White Rabbit Working Spec Notes
The Nanosat Fits in the class of small satellites called ‘picosatellites’. Launch costs about $10,000 for 1kg or less.
Typical missions are: IoT connectivity, using the globalstar relay network. Globalstar is a network of larger satellites which perform the job of relay for smaller satellites. Simple off the shelf radios which cost less that $6000 allow small satellites to easily communicate with users on the ground through relay services like Globalstar.
– End-to-end system
– Globstar connected
– Max 600 Kbytes/day
– 100% on-orbit success
– Flight ready
– TRL 9
– Coverage maps available
– Data cost table available
– Simplex inventory in stock – Compiant with new FCC requirements
One especially useful function of this relay network is for remote GPS location transmission. Say you are on a hike in an area without cell coverage. A SPOT radio will transmit your GPS signals to the Globalstar network and make your location available to the SPOT network.
Even smaller are Femtosatellites or ‘Chipsats’ which weigh less than 100 gram.
Picture of a 3U satellite deploying Chipsats.
Femto satellites can do things larger satellites cannot do like study the mesophere or areas where traditional
research satellites cannot operate successfully.
CubeSats can also function as inter-satellite relays as depicted above. Inter-satellite links (ISLs) are useful for exchanging information between satellites or to use satellites with more powerful radios to relay information to ground stations.
Sticking with the ‘Unit’ format the 3U cubesat provides much more room for batteries and instruments. It also has more surface area for mounting of antennas and the very important solar panel arrays. 3U Cubesats have performed some very important missions such as off grid asset monitoring and more detailed space and earth observation studies.
One challenge with radio frequency communications is the power requirement. The advantage of RF is that they do not require precise antenna pointing like lasers do but do require lots of power.
Managing power and communications is called Link Budget. There are uplink, downlink and satellite to satellite communications. The power that can be generated to support all these links is limited by the weight and size of the cubesat.
A link budget is the set of parameters that show the relationship between the power available and a reliable communications link. Some parameters that are used to calculate link budget:
– Transmit Power
– CubeSat antenna gain
– Ground station antenna gain
– Data rate
– Pointing and polarization loss/inaccuracies
– Atmospheric losses
Think about this. A cubesat used for environmental monitoring may generate a gigabit of data daily. The data rate of transmission is 50 kilobits per second. The spacecraft has a transmission duty cycle of 25% (which means it can only transmit a ¼ of the time it is operating e.g. less than 3 minutes out of a 10 minute pass over a ground station). Because of its particular orbit this satellite passes over a ground station for 10 minutes at a time and it sees 4 ground stations a day.
How many passes over the ground stations will be required to deliver one days worth of data? The answer is around 33 passes.
You can see that it would take over 8 days to transmit 1 days worth of data gathered to the ground stations. That means the mission controllers would need to plan when they want to collect data. This is part of the mission profile of the satellite.
This is a very simple example and there is consideration of the duty cycle related to heat generated and power drawn but we will just say the radio has a duty cycle of 25%. If the radio transmits longer than that it risks overheating and damaging itself and other components and also risks using too much power leaving the satellite batteries in a state where they cannot be recharged enough to handle eclipse (when the satellite is not illuminated by the sun).
New inventions like Optical Satellite Communications with Lasers can provide much faster data rates, the tradeoff however is that these devices must be pointed with high accuracy and precision. This requires additional power to keep the pointing of the laser stable. Another tradeoff is that optical transmissions suffer from losses when the link is obscured by clouds or other materials. However the time needed to transmit data will be much lower under normal conditions. Most satellites have multiple communications systems using various bands.
Bands describe the various frequencies the satellite can communicate over. Some popular bands are S-Band and Ku and Ka Bands. Ku is short for the ‘under’ side of the K band and Ka is naturally the ‘above’ side of the K band.
Below is a depiction of the tradeoffs between various bands. You can see the benefits of S-Band is that it is less susceptible to interference from rain and other environmental effects and requires less bandwidth. Some satellites use S-Band for command and control and health/status monitoring and K bands for large data transmission. You can imagine the engineers thought it was more important to keep command of the satellite and less important if some data was lost which could potentially be gathered in future passes.
With all this talk about communications you can see how important it is to understand the tradeoffs involved in picking the right type of Band for the right type of data and ensuring that whatever band you choose the radios can be powered sufficiently by the solar panels and batteries you pick which in turn is dictated by the size and shape of the satellite.
Code name: Moonlighter