side of these computing systems. Recently Xilinx 
Corporation, manufacturer of field-programmable gate arrays, 
has released their Virtex-II Pro series of FPGA chips, which 
not only is an FPGA but also a simple PowerPC processor 
bundled in one chip not much bigger than a thumbnail. These 
chips enable not only the use of advanced PowerPC processing 
capabilities in a very small area and with low power 
consumption, but also give the manufacturer the tool to 
customize the abilities of a designed and complete on-board 
computing system for a different mission without having to 
customize the PCB and its interfaces to the new mission 
thereby reducing development cost. In the case of Virtex-II Pro, 
Xilinx must have seen the opportunity to supply the spacecraft 
industry with the computing system they need, so they actually 
brought out a version of their system that has been targeted 
exactly for space applications and is not only flexible like its 
industrial counterpart but also radiation tolerant. - 
The second problem of spacecraft computers is the lack of the 
operator standing nearby. With modern day computers we are 
all far too familiar with the CTRL+ALT+DEL combination 
that saves our computer from a lock-up and sometimes and 
there is no other way of getting that computer running again 
other than rebooting by flicking the switch. This is a luxury 
many satellites don’t have. The solution again is the use of 
more than one processing unit. One can monitor the output of 
the second and should there be more than a glitch in the output 
of the second CPU the first one can instruct the power 
subsystem to recycle the power on the first CPU's backbone. 
This capability, combined with a regular watchdog timer to 
look for software glitches causing endless loops and an 
external software patching mechanism to reprogram the CPUs 
in case a fault is discovered on the on-board computer, would 
be as reliable as the more than million dollar expensive 
systems available on the market and can be developed in-house 
at the satellite building industry or at educational institutions 
for everyone to use on their satellite. 
3.3 Solutions in Communications 
Communications is one of the tougher of the systems to 
simplify and streamline than command and data handling. First 
of all, the basic laws of communication have not changed and 
laws of physics governing losses over distances will stay the 
same regardless of technological advances as long as we 
continue using RF links to transmit information. This does not 
mean however that simplifications can not be achieved or 
technological advances in other areas can not be put to use in 
communications subsystems. In the modern wireless world, 
power and space usage in RF link transceivers have been 
optimized and advances have been made in employing higher 
frequency data carriers that enable the use of smaller antennas. 
The use of wireless communication for voice and digital data 
has made many advances in terms of algorithms for error 
correction and data encryption and brought out commercial 
digital communication devices that would have taken years to 
produce for one small satellite system. 
Today, one can buy a wireless transceiver for as little as $700 
from Microhard Corporation with output power as much as 
IW, and with the proper authorization these devices can be 
modified to have even more output power and gain antennas 
can be used to deliver much better link margin results. Most of 
these devices will come with all the extra features that 
sometimes even the really expensive data transmission devices 
from aerospace companies will not offer. In case of the $700 
Microhard radio you will get data encryption, multi point 
networked communication and relay capabilities, and error 
correction built-in right into the system. And with 2.4 GHz or 
900 MHz frequency allocations these devices require much 
smaller antennas then their lower frequency counterparts. In 
addition these devices, not even 6cm wide and 9.5 cm long and 
2.5 cm high, take up almost no space in a satellite. Most of 
these devices are transparent to the command and data 
handling system and can be hooked up to any serial port of the 
command system of the satellite and treated no different than 
transferring data between your computer and your PDA. 
Miniaturization through newer advances in communications 
technology have yielded smaller and smaller patch antennas 
that not only are less massive and do not require big and 
expensive deploy mechanisms, but also provide better gains. 
They probably do not have enough high-bandwidth to provide 
communication capabilities for bigger links; however they can 
be used for telemetry and housekeeping antennas. In regard to 
the payload, a bigger antenna might be selected for the larger 
payload data generated. The receiving antennas for these 
smaller patch antennas can be made to be higher gain antennas 
with the appropriate authorization from the ITU and its 
national subsidiaries and whatever is lost in power dissipation 
on the spacecraft through miniaturization can be compensated 
for in the ground. 
3.4 Solutions in Power Systems 
Power systems are still considered to be one of the bigger parts 
of a satellite in terms of mass and thermal requirement 
generators. Not only are the batteries bulky and heavy but they 
also require a narrower range in temperature to operate 
efficiently. Most of the satellite surface is devoted to solar 
cells and when that surface is not enough today's satellites 
have to rely on heavy mechanical systems to deploy their solar 
arrays. In this respect smaller satellites have an advantage over 
larger ones: when scaling down a satellite, the ratio of 
projected area to inner volume is much increased and since the 
power consumption of the electronics remains proportional to 
their mass and volume the power generation needs of smaller 
satellites can be generally satisfied without the need of 
deployable solar cells (Wertz and Larson, 1999). 
Some progress has been made to achieve higher power 
densities in batteries enabling them to store more in less space 
and weight. The following table gives a good estimate on how 
some of the newer technologies like lithium-ion and nickel- 
hydrogen batteries compare to the older systems. 
  
  
  
  
Type Wh/kg Wh/l Voltage | Cycle Life 
Lead-acid | 35 80 2 400 
Nickel- AS 80 12 71000 
Cadmium 
  
  
  
  
  
  
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