First GNSS-R Sea Ice Level 2 Product Released

The first sea ice product on MERRByS from the TDS-1 satellite GNSS-R data catalogue has been released, and is now available on the FTP server. Surrey Satellite Technology Limited (SSTL) and the National Oceanography Centre (NOC) recently released v1.0 of the Calibrated Bistatic Radar Equation (C-BRE) processor, which provides a number of new and improved features including a novel GNSS-R Sea Ice Detection Algorithm (GSID). The output from this algorithm has been formatted into a Sea Ice product, and can be found here under “L2_CBRE_v1_0_GSID”. An updated product manual can be found here on the documentation page. We would be keen to hear feedback from users about this new capability.

A month by month availability web page has been set up to allow users to quickly view the historical ice data and can be found here.

Antarctic ice map generated using TDS-1 data between 31/10/2018 - 30/11/2019.
Antarctic ice map generated using TDS-1 data between 31/10/2018 – 30/11/2019.
Arctic ice map generated using TDS-1 data between 31/10/2018 - 30/11/2019.
Arctic ice map generated using TDS-1 data between 31/10/2018 – 30/11/2019.

NOC C-BRE v1.0 L2 Data Released

Cartographic plot showing C-BRE v0.5 and C-BRE v1.0 data availability (NOC 2019)

During an ESA-sponsored study in 2018-2019, Surrey Satellite Technology Limited (SSTL) and the National Oceanography Centre (NOC) have been collaborating to improve the L1B to L2 inversion accuracy for TDS-1 data.

The output of this work is an upgraded Calibrated Bistatic Radar Equation (C-BRE) processor (v1.0). This algorithm has been integrated into MERRByS and used to re-process the back catalogue of TDS-1 measurements.

The improvements over C-BRE v0.5 include:

  • Application of dual on-board / Antarctic radiometric calibration, increasing data coverage
  • An ice detection algorithm allowing data up to ice edges, and a new flag for presence of ice
  • Improved corrections for variation in GPS transmit powers
  • Improved flagging of processor and RFI anomalies

The data is available from the MERRByS FTP server in the data folder labelled “L2_CBRE_v1_0”. This folder can be found here.

An updated product manual describing the new C-BRE v1.0 Level 2 outputs can be found on the documentation page.

TDS-1 Drag Sail Deployment

Image of deployed drag sail taken by TDS-1 on-board inspection camera

The deployment of the Icarus-1 drag sail, which was supplied by Cranfield University, marks the end of mission operations for SSTL’s TechDemoSat-1.  This sail increases the cross-sectional area of the satellite and hence atmospheric drag to speed up reentry.

SSTL posted a press release announcing the deployment. Jonathan Amos from the BBC tweeted with a special mention of the achievements of TDS-1 in GNSS Reflectometry.

Meanwhile SSTL staff members are working on an upcoming satellite mission carrying an opportunistic GNSS reflectometry experiment. We hope to bring you news about this new project very soon.

April 2019 Update from TDS-1

Routine DDM collection from the SGR-ReSI on TechDemoSat-1 ceased in December 2018, and since then, operators have been making preparations for the drag-sail deployment. This has required controlled passivation of the satellite, which includes the slow expulsion of all remaining propellant from the propulsion tanks.

During gaps in this preparation period, we have been able to make a final handful of L0 raw data collections from TDS-1, to add to the collections we have gathered through its 5 year lifetime.

As well as C/A code signals from GPS, we have been targeting E1B/C reflections from the Galileo satellites, and other signals that fall within the SGR-ReSI’s frequency band. The Galileo signals are wider in bandwidth than GPS C/A code, and use BOC modulation. These new signals are somewhat more complex to process, but to a first order, we see similar behaviour in the DDMs to GPS C/A code reflections over ocean, land and ice. This is good news as it shows the practicality of using Galileo alongside GPS reflections in our upcoming GNSS-R missions.

Galileo E1(B+C) Delay Doppler Map from TDS-1 over ocean
Galileo E1(B+C) Delay Doppler Map from TDS-1 over ice

We at SSTL, together with partners from NOC, will be presenting recent results from TDS-1 at the GNSS+R 2019 workshop 20-22nd May 2019 in Benevento, Italy. We look forward to meeting many of you there.

End of MERRByS Operation Pilot Service

We are sad to announce that life extension of TechDemoSat-1 for supporting MERRByS is coming to an end. Routine GNSS-R data collection will cease this weekend (16th Dec 2018), and the GNSS reflectometry service will not resume in January. The SGR-ReSI was just one of the 8 technology experiments, and the final demonstration on TDS-1 is the de-orbit sail; this will be deployed early in 2019.

SSTL and NOC have plans for the flight of future GNSS Reflectometry instruments to continue what TDS-1 started, so watch this space!

In the meantime, we will keep maintaining the website and data repository.

Thank you for your ongoing interest in and participation with this GNSS-Reflectometry project. We continue to welcome feedback into a potential future GNSS-R service.

New Data Available

There are three significant updates to announce for the MERRByS service.

Mission extension – Continuous GNSS-R

The TDS-1 mission ended and since February 2018 it has been operating with a mission extension. The SGR-ReSI is now running 7/7 days, rather than the 2/8 as in the first 3 years. This is increasing the coverage and sampling by a factor of 4.

To support continuous operations the DDMs are now selected onboard to only those with antenna gain > 0dB, and compressed using a variable quantisation rate. We are not expecting users to see differences from this compression process. Further details are available on request and will be put in the Product Manual in due course. The L1b version has been increased to V0.71 to support the compression.

Data update every 6 hours

The 2nd announcement is that MERRByS is now being updated every 6 hours with new data to the FTP server. There are two streams of data available.

Standard Users:

  • 1 Month latency
  • Apply for account password, as before by the registration page

Fast Data Users:

  • 24-48 hour latency
  • Providing pilot data service – demo of data release within 1-2 days of measurement
    • Additional interaction with SSTL & NOC – support and feedback
    • Apply on the registration page with your request

New Ocean Wind Product

The 3rd announcement is that the L2_CBRE (Calibrated Bistatic Radar Equation) V0.5 Algorithm has been incorporated into MERRByS. The National Oceanography Centre (NOC) have developed this algorithm to use additional receiver and transmitter corrections to improves the wind-speed measurement. This is the algorithm as presented in GNSS+R 2017, incorporated into MERRByS.

It should be noted that this is currently preliminary work and is expected to be updated soon with a new version. Currently the coverage is sparse due to a known flagging issue.

We will currently be putting the CBRE and FDI ocean wind speed products alongside each other.

Direct Signal Power Update

The focus for GNSS-Reflectometry with the SGR-ReSI is to measure geophysical parameters such as ocean wind-speed and mean-squared-slope. This is performed by receiving the GPS navigation signals reflected from the Earth, measuring the power and signal distortion caused by the reflection then inferring back the parameters of the surface.

One of the factors that affect the reflection power measurement is the power level transmitted by each of the GNSS satellites. There is variation between each GPS transmitter and over their antenna patterns.

The SGR-ReSI uses the signals directly from the GPS transmitters for navigation and for the steering of the DDM processing to the location of the reflection. The power of the direct signals is measured onboard with a similar setup to that used for the reflections.

With the last update to MERRByS the direct signal power measurements were exported to NetCDF file and released. These contain the time-series of our direct signal power measurments. The files have now been replaced on MERRByS with version 0.3. Additional fields have been added and a number of corrections to the fields, so that now radiometric calibration can be performed on the data.

The Direct Signal Power file is in NetCDF format and contains self documenting headers that can be inspected from the file itself. There are some minor updates in a new version of the  MERRByS Product Manual V4.