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 DirectSignalPower.nc 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.

Update of Level 2 data

The Level 2 FDI wind-speed data has been updated to include measurements at high and low latitudes.

The processing remains FDI V1.11.

Originally a filter of 55 degree latitude was used to be sure that sea ice was excluded from the retrieval. The new approach filters using a sea-ice map so that our measurements continue closer to the Arctic and Antarctic.

Below are the availability of L2 wind-speed product,

With new sea ice filter:

Original sea ice filter by latitude:

TDS-1 Data release v0.7

We are updating the MERRByS service with two significant changes:

  • More data, lots more: From March 2016 to July 2017
    • Level1B and Level 2 (FDI windspeed)
  • New data format (NetCDF)

The new data format moves on from the XML / Tiff format that was used on the last release (Level 1B version 0.5). The new format is NetCDF and provides many advantages for distribution and processing. There is an updated Product Manual in Documentation.

To make it easier to get started with this new format, example code is provided in Matlab at: https://github.com/pjalesSSTL/GNSSR_MERRByS

All the old data will still be available on the FTP server at /DataV0p5 for the time-being.