In March 1961, a balloon was released to test the recently modified Mobile Tracking Station (MTS).  The MTS was originally built in 1958 to track the Pioneer space probes immediately after launch, along with the large tracking stations of the Deep Space Network. It was first used in Puerto Rico, returned to Goldstone Tracking Station in California to be modified and tested, then shipped to the DSN site in Johannesburg, South Africa in April 1961.
NASA
1961-03-01
<a rel="nofollow" class="external free" href="https://pub-lib.jpl.nasa.gov/docushare/dsweb/View/Collection-115">https://pub-lib.jpl.nasa.gov/docushare/dsweb/View/Collection-115</a>