# Implementation
## Implement your own custom signal
To implement a custom signal, you need to develop a C++ shared library (.dll on Windows, .so on linux). Any modern C++ IDE should be compatible. Everything you need to implement is defined in the header file **custom\_signal\_interface.h**.
All elapsed time arguments are in milliseconds, relative to the simulation start time. All PRN arguments are the legacy signal PRN number.
### Implement ICustomSignal
At minimum, you need to implement **ICustomSignal** for your shared library to be usable by Skydel. This class is instantiated at every simulation start and destroyed at every simulation stop. In your implementation .cpp file, you **must** add the following line and replace **MyCustomSignal** with your class name:
DEFINE\_CUSTOM\_SIGNAL(MyCustomSignal)
### Implement ICustomSignalCode
The key function of that class is **getChips**. It is called every millisecond for every visible satellite. Your implementation must copy the code chips potentially multiplied by a navigation message bit. The chips must be the ones at transmission time, Skydel will handle the propagation delay. The chips are uint8 values of either 1 or –1, anything else leads to an undefined behavior. The *chips* buffer is guaranteed to be **getNumberOfChipsPerMSec() + getExtraAllocSize()** long but only the **getNumberOfChipsPerMSec()** will be used for the simulation. **getExtraAllocSize()** is useful if you need some padding to optimize your chips generation algorithm.
### Implement ICustomSignalNavMsg
Your navigation message implementation will not return anything directly to Skydel (the navigation message is modulated with the ranging code inside the DLL). It is a facility to receive simulation data (such as satellite’s ephemerides, ionospheric terms, UTC terms and polar motion terms) and set some internal state accordingly. The **buildNavMsg** function is called every **getNavMsgDurationMs()** milliseconds, aligned to the constellation's time of week. You can offset this message by returning something other than 0 in **getTOWOffset**().