updated .ino code

// Current codeline for the elevation platforms (single MPL 3115A2 chip with a Feather board)

// xxx eta formula as a low pass filter ... average = (eta * float(reading) + ((1.0 - eta) * average))

// xxx change to a periodic polling of the sensor (fixed interval) and then when the ble call comes-in
//     return the current aggregated value

// xxx add battery level command

#include <Wire.h>
#include <SPI.h>

#include <Adafruit_Sensor.h>
#include <Adafruit_MPL3115A2.h>

#if not defined (_VARIANT_ARDUINO_DUE_X_) && not defined (_VARIANT_ARDUINO_ZERO_)
#include <SoftwareSerial.h>
#endif

#include "Adafruit_BLE.h"
#include "Adafruit_BluefruitLE_SPI.h"
// xxx remove #include "Adafruit_BluefruitLE_UART.h"
#include "BluefruitConfig.h"

Adafruit_MPL3115A2 baro = Adafruit_MPL3115A2();
Adafruit_BluefruitLE_SPI ble(BLUEFRUIT_SPI_CS, BLUEFRUIT_SPI_IRQ, BLUEFRUIT_SPI_RST);

const String PRES_HG, ELV_HR, TEMP_MPL;
static float baroCorrectionDistance, baroMeasuredAltitude;
int loopCount = 0; // currently not used anywhere
float eta = 0.8;
const float lowBattVolt = 3.0, highBattVolt = 4.3;
float filtered_pressure, filtered_temp, filtered_alt;
int check = 0;

// set initial to half-way between min and max (cf atlb)
float battVolt = lowBattVolt + (.5 * (highBattVolt - lowBattVolt));

void error(const __FlashStringHelper*err) {
  Serial.println(err);
  while (1);
}


int readBattery() {
  float rawBattVolt = 0.0;
  int _battPercent = 0;

  // this bit of magic code from https://learn.adafruit.com/pages/6437/elements/1753818/
  rawBattVolt = analogRead(A9); // the Feather's location for vBatt
  rawBattVolt *= 2;             // we divided by 2, so multiply back
  rawBattVolt *= 3.3;           // Multiply by 3.3V, our reference voltage
  rawBattVolt /= 1024;          // convert to voltage

  battVolt = (eta * float(rawBattVolt)) + ((1.0 - eta) * battVolt);

  _battPercent = map(battVolt, lowBattVolt, highBattVolt, 1, 100);

  return(_battPercent);
}


void setup() {
  Serial.begin(9600);

  if (!ble.begin(VERBOSE_MODE)){
    error(F("Couldn't find Bluefruit, make sure it's in CoMmanD mode & check wiring?"));
  }
  Serial.println(F("ok!"));

  while (!ble.isConnected()){
    if (check == 0) {
      //String* answer = whichDevice();
      //PRES_HG = answer[0];
      //ELV_HR = answer[1];
      //TEMP_MPL = answer[2];
      //ble.waitForOK();
      String name;
      ble.println("at+gapdevname");
      ble.readline();
      name = ble.buffer;
      name.toLowerCase();

      if (name == "maria"){
        PRES_HG = "77a";
        ELV_HR = "77b";
        TEMP_MPL = "77c";
      }

      else if (name == "wanda"){
        PRES_HG = "79a";
        ELV_HR = "79b";
        TEMP_MPL = "79c";
      }

      else if (name == "sadie"){
        PRES_HG = "80a";
        ELV_HR = "80b";
        TEMP_MPL = "80c";
      }

      else if (name == "gurdy"){
        PRES_HG = "81a";
        ELV_HR = "81b";
        TEMP_MPL = "81c";
      }

      else if (name == "buzz"){
        PRES_HG = "sa1";
        ELV_HR = "sa2";
        TEMP_MPL= "sa3";
      }

      else if (name == "ada"){
        PRES_HG = "82a";
        ELV_HR = "82b";
        TEMP_MPL= "82c";
      }

      else if (name == "laura"){
        PRES_HG = "83a";
        ELV_HR = "83b";
        TEMP_MPL= "83c";
      }

      // xxx add an else statement when we have an unrecognized device

      check = 1;
    }
    // xxx slow this down? it seems awfully frequent given the context
    delay(500);
  }

  Serial.println( F("Switching to DATA mode!") );
  ble.setMode(BLUEFRUIT_MODE_DATA);

  filtered_pressure = 1000;
  filtered_alt = 50;
  filtered_temp = 22;
}

String req = "";

String getSensors() {
  String MPL3115A2 = "MPL3115A2-Pressure," + PRES_HG + ",Hg;MPL3115A2-Altitude," + ELV_HR + ",m;MPL3115A2-Air Temperature,"
                     + TEMP_MPL + ",C;";
  return (MPL3115A2 + "done");
}

String getValues() {

  static char outstr[20];
  float sensor;
  String response;

  sensor = baro.getPressure();
  filtered_pressure = eta * float(sensor) + ((1.0 - eta) * filtered_pressure);
  response = "MPL3115A2-Pressure," + PRES_HG + "," + String(dtostrf(filtered_pressure, sizeof(filtered_pressure), 3, outstr)) + ";";

  sensor = baro.getAltitude();
  sensor = sensor - baroCorrectionDistance;
  filtered_alt = eta * float(sensor) + ((1.0 - eta) * filtered_alt);
  response += "MPL3115A2-Altitude," + ELV_HR + "," + String(dtostrf(filtered_alt, sizeof(filtered_alt), 3, outstr)) + ";";

  sensor = baro.getTemperature();
  filtered_temp = eta * float(sensor) + ((1.0 - eta) * filtered_temp);
  response += "MPL3115A2-Air Temperature," + TEMP_MPL + "," + String(dtostrf(filtered_temp, sizeof(filtered_temp), 3, outstr)) + ";";

  return (response + "done");
}

void loop() {
  String calSensorID;
  String calValue;
  int battPercent = 0;

  // check sensors
  if (!baro.begin()) {
    Serial.println("Could not find the MPL3115A2 sensor");
  }

  if (ble.isConnected()) {
    while (ble.available()) {
      // convert integer read to character
      char c = (char) ble.read();
      req += String(c);
    }

    Serial.println("Request:" + req);

    // Connected Field Day device asked for the sensors available on this platform.
    // xxx why doesn't this come from the database via an include file that is built
    // at compile time (or have FD and the platform build from a common place and then
    // just assume that it's so and not query the platform).

    // update - chau, eli and i are going to re-factor the interface between field day and the platform code,
    // association, data collection, dis-association
    if (req == "00") {
      String sensors = getSensors();
      String substr = "";
      for (int i = 0; i < sensors.length(); i++) {
        substr += sensors[i];
        if ((sensors[i] == ';') || (substr == "done")) {
          byte responseString[substr.length()];
          substr.getBytes(responseString, substr.length() + 1);
          ble.print(substr);
          Serial.println("00 Substring: " + substr);
          Serial.print("battVolt = "); Serial.println(battVolt);
          substr = "";
        }
      }

    // connected Field Day device asked for the current data values for the sensors on this platform.
    } else if (req == "vals") {
      String values = getValues();
      String substr = "";

      // xxx this code looks just like the stuff above
      for (int i = 0; i < values.length(); i++) {
        substr += values[i];
        if ((values[i] == ';') || (substr == "done")) {
          byte responseString[substr.length()];
          substr.getBytes(responseString, substr.length() + 1);
          ble.print(substr);
          Serial.println("vals Substring: " + substr);
          Serial.print("battVolt = "); Serial.println(battVolt);
          substr = "";
        }
      }

    } else if (req == "batt") {
      battPercent = readBattery();
      ble.print(battPercent);

    } else { // xxx what is req set to in this clause?
      int firstIndex = req.indexOf(";");
      String request = req.substring(0, firstIndex);

      if (request == "cal") {

        // figure-out which parameter to calibrate with the passed value
        int secondIndex = req.indexOf(";", firstIndex + 1);
        String sensorID = req.substring(firstIndex + 1, secondIndex);
        String calibValue = req.substring(secondIndex + 1);
        Serial.print("sensorID="); Serial.print(sensorID); Serial.print(", calibValue="); Serial.println(calibValue);

        // regardless of which sensor is chosen/passed, calibrate the altitude with the passed value
        baroMeasuredAltitude = baro.getAltitude();
        baroCorrectionDistance = baroMeasuredAltitude - calibValue.toFloat();
        Serial.print("baroMeasuredAltitude="); Serial.print(baroMeasuredAltitude);
        Serial.print(" baroCorrectionDistance="); Serial.println(baroCorrectionDistance);

        ble.print("calibrated");
      }
    }

    req = "";
    // xxx why sleep here? reduces energy consumption, makes typical wait time .5 * 5000 plus .5 * sample interval
    // determine a reasonable local collection interval (knob) and check for bt message (knob)
    delay(5000);
  }
}