DESIGN OF A TM/TC LINK BETWEEN SMALL AIRCRAFT AND GROUND STATION¶

WIKI TABLE OF CONTENTS¶

Research of a frequency band

INTRODUCTION¶

The aim of this project is the design of a Telemetry and Telecommand (TM/TC) link between small aircrafts (ie TB20) and an antenna on top of ISAE building. There are different aspects in this work:

• regulatory aspects : we must pay for rights of emission on a given frequency band
• link budget aspects : some parameters can be adjusted to reach the wanted performance
• flight constraints have to be considered in the choice of the equipment to allow a full time link

To summarize the system design constraints, we can fill the mission statement below:

In order to define the video bit rate required for our application, knowing the aspect ratio (AR=1,78) quality of the encoded video (L=720p) and its frame cadence (FPS=12), we used the following formula:

Bit rate = FPS*5,0692 * L ^1,391 /(1000*AR)

So

Bit rate=322 Kbps

As the link will allow data and video transmission, we choose to use the MPEG4 packet format, adapted for multiplexing different nature content. Therefore, we have to consider extra bits due to the encapsulation of data in MPEG4 frames, and we decide to define a bit rate of 500 kbps.

Research of a frequency band¶

When designing an RF link, one of the first tasks to realise is to define the frequency band that is adapted to the application foreseen. In fact, the frequency spectrum is shared among users in order to regulate RF emissions, and avoid interferences among various equipments.

There are international and national regulation agencies that allocate frequency band to users, with a limited bandwidth and a maximum emission power authorized for the equipment. In France, the ANFR (Agence Nationale des Fréquences) is the agency in charge of the frequency allocation and regulation, and the TNRBF (Tableau National de Répartition des Bandes de Fréquences), describes how the frequency spectrum is shared among services and users.

There are two approaches when researching a suitable frequency band for an application:

• Approach n°1 is the fastest, and consists in searching among “free bands” one that could fit our application, in term of bandwidth and emission power available.
• Approach n°2 requires more time, and consists in searching in the TNRBF an available bandwidth and sending a “request for allocation” to the ANFR.

Issue of a free band
System constraints and variable
Simulation of the system performance¶