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h1. Technical requirements

h2. Functional requirements

Functional requirements define what functions need to be done to accomplish the objectives of the mission. In this way, the following requirements are defined: 

* The satellite shall send one RF beacon regularly with a period time equal to 60 seconds = 1 minute.

* The satellite may send its GPS coordinates information inside the beacon frame.

* The satellite may send system state information (like temperature) inside the beacon frame.

* A ground station situated at the Télécom SatLab (Toulouse, France) shall be inside the satellite footprint at least twice a day.

h2. Performance requirements

Performance requirements define how well the system needs to perform the functions previously defined. Also they determine quantitative parameters that will set bases for the later technical measures specification. These concepts lead us to the performance requirements shown below:

* The beacon signal shall comply RTTY specifications.

* The communication time with the ground station shall be long enough in order to assure that one complete beacon frame can be received.

* The RF emitter device may be implemented with the low cost, 434 MHz, 2-FSK transmitter, available at the lab. Product information: Aurel, TX-4MAVPF10.

* If on board GPS reception is implemented, the device shall be accurate enough in order to assure, for each coordinate (latitude, longitude and attitude), an absolute error smaller than 50 m.

h2. Interface requirements

Once the system components are defined, a block diagram showing the main subsystems, their interconnections, and the external interfaces is a powerful tool to define the interfaces and interactions. By this way, the nodes represent each subsystem and the links represent each interface. There are two types of interfaces, internal and external. The internal interfaces are the links representing the interactions between all the subsystems. By the other hand, the external interfaces are the links representing the interactions between the main system and the rest of the world, outside the product boundaries. There are different types of interfaces: mechanical, electrical, data protocol, etc.

h3. System/subsystems interactions blocks diagram.

!{width: 70%}sys_interfaces.png!

The interface requirements are the set of specifications that both linked nodes need to meet in order to have a proper and coherent system behaviour and interactions. Thus, the next interface requirements are identified:

* Interface Launcher - Satellite (L1).

Type: Mechanical.

Since the nanosatellite is based on a Pumpkin Cubesat 2U module, it has to be compliant with Cubesat mechanical specifications.

* Interface Processor - Power supply (L2).

Type: Electrical.

The power supply shall feed the processor with a DC source. Voltage: +5V. Typical operating current: 0.5 mA.

A +3V backup battery shall be used.

*The other Power supply - Subsystem links shall be defined in the same way.*

* Interface Processor - GPS receiver (L3).

Type: Electrical, logical, protocol.

If onboard GPS reception is implemented, it may be used a SPI bus between the receiver and the processor. The decision is linked to the external interface of the GPS module chosen.

* Interface Processor - Beacon transmitter (L4).

Type: Electrical, logical, protocol.

The communication shall be in serial mode. Baud rate between 50 and 300 bauds.

Other type of interface may be for instance the RF modulation between two terminals.

* Interface Beacon transmitter - Ground station.

Type: RF modulation

Defined by functional requirements. It shall be used a simple 2-FSK modulation to transmit data as a serial stream of bits complying with RTTY specifications.

For the scope of this project, there is no need of further interfaces analysis. Obviously, all the inter-system connections shall be designed to be compatible for both linked subsystems.

h2. Environmental, reliability and safety requirements

Functional, performance and interface requirements are important but they don't constitute the whole set of requirements for the mission. For instance, there are several other particular constraints for space segment systems. These constraints are environmental requirements, reliability requirements (robustness, failure tolerance, redundancy, etc.) and safety requirements.

h3. Environmental requirements

A complete study of environmental requirements are out of the scope of this project. Nevertheless, it can be introduced the main idea.

Each space mission has a group of environmental requirements that apply to the flight segment elements. In order to specify these requirements, different factors should be considered, such as EMI/EMC, grounding, radiation, shielding and contamination protection. Once all the requirements are identified, then it should be defined the corresponding margins and design criterias to compensate them.

h3. Reliability

Reliability can be defined as the probability that a system will not fail for a given period of time under specified operating conditions. This is an inherent system design characteristic. Therefore, it is needed to specify all the system and subsystems requirements in order to ensure that they can perform properly their tasks and can handle a certain number of errors and failures. Reliability addresses design and verifications to meet the requested operational level as well as failure tolerance for the defined working conditions.

Additionally, a brief analysis of the operational lifetime over CubeSat projects will be helpful to well define the reliability requirements and the critical design criterias which are related.