# Indoor Signal Strength Evaluation of the Orbcomm Low Earth Orbit Satellite Constellation +
## 3 Material and Methodology
### The Orbcomm Constellation  
Since 1995, the Orbcomm satellite constellation has provided users with the ability to transmit and receive information in remote areas. Multiple sectors leverage this constellation, such as the maritime sector which uses the Orbcomm satellite constellation to transmit and receive Automatic Identification System (AIS) data. Furthermore, in the transport sector, the constellation is used to track and get status updates from vehicles [19].  
The Orbcomm system consists of UE, a space segment, and Earth stations. The space segment consists of two generations of satellites, namely, Orbcomm Generation 1 (OG1) and Orbcomm Generation 2 (OG2). The characteristics of the constellation as they were designed can be found in Table 1.  
The Orbcomm space segment functions as a link between the user and a gateway station on Earth. The satellites transmit a constant downlink signal that user devices can tune into. On this downlink channel, user devices can find the active uplink channels of that satellite. The user device can use these channels to transmit data to the satellite, which in turn relays it to the Earth gateway station. The downlink frequencies are positioned between 137 MHz and 138 MHz and the antenna subsystem of these Orbcomm satellites consists of a RHCP quadrifilar helix antenna which transmits with an Effective Isotropic Radiated Power (EIRP) of 12 dBW [20].  
The 137-138 MHz band is not exclusively used by Orbcomm for downlink communication. Multiple meteorological satellites use this frequency range to communicate,  
\begin{table}
\begin{tabular}{l l l l l l} \hline \hline \multirow{2}{*}{**Generation**} & **Plane** & **Number of** & \multirow{2}{*}{**Inclination**} & \multirow{2}{*}{**Altitude**} & **Planned no.** \\  & **Designation** & **Orbital Planes** & & & **of Satellites** \\ \hline OG1 & A, B, C, D & 4 & 45\({}^{\circ}\) & 825 km & 8 in each plane \\ OG1 & E & 1 & 0\({}^{\circ}\) & 1000 km & 7 \\ OG1 & F & 1 & 70\({}^{\circ}\) & 775 km & 4 \\ OG1 & G & 1 & 108\({}^{\circ}\) & 830 km & 4 \\ OG2 &? & 3 & 47\({}^{\circ}\) & 715 km & 4 in each plane \\ OG2 &? & 1 & 47\({}^{\circ}\) & 715 km & 3 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The key characteristics of the Orbcomm OG1 and OG2 constellation [4,20].  
Figure 1: The Diamond DPKE137 RHCP quadrifilar helix antenna in an outdoor environment.  
for instance, the National Oceanic and Atmospheric Administration (NOAA) satellites have multiple downlink channels in this band [20]. The band is also used by the recently launched SpaceBEE constellation [4]. Despite the shared use of this frequency band, there is no overlap between these systems. They each use their own well-defined frequency ranges within this band [20].  
Currently, the Orbcomm constellation does not exactly meet the specifications of Table 1. At the time of writing, the actual OG1 constellation consist of 22 functional satellites and one semi-operational satellite. Most of the OG1 satellites are currently also in a lower orbit which is between 750 km and 790 km [21]. The OG2 constellation consists of 12 functional satellites [21]. Some confusion surrounds this subject since at the moment, even the official Orbcomm website does not correctly display which satellites are operational [19]. However, 'operational' does not necessarily mean that the satellite is actively transmitting. During the measurement campaign, very few signals from OG1 satellites were received. This is in contrast with the operational OG2 satellites, which are all actively transmitting.