Architecture

Architecture

For existing GNSS signal generation, Skydel already knows all the signals specifications and how to generate the navigation message bits and the code chips. However, in order to generate a “custom signal”, users must provide information and compiled code to Skydel.

As it is shown on the diagram, the user needs to provide an XML file and a C++ shared library (.dll in Windows or .so in Linux) for each custom signal. Nevertheless, the same library can be shared by several custom signals.

As for the legacy signals, Skydel Engine automatically simulates the satellite visibility, signal dynamics and impairments (atmospheric delays, multipaths). The custom signal user just has to provide the signal data in the correct timeframe scheduled by the Skydel Engine.

A custom signal is composed of an arbitrary number of Subcarriers and Codes. The signal composition is described in an XML file. The codes' chips must be provided through a C++ shared library, as illustrated on the figure below.

The IQ samples are generated according to the following equation:

$$sample(t)=A× ∏^C_{i=1}C_i(t)×SC(t)$$

Where:

• $A$ a constant (in $C$) multiplication factor

• $C$ the number of codes

• $C_i(t)$ are the code chips value (-1 or 1) defined in the shared library (DLL)

• $SC(t)$ the subcarriers defined as:

Where:

$$SC(t)=1 \space if \space S=0, \space ∑^S_{j=1}SCj(t) \space otherwise$$

• $S$ the number of subcarriers

• $SC_j(t)$ are the subcarrier Real values defined in the XML file

With this approach it is possible to generate several GNSS modulation types, such as BPSK(N), BOC(M,N) and MBOC(M,N,P). Some examples of legacy GNSS signals are provided in Annex A. Tracking and accuracy of these signals have been validated on a GNSS receiver.

An evolution of the custom signal feature is already plan that will bring even more genericity. The objective is to have the possibility to generate (at least) all the legacy signals thanks to the custom signal feature. Description of these possible evolutions is given in Annex B.

XML file

CustomSignal

Version

Document interface version, curent version is "1.0".

NavMsg

Optional Boolean. If missing, it will try to get a navigation message from the shared library. If set to True, it will generate an error if the navigation message is missing from the shared library.

Constellation

The satellite constellation to which this custom signal will be attached to.

CentralFreq

Custom signal's central frequency, in Hertz.

Bandwidth

Custom signal's bandwidth, in Hertz.

DLLPath

Path to the shared library, relative to the XML.

SignalLevel

Default power offset to the constellation reference, in dB.

Subcarrier

Id

Subcarrier identifier.

Rate

Subcarrier rate, in Hertz.

Coef

An array of real numbers representing the subcarrier chip values. For example, a BOC would have the [-1, 1] coefficients. A CosBOC at half power would have the [-0.5, 0.5, 0.5, -0.5] coefficients.

Code

Id

Code identifier. Can be used to distinguish different custom signals in the same shared library.

ModulationCoef

Real

The real part of the multiplier applied to every samples.

Imag

The imaginary part of the multiplier applied to every samples.

Shared library

ICustomSignal

Constructor

Your implementation constructor must take a reference to a constant CSInitData structure, containing various simulation start information.

getNavMsg

Returns a pointer to the navigation message object or nullptr if this shared library does not define any.

getCode

Returns a pointer to the code object that match the null-terminated identifier name or nullptr if this is not a recognized name.

ICustomSignalCode

getNumberOfChipsPerMSec

Returns the code rate in chips per millisecond.

getChips

Copy your custom chips for elapsed time elapsed and PRN prn into the buffer chips. The buffer is guaranteed to be getNumberOfChipsPerMSec() + getExtraAllocSize() long.

getExtraAllocSize

Extra bytes to allocate for the chips buffer.

ICustomSignalNavMsg

getNavMsgDurationMs

Returns the navigation message duration in milliseconds.

buildNavMsg

Called every getNavMsgDurationMs() with updated simulation data.

getTOWOffset

Returns the navigation message's offset to the constellation's Time Of Week.