Ground-Based Altimetry Using a Single-Receiver Single-Frequency GNSS Phase

Ambiguity Resolution Technique

G. Stienne*S. ReboulJ.-B. ChoquelM. Benjelloun

SPACE REFLECTO 2013

System geometry Software receiver- Signal processing architecture- Phase processing in open loops

Altimetry measurement Ambiguity resolution Experiments Conclusion, prospectives

2

Overview

System geometry: ground-based applications

3

is the path difference between the direct and the reflected signal

4

Receiver architecture

Code generator Carrier replica (frequency)

Phase processing(POL)

Code generator Carrier replica (frequency, phase)

Code and phase processing (DLL-POL)

DirectsignalRHCP

ReflectedsignalLHCP

Sam

e os

cilla

tor f

or th

e di

gitiz

ing

Pse

udor

ange

var

iatio

ns

In open loops, phase measurements are angular

Additional code delay(s)

Filter defined as the Kalman filter but with the Circular Normal von Mises distribution

Prediction step:

Update step:

Phase tracking: circular filter (linear evolution case)

with

11

with

and

6

Phase tracking: circular change estimatorWhen a cycle slip occurs (high dynamics, low Signal to Noise Ratios), it can be detected and its amplitude estimated via a GLR change estimator defined following the von Mises distribution.

The estimations of (cycle slip position)and (cycle slip amplitude) are based on the inversion of (filter innovation)

7

Ranging: code vs phaseBoth the code and the phase of a GNSS signal are periodic

C/A code period: Phase period:

Range periods:

GNSS codes are square signals. The observed code delays are piecewise constant. The sampling frequency defines the measurements resolution. Ranging precision: several meters.

The carrier is continuous, and so the phase delays. Ranging precision: centimeter.

Phase ambiguity

8

Phase pseudorangingPseudorange variations

Received signal frequency

Replicated signal frequency

Phase delay between the received signal and its replica

Pseudorange at t=0 ambiguity

Local oscillator noises

9

Phase altimetry

Direct signal:

Reflected signal:

Choosen common Common for a GNSS-R receiver

Same receiver clock errors, atmospheric errors, orbit errors on both signals

10

Pseudorange at t=0The pseudorange at t=0 is obtained by dating the code using the data message

1 ms

Known emission date (TLM)

Received code

Known reception date

The telemetry word emission is dated, so the emission of each code period can be dated with the precision of the satellite atomic clock.

The first code delay, , has to be precisely estimated in order to get a precise datation at t=0.

11

Precise estimation of

Code delays

Phase variations applied to each code delay

Principle : averaging the code delays obtained over the whole acquired signalMethod : Bring each delay back to the origin using the estimated phase variations

12

Experiments : principle

Graduations for accurately known height modifications

13

Experiments : principle

Constant height for thereflecting water

Several acquisitions (7 seconds)Precisely known variations on the antenna height between two acquisitions

The variations of should be observed

Observation of several satellite footprints => same measured heights

First test:

Second test :

Experiments : observed footprints

14

Experiments : results

15

The height is constant over time: good estimation of the phase

The results on satellites 21 and 25 differ by up to 2 meters: wrong estimation of or with the signal of the satellite 21 or occurrence of a parasitic multipath

On satellite 25, the water level variations differ by up to 20 centimeter from what was expected

16

Conclusion & prospectives

Development of a mobile GNSS reflectometer Short signal durations, no double difference Robust and precise height variations estimations with 1 millisecond of coherent integration Still imprecisions on altimetry measurements linked to the phase ambiguity resolution Need for more experiments in order to find the limits of the proposed ambiguity resolution technique Airborne experiments

Thank you for your attention

17

GPS L1 signal structure

Emitted signal :

Received signal:

Code delay Phase delay

L1 carrier

C/A code

Data message

Modulation

Multiplexing

Emitted signal