The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
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Figure 1: An AIN’s UA with the IG’s PRS sensor and navigation-orientation payloads: GPS L1/L2 receiver, LN200 IMU, Hasselblad 
Biogon SWCE 903 and the PRS navigation-orientation Control Unit. (Photographs courtesy of AIN.) 
2. THE UA COMPONENTS OF AN UAS-BASED PRS 
SYSTEM 
In (Colomina et al., 2007) the classification of the components 
of an UAS-based PRS system is organized in two dimensions: 
the CS/UA dimension and the application 
dependent/independent one. In this paper we are interested in 
just describing the UA-PRS application item which contains 
the PRS sensor payload and the PRS navigation-orientation 
payload. The PRS sensor payload contains the set of all sensors 
with the exception of the navigation ones, their storage devices 
and the mechanical interfaces (sensor bay or platform). The 
PRS sensor payload may be assembled rigidly or with shock 
mounts, into the UA structural frame. Depending on the 
sensors, this payload may include a Control Unit (CU) in 
charge of spatio-temporal inter-sensor calibration, data storage, 
possibly real-time sensor data processing, and PRS mission 
control. 
The PRS navigation-orientation payload typically contains 
GPS receivers, inertial sensors, other navigation devices like 
barometric altimeters and magnetometers, and a Control Unit 
(CU). It provides a real-time navigation solution —including 
time synchronization signals and data— which may be used as 
an input to the UA Flight Control System (FCS) and to the PRS 
sensor payload. Usually, it stores observational data for a 
posteriori precise sensor orientation. The CU is a computer that 
synchronizes, reads and stores the measurements of the 
navigation and orientation instruments and that runs the real 
time navigation SW. In figure 2 the IG’s CU for small and 
tactical UAs can be seen. 
3. ON THE COMPATIBILITY OF HIGH- 
RESOLUTION AND LOW-COST IN UAS-BASED PRS 
High-resolution —and high-quality— at low cost is feasible 
thanks to the progress in sensor technology, HW 
“miniaturization” and SW or, more to the point, in computer 
models for navigation-orientation and remote sensing. 
HW miniaturization precisely means small volume, low weight 
and low power consumption. A PRS sensor payload based on 
optical cameras of 20 Mpx to 40 Mpx may take as little as 4 1 
of volume, 7 kg of weight and 30 W of power. The size of a 
GPS dual frequency board with WAAS/EGNOS navigation 
capability is about 8.5 x 12.5 x 1.7 cm 3 , weighs about 80 g and 
requires less than 5 W. A tactical grade IMU requires some 0.6 
1 of volume, 0.8 kg of weight and 16 W of power. The 
requirements of other secondary navigation sensors like 
barometric altimeters and magnetometers are almost negligible 
with respect to the previous amounts. A PRS navigation- 
orientation CU requires 3 1 of volume, some 1.8 kg of weight 
and less than 20W. If we take into account that medium-format 
cameras (figure 3) can be turned into metric cameras with the 
appropriate orientation and calibration SW tools, we are in 
front of high-resolution, high-quality, lightweight and moderate 
cost systems. 
The overall cost of the above configuration is around 70 k€. 
We are aware that the cost of the HW components of a system 
may represent as little as a third or less of the final cost for the 
final user. Moreover, the overall cost of a product or service 
based on a low-cost system may end up being higher than the 
cost of the same product or service based on more expensive 
infrastructures; at the end, it is overall price performance what 
counts. However, the low-cost and high-quality levels 
achievable with the previously described equipment on board 
of small UAs may capture some market segments and open 
new ones. 
An example of a PRS payload for a small UA 
In Figure 1 an experimental system consisting of a small UA 
(payload capacity up to approximately 10 kg), a navigation- 
orientation payload, its CU (figure 2) and a medium format 
camera (figure 3) can be seen. The system has been integrated 
by AIN and the IG within the frame of the uVISION project 
(Colomina et al., 2007). The navigation-orientation payload 
includes a geodetic grade GPS L1/L2 receiver Novatel OEMV, 
a tactical grade Northrop-Grumann Litton LN200 IMU, a 
Honeywell HPB barometric altimeter and a Leica Vectronix 
DMC-SX magnetometer. The navigation-orientation CU is 
based on a PC 104 architecture and a Linux operating system; 
besides the main board and standard communications boards it 
includes a time synchronization board. The sensor payload is 
composed of a Hasselblad Biogon SWCE 903 camera with a 
Kodak DCS Pro Back Plus digital backplane of 16.6 Mpx and 
time synchronization electronics. The camera and the IMU are 
rigidly assembled and isolated from the mechanical vibrations 
of the helicopter engine and rotor. Figure 4 shows two Siemens 
star targets photographed in static (engine off) and kinematic 
(engine on) modes. The system weighs less than 10 kg and 
requires some 50 W of power.