Now is the time for all users of GPS to "pay their dues" by lobbying for the continued availability of GPS signals usable by inexpensive GPS receivers. Without your input to your Congressional representatives, two proposed changes to the GPS system may render your inexpensive GPS receiver unreliable or even inoperative. The most recent proposal is a threat to the spectrum allocation, but there is also a previous proposal to reduce the signal output power in the next generation of satellites. First, here are two news stories from Aviation Week & Space Technology, September 29, 1997, page 56: "GPS users should be concerned about a proposal by Inmarsat to reallocate part of the L-1 signal for a communication downlink for Mobile Satellite Service (MSS). Protection of the 1559- 1610 MHz band for Global Navigation Satellite Systems is threatened by the MSS request to the International Tellecomunications Union to use 1559-1567. MSS would then have a 3.6 MHz direct GPS overlap and low-level "skirts" across the GPS receiver bands. The USAF's Joint Program Office sees this as the first step in a MSS bid to consolidate L-band use for MSS use. Current civil GPS receiver designs will not meet operating specifications if MSS makes full use of the requested band, according to the JPO, which could force a costly GPS redesign for the reduced bandwidth. The decision will be before the ITU's World Radiofrequency Conference '97 in Geneva next month. Users should advise Congress of their concerns before the delegation's position is set." "The community of civil GPS users must develop the options desired in the next generation of GPS by next March. The urgency is driven by the December 1998 procurement of the second lot of GPS Block 2F satellites. If not in that buy, the next chance for substantial changes will be 2020. A workshop is being sponsored by the Institute of Navigation (ION), GPS Interagency Advisory Council and NOAA National Geodetic Survey on Nov. 6 to develop otions. Participants include government, U.S. industry, commercial and private users. For registration information/agenda: ION home page: www.ion.org or FAX (703)683-7105" The older article is: "Atmospheric/Multipath Concerns for D-GPS", by Bruce D. Norwall, Aviation Week & Space Technology, October 14 1996, p.60. My concern is about the new GPS-2R satellites that will start replacing the existing GPS-2 and GPS-2A satellites in the year 2000. Here are relevant quotations: "...two potential threats to all GPS users - lower output power from the next generation of satellites and interference from the next peak solar cycle." "...2A satellites ... were designed to initially exceed the required power output, so that degradations over a satellite's life would still produce the minimum power at the end. But, when the output of the 2A satellites did not fall off as expected, RCA designed the 2R to the level in the specification. The drop in the level of the transmitted signal could be significant." "The problem is that manufacturers have been producing GPS receivers to receive signals at the current strength, not at the lower level which is actually specified for the satellite. The question is whether the receivers - particularly the lower cost models - will work with a signal/noise ratio that industry experts estimate may be lowered by 3-6 dB." "But compared with the initial output of a 2/2A, the 2R power level may be down from 2 to 4 dB." "Ionospheric variations generally are not a problem at GPS frequencies and in mid-latitudes, but that may not be the case during the next peak of solar activity - solar max - which occurs on an 11-year cycle." In my mind, the lower power output of the 2R satellites will exacerbate the solar max interference, in addition to greatly limiting the usefulness of inexpensive GPS receivers. Everyone who uses and wants to continue to use inexpensive GPS receivers should write a letter to their Congressional representative similar to the following. You may be able to send a message to your representative over the Internet via the web site: http://www.house.gov/ Congressman [name of your Congressman] [address of your Congressman] Dear Congressman [name of your Congressman]; I am a private user of the Global Positioning System that is operated by the U.S. government, and I am concerned that proposed changes may make this system unreliable or even unusable for inexpensive GPS receivers such as used in automobiles and by hikers and hunters. First, I am concerned about a proposal by Inmarsat to reallocate part of the GPS receiver frequency band for use as a communication downlink. Use of these frequencies for communications would render existing and near-term inexpensive GPS receiver unreliable or unusable. Please use your influence on the U.S. delegation to the ITU's World Radiofrequency Conference '97 in Geneva next month so that they will work to defeat the Inmarsat proposal and protect GPS spectrum. Second, I am concerned with plans to manufacture new GPS spacecraft with lower power output signals than the current satellites. With current inexpensive receivers designed to use the current signal power, and with the coming solar maximum increasing interference levels, the lower power satellites will not be useable except to the more expensive receivers employed by the military and commercial users. Please urge the Department of Defense to require that all new GPS satellites maintain or increase the signal power output of current satellites. References: "Filter Center", edited by Bruce D. Nordwall, Aviation Week & Space Technology, September 29, 1997, p.56. "Atmospheric/Multipath Concerns for D-GPS", by Bruce D. Norwall, Aviation Week & Space Technology, October 14 1996, p.60. Thank you. Your Constituent, [your name and address]