Highly accurate measurements of total electron content (TEC) of the ionosphere are made using beacon transmitters and a beacon receiver on low Earth orbit satellites. The Naval Research Laboratory Scintillation and Tomography Receiver in Space (CITRIS) instrument, in a nearly circular orbit at 560 km altitude and 35° inclination, uses signals from more than 12 VHF/UHF radio beacons also in low Earth orbits. CITRIS recordings of differential phase observations estimate TEC between two satellites using a differential phase technique and are limited by the 2π ambiguities of the measurements. During space-to-space conjunctions, nearby passes allow determination of absolute TEC by assuming that the TEC is zero for extrapolations to zero separation. Data from CITRIS conjunctions with the Coherent Electromagnetic Radio Tomography (CERTO) beacons on the Taiwanese COSMIC satellites provide absolute TEC and horizontal electron density gradients. Accurate absolute TEC measurements, though difficult to obtain, are important for successful space weather modeling. These types of high-resolution ionospheric observations have been routinely collected for midlatitude and low-latitude regions of the ionosphere with CITRIS for the past 2 years.