Monthly Archives: January 2015

ADS-B Lite Differences

As the name suggests ADS-B Lite is something less than the standard ADS-B 1090MHz-ES with transponder. It is also quite a bit more. In this blog we will explore how ADS-B Lite does more with less.

Less Functionality
If ADS-B is truly an alternative to radar surveillance, why not go all in and eliminate the transponder functionality: no IFF receiver, no Mode S; just the Extended Squitter broadcast.
Less Power
The rationale for eliminating the transponder is power. The DO-160B Minimum Operational Performance Standards (MOPS) specify the power requirements for a transponder. At high altitudes, detection beyond 200 miles is common. If cell phone range is all that is needed for slow, low-altitude operations we can cut the power requirement by a factor of over 10 (e.g. six watts and not 70 watts). Moreover, since we only need six watts every thousandth of a second, we can significantly reduce power consumption, an important factor for power-limited, battery operated drones or sailplanes (gliders) and an affordable alternative for light sport aircraft
Less Cost
The cost driver for a standard ADS-B system is the RF power amplifier. Quotes range from the thousands of dollars to the tens of thousands of dollars. Many manufacturers reply with no bid. Cell phone manufacturers have much more affordable power amplifiers because of market size (millions vs thousands) and a lower power requirement. Will a ten-fold reduction in power translate to a ten-fold reduction in price? The jury is out.
Less Weight
For electrically powered UAVs reducing the power requirement means a lighter power supply, less shielding of the RF section and a smaller RF power amplifier. Every ounce saved in avionics is an ounce that can be applied to a payload.
Less Range
In a previous post, you saw what FRUIT looks like on a radar screen. Now imagine hundreds of UAVs adding to that mix in a congested area such as the Los Angeles basin. Overlapping messages could result in garbled messages causing even more FRUIT and dropped updates. Reducing power to cell phone range limits the amount of bandwidth overload. Coordinating the inputs from multiple ADS-B “in” receivers in a mesh network would allow air traffic controllers to see all traffic in controlled airspace without overloading their radar bandwidth.

Now that you have seen what gets taken away with ADS-B Lite; let us talk about what gets added.

Option to Comply with Established Standards
For aircraft with reciprocating or gas turbine engines avionics power consumption is no factor. By simply enabling the IFF receiver and a Mode S software plug-in and by connecting the six watt output to an RF Power amplifier and its power supply, ADS-B Lite becomes a DO-160B MOPS compliant Mode S transponder with ADS-B 1090 MHz ES.
Option for Inertial Navigation System (INS) Integration
Using the Applications Programming Interface (API) of an INS system such as the Lord MicroStrain 3DM-GX4-45, ADS-B Lite will provide plug and play functionality over a USB connection. An INS provides an alternative source of location data in the event of loss of GPS. It also provides attitude and heading data that may be used for antenna steering, sensor pointing, stabilization and alignment. Click on the image below to download the manufacturer’s product data sheet.GX4-45ProductImage_1.00a

Waypoint Navigation
The intersection of a plane passing through the center of a sphere and a sphere is a great circle. The shortest distance between two points is an arc of a great circle, A great circle is fully defined by the latitude L, longitude λ and track TR being flown.

Great Circle (Latitude, Longitude, Track)

Great Circle (Latitude, Longitude, Track)

Let A be the present position and let B be a waypoint. Given the latitude L and longitude λ of points A and B, the following equations yield range and bearing from A to B. A two argument arctangent of the second and third elements yields the true bearing from A to B and the arc cosine of the first elements yields the great circle arc between A and B. Great circle arc in radians converts to nautical miles or kilometers using π radians = 10800 nautical miles = 20000 km. The API implements these equations for waypoint navigation and traffic conflict assessment.

Equation 1
Note that in spherical geometry the reciprocal bearing from B to A is generally not 180 degrees from the bearing from A to B. This can be readily seen by interchanging the roles of A and B.

Dead Reckoning
Dead reckoning predicts the latitude and longitude at a future time as a function of present latitude L, present longitude λ, present time t0, future time t, ground speed GS and track TR. The product of GS and t-t0 is a distance. On converting the distance in nautical miles or kilometers to a great circle arc γ in radians the sine and cosine of γ and the track TR are inserted into the dead reckoning equations below to solve for latitude and longitude. The two argument arctangent of the first two elements yields the longitude of point B and the arc sine of the third element yields the latitude of point B. Note that the product of the three by three matrix and its transpose is the identity matrix.
DR equations
Traffic Conflict Assessment
On each ADS-B “in” message containing GPS data ADS-B Lite calculates the intersection where one’s own trajectory crosses with the trajectory of the reporting aircraft. From this it derives the point of minimum separation and estimates the time of arrival and amount of separation both horizontally and vertically. If separation is below minimums, a maneuver is proposed to maximize separation. If coupled to the flight controller, the evasive maneuver is performed without human intervention; alternatively, a real-time process monitoring ADS-B Lite real-time data could display traffic conflicts on a horizontal situation indicator and/or collision avoidance system.
The boundary of a restricted area is treated the same as an aircraft trajectory except that the trajectory of the boundary is timeless and may span a large range of altitudes. Obstacles such as bridges, towers or power lines may be handled with geo-fences. Calculations are performed on each GPS update using an on-board geo-fence database amended as necessary by notices to airmen (NOTAMS).
Terrain Avoidance
Imagine an elevation contour on a map as a special case of a geo-fence. To avoid terrain, the aircraft must have the capability to overfly the bounded area. If it does not have this capability, the route must be replanned to circumnavigate the bounded area. Elevation contours would be extracted from an on-board database and a profile of minimum elevation versus range along the current trajectory would be updated at regular intervals where the interval length may vary depending on terrain steepness and aircraft maneuvers. In principle an aircraft could fly nap of the earth using GPS, ADS-B Lite and the on-board databases.
Time Division Multiple Access (TDMA)
TDMA forces aircraft to take turns broadcasting their ADS-B data. It is important that the timestamp applied to a position report be accurate; otherwise, the calculation of the point of intersection by the traffic conflict assessment process will be erroneous. Current practice is to minimize the latency between a fix and the corresponding position report. An alternative is to ignore latency and use a deterministic process to calculate the fix time. For example, let the position report begin at a fixed delay after the GPS pulse per second (PPS) signal. Subtracting the delay and rounding to the nearest second yields the exact time of the GPS fix. The advantage of this alternative is that each aircraft can be assigned a specific time slot where it can broadcast its position report. If each time slot is different, messages do not overlap one another and garbled messages become infrequent. When synchronized with GPS, the DO-160B MOPS require all broadcasts be 200 milliseconds after the GPS PPS signal. This approach will cause messages to be garbled at the worst possible moment, when two aircraft are about to collide.
Pulse on Pulse Logic
When messages are garbled, it may still be possible to decouple overlapping signals using pulse on pulse logic. Pulse-on-pulse logic will be tested on the breadboard ADS-B Lite system under development.

About ADS-B Lite

Automatic Dependent Surveillance – Broadcast (ADS-B) was conceived as an alternative to radar for tracking the location and movement of air traffic. Near airports Airborne Surveillance Radars (ASRs) scan the skies for aircraft. Identification Friend or Foe (IFF) interrogates the aircraft and a beacon on the aircraft called a transponder encodes a reply identifying itself to the radar operator.

ASR with IFF

ASR with IFF

Mode C transponders encode altitude and mode S transponders reply only when called. This helps in rejecting false replies unsynchronized in time (FRUIT). This is what a radar scope looks like before FRUIT has been removed.fruit_ppi

ADS-B broadcasts GPS location data twice per second on the radar’s 1090 MHz frequency in a reserved part of a transponder broadcast called Extended Squitter (ADS-B 1090 MHz ES). Since this location data is more accurate and more frequent than radar, the FAA has mandated that all aircraft operating within controlled airspace (altitudes above 18000 feet and close proximity to airports with control towers) have ADS-B by the year 2020. The FAA has also been directed to share the national airspace with UAVs. Their response to date has been to propose very restrictive rules that would make many commercial uses of drones unfeasible.

What if air traffic controllers could use voice commands to direct UAV use of controlled airspace, monitor compliance with ADS-B 1090 MHz knowing that the UAV would stay away from restricted airspace (geo-fencing), avoid collisions with structures and terrain and most importantly automatically avoid collisions with other ADS-B equipped manned or unmanned aircraft anywhere in the national airspace? Would that open the skies to commercially viable uses of UAVs? That is my vision for the ADS-B Lite project. It also overlaps NASA’s vision of an Unmanned Autonomous System (UAS) Traffic Management (UTM) system.