The relative accurate pose estimation problem of collaborating Unmanned Aerial Systems (UASs) in GPS-denied environments is addressed in this article. The on-board system uses fiducial markers attached in the facets of a truncated rhombicuboctahedron placed on top of the UAS. A visual marker-tracking mechanism using a global shutter camera is used to estimate the relative pose between the UASs under the Line-of-Sight (LoS) constraint. The pose estimator operates in parallel with an Ultra-wideband (UWB) system that transmits extremely short RF-pulses, spreading the radio energy over a wide frequency band, and measures the relative distance using Time Of Arrival (TOA) measurements from a set of off-board anchors. Experimental studies show that the UWB-system can provide satisfactory measurements at large distances (2m to 50m) but the resolution is not sufficient for operating the UASs in very short relative distances (under 5m). Therefore, the visual system is used to improve the relative position measurements at short distances (0.2m to 5m) in addition to providing orientation measurements.