Did your old etrex have WAAS enabled or was it capable of WAAS augmentation? Some of the earlier firmware on the earlier etrex's weren't WAAS capable.

It is actually very difficult to measure GPS accuracy, you would really need to connect the GPS to a PC via the serial or USB port and collect the output data i.e the GPS RTCM output messages then have some software measure statistically the Spherical Error Probability SEP (3D) or the Circular Error Probability CEP (2D) over a period of say 24 hrs. As the GPS satellite constellation is constantly moving or arcing across the sky. The geometry of the GPS constellation gives rise to something called Geometric Dilution of Precision GDOP. GDOP will have an effect on the accuracy of the GPS fix computation throughout the day. Good quality GPS receivers will have a CEP of around 20-25 metres without using differential means.

SEP and CEP is a function mainly of the accuracy of or lack of jitter in the Receivers clock Quartz TXCO and the sensitivity of the GPS antenna design (i.e. many survey antennas are designed to specifically reduce Multipath) and having the best low noise high gain front end amplifier for the L1 frequency (1575.42Mhz). Most of the worlds GPSs TXCO Quartz clocks are manufactured by a single company in New Zealand. Higher quality GPS receivers generally have a separate RF ASIC chip (front end RF PLL filter and RF low noise amplifier) for the front end rather than being integrated on to the same Digital Signal Processor Chip and microcontroller.

For the best improvement in accuracy, differential GPS needs to be employed. This can be achieved by turning on WAAS on the GPS or use something like an external 2 channel MBX-3 Automatic differential beacon receiver which picks up a radio broadcast signal from a differential GPS radio beacon. CEP with WAAS enabled should be around 3-5 metres with a good quality GPS receiver. CEPs of less than 2 metres are possible with a differential beacon reciever with the beacon being less than 100Km away. To improve PRN lock in a poor sky view environment i.e. foliage, being in a canyon or in a high rise area of a city, with a hand held commercial GPS then a secondary high gain, low multipath external antenna should be employed as different GPS chipsets i.e. SIRF or Trimble Lassen etc used internally will have very little noticeable effect unless an L1 L2 RTK GPS receiver is being employed (very expensive survey kit)

The SIRF chipset designs are excellent low cost, low power, efficient designs but the GPS chipset has very little to do with accuracy, speed of acquisition or signal strength as other factors inherent in the GPS system together with the other design choices in the overall GPS receiver design as a whole are much more influential in achieving accuracy and speed in a wide variety of environments. i.e. even the choice of battery in the GPS design will have a more of an impact in the overall GPS performance.



A full 12 channel GPS receiver, maybe Doug could have someone design the GR-10 GPS to fit into the Photon Freedom Micro light.