Balloon - Research or Radiosonde


On 1/23 about 0545 Denver time I noticed a rarely seen balloon icon in PiAware Skyview. It appeared the balloon took off from the Boulder area, was sending out ADS-B data and had both transponder squawk (4433) and an N registration (N231LB), which did not correspond to any recognized registration. The altitude as it traveled south along the Front Range was 61,000 feet, varying little. Speed also fairly stable at about 21kts. I tracked it until the ADB-S signal faded just north of Colorado Springs. What is this? Radiosondes generally use different (dedicated) frequencies, not 1090mhz. Research balloon? Anyone have a clue?