The "OpenGPS" receiver is a GFZ-developed research instrument focusing on the scientific exploitation of reflected GPS signals. Its hardware and software concept is based on the "OpenSourceGPS" project initiated by C. Kelley in 2001 [1].
The "OpenGPS" hardware utilizes the single frequency GPS module "Superstar II" (manufatured by CMC Electronics, now NovAtel) with GP2015/GP2021 chip set (Zarlink, formerly Mitel Semiconductor) whose processing and memory units were removed. A custom-designed interface logic allows the host CPU to directly access the registers of the 12 channel hardware correlator GP2021 via the PCI interface. The receiver board runs in a standard PC equipped with a real time operating system RTAI-Linux (www.rtai.org).
The "OpenGPS" receiver software is based on the GPL-licensed "OpenSourceGPS" software and has been extended according to specific requirements for GPS reflectometry. It features the following characteristics:
- The parameters of the carrier phase and code tracking loop may be adjusted during the measurement.
- Two or more detection channels can be aligned in phase and code space (channel cloning).
- The feed back parameters of the carrier phase as well as code loops can be detached from the tracking loops and replaced by corresponding feed back values extracted from model parametrizations (open-loop mode).
- Inphase- and quadphase correlation samples can be written to disk with sampling rates of up to 100 Hz.
- These correlation sums, as well as NCO values and other parameters may be displayed on the graphics screen.
During the last decade the "OpenGPS" receiver has been used successfully in a number of measurement campaigns [2, 3].
References
[1] Kelley, C.: Internet-based open source software for learning about GPS, GPS Solutions, 6, 201-205, doi:10.1007/s10291-002-0026-2, 2002.
[2] A. Helm: Ground-based GPS altimetry with the L1 OpenGPS receiver using carrier phase-delay observations of the reflected GPS signals, Scientific technical report STR08/10, Deutsches GeoForschungsZentrum, 2008.
[3] A. Helm et al., Detection of coherent reflections with GPS bipath interferometry, unpublished, http://arxiv.org/abs/physics/0407091
[4] G. Beyerle and F. Zus (2017): Open-loop GPS signal tracking at low elevation angles from a ground-based observation site. Atmos. Meas. Tech., 10, 15-34. doi:10.5194/amt-10-15-2017
[5] Beyerle, Georg (2016): Ground-based GPS observations at low elevation angles using
open-loop signal tracking. GFZ Data Services. doi:10.5880/GFZ.2016.1.1.002
