N24A-T010 TITLE: Subminiature Digital Pitot-static Sensor (SDPSS)

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Microelectronics; Trusted AI and Autonomy

OBJECTIVE: Develop a subminiature digital pitot-static sensor (SDPSS) that fully integrates all sensing and readout electronics within the body of the probe, improving GPS-denied navigation on small unmanned aircraft systems (UASs).

DESCRIPTION: Aircraft have used pitot-static air-data systems for measurement of airspeed and altitude since the dawn of the aviation era and are essential to safe navigation in US / international airspace as well as operation in GPS-denied environments. Technical limitations of current air-data system create integration challenges that degrade their utility in important DoD and civilian applications, including guidance, navigation, and control of small UASs.

The conventional approach to integration of air data system typically pairs a centralized air-data measurement system, containing mechanical or digital pressure transducers, plumbed with tubing to a relatively simple mechanical probe. Pitot-static probes provide pressure-tight connections to a small hole in the center of the rounded tip (pitot port) and to a ring of holes around the periphery of the probe (static pressure port), usually located about 5 probe diameters aft of the tip.

Pressure altitude is calculated using comparison of the absolute (vacuum referenced) static pressure measurement to standard conditions of the International Standard Atmosphere. Correction for ground-level barometric pressure (e.g., as measured at the local airport) permits estimation of the local altitude above ground level – essential for altitude deconfliction of aircraft, as well as terrain and obstacle avoidance. Correction for non-standard temperature and dew point permits computation of the density altitude, which is an important factor in aircraft performance (e.g., lift, drag, and power).

Using Bernoulli’s principle, a differential pressure measurement between pitot (ram air) pressure and static (ambient) pressure indicates the airspeed in the direction of the probe. Correction for installation effects of the probe mounted to the aircraft, compressibility effects at high speed, and air density yields the calibrated, equivalent, and true airspeed, respectively.

Although the pitot-static system has been a reliable component of manned and unmanned aviation for more than a century, several drawbacks persist, especially for integration on very small aircraft, including compact drones in wide usage by DoD and civilian operators. In many cases, these small UASs operate without airspeed / altitude sensing entirely and can only navigate with GPS, leaving them susceptible to GPS interference (intentional or unintentional). Furthermore, operation without (or with a sub-par) pitot-static system can reduce the robustness of the flight controls due to adverse weather and limit the safe flight envelope (reducing efficiency / loiter time or maximum climb / dash performance). These challenges include:

• Physical space occupied by centralized air data computer systems, including individually packaged absolute and differential pressure transducers, signal conditioning, analog-to-digital conversion, and microprocessor, as well as their accompanying mechanical interfaces (tubing, quick disconnects, strain relief bracketry, etc.)

• Hidden failures within pressure plumbing (e.g., leaks or obstructions) due to human error (e.g., pinched tube, contaminated connectors) or operations in a severe environment (e.g., icing, thermal / chemical / UV degradation of plastics)

• Need to provide separate static pressure connection to absolute sensor and reference port of differential pressure sensor, requiring a splitter and additional tubing bulk

• Pressure lags due to long tubing runs in small diameter tubing, degrading the speed, stability, and accuracy of key navigation inputs

• Limited availability of commercial sensors in miniature packages with required accuracy, stability, speed, resolution, and port configuration

• Reliance on separate sensors (or operator inputs) [e.g., outside air temperature] to apply proper correction factors – potentially introducing new sources of error

• Scalability (number of sensors) - challenges above are compounded when multiple sources of air-data measurements are desired or required to achieve desired reliability levels

• Scalability (physical size) – traditional construction relying on physical tubing interconnects between probe and readout device imposes scalability limitations preventing scaling to very small sizes, such as for integration on micro-air vehicles

• Calibration of probes separately from readout equipment creates challenges for traceability and integration

The Navy requires development of a new class of digital pitot-static sensors that sidesteps or mitigates each of these problems. We note that the current state of practice / art in related fields – such as high-performance micro-electromechanical systems (MEMS) manufacturing, widespread commercialization of multi-chip/die integration of heterogenous elements within the same mechanical packages, and advancements in precision additive manufacturing and computer-aided co-design of electronic and mechanical assemblies – could readily be applied to this problem space to offer compelling solutions.

The sensor assembly should:

• Have an overall outer probe tip shape consistent in proportion with accepted practice for construction of pitot-static tubes

• Fit within a cylindrical volume 0.25 inches in diameter and 5 inches long. Preferably, fit within an objective cylindrical volume 0.125 inches in diameter and 2.5 inches long (or smaller)

• Provide a mating surface / fixturing interface, preferably to a hollow support / extension shaft that adapts to the customer’s application (e.g., unmanned aircraft wing, nose, or fuselage mount, wind tunnel fixture, etc.)

• Contain all readout electronics, including but not limited to absolute/ differential pressure / temperature / humidity sensing elements, power / signal conditioning, analog-to-digital conversion, microprocessor, etc.

• Operate off a single, common, coarsely regulated low voltage supply, at low supply current – preferably < 20 mA at 3.3 V ± 0.3 V (not including anti-icing provisions)

• Produce pressure altitude measurements over a full-scale range of at least 1000 to 53,000 feet with an accuracy better than ±10 feet (-1000 to 20,000 feet) ±20 feet (20,000 to 29,0000 feet), ±30 feet (29,000 to 41,000 feet), and ±50 feet (41,000 to 53,000 feet)

• Produce indicated airspeed measurements with an accuracy better than ± 5% of the actual indicated airspeed over the full-scale range from 10 knots to 60 knots (low-speed configuration) or over the full-scale range from 30 knots to 180 knots (high-speed configuration)

• Produce outside air temperature (OAT) measurements over a full-scale range of at least -84 °C to +45 °C with an accuracy better than 1 °C

• Contain design features (e.g., heater, drain holes, coatings) to prevent rain / ice from interfering with proper operation

• Implement a bidirectional digital serial interface capable of supporting multiple addressable sensors on the same bus such as I2C, daisy-chained SPI or UART, CAN, etc.

• Permit addressing of individual sensors (e.g., for configuration / calibration) when sending command or the complete chain (e.g., to poll all sensors for a simultaneous on-demand measurement, or to ‘broadcast’ a shared configuration parameter)

• Digitally transmit commonly required information (e.g., sensor identification, sensor / correction status, altitude, airspeed, outside air temperature, etc.,) as a single serial data packet

• Transmit and receive data fields as appropriately scaled and signed integers, in binary representation of appropriate bit width, preferably in multiples of 8 bits (one byte)

• Encode / decode multibyte fields with uniform endianness. Endianness may be fixed or selectable by non-volatile configuration parameter.

• Utilize a Fletcher-16 checksum (or any alternative appropriate position-dependent binary checksum) to support the verification of encoded / decoded serial messages

• Respond to serial commands with appropriate acknowledgment / rejection (ACK / NACK) messages

• Provide a mechanism to unambiguously flag transmitted data that may be invalid (e.g., during execution of built-in-test or after detection of an out-of-range condition)

• Provide "factory-programmable" non-volatile storage (e.g., internal sensing element / signal conditioning calibration tables, operating software, and other data as required for operation and evaluation)

• Provide "user-programmable" non-volatile storage for correction coefficients (e.g., sensor installation effects, if known) and other operating parameters (e.g., reference barometric pressure, filtering / smoothing settings, correction types enabled, selection of output units, protocol / bus configurations, etc.)

• Implement a built-in-self-test function, including coverage of hardware, software, and status of non-volatile memory, where feasible

• Provide a method to reset configuration to factory settings

• Support operation in either polled (transmit on demand) or automatic (transmit at a fixed update rate) modes. Preferably, sampling should be time-aligned in either mode, when multiple sensors are daisy chained on the same bus

• Support a standard output data rate of at least 20 Hz. Preferably, higher / lower configurable output data rates may be configurable by the user

• Provide internal analog / digital filtering as required to ensure Nyquist sampling criteria are satisfied at the selected output data rate. Filter designs shall not exhibit overshoot

Note: Digital sensor concepts for measuring aerodynamic parameters typically derived from pitot-static probes using non-pressure based phenomenology (e.g., ultrasonic wave propagation) will be considered.

PHASE I: Define and develop a technical concept that can meet the SDPSS measurement performance and size, weight, and power constraints listed in the Description. Conduct modeling, simulation in order to provide an initial analytical assessment of concept performance. Develop conceptual options for both low-speed and high-speed configurations.

The Phase I Option, if exercised, includes further refinement and/or validation of one conceptual design configuration towards a manufacturable state. This effort may include risk reduction experimentation in a lab environment, in order to address residual feasibility concerns and validate novel aspects of the concept.

PHASE II: Finalize developments of the Phase I SDPSS design and fabricate prototypes for demonstration and validation. Deliver six calibrated prototypes to the government for wind-tunnel verification and/or low-risk flight evaluation (data logging only).

The Phase II Option, if exercised, would mature prototypes further for experimental integration as a primary air data sensor for an unmanned system. Enhancements may include redesign to tailor specifications for the application and/or to meet additional qualifications required for integration (shock, vibe, EMI, etc.).

PHASE III DUAL USE APPLICATIONS: Support the Navy/government to transition the SDPSS technology / units into various unmanned aviation programs within the DoD, with initial emphasis on small aircraft < 55 lbs. Once the technology is proven it may find use a primary or secondary (redundant) air data source on larger and even future manned aircraft.

In the commercial sector, SDPSS is likely to find a market both in civilian unmanned vehicle operations as well as in the general aviation / light sport / experimental aircraft industry due to the reliability and weight savings advantages of a "solid-state" digital air data system.

 

REFERENCES:

1. Pilot’s Handbook of Aeronautical Knowledge. FAA-H-8083-25C. Federal Aviation Administration
2. Merriam, Kenneth and Spaulding, Ellis. "COMPARATIVE TESTS OF PITOT-STATIC TUBES. NACA-TN-546." National Advisory Committee for Aeronautics, November 1935.

KEYWORDS: SENSORS; MEMS; ELECTRONICS; AIRSPEED; ALTITUDE; ALTIMETER; AIR; DATA; PRESSURE; DIGITAL; TRANSDUCER; PITOT; PITOT-STATIC; AIRSPEED INDICATOR; AUTOPILOT; UNMANNED; AIRCRAFT; SAFETY; NAVIGATION; UAS

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