Ground-Based Altimetry Using a Single- Receiver Single-Frequency GNSS Phase Ambiguity Resolution Technique G. Stienne* S. Reboul J.-B. Choquel M. 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) Direct signal RHCP Reflected signal LHCP Same oscillator for the digitizing Pseudorange variations 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 estimator When 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 phase Both 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 pseudoranging Pseudorange 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=0 The 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 signal Method : 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 the reflecting 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