The CanX-7 Nanosatellite ADS-B Mission: A Preliminary Assessment

The development of space-based Automatic Dependent Surveillance-Broadcast (ADS-B) will allow surveillance of aircraft in areas not covered by radar or ground-based ADS-B systems. In September 2016, the Canadian Advanced Nanospace eXperiment-7 (CanX-7) satellite was launched into a 690 km sun synchronous orbit with an ADS-B receiver payload. The first phase of ADS-B data collection took place over the North Atlantic between 4 and 31 October. A preliminary assessment of the data indicates that the average ADS-B signal strength is close to the calculated receiver detection threshold of −94.5 ± 0.5 dBm. The pattern of received ADS-B reception appears to be consistent with a signal propagation model developed for the CanX-7 mission. Future work includes the comparison of coincidental flight plan data for the operations area and an analysis of the payload antenna pattern.

Currently only 30% of the Earth is covered by the combination of radar and ADS-B. In the absence of aircraft surveillance, existing air traffic procedures use standardized routes and large inter-aircraft spacing to provide aircraft separation. ADS-B coverage is limited by the placement of ground stations, which cannot be installed in mid-ocean and are difficult to maintain in remote areas. A potential solution for the surveillance of aircraft anywhere in the world is through the monitoring of ADS-B transmissions using orbital platforms. The propagation modelling [2]- [8]. This paper gives a preliminary assessment of the CanX-7 ADS-B mission. Section 2 describes the satellite and payload parameters; Section 3 outlines the first phase of operations; Section 4 discusses the preliminary findings and Section 5 includes a summary and future work. primary payload consists of four deployable drag sails that will demonstrate passive de-orbiting from LEO. The Inter-Agency Space Debris Coordination Committee recommends de-orbiting of spacecraft in LEO within 25 years of mission completion, which is problematic for small satellites without propulsion systems.

CanX-7 Satellite and ADS-B Payload
The drag sail, with a total area of 4 m 2 , is scheduled for activation approximately six months after launch. Deployment of the sail will be captured with onboard cameras. The secondary payload is an ADS-B receiver that will collect transmissions prior to drag sail initiation. Raw ADS-B data will be stored onboard CanX-7 and downlinked later to the UTIAS ground station. Attitude determination is accomplished with a magnetometer, while a set of three magnetic torquers provide 2-axis attitude control by aligning a primary axis with the local magnetic field. Solar cells generate power with a lithium ion battery used for energy storage. Thermal tapes provide passive temperature control for the spacecraft. Table 1 lists specifications of CanX-7, while Figure 1 and Figure 2 show the major components of the satellite.     [9]. A schematic of the ADS-B payload is shown in Figure 3 [9].

ADS-B Operations
CanX-7 was launched into a sun synchronous orbit of 690 km on 28 September 2016, resulting in approximately 15 orbits per day. Following a satellite-commissioning period, the first phase of operations began on 04 October and ended on 31 October. During this time, the ADS-B receiver was activated for 18 minutes in the Northern Hemisphere during each orbit, representing a latitudinal coverage between 12˚N and 78˚N. There were 381 collection periods in which a total of 776,584 call sign, position and velocity messages were received. Status messages were not decoded and are not included here. Overall statistics for the first phase of the mission are shown in Table 2.  day. There were as many as four passes over the operations area per day, but typically a descending pass between 1100 UTC and 1300 UTC and an ascending pass between 2100 UTC and 2300 UTC provided the best coverage. Figure 4 shows the CanX-7 ground track for a 24-hour period, with favorable descending and ascending passes identified. Data provided by NAV Canada indicated that air traffic in the Ganderand Shanwick OCAs experience two peaks every day. As seen in Figure 5 there is a maximum of about 220 aircraft at 0300 UTC representing the eastward flow of aircraft and a similar peak at 1400 UTC representing the westward flow of aircraft [6]. During CanX-7 passage the number of   Greenland the operations area during a descending pass on 29 October. The satellite position is shown in 10-second increments with red dots representing ADS-B messages received during the entire pass, while aircraft symbols indicate signal reception during the shown 10-second interval. In Figure 6(a) the satellite, represented by the orange symbol, did not detect any of the aircraft. As the satellite transits southward in Figures 6(b)-(d), an increasing number of contacts is evident. The sequence, which is typical of the descending passes, implies a bias of the satellite antenna to the northwest because of the pointing vector. Contacts to the east of the satellite track do not appear until the satellite is near the southern boundary of the operations area. Figure 7 shows the local magnetic field for the 29 October descending pass in one-minute increments based on the International Geomagnetic Reference Field (IGRF). The theoretical offset of the nadir point is shown to the northwest of the satellite ground track.

Signal Levels
ADS-B signals received by the payload are given an integer RSSI value between 0 and 255. The average RSSI value for all signals received was 28, which is close the calculated −94.5 ± 0.5 dBm Minimum Detectable Signal (MDS) of the payload receiver. In accordance with Van Der Pryt and Vincent [8], Figure 8 shows the ADS-B signal propagation model for a satellite altitude of 690 km based on a 500 W transmitter and a typical aircraft antenna radiation pattern for ADS-B transmissions. Taking into account the calculated payload MDS, represented by the dashed line in Figure 8, the model implies that aircraft 5˚ to 8˚ and 42˚ to 60˚ from nadir should be detected. There is a null 0˚ to 5˚ because of the aircraft quarter-wave monopole radiation pattern, while signals between 8˚ and 42˚ are  predicted to fall below the payload detection threshold. Spurious contact was observed for all passes. This is likely because signal strength may be near the threshold of payload sensitivity depending on aircraft-satellite orientation. While there are many instances of single message contact in the database, there are also examples of hundreds of messages received from the same aircraft in a single pass. With respect to multiple message contact, the data stream could be near continuous or experience significant intervals between messages. For the descending pass of 29 October ( Figure 6) there were 60 different aircraft detected in the operations area, representing 996 position messages. During this pass, there were seven instances of 40 or more position messages received from a single aircraft with a maximum of 106 messages from the same aircraft. Figure 9(a) illustrates the breakdown of the position messages received for each aircraft in the operations areas during the 12 UTC descending pass on 29 October. Figure 9(b) offers a comparison to the 12 UTC descending pass of 16 October to demonstrate the observed consistency between similarly timed passes. The average RSSI value for 40-plus multiple message contact shown in Figure 9(a) and Figure 9(b) is not significantly different from the overall average RSSI value of 28 and remains close the payload MDS. This implies that the disparity between the number and consistency of received signals from individual aircraft may be a function of aircraft antenna radiation pattern rather than transmitter power. For 16 and 29 October, the greatest number of received messages originated from a Boeing 777 (KLM) and a Boeing 767 (Delta) respectively, which could indicate a superior antenna configuration for space-based ADS-B surveillance.

Signal Propagation Model
The tilt of the antenna boresight as a function of the magnetic field may result in contact at extended ranges from the satellite. Figure 10   are few contacts in the operations area, however a grouping of aircraft are detected on the east coast of North America. The slant range to these aircraft is 2500 to 3000 km. During this timeframe, there is evidence that the payload did not detect a number of aircraft between CanX-7 and the coastal region. Figure   11 shows flights reported during the satellite pass by Flightradar24, a flight tracking application that combines data from several sources including groundbased ADS-B and radar. Considering the calculated MDS of the payload, the signal propagation model in Figure 8 predicts the potential of missed contacts in the medium range as suggested by Figure 11.

Summary and Future Work
The CanX-7 ADS-B receiver collected data over the Gander and Shanwick OCAs high-density air traffic areas. The drag sail is scheduled for deployment at the beginning of May 2017, at which time ADS-B operations shall be terminated.