8/17/2019 Poster LIMB workshop

http://slidepdf.com/reader/full/poster-limb-workshop 1/1

An End-to-End GNSS Radio Occultation SimulatorJoel Rasch, Molflow

Anders Carlström RUAG Space AB

Patrick Eriksson, Chalmers

GNSS radio occultation (RO) is a technique where one

measures the refraction of GNSS signals as they pass

through the atmosphere using receivers on LEO satellites.

From the bending of the signals one can deduce

atmospheric parameters such as pressure, temperature,

humidity, and electron content. The technique is very

reliable and requires little calibration, and holds the

promise of being able to deliver important data for NWP.

In order to be able to simulate how the next generation of

receivers for RO will perform, and what the fundamental

limitations of the technique are, it is necessary to be able

to simulate the microwave field propagation through the

atmosphere and into LEO, as well as adding noise and

applying inversion methods to the signal. For this reason

we have constructed a RO simulator which uses the

multiple phase screen technique to propagate the field in

the atmosphere, a diffractive integral to propagate it to

LEO, and the phase matching technique to invert the

signal.

Introduction

Principles of GNSS Radio

Occultation

When a GNSS microwave signals passes through the Earth

atmosphere its wavefront is distorted due to refractivity

gradients in the atmosphere. At each position where the

wavefront is encountered by a LEO satellite one can

identify a ray (or several) having a bending angle () and

an impact height ().

From the signal phase and amplitude we can deduce the

bending angle vs. impact height for a vertical column in

the atmosphere in the vicinity of the tangent point.

From the  vs.  diagram one can (via the so-called Abel

transform) find the refractivity vs. height diagram.

Using sophisticated methods, atmospheric parameters

such as temperature, pressure and humidity can be

deduced from the refractivity curve.

The Wave Optics

Propagator

To be able to make realistic simulations of what the

microwave field will be in LEO, and to judge the

performance of receivers etc., it is necessary to simulate

the propagation of the field from GNSS to the

atmosphere, through the atmosphere, and into LEO.

1) Propagation from GNSS to atmosphere

2) Propagation through the atmosphere

3) Propagation from atmosphere to LEO

This step is easy, the field is modelled as a cylindrical

wave, so the amplitude is given by

= exp 0  

Where 0 is the wavenumber, and a position vector with

respect to the Earth center.

The propagation through the atmosphere is achieved with

the Multiple Phase Screen (MPS) technique. Using a

spatial Fourier in the direction perpendicular (y) to the

propagation direction (z)

, = ℱ (, ) = , exp ∞

−∞ 

One can solve the Helmholtz equation approximately, and

the wavefront can be propagated a small step (∆) in the

propagation direction using

, + ∆ = exp 02 () 1  

× ℱ− exp

20 ℱ ,()  

Where   is the refractive index. The influence of theatmosphere is thus taken into account on phase screens in

the vertical direction separated by the distance ∆. Using

some 1000 screens one can propagate through the entire

atmosphere.

The propagation from the last phase screen to LEO is

achieved using a diffractive integral

== 0

2 cos exp 0 4

0

 

Where   is the height of the screen, and   the angle

between the screen normal and the vector between the

screen and the LEO position.

Signal InversionIn LEO the signal phase and amplitude are received. The

problem is now how to interpret the signal in terms of

bending angle and impact height. There are several

methods, e.g.: Back-Propagation, Canonical Transform,

Full Spectrum Inversion, and Phase Matching (PM). The

main problem is how to resolve multipath regions. PM is

generally considered to be the most accurate, and that is

what we have used.